Species and VarietiesTheir Origin by Mutation

Lectures delivered at the University of California

ByHugo DeVriesProfessor of Botany in the University of Amsterdam

Edited byDaniel Trembly MacDougalDirector Department of Botanical ResearchCarnegie Institution of Washington

Second EditionCorrected and Revised

CHICAGOThe Open Court Publishing CompanyLONDONKegan Paul, Trench, Trubner and Co. , Ltd. 1906

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COPYRIGHT 1904BYThe Open Court Pub. Co. CHICAGO

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THE ORIGIN OF SPECIES

The origin of species is a natural phenomenon. LAMARCK

The origin of species is an object of inquiry. DARWIN

The origin of species is an object of experimental investigation. DeVRIES. 

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PREFACE BY THE AUTHOR

THE purpose of these lectures is to point out the means and methods bywhich the origin of species and varieties may become an object forexperimental inquiry, in the interest of agricultural and horticulturalpractice as well as in that of general biologic science. Comparativestudies have contributed all the evidence hitherto adduced for thesupport of the Darwinian theory of descent and given us some generalideas about the main lines of the pedigree of the vegetable kingdom, butthe way in which one species originates from another has not beenadequately explained. The current belief assumes that species are slowlychanged into new types. In contradiction to this conception the theoryof mutation assumes that new species and varieties are produced fromexisting forms by sudden leaps. The parent-type itself remains unchangedthroughout this process, and may repeatedly give birth to new forms. These may arise simultaneously and in groups or separately at more orless widely distant periods. 

The principal features of the theory of mutation have been dealt with atlength in my book "Die Mutationstheorie" (Vol. I. , 1901, Vol. II. , 1903. Leipsic, Veit & Co. ), in which I have endeavored to present ascompletely as possible the detailed evidence obtained from trustworthyhistorical records, and from my own experimental researches, upon whichthe theory is based. 

The University of California invited me to deliver a series of lectureson this subject, at Berkeley, during the [vii] summer of 1904, and theselectures are offered in this form to a public now thoroughly interestedin the progress of modern ideas on evolution. Some of my experiments andpedigree-cultures are described here in a manner similar to that used inthe "Mutationstheorie, " but partly abridged and partly elaborated, inorder to give a clear conception of their extent and scope. Newexperiments and observations have been added, and a wider choice of thematerial afforded by the more recent current literature has been made inthe interest of a clear representation of the leading ideas, leaving theexact and detailed proofs thereof to the students of the larger book. 

Scientific demonstration is often long and encumbered with difficultpoints of minor importance. In these lectures I have tried to devoteattention to the more important phases of the subject and have avoidedthe details of lesser interest to the general reader. 

Considerable care has been bestowed upon the indication of the lacunaein our knowledge of the subject and the methods by which they may befilled. Many interesting observations bearing upon the little knownparts of the subject may be made with limited facilities, either in thegarden or upon the wild flora. Accuracy and perseverance, and a warmlove for Nature's children are here the chief requirements in suchinvestigations. 

In his admirable treatise on Evolution and Adaptation (New York, Macmillan & Co. , 1903), Thomas Hunt Morgan has dealt in a criticalmanner with many of the speculations upon problems subsidiary to thetheory of descent, in so convincing and complete a manner, that I thinkmyself justified in neglecting these questions here. His book gives anaccurate survey of them all, and is easily understood by the generalreader. 

In concluding I have to offer my thanks to Dr. D. T. MacDougal and MissA. M. Vail of the New York Botanical Garden for their painstaking work inthe preparation of the manuscript for the press. Dr. MacDougal, by[viii] his publications, has introduced my results to his Americancolleagues, and moreover by his cultures of the mutative species of thegreat evening-primrose has contributed additional proof of the validityof my views, which will go far to obviate the difficulties, which arestill in the way of a more universal acceptation of the theory ofmutation. My work claims to be in full accord with the principles laiddown by Darwin, and to give a thorough and sharp analysis of some of theideas of variability, inheritance, selection, and mutation, which werenecessarily vague at his time. It is only just to state, that Darwinestablished so broad a basis for scientific research upon thesesubjects, that after half a century many problems of major interestremain to be taken up. The work now demanding our attention ismanifestly that of the experimental observation and control of theorigin of species. The principal object of these lectures is to secure amore general appreciation of this kind of work. 

HUGO DE VRIES. Amsterdam, October, 1904. 

[ix]

PREFACE BY THE EDITOR

PROFESSOR DE VRIES has rendered an additional service to all naturalistsby the preparation of the lectures on mutation published in the presentvolume. A perusal of the lectures will show that the subject matter of"Die Mutationstheorie" has been presented in a somewhat condensed form, and that the time which has elapsed since the original was prepared hasgiven opportunity for the acquisition of additional facts, and are-examination of some of the more important conclusions with the resultthat a notable gain has been made in the treatment of some complicatedproblems. 

It is hoped that the appearance of this English version of the theory ofmutation will do much to stimulate investigation of the various phasesof the subject. This volume, however, is by no means intended toreplace, as a work of reference, the larger book with its detailedrecital of facts and its comprehensive records, but it may prove asubstitute for the use of the general reader. 

The revision of the lectures has been a task attended with no littlepleasure, especially since it has given the editor the opportunity foran advance consideration of some of the more recent results, thusmaterially facilitating investigations which have been in progress atthe New York Botanical Garden for some time. So far as the ground hasbeen covered the researches in question corroborate the conclusions ofde Vries in all important particulars. The preparation of the manuscriptfor the printer has consisted chiefly in the adaptation of oral [xii]discussions and demonstrations to a form suitable for permanent record, together with certain other alterations which have been duly submittedto the author. The original phraseology has been preserved as far aspossible. The editor wishes to acknowledge material assistance in thiswork from Miss A. M. Vail, Librarian of the New York Botanical Garden. 

D. T. MacDougal. New York Botanical Garden, October, 1904. 

PREFACE TO THE SECOND EDITION. 

THE constantly increasing interest in all phases of evolution has madenecessary the preparation of a second edition of this book within a fewmonths after the first appeared. The opportunity has been used toeliminate typographical errors, and to make alterations in the form of afew sentences for the sake of clearness and smoothness. The subjectmatter remains practically unchanged. An explanatory note has been addedon page 575 in order to avoid confusion as to the identity of some ofthe plants which figure prominently in the experimental investigationsin Amsterdam and New York. 

The portrait which forms the frontispiece is a reproduction of aphotograph taken by Professor F. E. Lloyd and Dr. W. A. Cannon during thevisit of Professor de Vries at the Desert Botanical Laboratory of theCarnegie Institution, at Tucson, Arizona, in June, 1904. 

D. T. MACDOUGAL. December 15, 1905. 

CONTENTS

A. INTRODUCTION. 

LECTURE PAGE

I. Descent: theories of evolution and methods of investigation. 1 The theory of descent and of natural selection. Evolution andadaptation. Elementary species and varieties. Methods of scientificpedigree-culture. 

B. ELEMENTARY SPECIES. 

II. Elementary species in nature. 32 _Viola tricolor_, _Draba verna_, _Primula acaulis_, and otherexamples. _Euphorbia pecacuanha_. _Prunus maritima_. _Taraxacum_ and_Hieracium_. 

III. Elementary species of cultivated plants. 63 Beets, apples, pears, clover, flax and coconut. 

IV. Selection of elementary species. 92 Cereals. Le Couteur. Running out of varieties. Rimpau andRisler, _Avena fatua_. Meadows. Old Egyptian cereals. Selection by theRomans. Shirreff. Hays. 

C. RETROGRADE VARIETIES. 

V. Characters of retrograde varieties. 121 Seed varieties of pure, not hybrid origin. Differences fromelementary species. Latent characters. Ray-florets of composites. [xiii] Progressive red varieties. Apparent losses. _Xanthiumcanadense_. Correlative variability. Laciniate leaves and petals. Compound characters. 

VI. Stability and real atavism. 154 Constancy of retrograde varieties. Atavism in _Ribes sanguineumAlbidum_, in conifers, in _Iris pallida_. Seedlings of _Acacia_. Reversion by buds. 

VII. Ordinary or false atavism. 185 Vicinism or variation under the influence of pollination byneighboring individuals. Vicinism in nurseries. Purifying new andold varieties. A case of running out of corn in Germany. 

VIII. Latent characters. 216 Leaves of seedlings, adventitious buds, systematic latency andretrogressive evolution. Degressive evolution. Latency of specificand varietal characters in wheat-ear carnation, in the green dahlias, in white campanulas and others. Systematic latency of flower colors. 

IX. Crossing of species and varieties. 247 Balanced and unbalanced, or species and variety crosses. Constant hybrids of _Oenothera muricata_ and _O. Biennis_. _Aegilops_, _Medicago_, brambles and other instances. 

X. Mendel's law of balanced crosses. 276 Pairs of antagonistic characters, one active and one latent. _Papaver somniferum_. [xiv] _Mephisto Danebrog_. Mendel's laws. Unit-characters. 

D. EVERSPORTING VARIETIES. 

XI. Striped flowers. 309 _Antirrhinum majus luteum rubro-striatum_ with pedigree. Stripedflowers, fruits and radishes. Double stocks. 

XII. "Five leaved" clover. 340 Origin of this variety. Periodicity of the anomaly. Pedigree-cultures. Ascidia. 

XIII. Polycephalic poppies. 369 Permanency and high variability. Sensitive period of theanomaly. Dependency on external conditions. 

XIV. Monstrosities. 400 Inheritance of monstrosities. Half races and middle races. Hereditary value of atavists. Twisted stems and fasciations. Middleraces of tricotyls and syncotyls. Selection by the hereditarypercentage among the offspring. 

XV. Double adaptations. 430 Analogy between double adaptations and anomalous middle races. _Polygonum amphibium_. Alpine plants. _Othonna crassifolia_. Leavesin sunshine and shadow. Giants and dwarfs. Figs and ivy. Leaves ofseedlings. 

E. MUTATIONS. 

XVI. Origin of the peloric toad-flax. 459 Sudden and frequent origin in the wild state. Origin in theexperiment-garden. Law of repeated mutations. Probable origin ofother pelories. 

[xv]XVII. The production of double flowers. 488 Sudden appearance of double flowers in horticulture. Historicalevidence. Experimental origin of _Chrysanthemum segetum plenum_. Dependency upon nourishment. Petalody of stamens. 

XVIII New species of _Oenothera_. 516 Mutations of _Oenothera lamarckiana_ in the wild state nearHilversum. New varieties of _O. Laevifolia_, _O. Brevistylis_, and_O. Nanella_. New elementary species, _O. Gigas_, _O. Rubrinervis_, _albida_, and _oblonga_. _O. Lata_, a pistillate form. Inconstancy of _O. Scintillans_. 

XIX. Experimental pedigree-cultures. 547 Pedigree of the mutative products of _Oenothera lamarckiana_ inthe Botanical Garden at Amsterdam. Laws of mutability. Sudden andrepeated leaps from an unchanging main strain. Constancy of the newforms. Mutations in all directions. 

XX. Origin of wild species and varieties. 576 Problems to solve. _Capsella heegeri_. _Oenothera biennis cruciata_. _Epilobium hirsutum cruciatum_. _Hibiscus Moscheutos_. Purple beech. Monophyllous strawberries. Chances of success with new mutations. 

XXI. Mutations in horticulture. 604 _Chelidonium majus lacinatum_. Dwarf and spineless varieties. Laciniate leaves. Monophyllous and broom-like varieties. [xvi] Purpleleaves. _Celosia_. Italian poplar. Cactus dahlia. Mutative origin of_Dahlia fistulosa_, and _Geranium praetense_ in the experiment-garden. 

XXII. Systematic atavism. 630Reappearance of ancestral characters. _Primula acaulis umbellata_. Bracts of crucifers. _Zea Mays cryptosperma_. Equisetum, _Dipsacussylvestris torsus_. Tomatoes. 

XXIII. Taxonomic anomalies. 658 Specific characters occurring in other cases as casualanomalies. _Papaver bracteatum monopetalum_. _Desmodium gyrans_ andmonophyllous varieties. Peltate leaves and ascidia. Flowers onleaves. Leaves. _Hordeum trifurcatum_. 

XXIV. Hypothesis of periodical mutations. 686 Discovering mutable strains. Periods of mutability and constancy. Periods of mutations. Genealogical trees. Limited life-time of theorganic kingdom. 

F. FLUCTUATIONS. 

XXV. General laws of fluctuations. 715 Fluctuating variability. Quetelet's law. Individual and partialfluctuations. Linear variability. Influence of nutrition. Periodicity curves. 

XXVI. Asexual multiplication of extremes. 742 Selection between species and intra-specific selection. Excluding individual [xvii] embryonic variability. Sugar-canes. Flowering cannas. Double lilacs. Other instances. Burbank's methodof selection. 

XXVII. Inconstancy of improved races 770 Larger variability in the case of propagation by seed, progression and regression after a single selection, and afterrepeated selections. Selection experiments with corn. Advantagesand effect of repeated selection. 

XXVIII. Artificial and natural selection. 798 Conclusions. Specific and intra-specific selection. Naturalselection in the field. Acclimatization. Improvement-selection ofsugar-beets by various methods. Rye. Hereditary percentage andcentgener power as marks by which intraspecific selection may beguided. 

Index 827

[1]A. INTRODUCTION

LECTURE I

DESCENT: THEORIES OF EVOLUTIONAND METHODS OF INVESTIGATION

Newton convinced his contemporaries that natural laws rule the wholeuniverse. Lyell showed, by his principle of slow and gradual evolution, that natural laws have reigned since the beginning of time. To Darwin weowe the almost universal acceptance of the theory of descent. 

This doctrine is one of the most noted landmarks in the advance ofscience. It teaches the validity of natural laws of life in its broadestsense, and crowns the philosophy founded by Newton and Lyell. 

Lamarck proposed the hypothesis of a common origin of all living beingsand this ingenious and thoroughly philosophical conception was warmlywelcomed by his partisans, but was not widely accepted owing to lack ofsupporting evidence. To Darwin was reserved the task of [2] bringing thetheory of common descent to its present high rank in scientific andsocial philosophy. 

Two main features in his work have contributed to this early andunexpected victory. One of them is the almost unlimited amount ofcomparative evidence, the other is his demonstration of the possibilityof a physiological explanation of the process of descent itself. 

The universal belief in the independent creation of living organisms wasrevised by Linnaeus and was put upon a new foundation. Before him thegenera were supposed to be created, the species and minor forms havingarisen from them through the agency of external conditions. In his firstbook Linnaeus adhered to this belief, but later changed his mind andmaintained the principle of the separate creation of species. The weightof his authority soon brought this conception to universal acceptance, and up to the present time the prevailing conception of a species hasbeen chiefly based on the definition given by Linnaeus. His speciescomprised subspecies and varieties, which were in their turn, supposedto have evolved from species by the common method. 

Darwin tried to show that the links which bind species to genera are ofthe same nature as those which determine the relationship of [3]subspecies and varieties. If an origin by natural laws is conceded forthe latter, it must on this ground be granted for the first also. Inthis discussion he simply returned to the pre-Linnean attitude. But hismaterial was such as to allow him to go one step further, and this stepwas an important and decisive one. He showed that the relation betweenthe various genera of a family does not exhibit any features of a natureother than that between the species of a genus. What has been concededfor the one must needs be accepted for the other. The same holds goodfor the large groups. 

The conviction of the common origin of closely allied forms necessarilyleads to the conception of a similar descent even in remoterelationships. 

The origin of subspecies and varieties as found in nature was notproved, but only generally recognized as evident. A broader knowledgehas brought about the same state of opinion for greater groups ofrelationships. Systematic affinities find their one possible explanationby the aid of this principle; without it, all similarity is onlyapparent and accidental. Geographic and paleontologic facts, broughttogether by Darwin and others on a previously unequalled scale, pointclearly in the same direction. The vast amount of evidence of all [4]comparative sciences compels us to accept the idea. To deny it, is togive up all opportunity of conceiving Nature in her true form. 

The general features of the theory of descent are now accepted as thebasis of all biological science. Half a century of discussion andinvestigation has cleared up the minor points and brought out anabundance of facts; but they have not changed the principle. Descentwith modification is now universally accepted as the chief law of naturein the organic world. In honor of him, who with unsurpassed genius, andby unlimited labor has made it the basis of modern thought, this law iscalled the "Darwinian theory of descent. "

Darwin's second contribution to this attainment was his proof of thepossibility of a physiological explanation of the process of descentitself. Of this possibility he fully convinced his contemporaries, butin indicating the particular means by which the change of species hasbeen brought about, he has not succeeded in securing universalacceptation. Quite on the contrary, objections have been raised from thevery outset, and with such force as to compel Darwin himself to changehis views in his later writings. This however, was of no avail, andobjections and criticisms have since steadily accumulated. Physiologicfacts concerning the origin of [5] species in nature were unknown in thetime of Darwin. It was a happy idea to choose the experience of thebreeders in the production of new varieties, as a basis on which tobuild an explanation of the processes of nature. In my opinion Darwinwas quite right, and he has succeeded in giving the desired proof. Butthe basis was a frail one, and would not stand too close an examination. Of this Darwin was always well aware. He has been prudent to the utmost, leaving many points undecided, and among them especially the range ofvalidity of his several arguments. Unfortunately this prudence has notbeen adopted by his followers. Without sufficient warrant they have laidstress on one phase of the problem, quite overlooking the others. Wallace has even gone so far in his zeal and ardent veneration forDarwin, as to describe as Darwinism some things, which in my opinion, had never been a part of Darwin's conceptions. 

The experience of the breeders was quite inadequate to the use whichDarwin made of it. It was neither scientific, nor critically accurate. Laws of variation were barely conjectured; the different types ofvariability were only imperfectly distinguished. The breeders'conception was fairly sufficient for practical purposes, but scienceneeded a clear understanding of the [6] factors in the general processof variation. Repeatedly Darwin tried to formulate these causes, but theevidence available did not meet his requirements. 

Quetelet's law of variation had not yet been published. Mendel's claimof hereditary units for the explanation of certain laws of hybridsdiscovered by him, was not yet made. The clear distinction betweenspontaneous and sudden changes, as compared with the ever-presentfluctuating variations, is only of late coming into recognition byagriculturists. Innumerable minor points which go to elucidate thebreeders' experience, and with which we are now quite familiar, wereunknown in Darwin's time. No wonder that he made mistakes, and laidstress on modes of descent, which have since been proved to be of minorimportance or even of doubtful validity. 

Notwithstanding all these apparently unsurmountable difficulties, Darwindiscovered the great principle which rules the evolution of organisms. It is the principle of natural selection. It is the sifting out of allorganisms of minor worth through the struggle for life. It is only asieve, and not a force of nature, not a direct cause of improvement, asmany of Darwin's adversaries, and unfortunately many of his followersalso, have so often asserted. 

It is [7] only a sieve, which decides what is to live, and what is todie. But evolutionary lines are of great length, and the evolution of aflower, or of an insectivorous plant is a way with many sidepaths. It isthe sieve that keeps evolution on the main line, killing all, or nearlyall that try to go in other directions. By this means natural selectionis the one directing cause of the broad lines of evolution. 

Of course, with the single steps of evolution it has nothing to do. Onlyafter the step has been taken, the sieve acts, eliminating the unfit. The problem, as to the manner in which the individual steps are broughtabout, is quite another side of the question. 

On this point Darwin has recognized two possibilities. One means ofchange lies in the sudden and spontaneous production of new forms fromthe old stock. The other method is the gradual accumulation of thosealways present and ever fluctuating variations which are indicated bythe common assertion that no two individuals of a given race are exactlyalike. The first changes are what we now call "mutations, " the secondare designated as "individual variations, " or as this term is often usedin another sense, as "fluctuations. " Darwin recognized both lines ofevolution; Wallace disregarded the sudden changes and proposedfluctuations [8] as the exclusive factor. Of late, however, this pointof view has been abandoned by many investigators, especially in America. 

The actual occurrence of mutations is recognized, and the battle ragesabout the question, as to whether they are be regarded as the principalmeans of evolution, or whether slow and gradual changes have not alsoplayed a large and important part. 

The defenders of the theory of evolution by slow accumulation of slightfluctuations are divided into two camps. One group is called theNeo-Lamarckians; they assume a direct modifying agency of theenvironment, producing a corresponding and useful change in theorganization. The other group call themselves Darwinians orselectionists, but to my mind with no other right beyond the arbitraryrestriction of the Darwinian principles by Wallace. They assumefluctuating variations in all directions and leave the choice betweenthem to the sieve of natural selection. 

Of course we are far from a decision between these views, on the soleground of the facts as known at present. Mutations under observation areas yet very rare; enough to indicate the possible and most probableways, but no more. On the other hand the accumulation of fluctuationsdoes not transgress relatively narrow [9] limits as far as the presentmethods of selection go. But the question remains to be solved, whetherour methods are truly the right ones, and whether by the use of newprinciples, new results might not cause the balance of opinion to favorthe opposite side. 

Of late, a thorough and detailed discussion of the opposing views hasbeen given by Morgan in his valuable book on evolution and adaptation. He has subjected all the proposed theories to a severe criticism both onthe ground of facts and on that of their innate possibility and logicalvalue. He decides in favor of the mutation theory. His arguments areincisive and complete and wholly adapted to the comprehension of allintelligent readers, so that his book relieves me entirely of thenecessity of discussing these general questions, as it could not be donein a better or in a clearer way. 

I intend to give a review of the facts obtained from plants which go toprove the assertion, that species and varieties have originated bymutation, and are, at present, not known to originate in any other way. This review consists of two parts. One is a critical survey of the factsof agricultural and horticultural breeding, as they have accumulatedsince the time of Darwin. This body of evidence is to be combined withsome corresponding experiments [10] concerning the real nature ofspecies in the wild state. The other part rests on my own observationsand experiments, made in the botanical garden of the University ofAmsterdam. 

For many years past I have tried to elucidate the hereditary conditionsof species and varieties, and the occasional occurrence of mutations, that suddenly produce new forms. 

The present discussion has a double purpose. On one side it will givethe justification of the theory of mutations, as derived from the factsnow at hand. On the other hand it will point out the deficiencies ofavailable evidence, and indicate the ways by which the lacunae maygradually be filled. Experimental work on heredity does not require vastinstallments or costly laboratory equipment. It demands chieflyassiduity and exactitude. Any one who has these two qualities, and whohas a small garden at his disposal is requested to take part in thisline of investigation. 

In order to observe directly the birth of new forms it is necessary, inthe first place, to be fully clear concerning the question as to whatforms are to be expected to arise from others, and before proceeding toa demonstration of the origin of species, it is pertinent to raise thequestion as to what constitutes a species. 

Species is a word, which always has had a [11] double meaning. One isthe systematic species, which is the unit of our system. But these unitsare by no means indivisible. Long ago Linnaeus knew them to be compoundin a great number of instances, and increasing knowledge has shown thatthe same rule prevails in other instances. Today the vast majority ofthe old systematic species are known to consist of minor units. Theseminor entities are called varieties in systematic works. However, thereare many objections to this usage. First, the term variety is applied inhorticulture and agriculture to things so widely divergent as to conveyno clear idea at all. Secondly, the subdivisions of species are by nomeans all of the same nature, and the systematic varieties include unitsthe real value of which is widely different in different cases. Some ofthese varieties are in reality as good as species, and have been"elevated, " as it is called by some writers, to this rank. Thisconception of the elementary species would be quite justifiable, andwould at once get rid of all difficulties, were it not for one practicalobstacle. The number of the species in all genera would be doubled andtripled, and as these numbers are already cumbersome in many cases, thedistinction of the native species of any given country would lose mostof its charm and interest. 

[12] In order to meet this difficulty we must recognize two sorts ofspecies. The systematic species are the practical units of thesystematists and florists, and all friends of wild nature should dotheir utmost to preserve them as Linnaeus has proposed them. These unitshowever, are not really existing entities; they have as little claim tobe regarded as such as genera and families. The real units are theelementary species; their limits often apparently overlap and can onlyin rare cases be determined on the sole ground of field observations. Pedigree-culture is the method required and any form which remainsconstant and distinct from its allies in the garden is to be consideredas an elementary species. 

In the following lectures we shall consider this point at length, toshow the compound nature of systematic species in wild and in cultivatedplants. In both cases, the principle is becoming of great importance, and many papers published recently indicate its almost universalacceptation. 

Among the systematic subdivisions of species, not all have the sameclaim to the title of elementary species. In the first place the casesin which the differences may occur between parts of the same individualare to be excluded. Dividing an alpine plant into two halves and [13]planting one in a garden, varietal differences at once arise and areoften designated in systematic works under different varietal names. Secondly all individual differences which are of a fluctuating natureare to be combined into a group. But with these we shall deal later. 

Apart from these minor points the subdivisions of the systematic speciesexhibit two widely different features. I will now try to make this clearin a few words, but will return in another lecture to a fullerdiscussion of this most interesting contrast. 

Linnaeus himself knew that in some cases all subdivisions of a speciesare of equal rank, together constituting the group called species. Noone of them outranks the others; it is not a species with varieties, buta group, consisting only of varieties. A closer inquiry into the casestreated in this manner by the great master of systematic science, showsthat here his varieties were exactly what we now call elementaryspecies. 

In other cases the varieties are of a derivative nature. The speciesconstitutes a type that is pure in a race which ordinarily is stillgrowing somewhere, though in some cases it may have died out. From thistype the varieties are derived, and the way of this derivation isusually quite manifest to the botanist. It is ordinarily [14] by thedisappearance of some superficial character that a variety isdistinguished from its species, as by the lack of color in the flowers, of hairs on stems and foliage, of the spines and thorns, &c. Suchvarieties are, strictly speaking, not to be treated in the same way aselementary species, though they often are. We shall designate them bythe term of "retrograde varieties, " which clearly indicates the natureof their relationship to the species from which they are assumed to havesprung. In order to lay more stress on the contrast between elementaryspecies and retrograde varieties, it should be stated at once, that thefirst are considered to have originated from their parent-form in aprogressive way. They have succeeded in attaining something quite newfor themselves, while retrograde varieties have only thrown off somepeculiarity, previously acquired by their ancestors. 

The whole vegetable kingdom exhibits a constant struggle betweenprogression and retrogression. Of course, the great lines of the generalpedigree are due to progression, many single steps in this directionleading together to the great superiority of the flowering plants overtheir cryptogamous ancestors. But progression is nearly alwaysaccompanied by retrogression in the principal lines of evolution, [15]as well as in the collateral branches of the genealogical tree. Sometimes it prevails, and the monocotyledons are obviously a reducedbranch of the primitive dicotyledons. In orchids and aroids, in grassesand sedges, reduction plays a most important part, leaving its traces onthe flowers as well as on the embryo of the seed. Many instances couldbe given to prove that progression and retrogression are the two mainprinciples of evolution at large. Hence the conclusion, that ouranalysis must dissect the complicated phenomena of evolution so far asto show the separate functions of these two contrasting principles. Hundreds of steps were needed to evolve the family of the orchids, butthe experimenter must take the single steps for the object of hisinquiry. He finds that some are progressive and others retrogressive andso his investigation falls under two heads, the origin of progressivecharacters, and the subsequent loss of the same. Progressive steps arethe marks of elementary species, while retrograde varieties aredistinguished by apparent losses. They have equal claim to our interestand our study. 

As already stated I propose to deal first with the elementary speciesand afterwards with the retrograde varieties. I shall try to depict themto you in the first place as they are seen in [16] nature and inculture, leaving the question of their origin to a subsequentexperimental treatment. 

The question of the experimental origin of new species and varieties hasto be taken up from two widely separated starting points. This may beinferred from what we have already seen concerning the two opposingtheories, derived and isolated from Darwin's original broad conception. One of them considers mutations as the origin of new forms, while theother assumes fluctuations to be the source of all evolution. 

As mentioned above, my own experience has led me to accept the firstview. Therefore I shall have to show that mutations do yield new andconstant forms, while fluctuations are not adequate to do so. Retrogradevarieties and elementary species may both be seen to be produced bysudden mutations. Varieties have often been observed to appear at onceand quite unexpectedly in horticulture and agriculture, and a survey ofthese historical facts will be the subject of one of my lectures. Insome instances I have succeeded in repeating these observations in mygarden under the strict conditions of a scientific experiment, and theseinstances teach us the real nature of the process of mutation in all itsvisible features. New elementary [17] species are far more rare, but Ihave discovered in the great evening-primrose, or _Oenotheralamarckiana_ a strain which is producing them yearly in the wild stateas well as in my garden. These observations and pedigree-experimentswill be dealt with at due length in subsequent lectures. 

Having proved the existence and importance of mutations, it remains toinquire how far the improvements may go which are due only tofluctuating variability. As the term indicates, this variability isfluctuating to and fro, oscillating around an average type. It neverfails nor does it, under ordinary circumstances, depart far from thefixed average. 

But the deviation may be enlarged by a choice of extremes. In sowingtheir seed, the average of the strain is seen to be changed, and inrepeating the experiment the change may be considerable. It is notclear, whether theoretically by such an accumulation, deviations mightbe reached which could not be attained at once in a single sowing. Thisquestion is hardly susceptible of an experimental answer, as it wouldrequire such an enormous amount of seed from a few mother plants as canscarcely ever be produced. 

The whole character of the fluctuations shows them to be of an oppositenature, contrasting [18] manifestly with specific and varietalcharacters. By this method they may be proved to be inadequate ever tomake a single step along the great lines of evolution, in regard toprogressive as well as to retrograde development. 

First of all fluctuations are linear, amplifying or lessening theexisting qualities, but not really changing their nature. They are notobserved to produce anything quite new, and evolution of course, is notrestricted to the increase of the already existing peculiarities, butdepends chiefly on the continuous addition of new characters to thestock. Fluctuations always oscillate around an average, and if removedfrom this for some time, they show a tendency to return to it. Thistendency, called retrogression, has never been observed to fail, as itshould, in order to free the new strain from the links with the average, while new species and new varieties are seen to be quite free from theirancestors and not linked to them by intermediates. 

The last few lectures will be devoted to questions concerning the greatproblem of the analogy between natural and artificial selection. Asalready stated, Darwin made this analogy the foundation stone of histheory of descent, and he met with the severest objections andcriticisms precisely on this point. But I hope to [19] show that he wasquite right, and that the cause of the divergence of opinions is duesimply to the very incomplete state of knowledge concerning bothprocesses. If both are critically analyzed they may be seen to comprisethe same factors, and further discussion may be limited to theappreciation of the part which each of them has played in nature andamong cultivated plants. 

Both natural and artificial selection are partly specific, and partlyintra-specific or individual. Nature of course, and intelligent menfirst chose the best elementary species from among the swarms. Incultivation this is the process of variety-testing. In nature it is thesurvival of the fittest species, or, as Morgan designates it, thesurvival of species in the struggle for existence. The species are notchanged by this struggle, they are only weighed against each other, theweak being thrown aside. 

Within the chosen elementary species there is also a struggle. It isobvious, that the fluctuating variability adapts some to the givencircumstances, while it lessens the chances of others. A choice results, and this choice is what is often exclusively called selection, eithernatural or artificial. In cultivation it produces the improved and thelocal races; in nature little is known about improvement in this way, but [19] local adaptations with slight changes of the average characterin separate localities, seem to be of quite normal occurrence. 

A new method of individual selection has been used in recent years inAmerica, especially by W. M. Hays. It consists in judging the hereditaryworth of a plant by the average condition of its offspring, instead ofby its own visible characters. If this determination of the "centgenerpower, " as Hays calls it, should prove to be the true principle ofselection, then indeed the analogy between natural and artificialselection would lose a large part of its importance. We will reservethis question for the last lecture, as it pertains more to the future, than to our present stock of knowledge. 

Something should be said here concerning hybrids and hybridism. Thisproblem has of late reached such large proportions that it cannot bedealt with adequately in a short survey of the phenomena of heredity ingeneral. It requires a separate treatment. For this reason I shall limitmyself to a single phase of the problem, which seems to be indispensablefor a true and at the same time easy distinction between elementaryspecies and retrograde varieties. According to accepted terminology, some crosses are to be considered as unsymmetrical, while others aresymmetrical. The first are one-sided, [21] some peculiarity being foundin one of the parents and lacking in the other. The second are balanced, as all the characters are present in both parents, but are found in adifferent condition. Active in one of them, they are concealed orinactive in the other. Hence pairs of contrasting units result, while inunbalanced crosses no pairing of the particular character underconsideration is possible. This leads to the principal differencebetween species and varieties, and to an experimental method of decidingbetween them in difficult and doubtful cases. 

Having thus indicated the general outlines of the subjects I shall dealwith, something now may be said as to methods of investigation. 

There are two points in which scientific investigation differs fromordinary pedigree-culture in practice. First the isolation of theindividuals and the study of individual inheritance, instead ofaverages. Next comes the task of keeping records. Every individual mustbe entered, its ancestry must be known as completely as possible, andall its relations must be noted in such a form, that the most completereference is always possible. Mutations may come unexpectedly, and whenonce arisen, their parents and grand-parents should be known. Recordsmust be available which will allow of a most complete knowledge of thewhole ancestral [22] line. This, and approximately this only, is theessential difference between experimental and accidental observation. 

Mutations are occurring from time to time in the wild state as well asin horticulture and agriculture. A selection of the most interestinginstances will be given later. But in all such cases the experimentalproof is wanting. The observations as a rule, only began when themutation had made its appearance. A more or less vague remembrance aboutthe previous state of the plants in question might be available, thougheven this is generally absent. But on doubtful points, concerningpossible crosses or possible introduction of foreign strains, mererecollection is insufficient. The fact of the mutation may be veryprobable, but the full proof is, of course, wanting. Such is the casewith the mutative origin of _Xanthium commune_ Wootoni from New Mexicoand of _Oenothera biennis cruciata_ from Holland. The same doubt existsas to the origin of the _Capsella heegeri_ of Solms-Laubach, and of theoldest recorded mutation, that of _Chelidonium laciniatum_ in Heidelbergabout 1600. 

First, we have doubts about the fact itself. These, however, graduallylose their importance in the increasing accumulation of evidence. Secondly, the impossibility of a closer [23] inquiry into the realnature of the change. For experimental purposes a single mutation doesnot suffice; it must be studied repeatedly, and be produced more or lessarbitrarily, according to the nature of the problems to be solved. Andin order to do this, it is evidently not enough to have in hand themutated individual, but it is indispensable to have also the mutableparents, or the mutable strain from which it sprang. 

All conditions previous to the mutation are to be considered as of farhigher importance than all those subsequent to it. 

Now mutations come unexpectedly, and if the ancestry of an accidentalmutation is to be known, it is of course necessary to keep accounts ofall the strains cultivated. It is evident that the required knowledgeconcerning the ancestry of a supposed mutation, must necessarily nearlyall be acquired from the plants in the experimental garden. 

Obviously this rule is as simple in theory, as it is difficult to carryout in practice. First of all comes the book-keeping. The parents, grandparents and previous ancestors must be known individually. Accountsof them must be kept under two headings. A full description of theirindividual character and peculiarities must always be available on theone hand, and on the other, all facts concerning their hereditary [24]qualities. These are to be deduced from the composition of the progeny, and in order to obtain complete evidence on this point, two successivegenerations are often required. The investigation must ascertain theaverage condition of this offspring and the occurrence of any deviatingspecimens, and for both purposes it is necessary to cultivate them inrelatively large numbers. It is obvious that, properly speaking, thewhole family of a mutated individual, including all its nearer and moreremote relatives, should be known and recorded. 

Hence pedigree-book-keeping must become the general rule. Subordinate tothis are two further points, which should likewise be stated here. Onepertains to the pure or hybrid nature of the original strain, and theother to the life-conditions and all other external influences. It ismanifest that a complete understanding of a mutation depends upon fullinformation upon these points. 

All experiments must have a beginning. The starting-point may be asingle individual, or a small group of plants, or a lot of seeds. Inmany cases the whole previous history is obscure, but sometimes a littlehistorical evidence is at hand. Often it is evident that the initialmaterial belongs to a pure species, but with respect to the question ofelementary species it is [25] not rarely open to doubt. Large numbers ofhybrid plants and hybrid races are in existence, concerning the originof which it is impossible to decide. It is impossible in many instancesto ascertain whether they are of hybrid or of pure origin. Often thereis only one way of determining the matter; it is to guess at theprobable parents in case of a cross and to repeat the cross. This is apoint which always requires great care in the interpretation of unusualfacts. 

Three cases are to be distinguished as to heredity. Many plants are soconstituted as to be fertilized with their own pollen. In this case thevisits of insects have simply to be excluded, which may be done bycovering plants with iron gauze or with bags of prepared paper. Sometimes they fertilize themselves without any aid, as for instance, the common evening-primrose; in other cases the pollen has to be placedon the stigma artificially, as with Lamarck's evening-primrose and itsderivatives. Other plants need cross-fertilization in order to produce anormal yield of seeds. Here two individuals have always to be combined, and the pedigree becomes a more complicated one. Such is the case withthe toad-flax, which is nearly sterile with its own pollen. But even inthese cases the visits of insects bringing pollen [26] from otherplants, must be carefully excluded. A special lecture will be devoted tothis very interesting source of impurity and of uncertainty in ordinarycultures. 

Of course, crosses may lie in the proposed line of work, and this is thethird point to be alluded to. They must be surrounded with the samecareful isolation and protection against bees, as any otherfertilizations. And not only the seed-parent, but also the pollen mustbe kept pure from all possible foreign admixtures. 

A pure and accurately recorded ancestry is thus to be considered as themost important condition of success in experimental plant breeding. Nextto this comes the gathering of the seeds of each individual separately. Fifty or sixty, and often more, bags of seeds are by no means uncommonfor a single experiment, and in ordinary years the harvest of my gardenis preserved in over a thousand separate lots. 

Complying with these conditions, the origin of species may be seen aseasily as any other phenomenon. It is only necessary to have a plant ina mutable condition. Not all species are in such a state at present, andtherefore I have begun by ascertaining which were stable and which werenot. These attempts, of course, had to be made in the experimentalgarden, and large quantities of seed had to be procured and [27] sown. Cultivated plants of course, had only a small chance to exhibit newqualities, as they have been so strictly controlled during so manyyears. Moreover their purity of origin is in many cases doubtful. Amongwild plants only those could be expected to reward the investigatorwhich were of easy cultivation. For this reason I have limited myself tothe trial of wild plants of Holland, and have had the good fortune tofind among them at least one species in a state of mutability. It wasnot really a native plant, but one that had been introduced from Americaand belongs to an American genus. I refer to the great evening-primroseor the evening-primrose of Lamarck. A strain of this beautiful speciesis growing in an abandoned field in the vicinity of Hilversum, at ashort distance from Amsterdam. Here it has escaped from a park andmultiplied. In doing so it has produced and is still producing quite anumber of new types, some of which may be considered as retrogradevarieties, while others evidently are of the nature of progressiveelementary species. 

This interesting plant has afforded me the means of observing directlyhow new species originate, and of studying the laws of these changes. Myresearches have followed a double line of inquiry. On one side, I havelimited [28] myself to direct field observations, and to tests of seed, collected from the wild plants in their native locality. Obviously themutations are decided within the seed, and the culture of young plantsfrom them had no other aim than that of ascertaining what had occurredin the field. And then the many chances of destruction that threatenyoung plants in a wild state, could be avoided in the garden, whereenvironmental factors can be controlled. 

My second line of inquiry was an experimental repetition of thephenomena which were only partly discerned at the native locality. Itwas not my aim to intrude into the process, nor to try to bring out newfeatures. My only object was to submit to the precepts just givenconcerning pure treatment, individual seed gathering, exclusion ofcrosses and accurate recording of all the facts. The result has been apedigree which now permits of stating the relation between all thedescendants of my original introduced plant. This pedigree at onceexhibits the laws followed by the mutating species. The main fact is, that it does not change itself gradually, but remains unaffected duringall succeeding generations. It only throws off new forms, which aresharply contrasted with the parent, and which are from the verybeginning as perfect and as constant, as narrowly [29] defined and aspure of type as might be expected of any species. 

These new species are not produced once or in single individuals, butyearly and in large numbers. The whole phenomenon conveys the idea of aclose group of mutations, all belonging to one single condition ofmutability. Of course this mutable state must have had a beginning, asit must sometime come to an end. It is to be considered as a periodwithin the life-time of the species and probably it is only a small partof it. 

The detailed description of this experiment, however, I must delay to asubsequent lecture, but I may be allowed to state, that the discovery ofthis period of mutability is of a definite theoretical importance. Oneof the greatest objections to the Darwinian theory of descent arose fromthe length of time it would require, if all evolution was to beexplained on the theory of slow and nearly invisible changes. Thisdifficulty is at once met and fully surmounted by the hypothesis ofperiodical but sudden and quite noticeable steps. This assumptionrequires only a limited number of mutative periods, which might welloccur within the time allowed by physicists and geologists for theexistence of animal and vegetable life on the earth. 

[30] Summing up the main points of these introductory remarks, I proposeto deal with the subjects mentioned above at some length, devoting toeach of them, if possible at least an entire lecture. The decisive factsand discussions upon which the conclusions are based will be given inevery case. Likewise I hope to point out the weak places and the lacunaein our present knowledge, and to show the way in which each of you maytry to contribute his part towards the advancement of science in thissubject. Lastly I shall try to prove that sudden mutation is the normalway in which nature produces new species and new varieties. Thesemutations are more readily accessible to observation and experiment thanthe slow and gradual changes surmised by Wallace and his followers, which are entirely beyond our present and future experience. 

The theory of mutations is a starting-point for direct investigation, while the general belief in slow changes has held back science from suchinvestigations during half a century. 

Coming now to the subdivisions and headings under which my material isto be presented, I propose describing first the real nature of theelementary species and retrograde varieties, both in normal form and inhybridizations. A discussion of other types of varieties, including [31]monstrosities will complete the general plan. The second subdivisionwill deal with the origin of species and varieties as taught byexperiment and observation, treating separately the sudden variationswhich to my mind do produce new forms, and subsequently the fluctuationswhich I hold to be not adequate to this purpose. 

[32]B. ELEMENTARY SPECIES

LECTURE II

ELEMENTARY SPECIES IN NATURE

What are species? Species are considered as the true units of nature bythe vast majority of biologists. They have gained this high rank in ourestimation principally through the influence of Linnaeus. They havesupplanted the genera which were the accepted units before Linnaeus. They are now to be replaced in their turn, by smaller types, for reasonswhich do not rest upon comparative studies but upon direct experimentalevidence. 

Biological studies and practical interests alike make new demands uponsystematic botany. Species are not only the subject-material of herbariaand collections, but they are living entities, and their life-historyand life-conditions command a gradually increasing interest. One phaseof the question is to determine the easiest manner to deal with thecollected forms of a country, and another feature is the problem [33] asto what groups are real units and will remain constant and unchangedthrough all the years of our observations. 

Before Linnaeus, the genera were the real units of the system. DeCandolle pointed out that the old common names of plants, such as rosesand clover, poplars and oaks, nearly all refer to genera. The type ofthe clovers is rich in color, and the shape of the flower-heads and thesingle flowers escape ordinary observation; but notwithstanding this, clovers are easily recognized, even if new types come to hand. White andred clovers and many other species are distinguished simply byadjectives, the generic name remaining the same for all. 

Tournefort, who lived in the second half of the 17th century(1656-1708), is generally considered as the author of genera insystematic botany. He adopted, what was at that time the generalconception and applied it throughout the vegetable kingdom. He groupedthe new and the rare and the previously overlooked forms in the samemanner in which the more conspicuous plants were already arranged byuniversal consent. Species were distinguished by minor marks and oftenindicated by short descriptions, but they were considered of secondaryimportance. 

Based on the idea of a direct creation of all [34] living beings, thegenera were then accepted as the created forms. They were thereforeregarded as the real existing types, and it was generally surmised thatspecies and varieties owed their origin to subsequent changes under theinfluence of external conditions. Even Linnaeus agreed with this view inhis first treatises and in his "Philosophical Botany" he still kept tothe idea that all genera had been created at once with the beginning oflife. 

Afterwards Linnaeus changed his opinion on this important point, andadopted species as the units of the system. He declared them to be thecreated forms, and by this decree, at once reduced the genera to therank of artificial groups. Linnaeus was well aware that this conceptionwas wholly arbitrary, and that even the species are not real indivisibleentities. But he simply forbade the study of lesser subdivisions. At histime he was quite justified in doing so, because the first task of thesystematic botanists was the clearing up of the chaos of forms and thebringing of them into connection with their real allies. 

Linnaeus himself designated the subdivisions of the species asvarieties, but in doing so he followed two clearly distinct principles. In some cases his species were real plants, and the varieties seemed tobe derived from them by [35] some simple changes. They were subordinatedto the parent-species. In other cases his species were groups of lesserforms of equal value, and it was not possible to discern which was theprimary and which were the derivatives. 

These two methods of subdivision seem in the main, and notwithstandingtheir relatively imperfect application in many single examples, tocorrespond with two really distinct cases. The derivative varieties aredistinguished from the parent-species by some single, but striking mark, and often this attribute manifests itself as the loss of some apparentquality. The loss of spines and of hairs and the loss of blue and redflower-colors are the most notorious, but in rarer cases many singlepeculiarities may disappear, thereby constituting a variety. Thisrelation of varieties to the parent-species is gradually increasing inimportance in the estimation of botanists, sharply contrasting withthose cases, in which such dependency is not to be met with. 

If among the subdivisions of a species, no single one can be pointed outas playing a primary part, and the others can not be traced back to it, the relation between these lesser units is of course of anothercharacter. They are to be considered of equal importance. They aredistinguished from each other by more than [36] one character, often byslight differences in nearly all their organs and qualities. Such formshave come to be designated as "elementary species. " They are onlyvarieties in a broad and vague systematic significance of the word, notin the sense accorded to this term in horticultural usage, nor in asharper and more scientific conception. 

Genera and species are, at the present time, for a large partartificial, or stated more correctly, conventional groups. Everysystematist is free to delimit them in a wider or in a narrower sense, according to his judgment. The greater authorities have as a rulepreferred larger genera, others of late have elevated innumerablesubgenera to the rank of genera. This would work no real harm, ifunfortunately, the names of the plants had not to be changed each time, according to current ideas concerning genera. Quite the same inconstancyis observed with species. In the Handbook of the British Flora, Benthamand Hooker describe the forms of brambles under 5 species, whileBabington in his Manual of British Botany makes 45 species out of thesame material. So also in other cases. For instance, the willows whichhave 13 species in one and 31 species in the other of these manuals, andthe hawkweeds for which the figures are 7 and 32 [37] respectively. Other authors have made still greater numbers of species in the samegroups. 

It is very difficult to estimate systematic differences on the ground ofcomparative studies alone. All sorts of variability occur, and noindividual or small group of specimens can really be considered as areliable representative of the supposed type. Many original diagnoses ofnew species have been founded on divergent specimens and of course, thetype can afterwards neither be derived from this individual, nor fromthe diagnosis given. 

This chaotic state of things has brought some botanists to theconviction that even in systematic studies only direct experimentalevidence can be relied upon. This conception has induced them to testthe constancy of species and varieties, and to admit as real units onlysuch groups of individuals as prove to be uniform and constantthroughout succeeding generations. The late Alexis Jordan, of Lyons inFrance, made extensive cultures in this direction. In doing so, hediscovered that systematic species, as a rule, comprise some lesserforms, which often cannot easily be distinguished when grown indifferent regions, or by comparing dried material. This fact was, ofcourse, most distasteful to the systematists of his time and even for along period afterwards [38] they attempted to discredit it. Milde andmany others have opposed these new ideas with some temporary success. Only of late has the school of Jordan received due recognition, afterThuret, de Bary, Rosen and others tested its practices and openlypronounced for them. Of late Wittrock of Sweden has joined them, makingextensive experimental studies concerning the real units of some of thelarger species of his country. 

From the evidence given by these eminent authorities, we may concludethat systematic species, as they are accepted nowadays, are as a rulecompound groups. Sometimes they consist of two or three, or a fewelementary types, but in other cases they comprise twenty, or fifty, oreven hundreds of constant and well differentiated forms. 

The inner constitution of these groups is however, not at all the samein all cases. This will be seen by the description of some of the moreinteresting of them. The European heartsease, from which ourgarden-pansies have been chiefly derived, will serve as an example. Thegarden-pansies are a hybrid race, won by crossing the _Viola tricolor_with the large flowered and bright yellow _V. Lutea_. They combine, aseveryone knows, in their wide range of [39] varieties, the attributes ofthe latter with the peculiarities of the former species. 

Besides the _lutea_, there are some other species, nearly allied totricolor, as for instance, _cornuta_, _calcarata_, and _altaica_, whichare combined with it under the head of _Melanium_ as a subgenus, andwhich together constitute a systematic unity of undoubted value, butranging between the common conceptions of genus and species. These formsare so nearly allied to the heartsease that they have of late been madeuse of in crosses, in order to widen the range of variability ofgarden-pansies. 

_Viola tricolor_ is a common European weed. It is widely dispersed andvery abundant, growing in many localities in large numbers. It is anannual and ripens its seeds freely, and if opportunity is afforded, itmultiplies rapidly. 

_Viola tricolor_ has three subspecies, which have been elevated to therank of species by some authors, and which may here be called, forbrevity's sake, by their binary names. One is the typical _V. Tricolor_, with broad flowers, variously colored and veined with yellow, purple andwhite. It occurs in waste places on sandy soil. The second is called _V. Arvensis_ or the field-pansy; it has small inconspicuous flowers, withpale-yellowish petals which are shorter than the sepals. It pollinatesitself without the [40] aid of insects, and is widely dispersed incultivated fields. The third form, _V. Alpestris_, grows in the Alps, but is of lesser importance for our present discussion. 

Anywhere throughout the central part of Europe _V. Tricolor_ and _V. Arvensis_ may be seen, each occupying its own locality. They may beconsidered as ranging among the most common native plants of theparticular regions they inhabit. They vary in the color of the flowers, branching of the stems, in the foliage and other parts, but not to suchan extent as to constitute distinct strains. They have been brought intocultivation by Jordan, Wittrock and others, but throughout Europe eachof them constitutes a single type. 

These types must be very old and constant, fluctuating always within thesame distinct and narrow limits. No slow, gradual changes can have takenplace. In different countries their various habitats are as old as thehistorical records, and probably many centuries older. They are quiteindependent of one another, the distance being in numerous cases far toogreat for the exchange of pollen or of seeds. If slow and gradualchanges were the rule, the types could not have remained so uniformthroughout the whole range of these two species. They would necessarilyhave split up into thousands [41] and thousands of minor races, whichwould show their peculiar characteristics if tested by cultures inadjacent beds. This however, is not what happens. As a matter of fact_V. Tricolor_ and _V. Arvensis_ are widely distributed but whollyconstant types. 

Besides these, there occur distinct types in numerous localities. Someof them evidently have had time and opportunity to spread more or lesswidely and now occupy larger regions or even whole countries. Others arenarrowly limited, being restricted to a single locality. Wittrockcollected seeds or plants from as many localities as possible indifferent parts of Sweden and neighboring states and sowed them in hisgarden near Stockholm. He secured seeds from his plants, and grew fromthem a second, and in many cases a third generation in order to estimatethe amount of variability. As a rule the forms introduced into hisgarden proved constant, notwithstanding the new and abnormal conditionsunder which they were propagated. 

First of all we may mention three perennial forms called by him _Violatricolor ammotropha_, _V. Tricolor coniophila_ and _V. Stenochila_. Thetypical _V. Tricolor_ is an annual plant; sowing itself in summer andgerminating soon afterwards. The young plants thrive throughout [42] thelatter part of the summer and during the fall, reaching an advancedstage of development of the branched stems before winter. Early in thespring the flowers begin to open, but after the ripening of the seedsthe whole plant dies. 

The three perennial species just mentioned develop in the same manner inthe first year. During their flowering period, however, and afterwards, they produce new shoots from the lower parts of the stem. They preferdry and sandy soils, often becoming covered with the sand that is blownon them by the winds. They are prepared for such seemingly adversecircumstances by the accumulation of food in the older stems and by thecapacity of the new shoots to thrive on this food till they have becomelong enough to reach the light. _V. Tricolor ammotropha_ is native nearYstad in Sweden, and the other two forms on Gotland. All three havenarrowly limited habitats. 

The typical tricolored heartsease has remained annual in all its othersubspecies. It may be divided into two types in the first place, _V. Tricolor genuina_ and _V. Tricolor versicolor_. Both of them have a widedistribution and seem to be the prototypes from which the rarer formsmust have been derived. Among these latter Wittrock describes sevenlocal types, which [43] proved to be constant in his pedigree-cultures. Some of them have produced other forms, related to them in the way ofvarieties. They all have nearly the same general habit and do notexhibit any marked differences in their growth, in the structure andbranching of the stems, or in the character of their foliage. Differentiating points are to be found mainly in the colors and patternsof the flowers. The veins, which radiate from the centre of the corollaare branched in some and undivided in others; in one elementary speciesthey are wholly lacking. The purple color may be absent, leaving theflowers of a pale or a deep yellow. Or the purple may be reddish orbluish. Of the petals all five may have the purple hue on their tips, orthis attribute may be limited to the two upper ones. Contrasting withthis wide variability is the stability of the yellow spot in the centre, which is always present and becomes inconspicuous only, when the wholepetals are of the same hue. It is a general conception that colors andcolor-markings are liable to great variability and do not constitutereliable standards. But the cultures of Wittrock have proved thecontrary, at least in the case of the violets. No pattern, howeverquaint, appears changeable, if one elementary species only isconsidered. Hundreds of plants from seeds [44] from one locality may begrown, and all will exhibit exactly the same markings. Most of theseforms are of very local occurrence. The most beautiful of all, the_ornatissima_, is found only in Jemtland, the _aurobadia_ only inSodermanland, the anopetala_ in other localities in the same country, the _roseola_ near Stockholm, and the yellow _lutescens_ in Finmarken. 

The researches of Wittrock included only a small number of elementaryspecies, but every one who has observed the violets in the central partsof Europe must be convinced that many dozens of constant forms of thetypical _Viola tricolor_ might easily be found and isolated. 

We now come to the field pansy, the _Viola arvensis_, a very common weedin the grain-fields of central Europe. I have already mentioned itssmall corolla, surpassed by the lobes of the calyx and its capacity ofself-fertilization. It has still other curious differentiatingcharacters; the pollen grains, which are square in _V. Tricolor_, arefive-sided in _V. Arvensis_. Some transgressive fluctuating variabilitymay occur in both cases through the admixture of pollen-grains. Eventhree-angled pollen grains are seen sometimes. Other marks are observedin the form of the anthers and the spur. 

There seem to be very many local subspecies [45] of the field-pansy. Jordan has described some from the vicinity of Lyons, and Wittrockothers from the northern parts of Europe. They diverge from their commonprototype in nearly all attributes, the flowers not showing theessential differentiating characters as in the _V. Tricolor_. Some havetheir flower-stalks erect, and in others the flowers are held nearly atright angles to the stem. _V. Pallescens_ is a small, almost unbranchedspecies with small pale flowers. _V. Segetalis_ is a stouter specieswith two dark blue spots on the tips of the upper petals. _V. Agrestis_is a tall and branched, hairy form. _V. Nemausensis_ attains a height ofonly 10 cm. , has rounded leaves and long flower-stalks. Even the seedsafford characters which may be made use of in isolating the variousspecies. 

The above-mentioned elementary forms belong to the flora of southernFrance, and Wittrock has isolated and cultivated a number of others fromthe fields of Sweden. A species from Stockholm is called _Viola patens_;_V. Arvensis curtisepala_ occurs in Gotland, and _V. Arvensis striolata_is a distinct form, which has appeared in his cultures without its trueorigin being ascertained. 

The alpine violets comprise a more widespread type with some localelementary species [46] derived exactly in the same way as thetricolored field pansies. 

Summarizing the general result of this description we see that theoriginal species _Viola tricolor_ may be split up into larger and lessergroups of separate forms. These last prove to be constant inpedigree-cultures, and therefore are to be considered as really existentunits. They are very numerous, comprising many dozens in each of the twolarger subdivisions. 

All systematic grouping of these forms, and their combination intosubspecies and species rests on the comparative study of theircharacters. The result of such studies must necessarily depend onprinciples which underlie them. According to the choice of theseprinciples, the construction of the groups will be found to bedifferent. Wittrock trusts in the first place to morphologic characters, and considers the development as passing from the more simple to themore complex types. On the other hand the geographic distribution may beconsidered as an indication of the direction of evolution, thewide-spread forms being regarded as the common parents of the minorlocal species. 

However, such considerations are only of secondary importance. It mustbe borne in mind that an ordinary systematic species may include [47]many dozens of elementary forms, each of which remains constant andunchanged in successive generations, even if cultivated in the samegarden and under similar external conditions. 

Leaving the violets, we may take the vernal whitlow-grass or _Drabaverna_ for a second illustration. This little annual cruciferous plantis common in the fields of many parts of the United States, thoughoriginally introduced from Europe. It has small basal rosettes whichdevelop during summer and winter, and produce numerous leaflessflowering stems early in the spring. It is a native of central Europeand western Asia, and may be considered as one of the most commonplants, occurring anywhere in immense numbers on sandy soils. Jordan wasthe first to point out that it is not the same throughout its entirerange. Although a hasty survey does not reveal differences, they showthemselves on closer inspection. De Bary, Thuret, Rosen and many othersconfirmed this result, and repeated the pedigree-cultures of Jordan. Every type is constant and remains unchanged in successive generations. The anthers open in the flower-buds and pollinate the stigmas before theexpansion of the flowers, thus assuring self-fertilization. Moreover, these inconspicuous little flowers are only sparingly visited byinsects. Dozens of subspecies [48] may be cultivated in the same gardenwithout any real danger of their intercrossing. They remain as pure asunder perfect isolation. 

It is very interesting to observe the aspect of such types, when growingnear each other. Hundreds of rosettes exhibit one type, and areundoubtedly similar. The alternative group is distinguishable at firstsight, though the differentiating marks are often so slight as to betraceable with difficulty. Two elementary species occur in Holland, onewith narrow leaves in the western provinces and one with broader foliagein the northern parts. I have cultivated them side by side, and was asmuch struck with the uniformity within each group, as with the contrastbetween the two sets. 

Nearly all organs show differences. The most marked are those of theleaves, which may be small or large, linear or elliptic or oblong andeven rhomboidal in shape, more or less hairy with simple or withstellate branched hairs, and finally of a pure green or of a glaucouscolor. The petals are as a rule obcordate, but this type may be combinedwith others having more or less broad emarginations at the summit, andwith differences in breadth which vary from almost linear types toothers which touch along their margins. The pods are short and broad, orlong and narrow, or varying in sundry other [49] ways. All in all thereare constant differences which are so great that it has been possible todistinguish and to describe large numbers of types. 

Many of them have been tested as to their constancy from seed. Jordanmade numerous cultures, some of which lasted ten or twelve years; Thurethas verified the assertion concerning their constancy by culturesextending over seven years in some instances; Villars and de Bary madenumerous trials of shorter duration. All agree as to the main points. The local races are uniform and come true from seed; the variability ofthe species is not of a fluctuating, but of a polymorphous nature. Agiven elementary species keeps within its limits and cannot vary beyondthem, but the whole group gives the impression of variability by itswide range of distinct, but nearly allied forms. 

The geographic distribution of these elementary species of thewhitlow-grass is quite distinct from that of the violets. Herepredominant species are limited to restricted localities. Most of themoccupy one or more departments of France, and in Holland two of them arespread over several provinces. An important number are native in thecentre of Europe, and from the vicinity of Lyons, Jordan succeeded inestablishing about fifty elementary [50] species in his garden. In thisregion they are crowded together and not rarely two or even more quitedistinct forms are observed to grow side by side on the same spot. Farther away from this center they are more widely dispersed, eachholding its own in its habitat. In all, Jordan has distinguished abouttwo hundred species of _Draba verna_ from Europe and western Asia. Subsequent authors have added new types to the already existing numberfrom time to time. 

The constancy of these elementary species is directly proven by theexperiments quoted above, and moreover it may be deduced from theuniformity of each type within its own domain. These are so large thatmost of the localities are practically isolated from one another, andmust have been so for centuries. If the types were slowly changing suchlocalities would often, though of course not always, exhibit slighterdifferences, and on the geographic limits of neighboring speciesintermediates would be found. Such however, are not on record. Hence theelementary species must be regarded as old and constant types. 

The question naturally arises how these groups of nearly allied formsmay originally have been produced. Granting a common origin for all ofthem, the changes may have been [51] simultaneous or successive. According to the geographic distribution, the place of common originmust probably be sought in the southern part of central Europe, perhapseven in the vicinity of Lyons. Here we may assume that the old _Drabaverna_ has produced a host or a swarm of new types. Thence they musthave spread over Europe, but whether in doing so they have remainedconstant, or whether some or many of them have repeatedly undergonespecific mutations, is of course unknown. 

The main fact is, that such a small species as _Draba verna_ is not atall a uniform type, but comprises over two hundred well distinguishedand constant forms. 

It is readily granted that violets and whitlowgrasses are extremeinstances of systematic variability. Such great numbers of elementaryspecies are not often included in single species of the system. But thenumbers are of secondary importance, and the fact that systematicspecies consist, as a rule, of more than one independent and constantsubspecies, retains its almost universal validity. 

In some cases the systematic species are manifest groups, sharplydifferentiated from one another. In other instances the groups ofelementary forms as they are shown by direct observation, have beenadjudged by many authors [52] to be too large to constitute species. Hence the polymorphous genera, concerning the systematic subdivisions ofwhich hardly two authors agree. Brambles and roses are widely knowninstances, but oaks, elms, apples, and pears, _Mentha_, _Prunu_s, _Vitis_, _Lactuca_, _Cucumis_, _Cucurbita_ and numerous others are inthe same condition. 

In some instances the existence of elementary species is so obvious, that they have been described by taxonomists as systematic varieties oreven as good species. The primroses afford a widely known example. Linnaeus called them _Primula veris_, and recognized three types aspertaining to this species, but Jacquin and others have elevated thesesubspecies to the full rank of species. They now bear the names of_Primula elatior_ with larger, _P. Officinalis_ with smaller flowers, and _P. Acaulis_. In the last named the common flower-stalk is lackingand the flowers of the umbel seem to be borne in the arils of the basalleaves. 

In other genera such nearly allied species are more or less universallyrecognized. _Galium Mollugo_ has been divided into _G. Elatum_ with along and weak stem, and _G. Erectum_ with shorter and erect stems;_Cochlearia danica_, _anglica_ and _officinalis_ are so nearly allied asto be hardly distinguishable. _Sagina apetala_ and _patula_, [53]_Spergula media_ and _salina_ and many other pairs of allied specieshave differentiating characters of the same value as those of theelementary species of _Draba verna_. _Filago_, _Plantago_, _Carex_, _Ficaria_ and a long series of other genera afford proofs of the sameclose relation between smaller and larger groups of species. TheEuropean frost-weeds or _Helianthemum_ include a group of species whichare so closely allied, that ordinary botanical descriptions are notadequate to give any idea of their differentiating features. It isalmost impossible to determine them by means of the common analyticalkeys. They have to be gathered from their various native localities andcultivated side by side in the garden to bring out their differences. Among the species of France, according to Jordan, _Helianthemumpolifolium_, _H. Apenninum_, _H. Pilosum_ and _H. Pulverulentum_ are ofthis character. 

A species of cinquefoil, _Potentilla Tormentilla_, which isdistinguished by its quaternate flowers, occurs in Holland in twodistinct types, which have proved constant in my cultural experiments. One of them has, broad petals, meeting together at the edges, andconstituting rounded saucer without breaks. The other has narrow petals, which are strikingly separated from one another and show the sepalsbetween them. [54] In the same manner bluebells vary in the size andshape of the corolla, which may be wide or narrow, bell-shaped orconical, with the tips turned downwards, sidewards or backwards. 

As a rule all of the more striking elementary types have been describedby local botanists under distinct specific names, while they are throwntogether into the larger systematic species by other authors, who studythe distribution of plants over larger portions of the world. Everythingdepends on the point of view taken. Large floras require large species. But the study of local floras yields the best results if the many formsof the region are distinguished and described as completely as possible. And the easiest way is to give to each of them a specific name. If twoor more elementary species are united in the same district, they areoften treated in this way, but if each region had its own type of somegiven species, commonly the part is taken for the whole, and the sundryforms are described under the same name, without further distinctions. 

Of course these questions are all of a practical and conventionalnature, but involve the different methods in which different authorsdeal with the same general fact. The fact is that systematic species arecompound groups, exactly like the genera and that their real units [55]can only be recognized by comparative experimental studies. 

Though the evidence already given might be esteemed to be sufficient forour purpose, I should like to introduce a few more examples; two of thempertain to American plants. 

The Ipecac spurge or _Euphorbia Ipecacuanha_ occurs from Connecticut toFlorida, mainly near the coast, preferring dry and sandy soil. It isoften found by the roadsides. According to Britton and Brown's"Illustrated Flora" it is glabrous or pubescent, with several or manystems, ascending or nearly erect; with green or red leaves, which arewonderfully variable in outline, from linear to orbicular, mostlyopposite, the upper sometimes whorled, the lower often alternate. Theglands of the involucres are elliptic or oblong, and even the seeds varyin shape. 

Such a wide range of variability evidently points to the existence ofsome minor types. Dr. John Harshberger has made a study of those whichoccur in the vicinity of Whitings in New Jersey. His types agree withthe description given above. Others were gathered by him at Brown'sMills in the pinelands, New Jersey, where they grew in almost pure sandin the bright sunlight. He observed still other differentiatingcharacters. The amount of seed [56] produced and the time of floweringwere variable to a remarkable degree. 

Dr. Harshberger had the kindness to send me some dried specimens of themost interesting of these types. They show that the peculiarities areindividual, and that each specimen has its own characters. It is veryprobable that a comparative experimental study will prove the existenceof a large number of elementary species, differing in many points; theywill probably also show differences in the amount of the active chemicalsubstances, especially of emetine, which is usually recorded as presentin about 1%, but which will undoubtedly be found in larger quantities insome, and in smaller quantities in other elementary species. In this waythe close and careful distinction of the really existing units mightperhaps prove of practical importance. 

MacFarlane has studied the beach-plum or _Prunus maritima_, which isabundant along the coast regions of the Eastern States from Virginia toNew Brunswick. It often covers areas from two to two hundred acres inextent, sometimes to the exclusion of other plants. It is most prolificon soft drifting sand near the sea or along the shore, where it may attimes be washed with ocean-spray. The fruit usually become ripe aboutthe middle of August, and show extreme [57] variations in size, shape, color, taste, consistency and maturation period, indicating theexistence of separate races or elementary species, with widely differingqualities. The earlier varieties begin to ripen from August 10 to 20, and a continuous supply can be had till September 10, while a few goodvarieties continue to ripen till September 20. But even late in Octobersome other types are still found maturing their fruits. 

Exact studies were made of fruit and stone variations, and theircharacteristics as to color, weight, size, shape and consistency werefully described. Similar variations have been observed, as is wellknown, in the cultivated plums. Fine blue-black fruits were seen on someshrubs and purplish or yellow fruits on others. Some exhibit a firmertexture and others a more watery pulp. Even the stones show differenceswhich are suggestive of distinct races. 

Recently Mr. Luther Burbank of Santa Rosa, California, has made use ofthe beach-plum to produce useful new varieties. He observed that it is avery hardy species, and never fails to bear, growing under the mosttrying conditions of dry and sandy, or of rocky and even of heavy soil. The fruits of the wild shrubs are utterly worthless for anything butpreserving. [58] But by means of crossing with other species andespecially with the Japanese plums, the hardy qualities of thebeach-plum have been united with the size, flavor and other valuablequalities of the fruit, and a group of new plums have been produced withbright colors, ovoid and globular forms which are never flattened andhave no suture. The experiments were not finished, when I visited Mr. Burbank in July, 1904, and still more startling improvements were saidto have been secured. 

I may perhaps be allowed to avail myself of this opportunity to pointout a practical side of the study of elementary species. This alwaysappears whenever wild plants are subjected to cultivation, either inorder to reproduce them as pure strains, or to cross them with otheralready cultivated species. The latter practice is as a rule made use ofwhenever a wild species is found to be in possession of some qualitywhich is considered as desirable for the cultivated forms. In the caseof the beach-plum it is the hardiness and the great abundance of fruitsof the wild species which might profitably be combined with therecognized qualities of the ordinary plums. Now it is manifest, that inorder to make crosses, distinct individual plants are to be chosen, andthat the variability of the wild species may be of very greatimportance. [59] Among the range of elementary species those should beused which not only possess the desired advantages in the highestdegree, but which promise the best results in other respects or theirearliest attainment. The fuller our knowledge of the elementary speciesconstituting the systematic groups, the easier and the more reliablewill be the choice for the breeder. Many Californian wild flowers withbright colors seem to consist of large numbers of constant elementaryforms, as for instance, the lilies, godetias, eschscholtias and others. They have been brought into cultivation many times, but the minutestdistinction of their elementary forms is required to attain the highestsuccess. 

In concluding, I will point out a very interesting difficulty, which insome cases impedes the clear understanding of elementary species. It isthe lack of self-fertilization. It occurs in widely distant families, but has a special interest for us in two genera, which are generallyknown as very polymorphous groups. 

One of them is the hawkweed or _Hieracium_, and the other is thedandelion or _Taraxacum officinale_. Hawkweeds are known as a genus inwhich the delimitation of the species is almost impossible, Thousands offorms may be cultivated side by side in botanical gardens, exhibiting[60] slight but undoubted differentiating features, and reproducethemselves truly by seed. Descriptions were formerly difficult and socomplicated that the ablest writers on this genus, Fries and Nageli aresaid not to have been able to recognize the separate species by thedescriptions given by each other. Are these types to be considered aselementary species, or only as individual differences? The decision ofcourse, would depend upon their behavior in cultures. Such tests havebeen made by various experimenters. In the dandelion the bracts of theinvolucre give the best characters. The inner ones may be linear orlinear-lanceolate, with or without appendages below the tip; the outerones may be similar and only shorter, or noticeably larger, erect, spreading or even reflexed, and the color of the involucre may be a puregreen or glaucous; the leaves may be nearly entire or pinnatifid, orsinuate-dentate, or very deeply runcinate-pinnatifid, or even pinnatelydivided, the whole plant being more or less glabrous. 

Raunkiaer, who has studied experimentally a dozen types from Denmark, found them constant, but observed that some of them have no pollen atall, while in others the pollen, though present, is impotent. It doesnot germinate on the stigma, cannot produce the ordinary tube, [61] andhence has no fertilizing power. But the young ovaries do not need suchfertilization. They are sufficient unto themselves. One may cut off allthe flowers of a head before the opening of the anthers, and leave theovaries untouched, and the head will ripen its seeds quite as well. Thesame thing occurs in the hawkweeds. Here, therefore, we have nofertilization and the extensive widening of the variability, whichgenerally accompanies this process is, of course, wanting. Only partialor vegetative variability is present. Unfertilized eggs when developinginto embryos are equivalent to buds, separated from the parent-plant andplanted for themselves. They repeat both the specific and the individualcharacters of the parent. In the case of the hawkweed and the dandelionthere is at present no means of distinguishing between these twocontrasting causes of variability. But like the garden varieties whichare always propagated in the vegetative way, their constancy anduniformity are only apparent and afford no real indication of hereditaryqualities. 

In addition to these and other exceptional cases, seed-cultures arehenceforth to be considered as the sole means of recognizing the reallyexisting systematic units of nature. All other groups, includingsystematic species and [62] genera, are equally artificial orconventional. In other words we may state "that current misconceptionsas to the extreme range of fluctuating variability of many nativespecies have generally arisen from a failure to recognize the compositenature of the forms in question, " as has been demonstrated by MacDougalin the case of the common evening-primrose, _Oenothera biennis_. "It isevident that to study the behavior of the characters of plants we musthave them in their simplest combinations; to investigate the origin andmovements of species we must deal with them singly and uncomplicated. "

[63]

LECTURE III

ELEMENTARY SPECIES OF CULTIVATED PLANTS

Recalling the results of the last lecture, we see that the species ofthe systematists are not in reality units, though in the ordinary courseof floristic studies they may, as a rule, seem to be so. In some casesrepresentatives of the same species from different countries or regions, when compared with one another do not exactly agree. Many species offerns afford instances of this rule, and Lindley and other greatsystematists have frequently been puzzled by the wide range ofdifferences between the individuals of a single species. 

In other cases the differing forms are observed to grow near each other, sometimes in neighboring provinces, sometimes in the same locality, growing and flowering in mixtures of two or three or even moreelementary types. The violets exhibit widespread ancient types, fromwhich the local species may be taken to have arisen. The commonancestors of the Whitlow-grasses are probably not to be found [64] amongexisting forms, but numerous types are crowded together in the southernpart of central Europe and more thinly scattered elsewhere, even as faras western Asia. There can be little doubt that their common origin isto be sought in the center of their geographic distribution. 

Numerous other cases exhibit smaller numbers of elementary units withina systematic species; in fact purely uniform species seem to berelatively rare. But with small numbers there are of course noindications to be expected concerning their common origin or thestarting point of their distribution. 

It is manifest that these experiences with wild species must find aparallel among cultivated plants. Of course cultivated plants wereoriginally wild and must have come under the general law. Hence we mayconclude that when first observed and taken up by man, they must alreadyhave consisted of sundry elementary subspecies. And we may confidentlyassert that some must have been rich and others poor in such types. 

Granting this state of things as the only probable one, we can easilyimagine what must have been the consequences. If a wild species had beentaken into cultivation only once, the cultivated form would have been asingle elementary [65] type. But it is not very likely that suchpartiality would occur often. The conception that different tribes atdifferent times and in distant countries would have used the wild plantsof their native regions seems far more natural than that all should haveobtained plants for cultivation from the same source or locality. Ifthis theory may be relied upon, the origin of many of the more widelycultivated agricultural plants must have been multiple, and the numberof the original elementary species of the cultivated types must havebeen so much the larger, the more widely distributed and variable theplants under consideration were before the first period of cultivation. 

Further it would seem only natural to explain the wide variability ofmany of our larger agricultural and horticultural stocks by such anincipient multiformity of the species themselves. Through commercialintercourse the various types might have become mixed so as to make itquite impossible to point out the native localities for each of them. 

Unfortunately historical evidence on this point is almost whollylacking. The differences in question could not have been appreciated atthat remote period, and interest the common observer but little eventoday. The history of most of the cultivated plants is very obscure, [66] and even the most skillful historians, by sifting the evidenceafforded by the older writers, and that obtained by comparativelinguistic investigations have been able to do little more than framethe most general outline of the cultural history of the most common andmost widely used plants. 

Some authors assume that cultivation itself might have been theprincipal cause of variability, but it is not proved, nor even probable, that cultivated plants are intrinsically more variable than their wildprototypes. Appearances in this case are very deceptive. Of coursewidely distributed plants are as a rule richer in subspecies than formswith limited distribution, and the former must have had a better chanceto be taken into cultivation than the latter. In many cases, especiallywith the more recent cultivated species, man has deliberately chosenvariable forms, because of their greater promise. Thirdly, widevariability is the most efficient means of acclimatization, and onlyspecies with many elementary units would have offered the adequatematerial for introduction into new countries. 

From this discussion it would seem that it is more reasonable to assertthat variability is one of the causes of the success of cultivation, than to assume that cultivation is a cause of variability [67] at large. And this assumption would be equally sufficient to explain the existingconditions among cultivated plants. 

Of course I do not pretend to say that cultivated plants should beexpected to be less variable than in the wild state, or that swarms ofelementary species might not be produced during cultivation quite aswell as before. However the chance of such an event, as is easily seen, cannot be very great, and we shall have to be content with a fewexamples of which the coconut is a notable one. 

Leaving this general discussion of the subject, we may take up theexample of the beets. The sugar-beet is only one type from among a hordeof others, and though the origin of all the single types is nothistorically known, the plant is frequently found in the wild state evenat the present time, and the native types may be compared with thecorresponding cultivated varieties. 

The cultivation of beets for sugar is not of very ancient date. TheRomans knew the beets and used them as vegetables, both the roots andthe leaves. They distinguished a variety with white and one with redflesh, but whether they cultivated them, or only collected them fromwhere they grew spontaneously, appears to be unknown. 

[68] Beets are even now found in large quantities along the shores ofItaly. They prefer the vicinity of the sea, as do so many other membersof the beet family, and are not limited to Italy, but are found growingelsewhere on the littoral of the Mediterranean, in the Canary Islandsand through Persia and Babylonia to India. In most of their nativelocalities they occur in great abundance. 

The color of the foliage and the size of the roots are extremelyvariable. Some have red leafstalks and veins, others a uniform red orgreen foliage, some have red or white or yellow roots, or exhibitalternating rings of a red and of a white tinge on cut surfaces. Itseems only natural to consider the white and the red, and even thevariegated types as distinct varieties, which in nature do nottransgress their limits nor change into one another. In a subsequentlecture I will show that this at least is the rule with thecorresponding color-varieties in other genera. 

The fleshiness or pulpiness of the roots is still more variable. Someare as thick as the arm and edible, others are not thicker than a fingerand of a woody composition, and the structure of this woody variety isvery interesting. The sugar-beet consists, as is generally known, ofconcentric layers of sugar-tissue and of vascular [69] strands; thelarger the first and the smaller the latter, the greater is, as a rule, the average amount of sugar of the race. Through the kindness of thelate Mr. Rimpau, a well known German breeder of sugar-beet varieties, Iobtained specimens from seed of a native wild locality near Bukharest. The plants produced quite woody roots, showing almost no sugar tissue atall. Woody layers of strongly developed fibrovascular strands were seento be separated one from another only by very thin layers ofparenchymatous cells. Even the number of layers is variable; it wasobserved to be five in my plants; but in larger roots double this numberand even more may easily be met with. 

Some authors have distinguished specific types among these wild forms. While the cultivated beets are collected under the head of _Betavulgaris_, separate types with more or less woody roots have beendescribed as _Beta maritima_ and _Beta patula_. These show differencesin the habit of the stems and the foliage. Some have a strong tendencyto become annual, others to become biennial. The first of course do notstore a large quantity of food in their roots, and remain thin, even atthe time of flowering. The biennial types occur in all sizes of roots. In the annuals the stems may vary from [70] erect to ascending, and thename _patula_ indicates stems which are densely branching from the basewith widely spreading branches throughout. Mr. Em. Von Proskowetz ofKwassitz, Austria, kindly sent me seeds of this _Beta patula_, thevariability of which was so great in my cultures as to range from nearlytypical sugar-beets to the thin woody type of Bukharest. 

Broad and narrow leaves are considered to be differentiating marksbetween _Beta vulgaris_ and _Beta patula_, but even here a wide range offorms seem to occur. 

Rimpau, Proskowetz, Schindler and others have made cultures of beetsfrom wild localities in order to discover a hypothetical common ancestorof all the present cultivated types. These researches point to the _B. Patula_ as the probable ancestor, but of course they were not made todecide the question as to whether the origination of the several nowexisting types had taken place before or during culture. From a generalpoint of view the variability of the wild species is parallel to that ofthe cultivated forms to such a degree as to suggest the multiple originof the former. But a close investigation of this highly importantproblem has still to be made. 

The varieties of the cultivated beets are commonly [71] included in foursubspecies. The two smallest are the salad-beets and the ornamentalforms, the first being used as food, and ordinarily cultivated in redvarieties, the second being used as ornamental plants during the fall, when they fill the beds left empty by summer flowers, with a brightfoliage that is exceedingly rich in form and color. Of the remainingsubspecies, one comprises the numerous sorts cultivated as forage-cropsand the other the true sugar-beets. Both of them vary widely as to theshape and the size of the roots, the quality of the tissue, the foliageand other characteristics. 

Some of these forms, no doubt, have originated during culture. Most ofthem have been improved by selection, and no beet found in the wildstate ever rivals any cultivated variety. But the improvement chieflyaffects the size, the amount of sugar and nutrient substances and someother qualities which recur in most of the varieties. The varietalattributes themselves however, are more or less of a specific nature, and have no relation to the real industrial value of the race. Theshort-rooted and the horn-shaped varieties might best be cited asexamples. 

The assertion that the sundry varieties of forage-beets are not theresult of artificial selection, [72] is supported in a large measure bythe historic fact that the most of them are far older than the method ofconscious selection of plants itself. This method is due to LouisVilmorin and dates from the middle of the last century. But in thesixteenth century most of our present varieties of beets were already incultivation. Caspar Bauhin gives a list of the beets of his time and itis not difficult to recognize in it a large series of subspecies andvarieties and even of special forms, which are still cultivated. A morecomplete list was published towards the close of the same century byOlivier de Serres in his world-renowned "Theatre d'Agriculture" (Paris, 1600). 

The red forage-beets which are now cultivated on so large a scale, hadbeen introduced from Italy into France only a short time before. 

From this historic evidence, the period during which the beets werecultivated from the time of the Romans or perhaps much later, up to thetime of Bauhin and De Serres, would seem far too short for theproduction by the unguided selection of man of all the now existingtypes. On the other hand, the parallelism between the characters of somewild and some cultivated varieties goes to make it very probable thatother varieties have been found in the same way, some in this countryand others in that, [73] and have been taken into cultivationseparately. Afterwards of course all must have been improved in thedirection required by the needs of man. 

Quite the same conclusion is afforded by apples. The facts are to someextent of another character, and the rule of the derivation of thepresent cultivated varieties from original wild forms can be illustratedin this case in a more direct way. Of course we must limit ourselves tothe varieties of pure ancestry and leave aside all those which are ofhybrid or presumably hybrid origin. 

Before considering their present state of culture, something must be, said about the earlier history and the wild state of the apples. 

The apple-tree is a common shrub in woods throughout all parts ofEurope, with the only exception of the extreme north. Its distributionextends to Anatolia, the Caucasus and Ghilan in Persia. It is found innearly all forests of any extent and often in relatively large numbersof individuals. It exhibits varietal characters, which have led to therecognition of several spontaneous forms, especially in France and inGermany. 

The differentiating qualities relate to the shape and indumentum of theleaves. Nothing is known botanically as to differences between [74] thefruits of these varieties, but as a matter of fact the wild apples ofdifferent countries are not at all the same. 

Alphonse De Candolle, who made a profound study of the probable originof most of our cultivated plants, comes to the conclusion that the appletree must have had this wide distribution in prehistoric times, and thatits cultivation began in ancient times everywhere. 

This very important conclusion by so high an authority throwsconsiderable light on the relation between cultivated and wild varietiesat large. If the historic facts go to prove a multiple origin for thecultivation of some of the more important useful plants, the probabilitythat different varieties or elementary species have been the startingpoints for different lines of culture, evidently becomes stronger. 

Unfortunately, this historic evidence is scanty. The most interestingfacts are those concerning the use of apples by the Romans and by theircontemporaries of the Swiss and middle European lake-dwellings. OswaldHeer has collected large numbers of the relics of this prehistoricperiod. Apples were found in large quantities, ordinarily cut intohalves and with the signs of having been dried. Heer distinguished twovarieties, one with large and one with small fruits. The first about 3and [75] the other about 1. 5-2 cm. In diameter. Both are therefore verysmall compared with our present ordinary varieties, but of the samegeneral size as the wild forms of the present day. Like these, they musthave been of a more woody and less fleshy tissue. They would scarcelyhave been tasteful to us, but in ancient times no better varieties wereknown and therefore no comparison was possible. 

There is no evidence concerning the question, as to whether during theperiods mentioned apples were cultivated or only collected in the wildstate. The very large numbers which are found, have induced some writersto believe in their culture, but then there is no reason why they shouldnot have been collected in quantity from wild shrubs. The main fact isthat the apple was not a uniform species in prehistoric times but showedeven then at least some amount of variability. 

At the present day the wild apples are very rich in elementary species. Those of Versailles are not the same as those of Belgium, and stillothers are growing in England and in Germany. The botanical differencesderived from the blossoms and the leaves are slight, but the flavor, size and shape of the fruits diverge widely. Two opinions have beenadvanced to explain this high degree of variability, but [76] neither ofthem conveys a real explanation; their aim is chiefly to supportdifferent views as to the causes of variability, and the origin ofelementary species at large. 

One opinion, advocated by De Candolle, Darwin and others, claims thatthe varieties owe their origin to the direct influence of cultivation, and that the corresponding forms found in the wild state, are not at alloriginal, but have escaped from cultivation and apparently become wild. Of course this possibility cannot be denied, at least in any singleinstance, but it seems too sweeping an assertion to make for the wholerange of observed forms. 

The alternative theory is that of van Mons, the Belgian originator ofcommercial varieties of apples, who has published his experiments in alarge work called "Arbres fruitiers ou Pomonomie belge. " Most of themore remarkable apples of the first half of the last century wereproduced by van Mons, but his greatest merit is not the directproduction of a number of good varieties, but the foundation of themethod, by which new varieties may be obtained and improved. 

According to van Mons, the production of a new variety consists chieflyof two parts. The first is the discovery of a subspecies with newdesirable qualities. The second is the transformation [77] of theoriginal small and woody apple into a large, fleshy and palatablevariety. Subspecies, or what we now call elementary species were notproduced by man; nature alone creates new forms, as van Mons has it. Heexamined with great care the wild apples of his country, and especiallythose of the Ardennes, and found among them a number of species withdifferent flavors. For the flavor is the one great point, which must befound ready in nature and which may be improved, but can never becreated by artificial selection. The numerous differences in flavor arequite original; all of them may be found in the wild state and most ofthem even in so limited a region as the Ardennes Mountains. Of coursevan Mons preferred not to start from the wild types themselves, when thesame flavor could be met with in some cultivated variety. His generalmethod was, to search for a new flavor and to try to bring the bearer ofit up to the desired standard of size and edibility. 

The latter improvement, though it always makes the impression of anachievement, is only the last stone to be added to the building up ofthe commercial value of the variety. Without it, the best flavored appleremains a crab; with it, it becomes a conquest. According to the methodof van Mons it may be reached within [78] two or three generations, anda man's life is wholly sufficient to produce in this way many new typesof the very best sorts, as van Mons himself has done. It is done in theusual way, sowing on a large scale and selecting the best, which are intheir turn brought to an early maturation of their fruit by grafting, because thereby the life from seed to seed may be reduced to a fewyears. 

Form, taste, color, flavor and other valuable marks of new varieties arethe products of nature, says van Mons, only texture, fleshiness and sizeare added by man. And this is done in each new variety by the samemethod and according to the same laws. The richness of the cultivatedapples of the present day was already present in the large range oforiginal wild elementary species, though unobserved and requiringimprovement. 

An interesting proof of this principle is afforded by the experience ofMr. Peter M. Gideon, as related by Bailey. Gideon sowed large quantitiesof apple-seeds, and one seed produced a new and valuable variety calledby him the "Wealthy" apple. He first planted a bushel of apple-seeds, and then every year, for nine years, planted enough seeds to produce athousand trees. At the end of ten years all seedlings had perishedexcept one hardy seedling [79] crab. This experiment was made inMinnesota, and failed wholly. Then he bought a small lot of seeds ofapples and crab-apples in Maine and from these the "Wealthy" came. Therewere only about fifty seeds in the lot of crab-apple seed which producedthe "Wealthy, " but before this variety was obtained, more than a bushelof seed had been sown. Chance afforded a species with an unknown taste;but the growing of many thousands of seedlings of known varieties wasnot the best means to get something really new. 

Pears are more difficult to improve than apples. They often require sixor more generations to be brought from the wild woody state to theordinary edible condition. But the varieties each seem to have aseparate origin, as with apples, and the wide range of form and of tastemust have been present in the wild state, long before cultivation. Onlyrecently has the improvement of cherries, plums, currants andgooseberries been undertaken with success by Mr. Burbank, and thedifference between the wild and cultivated forms has hitherto been verysmall. All indications point to the existence, before the era ofcultivation, of larger or smaller numbers of elementary species. 

The same holds good with many of the larger forage crops and otherplants of great industrial [80] value. Clover exhibits many varieties, which have been cultivated indiscriminately, and often in motleymixtures. The flower heads may be red or white, large or small, cylindric or rounded, the leaves are broader or narrower, with orwithout white spots of a curious pattern. They may be more or less hairyand so forth. Even the seeds exhibit differences in size, shape orcolor, and of late Martinet has shown, that by the simple means ofpicking out seeds of the same pattern, pure strains of clover may beobtained, which are of varying cultural value. In this way the bestsubspecies or varieties may be sought out for separate cultivation. Eventhe white spots on the leaflets have proved to be constant characterscorresponding with noticeable differences in yield. 

Flax is another instance. It was already cultivated, or at least madeuse of during the period of the lake-dwellers, but at that time it was aspecies referred to as _Linum angustifolium_, and not the _Linumusitatissimum_, which is our present day flax. There are now manysubspecies, elementary species, and varieties under cultivation. Theoldest of them is known as the "springing flax, " in opposition to theordinary "threshing flax. " It has capsules which open of themselves, inorder to disseminate the seeds, while the ordinary heads of the [81]flax remain closed until the seeds are liberated by threshing. It seemsprobable that the first form or _Linum crepitans_ might thrive in thewild state as well as any other plant, while in the common species thosequalities are lacking which are required for a normal dissemination ofthe seeds. White or blue flowers, high or dwarf stems, more or lessbranching at the base and sundry other qualities distinguish thevarieties, aside from the special industrial difference of the fibres. Even the life-history varies from annual and biennial, to perennial. 

It would take us too long to consider other instances. It is well knownthat corn, though considered as a single botanical species, isrepresented by different subspecies and varieties in nearly every regionin which it is grown. Of course its history is unknown and it isimpossible to decide whether all the tall and dwarf forms, or starchyand sweet varieties, dented or rounded kernels, and hundreds of othersare older than culture or have come into existence during historictimes, or as some assume, through the agency of man. But our main pointnow is not the origin, but only the existence of constant and sharplydifferentiated forms within botanical species. Nearly every cultivatedplant affords instances of such diversity. Some include a few typesonly, while [82] others show, a large number of forms clearly separatedto a greater or lesser degree. 

In some few instances it is obvious that this variability is of laterdate than culture. The most conspicuous case is that of the coconut. This valuable palm is found on nearly all tropical coasts, in America, as well as in Asia, but in Africa and Australia there are many hundredsof miles of shore line, where it is not found. Its importance is not atall the same everywhere. On the shores and islands of the Indian Oceanand the Malay Archipelago, man is chiefly dependent upon it, but inAmerica it is only of subordinate usefulness. 

In connection with these facts, it abounds in subspecies and varietiesin the East Indian regions, but on the continent of America littleattention has as yet been given to its diverging qualities. In theMalayan region it affords nearly all that is required by theinhabitants. The value of its fruit as food, and the delicious beveragewhich it yields, are well known. The fibrous rind is not less useful; itis manufactured into a kind of cordage, mats and floor-cloths. Anexcellent oil is obtained from the kernel by compression. The hardcovering of the stem is converted into drums and used in theconstruction of huts; the lower part is so hard as to take on abeautiful polish [83] when it resembles agate. Finally the unexpandedterminal bud is a delicate article of food. Many other uses could bementioned, but these may suffice to indicate how closely the life of theinhabitants is bound up with the culture of this palm, and how sharply, in consequence, its qualities must have been watched by early man. Anydivergence from the ordinary type must have been noted; those which wereinjurious must have been rejected, but the useful ones must have beenappreciated and propagated. In a word any degree of variability affordedby nature must have been noticed and cultivated. 

More than fifty different sorts of the coconut are described from theIndian shores and islands, with distinct local and botanical names. Miquel, who was one of the best systematists of tropical plants, of thelast century, described a large number of them, and since, more havebeen added. Nearly all useful qualities vary in a higher or lesserdegree in the different varieties. The fibrous strands of the rind ofthe nut are developed in some forms to such a length and strength as toyield the industrial product known as the coir-fibre. Only three of themare mentioned by Miquel that have this quality, the _Cocos nuciferarutila_, _cupuliformis_ and _stupposa_. Among them the _rutila_ [84]yields the best and most supple fibres, while those of the _stupposa_are stiff and almost unbending. 

The varieties also differ greatly in size, color, shape and quality, andthe trees have also peculiar characteristics. One variety exhibitsleaves which are nearly entire, the divisions being only imperfectlyseparated, as often occurs in the very first leaves of the seedlings ofother varieties. The flavor of the flesh, oil and milk likewise yieldmany good varietal marks. 

In short, the coconut-palm comes under the general rule, that botanicalspecies are built up of a number of sharply distinguishable types, whichprove their constancy and relative independence by their widedistribution in culture. In systematic works all these forms are calledvarieties, and a closer investigation of their real systematic value hasnot yet been made. But the question as to the origin of the varietiesand of the coconut itself has engrossed the attention of many botanists, among whom are De Candolle in the middle of the last century, and Cookat its close. 

Both questions are closely connected. De Candolle claimed an Asiaticorigin for the whole species, while Cook's studies go to prove that itsoriginal habitat is to be sought in the northern countries of SouthAmerica. Numerous [85] varieties are growing in Asia and have as yet notbeen observed to occur in America, where the coconut is only ofsubordinate importance, being one of many useful plants, and not theonly one relied upon by the natives for their subsistence. If therefore, De Candolle's opinion is the right one, the question as to whether thevarieties are older or younger than the cultivated forms of the species, must always remain obscure. But if the proofs of an American originshould be forthcoming, the possibility, and even the probability thatthe varieties are of later date than the beginning of their culture, andhave originated while in this condition must at once be granted. Animportant point in the controversy is the manner in which the coconutswere disseminated from shore to shore, from island to island. DeCandolle, Darwin and most of the European writers claim that thedispersal was by natural agencies, such as ocean-currents. They pointout that the fibrous rind or husk would keep the fruits afloat, anduninjured, for many days or even many weeks, while being carried fromone country to another in a manner that would explain their geographicdistribution. But the probability of the nuts being thrown upon thestrand, and far enough from the shore to find suitable conditions fortheir germination, is a very small one. To insure [86] healthy andvigorous seedlings the nuts must be fully ripe, after which plantingcannot be safely delayed for more than a few weeks. If kept too moistthe nuts rot. If once on the shore, and allowed to lie in the sun, theybecome overheated and are thereby destroyed; if thrown in the shade ofother shrubs and trees, the seedlings do not find the requiredconditions for a vigorous growth. 

Some authors have taken the fibrous rind to be especially adapted totransport by sea, but if this were so, this would argue that water isthe normal or at least the very frequent medium of dissemination, whichof course it is not. We may, claim with quite as much right that thethick husk is necessary to enable the heavy fruit to drop from talltrees with safety. But even for this purpose the protection is notsufficient, as the nuts often suffer from falling to such a degree as tobe badly injured as to their germinating qualities. It is well knownthat nuts, which are destined for propagation, are as a rule not allowedto fall off, but are taken from the trees with great care. 

Summing up his arguments, Cook concludes that there is little in the wayof known facts to support the poetic theory of the coconut palm droppingits fruits into the sea to float away to barren islands and prepare themfor [87] human habitation. Shipwrecks might furnish a successful methodof launching viable coconuts, and such have no doubt sometimescontributed to their distribution. But this assumption implies adissemination of the nuts by man, and if this principal fact is granted, it is far more natural to believe in a conscious intelligentdissemination. 

The coconut is a cultivated tree. It may be met with in some spotsdistant from human dwellings, but whenever such cases have beensubjected to a closer scrutiny, it appears that evidently, or at leastprobably, huts had formerly existed in their neighborhood, but havingbeen destroyed by some accident, had left the palm trees uninjured. Evenin South America, where it may be found in forests at great distancesfrom the sea-shore, it is not at all certain that true native localitiesoccur, and it seems to be quite lost in its natural condition. 

Granting the cultivated state of the palms as the only really importantone, and considering the impossibility or at least great improbabilityof its dissemination by natural means, the distribution by man himself, according to his wants, assumes the rank of an hypothesis fully adequateto the explanation of all the facts concerning the life-history of thetree. 

We now have to inquire into the main question, [88] whether it isprobable that the coconut is of American or of Asiatic origin, leavingaside the historic evidence which goes to prove that nothing is knownabout the period in which its dissemination from one hemisphere toanother took place, we will now consider only the botanic and geographicevidence, brought forward by Cook. He states that the whole family ofcoconut-palms, consisting of about 20 genera and 200 species, are allstrictly American with the exception of the rather aberrant Africanoilpalm, which has, however, an American relative referred to the samegenus. The coconut is the sole representative of this group which isconnected with Asia and the Malayan region, but there is no manifestreason why other members of the same group could not have establishedthemselves there, and maintained an existence under conditions, whichare not at all unfavorable to them. The only obvious reason is theassumption already made, that the distribution was brought about by man, and thus only affected the species, chosen by him for cultivation. Thatthe coconut cannot have been imported from Asia into America seems to bethe most obvious conclusion from the arguments given. It should bebriefly noted, that it was known and widely distributed in tropicalAmerica at the time of the discovery of that continent [89] by Columbus, according to accounts of Oviedo and other contemporary Spanish writers. 

Concluding we may state that according to the whole evidence as it hasbeen discussed by De Candolle and especially by Cook, the coconut-palmis of American origin and has been distributed as a cultivated tree byman through the whole of its wide range. This must have happened in aprehistoric era, thus affording time enough for the subsequentdevelopment of the fifty and more known varieties. But the possibilitythat at least some of them have originated before culture and have beendeliberately chosen by man for distribution, of course remainsunsettled. 

Coconuts are not very well adapted for natural dispersal on land, andthis would rather induce us to suppose an origin within the period ofcultivation for the whole group. There are a large number of cultivatedvarieties of different species which by some peculiarity do not seemadapted for the conditions of life in the wild state. These last haveoften been used to prove the origin of varietal forms during culture. One of the oldest instances is the variety or rather subspecies of theopium-poppy, which lacks the ability to burst open its capsules. Theseeds, which are thrown out by the wind, in the common forms, throughthe apertures underneath [90] the stigma, remain enclosed. This ismanifestly a very useful adaptation for a cultivated plant, as by thismeans no seeds are lost. It would be quite a disadvantage for a wildspecies, and is therefore claimed to have been connected from thebeginning with the cultivated form. 

The large kernels of corn and grain, of beans and peas, and even of thelupines were considered by Darwin and others to be unable to cope withnatural conditions of life. Many valuable fruits are quite sterile, orproduce extremely few seeds. This is notoriously the case with some ofthe best pears and grapes, with the pine-apples, bananas, bread-fruits, pomegranate and some members of the orange tribe. It is open todiscussion as to what may be the immediate cause of this sterility, butit is quite evident, that all such sterile varieties must haveoriginated in a cultivated condition. Otherwise they would surely havebeen lost. 

In horticulture and agriculture the fact that new varieties arise fromtime to time is beyond all doubt, and it is not this question with whichwe are now concerned. Our arguments were only intended to prove thatcultivated species, as a rule, are derived from wild species, which obeythe laws discussed in a previous lecture. The botanic units are compoundentities, and [91] the real systematic units in elementary species playthe same part as in ordinary wild species. The inference that the originof the cultivated plants is multiple, in most cases, and that more thanone, often many separate elementary forms of the same species mustoriginally have been taken into cultivation, throws much light upon manyhighly important problems of cultivation and selection. This aspect ofthe question will therefore be the subject of the next lecture. 

[92]

LECTURE IV

SELECTION OF ELEMENTARY SPECIES

The improvement of cultivated plants must obviously begin with alreadyexisting forms. This is true of old cultivated sorts as well as forrecent introductions. In either case the starting-point is as importantas the improvement, or rather the results depend in a far higher degreeon the adequate choice of the initial material than on the methodicaland careful treatment of the chosen varieties. This however, has notalways been appreciated as it deserves, nor is its importance at presentuniversally recognized. The method of selecting plants for theimprovement of the race was discovered by Louis Vilmorin about themiddle of the last century. Before his time selection was applied todomestic animals, but Vilmorin was the first to apply this principle toplants. As is well known, he used this method to increase the amount ofsugar in beets and thus to raise their value as forage-crops, with suchsuccess, that his plants have since been used for the production [93] ofsugar. He must have made some choice among the numerous available sortsof beets, or chance must have placed in his hands one of the mostappropriate forms. On this point however, no evidence is at hand. 

Since the work of Vilmorin the selection-principle has increasedenormously in importance, for practical purposes as well as for thetheoretical aspect of the subject. It is now being applied on a largescale to nearly all ornamental plants. It is the one great principle nowin universal practice as well as one of preeminent scientific value. Ofcourse, the main arguments of the evolution theory rest uponmorphologic, systematic, geographic and paleontologic evidence. But thequestion as to how we can coordinate the relation between existingspecies and their supposed ancestors is of course one of a physiologicnature. Direct observation or experiments were not available for Darwinand so he found himself constrained to make use of the experience ofbreeders. This he did on a broad scale, and with such success that itwas precisely this side of his arguments that played the major part inconvincing his contemporaries. 

The work of the breeders previous to Darwin's time had not been verycritically performed. Recent analyses of the evidence obtained [94] fromthem show that numerous types of variability were usually throwntogether. What type in each case afforded the material, which thebreeder in reality made use of, has only been inquired into in the lastfew decades. Among those who have opened the way for thorough and morescientific treatment are to be mentioned Rimpau and Von Rumker ofGermany and W. M. Hays of America. 

Von Rumker is to be considered as the first writer, who sharplydistinguished between two phases of methodical breeding-selection. Oneside he calls the production of new forms, the other the improvement ofthe breed. He dealt with both methods extensively. New forms areconsidered as spontaneous variations occurring or originating withouthuman aid. They have only to be selected and isolated, and their progenyat once yields a constant and pure race. This race retains its characteras long as it is protected against the admixture of other minorvarieties, either by cross-pollination, or by accidental seeds. 

Improvement, on the other hand, is the work of man. New varieties ofcourse can only be isolated if chance offers them; the improvement isnot incumbent on chance. It does not create really anything new, butdevelops characters, which were already existing. It brings [95] therace above its average, and must guard constantly against the regressiontowards this average which usually takes place. 

Hays has repeatedly insisted upon the principle of the choice of themost favorable varieties as the foundation for all experiments inimproving races. He asserts that half the battle is won by choosing thevariety which is to serve as a foundation stock, while the other halfdepends upon the selection of parent-plants within the chosen variety. Thus the choice of the variety is the first principle to be applied inevery single case; the so-called artificial selection takes only asecondary place. Calling all minor units within the botanic species bythe common name of varieties, without regard to the distinction betweenelementary species and retrograde varieties, the principle is designatedby the term of "variety-testing. " This testing of varieties is now, asis universally known, one of the most important lines of work of theagricultural experiment stations. Every state and every region, in someinstances even the larger farms, require a separate variety of corn, orwheat, or other crops. They must be segregated from among the hundredsof generally cultivated forms, within each single botanic species. Oncefound, the type may be ameliorated according to the local conditions[96] and needs, and this is a question of improvement. 

The fact that our cultivated plants are commonly mixtures of differentsorts, has not always been known. The first to recognize it seems tohave been the Spanish professor of botany, Mariano Lagasca, whopublished a number of Spanish papers dealing with useful plants andbotanical subjects between 1810 and 1830, among them a catalogue ofplants cultivated in the Madrid Botanical Garden. Once when he was on avisit to Colonel Le Couteur on his farm in Jersey, one of the ChannelIslands off the coast of France, in discussing the value of the fieldsof wheat, he pointed out to his host, that they were not really pure anduniform, as was thought at that time, and suggested the idea that someof the constituents might form a larger part in the harvest than others. In a single field he succeeded in distinguishing no less than 23varieties, all growing together. Colonel Le Couteur took the hint, andsaved the seeds of a single plant of each supposed variety separately. These he cultivated and multiplied till he got large lots of each andcould compare their value. From among them he then chose the varietyproducing the greatest amount of the finest, whitest and most nutritiousflour. This he eventually placed in the [97] market under the name of"Talavera de Bellevue. " It is a tall, white variety, with long andslender white heads, almost without awns, and with fine white pointedkernels. It was introduced into commerce about 1830, and is still one ofthe most generally cultivated French wheats. It was highly prized in themagnificent collection of drawings and descriptions of wheats, publishedby Vilmorin under the title "Les meilleurs bles" and is said to havequite a number of valuable qualities, branching freely and producing anabundance of good grain and straw. It is however, sensitive to coldwinters in some degree and thereby limited in its distribution. Hallett, the celebrated English wheat-breeder, tried in vain to improve thepeculiar qualities of this valuable production of Le Couteur's. 

Le Couteur worked during many years along this line, long before thetime when Vilmorin conceived the idea of improvement by race selections, and he used only the simple principle of distinguishing and isolatingthe members of his different fields. Later he published his results in awork on the varieties, peculiarities and classification of wheat (1843), which though now very rare, has been the basis and origin of theprinciple of variety-testing. 

The discovery of Lagasca and Le Couteur was [98] of course notapplicable to the wheat of Jersey alone. The common cultivated sorts ofwheat and other grains were mixtures then as they are even now. Improvedvarieties are, or at least should be, in most cases pure and uniform, but ordinary sorts, as a rule, are mixtures. Wheat, barley and oats areself-fertile and do not mix in the field through cross-pollination. Every member of the assemblage propagates itself, and is only checked byits own greater or less adaptation to the given conditions of life. Rimpau has dealt at large with the phenomenon as it occurs in thenorthern and middle parts of Germany. Even Rivett's "Bearded wheat, "which was introduced from England as a fine improved variety, and hasbecome widely distributed throughout Germany, cannot keep itself pure. It is found mingled almost anywhere with the old local varieties, whichit was destined to supplant. Any lot of seed exhibits such impurities, as I have had the opportunity of observing myself in sowings in theexperimental-garden. But the impurities are only mixtures, and all theplants of Rivett's "Bearded wheat, " which of course constitute the largemajority, are of pure blood. This may be confirmed when the seeds arecollected and sown separately in cultures that can be carefully guarded. 

[99] In order to get a closer insight into the causes of this confusedcondition of ordinary races, Rimpau made some observations on Rivett'swheat. He found that it suffers from frost during winter more than thelocal German varieties, and that from various causes, alien seeds mayaccidentally, and not rarely, become mixed with it. Thethreshing-machines are not always as clean as they should be and may bethe cause of an accidental mixture. The manure comes from stables, wherestraw and the dust from many varieties are thrown together, andconsequently living kernels may become mixed with the dung. Such straygrains will easily germinate in the fields, where they find morecongenial conditions than does the improved variety. If winter arrivesand kills quantities of this latter, the accidental local races willfind ample space to develop. Once started, they will be able to multiplyso rapidly, that in one or two following generations they willconstitute a very considerable portion of the whole harvest. In this waythe awnless German wheat often prevails over the introduced Englishvariety, if the latter is not kept pure by continuous selection. 

The Swiss wheat-breeder Risler made an experiment which goes to provethe certainty of the explanation given by Rimpau. He observed on hisfarm at Saleves near the lake of Geneva that after a lapse of time the"Galland wheat" deteriorated and assumed, as was generally believed, thecharacters of the local sorts. In order to ascertain the real cause ofthis apparent change, he sowed in alternate rows in a field, the"Galland" and one of the local varieties. The "Galland" is a race withobvious characters and was easily distinguished from the other at thetime when the heads were ripe. They are bearded when flowering, butafterwards throw off the awns. The kernels are very large and yield anextraordinarily good, white flour. 

During the first summer all the heads of the "Galland" rows had thedeciduous awns but the following year these were only seen on half ofthe plants, the remainder having smooth heads, and the third year the"Galland" had nearly disappeared, being supplanted by the competinglocal race. The cause of this rapid change was found to be twofold. First the "Galland, " as an improved variety, suffers from the winter ina far higher degree than the native Swiss sorts, and secondly it ripensits kernels one or two weeks later. At the time of harvest it may nothave become fully ripe, while the varieties mixed with it had reachedmaturity. The wild oat, _Avena fatua_, is very common in [101] Europefrom whence it has been introduced in the United States. In summerswhich are unfavorable to the development of the cultivated oats it maybe observed to multiply with an almost incredible rapidity. It does notcontribute to the harvest, and is quite useless. If no selection weremade, or if selection were discontinued, it would readily supplant thecultivated varieties. 

From these several observations and experiments it may be seen, that itis not at all easy to keep the common varieties of cereals pure and thateven the best are subject to the encroachment of impurities. Hence it isonly natural that races of cereals, when cultivated without the utmostcare, or even when selected without an exact knowledge of their singleconstituents, are always observed to be more or less in a mixedcondition. Here, as everywhere with cultivated and wild plants, thesystematic species consist of a number of minor types, which pertain todifferent countries and climates, and are growing together in the sameclimate and under the same external conditions. They do not mingle, norare their differentiating characters destroyed by intercrossing. Theyeach remain pure, and may be isolated whenever and wherever thedesirability for such a proceeding should arise. The purity of [102] theraces is a condition implanted in them by man, and nature always strivesagainst this arbitrary and one-sided improvement. Numerous slightdifferences in characters and numerous external influences benefit theminor types and bring them into competition with the better ones. Sometimes they tend to supplant the latter wholly, but ordinarily sooneror later a state of equilibrium is reached, in which henceforth thedifferent sorts may live together. Some are favored by warm and othersby cool summers, some are injured by hard winters while others thrivethen and are therefore relatively at an advantage. The mixed conditionis the rule, purity is the exception. 

Different sorts of cereals are not always easily distinguishable by thelayman and therefore I will draw your attention to conditions inmeadows, where a corresponding phenomenon can be observed in a muchsimpler way. 

Only artificial pasture-grounds are seen to consist of a single speciesof grass or clover. The natural condition in meadows is the occurrenceof clumps of grasses and some clovers, mixed up with perhaps twenty ormore species of other genera and families. The numerical proportion ofthese constituents is of great interest, and has been studied atRothamstead in England and on a number of other farms. It is [103]always changing. No two successive years show exactly the sameproportions. At one time one species prevails, at another time one ortwo or more other species. The weather during the spring and summerbenefits some and hurts others, the winter may be too cold for some, butagain harmless for others, the rainfall may partly drown some species, while others remain uninjured. Some weeds may be seen floweringprofusely during some years, while in other summers they are scarcely tobe found in the same meadow. The whole population is in a fluctuatingstate, some thriving and others deteriorating. It is a continuousresponse to the ever changing conditions of the weather. Rarely aspecies is wholly annihilated, though it may apparently be so for years;but either from seeds or from rootstocks, or even from neighboringlands, it may sooner or later regain its foothold in the generalstruggle for life. 

This phenomenon is a very curious and interesting one. The struggle forlife, which plays so considerable a part in the modern theories ofevolution, may be seen directly at work. It does not alter the speciesthemselves, as is commonly supposed, but it is always changing theirnumerical proportion. Any lasting change in the external conditions willof course alter the average oscillation and the influence [104] of suchalterations will manifest itself in most cases simply in new numericalproportions. Only extremes have extreme effects, and the chance for theweaker sorts to be completely overthrown is therefore very small. 

Any one, who has the opportunity of observing a waste field during aseries of years, should make notes concerning the numerical proportionsof its inhabitants. Exact figures are not at all required; approximateestimates will ordinarily prove to be sufficient, if only the standardremains the same during the succeeding years. 

The entire mass of historic evidence goes to prove that the sameconditions have always prevailed, from the very beginning of cultivationup to the present time. The origin of the cultivation of cereals is tobe sought in central Asia. The recent researches of Solms Laubach showit to be highly probable that the historic origin of the wheatcultivated in China, is the same as that of the wheat of Egypt andEurope. Remains of cereals are found in the graves of Egyptian mummies, in the mounds of waste material of the lake-dwellings of Central Europe, and figures of cereals are to be seen on old Roman coins. In thesepulchre of King Ra-n-Woser of the Fifth Dynasty of Egypt, who livedabout 2000 years B. C. , two [105] tombs have recently been opened by theGerman Oriental Society. In them were found quantities of the tares ofthe _Triticum dicoccum_, one of the more primitive forms of wheat. Inother temples and pyramids and among the stones of the walls of Dashurand El Kab studied by Unger, different species and varieties of cerealswere discovered in large quantities, that showed their identity with thepresent prevailing cultivated races of Egypt. 

The inhabitants of the lake-dwellings in Switzerland possessed somevarieties of cereals, which have entirely disappeared. They aredistinguished by Heer under special names. The small barley and thesmall wheat of the lake-dwellers are among them. All in all there wereten well distinguished varieties of cereals, the Panicum and the Setariaor millet being of the number. Oats were evidently introduced onlytoward the very last of the lake-dwelling period, and rye is of farlater introduction into western Europe. Similar results are attained bythe examination of the cereals figured by the Romans of the same period. 

All these are archaeologic facts, and give but slight indicationsconcerning the methods of cultivation or the real condition of thecultivated races of that time. Virgil has left us some knowledge of therequirements of methodical [106] culture of cereals of his time. In hispoem _Georgics_ (I. 197) the following lines are found:

 _Vidi lecta din, et multo spectata labore Degenerare tamen, ni vis humana quotannis Maxima quaeque manu legeret_. 

 (The chosen seed, through years and labor improved, Was seen to run back, unless yearly Man selected by hand the largest and fullest of ears. )

Elsewhere Virgil and also some lines of Columella and Varro go to provein the same way that selection was applied by the Romans to theircereals, and that it was absolutely necessary to keep their races pure. There is little doubt, but that it was the same principle as that whichhas led, after many centuries, to the complete isolation and improvementof the very best races of the mixed varieties. It further proves thatthe mixed conditions of the cereals was known to man at that time, although distinct ideas of specific marks and differences were of coursestill wholly lacking. It is proof also that cultivated cereals from theearliest times must have been built up of numerous elementary forms. Moreover it is very probable, that in the lapse of centuries a goodlynumber of such types must have disappeared. [107] Among the vanishedforms are the special barley and wheat of the lake-dwellings, theremains of which have been accidentally preserved, but most of the formsmust have disappeared without leaving any trace. 

This inference is supported by the researches of Solms-Laubach, whofound that in Abyssinia numerous primitive types of cereals are still inculture. They are not adequate to compete with our present varieties, and would no doubt also have disappeared, had they not been preserved bysuch quite accidental and almost primitive isolation. 

Closing this somewhat long digression into history we will now resumeour discussion concerning the origin of the method of selecting cerealsfor isolation and segregate-cultivation. Some decades after Le Couteur, this method was taken up by the celebrated breeder Patrick Sheriff ofHaddington in Scotland. His belief, which was general at that time, was"That cultivation has not been found to change well defined kinds, andthat improvement can be best attained by selecting new and superiorvarieties, which nature occasionally produces, as if inviting thehusbandman to stretch forth his hand and cultivate them. "

Before going into the details of Sheriff's work it is as well to saysomething concerning [108] the use of the word "selection. " This wordwas used by Sheriff as seen in the quotation given, and it was obviouslydesigned to convey the same idea as the word "lecta" in the quotationfrom Virgil. It was a choice of the best plants from among known mixedfields, but the chosen individuals were considered to be representativesof pure and constant races, which could only be isolated, but notameliorated. Selection therefore, in the primitive sense of the word, isthe choice of elementary species and varieties, with no other purposethan that of keeping them as pure as possible from the admixture ofminor sorts. The Romans attained this end only imperfectly, simplybecause the laws governing the struggle for life and the competition ofnumerous sorts in the fields were unsuspected by them. 

Le Couteur and Sheriff succeeded in the solution of the problem, becausethey had discovered the importance of isolation. The combination of acareful choice with subsequent isolation was all they knew about it, andit was one of the great achievements to which modern agriculture owesits success. 

The other great principle was that of Vilmorin. It was the improvementwithin the race, or the "amelioration of the race" as it was termed byhim. It was introduced into [109] England by F. F. Hallett of Brighton inSussex, who at once called it "pedigree-culture, " and produced his firstnew variety under the very name of "Pedigree-wheat. " This principle, which yields improved strains, that are not constant but dependent onthe continued and careful choice of the best plants in each succeedinggeneration, is now generally called "selection. " But it should always beremembered that according to the historic evolution of the idea, theword has the double significance of the distinction and isolation ofconstant races from mixtures, and that of the choice of the bestrepresentatives of a race during all the years of its existence. Evensugar-beets, the oldest "selected" agricultural plants, are far fromhaving freed themselves from the necessity of continuous improvement. Without this they would not remain constant, but would retrograde withgreat rapidity. 

The double meaning of the word selection still prevailed when Darwinpublished his "Origin of Species. " This was in the year 1859, and atthat time Shirreff was the highest authority and the most successfulbreeder of cereals. Vilmorin's method had been applied only to beets, and Hallett had commenced his pedigree-cultures only a few years beforeand his first publication of the "Pedigree-wheat" [110] appeared someyears later at the International Exhibition of London in 1862. Hence, whenever Darwin speaks of selection, Shirreff's use of the word may aswell be meant as that of Vilmorin. 

However, before going deeper into such theoretical questions, we willfirst consider the facts, as given by Shirreff himself. 

During the best part of his life, in fact during the largest part of thefirst half of the nineteenth century, Shirreff worked according to avery simple principle. When quite young he had noticed that sometimessingle plants having better qualities than the average were seen in thefields. He saved the grains, or sometimes the whole heads of such plantsseparately, and tried to multiply them in such manner as to avoidintermixtures. 

His first result was the "Mungoswell's wheat. " In the spring of 1819 heobserved quite accidentally in a field of the farm of that name, asingle plant which attracted his attention by a deeper green and bybeing more heavily headed out. Without going into further details, he atonce chose this specimen as the starting point of a new race. Hedestroyed the surrounding plants so as to give it more space, appliedmanure to its roots, and tended it with special care. It yielded 63heads and nearly [111] 2500 grains. All of these were sown thefollowing fall, and likewise in the succeeding years the whole harvestwas sown in separate lots. After two years of rapid multiplication itproved to be a good new variety and was brought into commerce. It hasbecome one of the prominent varieties of wheat in East Lothian, thatcounty of Scotland of which Haddington is the principal borough. 

The grains of "Mungoswell's wheat" are whiter than those of the allied"Hunter's wheat, " more rounded but otherwise of the same size acidweight. The straw is taller and stronger, and each plant produces moreculms and more heads. 

Shirreff assumed, that the original plant of this variety was a sportfrom the race in which he had found it, and that it was the onlyinstance of this sport. He gives no details about this most interestingside of the question, omitting even to tell the name of the parentvariety. He only asserts that it was seen to be better, and afterwardsproved so by the appreciation of other breeders and its success intrade. He observed it to be quite constant from the beginning, nosubsequent selection being needed. This important feature was simplyassumed by him to be true as a matter of course. 

[112] Some years afterwards, in the summer of 1824, he observed a largespecimen of oats in one of the fields of the same farm. Being at thattime occupied in making a standard collection of oats for a closercomparison of the varieties, he saved the seeds of that plant and sowedthem in a row in his experiment-field. It yielded the largest culms ofthe whole collection and bore long and heavy kernels with a red streakon the concave side and it excelled all other sorts by the finequalities of its very white meal. In the unequal length of its stalks ithas however a drawback, as the field appears thinner and more meagerthan it is in reality. "Hopetown oats, " as it is called, has found itsway into culture extensively in Scotland and has even been introducedwith success into England, Denmark and the United States. It has beenone of the best Scottish oats for more than half a century. 

The next eight years no single plant judged worthy of selection on hisown farm attracted Shirreff's attention. But in the fall of 1832 he sawa beautiful plant of wheat on a neighboring farm and he secured a headof it with about 100 grains. From this he produced the "Hopetown wheat. "After careful separation from the kernels this original ear waspreserved, and was afterwards exhibited at the Stirling Agricultural[113] Museum. The "Hopetown wheat" has proved to be a constant variety, excelling the ordinary "Hunter's wheat" by larger grains and longerheads; it yields likewise a straw of superior quality and has becomequite popular in large districts of England and Scotland, where it isknown by the name of "White Hunter's" from its origin and the brilliantwhiteness of its heads. 

In the same way Shirreff's oats were discovered in a single plant in afield where it was isolated in order to be brought into commerce aftermultiplication. It has won the surname of "Make-him-rich. " Nothing is onrecord about the details of its origin. 

Four valuable new varieties of wheat and oats were obtained in this wayin less than forty years. Then Shirreff changed his ideas and his methodof working. Striking specimens appeared to be too rare, and theexpectation of a profitable result too small. Therefore he began work ona larger scale. He sought and selected during the summer of 1857 seventyheads of wheat, each from a single plant showing some marked andpresumably favorable peculiarity. These were not gathered on one field, but were brought together from all the fields to which he had access inhis vicinity. The grains of each of these selected heads were [114] sownseparately, and the lots compared during their whole life-period andchiefly at harvest time. Three of the lots were judged of highexcellence, and they alone were propagated, and proving to be constantnew varieties from the outset were given to the trade under the names of"Shirreff's bearded white, " "Shirreff's bearded red, " and "Pringle'swheat. " They have found wide acceptance, and the first two of them arestill considered by Vilmorin as belonging to the best wheats of France. 

This second method of Shirreff evidently is quite analogous to theprinciple of Lagasca and Le Couteur. The previous assumption that newvarieties with striking features were being produced by nature from timeto time, was abandoned, and a systematic inquiry into the worth of allthe divergent constituents of the fields was begun. Every single ear atonce proved to belong to a constant and pure race, but most of thesewere only of average value. Some few however, excelled to a degree, which made them worth multiplying, and to be introduced into trade asseparate varieties. 

Once started, this new method of comparison, selection and isolatedmultiplication was of course capable of many improvements. The culturein the experiment-field was improved, so as to insure a fuller and morerapid growth. 

[115] The ripe heads had to be measured and counted and compared withrespect to their size and the number of their kernels. Qualities ofgrain and of meal had to be considered, and the influence of climate andsoil could not be overlooked. 

Concerning the real origin of his new types Shirreff seems never to havebeen very inquisitive. He remarks that only the best cultivatedvarieties have a chance to yield still better types, and that it isuseless to select and sow the best heads of minor sorts. He furtherremarks that it is not probable that he found a new sport every time; onthe contrary he assumes that his selections had been present in thefield before, and during a series of succeeding generations. How manyyears old they were, was of course impossible to determine. But there isno reason to believe that the conditions in the fields of Scotland weredifferent from those observed on the Isle of Jersey by Le Couteur. 

In the year 1862 Shirreff devoted himself to the selection of oats, searching for the best panicles from the whole country, and comparingtheir offspring in his experimental garden. "Early Fellow, " "FineFellow, " "Longfellow" and "Early Angus" are very notable varietiesintroduced into trade in this way. 

[116] Some years later Patrick Shirreff described his experiments andresults in a paper entitled, "On the improvement of cereals, " but thedescriptions are very short, and give few details of systematic value. The leading principle, however, is clearly indicated, and anyone whostudies with care his method of working, may confidently attempt toimprove the varieties of his own locality in the same way. 

This great principle of "variety-testing, " as it has been founded by LeCouteur and Patrick Shirreff, has increased in importance ever since. Two main features are to be considered here. One is the production oflocal races, the other the choice of the best starting-point forhybridizing experiments, as is shown in California by the work of LutherBurbank in crossing different elementary species of _Lilium pardalinum_and others. 

Every region and locality has its own conditions of climate and soil. Any ordinary mixed race will contain some elementary forms which arebetter adapted to a given district, while others are more suitable todivergent conditions. Hence it can readily be inferred that the choicecannot be the same for different regions. Every region should select itsown type from among the various forms, and variety testing thereforebecomes a task which every [117] one must undertake under his ownconditions. Some varieties will prove, after isolation, to be profitablefor large districts and perhaps for whole states. Others will be foundto be of more local value, but in such localities to excel all others. 

As an example we may take one of the varieties of wheat originated bythe Minnesota Experiment Station. Hays described it as follows. It wasoriginated from a single plant. From among 400 plants of "Blue stem"several of the best were chosen, each growing separately, a foot apartin every direction. Each of the selected plants yielded 500 or moregrains of wheat, weighing 10 or more grams. The seeds from theseselected plants were raised for a few years until sufficient wasobtained to sow a plot. Then for several years the new strains weregrown in a field beside the parent-variety. One of them was so muchsuperior that all others were discarded. It was the one named "MinnesotaNo. 169. " For a large area of Minnesota this wheat seems capable ofyielding at least 1 or 2 bushels more grain per acre than its parentvariety, which is the best kind commonly and almost universally found onthe farms in southern and central Minnesota. 

It would be quite superfluous for our present purpose to give moreinstances. The fact of [118] the compound nature of so-called species ofcultivated plants seems to be beyond all doubt, and its practicalimportance is quite obvious. 

Acclimatization is another process, which is largely dependent on thechoice of adequate varieties. This is shown on a large scale by the slowand gradual dispersion of the varieties of corn in this country. Thelargest types are limited to temperate and subtropical regions, whilethe varieties capable of cultivation in more northern latitudes aresmaller in size and stature and require a smaller number of days toreach their full development from seed to seed. Northern varieties aresmall and short lived, but the "Forty-day-corn" or "Quarantino maize" isrecorded to have existed in tropical America at the time of Columbus. Inpreference, or rather to the entire exclusion of taller varieties, ithas thriven on the northern boundaries of the corn-growing states ofEurope since the very beginning of its cultivation. 

According to Naudin, the same rule prevails with melons, cucumbers andgherkins, and other instances could easily be given. 

Referring now to the inferences that may be drawn from the experience ofthe breeders in order to elucidate the natural processes, we will returnto the whitlow-grasses and pansies. 

[119] Nature has constituted them as groups of slightly differentconstant forms, quite in the same way as wheat and oats and corn. Assuming that this happened ages ago somewhere in central Europe, it isof course probable that the same differences in respect to the influenceof climatic conditions will have prevailed as with cereals. Subsequentto the period which has produced the numerous elementary species of thewhitlow-grass came a period of widespread distribution. The process musthave been wholly comparable with that of acclimatization. Some speciesmust have been more adapted to northern climates, others to the soils ofwestern or eastern regions and so on. These qualities must have decidedthe general lines of the distribution, and the species must have beensegregated according to their respective climatic qualities, and theiradaptability to soil and weather. A struggle for life and a naturalselection must have accompanied and guided the distribution, but thereis no reason to assume that the various forms were changed by thisprocess, and that we see them now endowed with other qualities than theyhad at the outset. 

Natural selection must have played, in this and in a large number ofother cases, quite the same part as the artificial method of varietytesting. 

[120] Indeed it may be surmised that this has been its chief andprominent function. Taking up again our metaphor of the sieve we canassert that in such cases climate and soil exercise sifting action andin this way the application of the metaphor becomes more definite. Ofcourse, next to the climate and soil in importance, come ecologicalconditions, the vegetable and animal enemies of the plants and otherinfluences of the same nature. 

In conclusion it is to be pointed out that this side of the problem ofnatural selection and the struggle for life appears to offer the bestprospects for experimental, or for continued statistical inquiry. Directobservations are possible and any comparison of numerical proportions ofspecies in succeeding years affords clear proof of the part it plays. And above all, such observations can be made quite independently ofdoubtful theoretical considerations about presumed changes of character. 

The fact of natural selection is plain and it should be studied in itsmost simple conditions. 

[121]C. RETROGRADE VARIETIES

LECTURE V

CHARACTERS OF RETROGRADE VARIETIES

Every one admires the luxuriance of garden-flowers, and their diversityof color and form. All parts of the world have contributed to theirnumber and every taste can find its preference among them. New formsproduced by the skill of the breeder are introduced every year. This hasbeen done mostly by crossing and intermingling the characters ofintroduced species of the same genus. In some of the cases the historyof our flowers is so old that their hybrid origin is forgotten, as inthe case of the pansies. Hybridizations are still going on in othergroups on a large scale, and new forms are openly claimed to be ofhybrid origin. 

Breeders and amateurs generally have more interest in the results thanin the way in which they have been brought about. Excellent flowers andfruit recommend themselves and there seems to be no reason for inquiring[122] about their origin. In some cases the name of the originator maybe so widely known that it adds weight to the value of the new form, andtherefore may advantageously be coupled with it. The origin and historyof the greater part of our garden-flowers, fruits and vegetables areobscure; we see them as they are, and do not know from whence they came. The original habitat for a whole genus or for a species at large, may beknown, but questions as to the origin of the single forms, of which itis built up, ordinarily remain unanswered. 

For these reasons we are restricted in most cases to the comparison ofthe forms before us. This comparison has led to the general use of theterm "variety" in opposition to "species. " The larger groups of forms, which are known to have been introduced as such are called species. Allforms which by their characters belong to such a species are designatedas varieties, irrespective of their systematic relation to the form, considered as the ancestor of the group. 

Hence, we distinguish between "hybrid varieties" and "pure varieties"according to their origin from different parents or from a single lineof ancestors. Moreover, in both groups the forms may be propagated byseeds, or in the vegetative way by buds, by grafting or [123] bycutting, and this leads to the distinction of "seed-varieties" and"vegetative varieties. " In the first case the inheritance of the specialcharacters through the seeds decides the status of the variety, in thelatter case this point is left wholly out of consideration. 

Leaving aside all these different types, we are concerned here only withthe "seed-varieties" of pure origin, or at least with those, that aresupposed to be so. Hybridization and vegetative multiplication of thehybrids no doubt occur in nature, but they are very rare, when comparedwith the ordinary method of propagation by seed. "Seed-varieties" mayfurther be divided into constant and inconstant ones. The difference isvery essential, but the test is not always easy to apply. Constantvarieties are as sharply defined and as narrowly limited as are the bestwild species, while inconstant types are cultivated chiefly on accountof their wide range of form and color. This diversity is repeatedyearly, even from the purest seed. We will now discuss the constantseed-varieties, leaving the inconstant and eversporting types to asubsequent lecture. 

In this way we may make an exact inquiry into the departures from thespecies which are ordinarily considered to constitute the essentialcharacter of such a constant and pure seed-variety [124] and need onlycompare these differences with those that distinguish the elementaryspecies of one and the same group from each other. 

Two points are very striking. By far the greatest part of the ordinarygarden-varieties differ from their species by a single sharp characteronly. In derivative cases two, three or even more such characters may becombined in one variety, for instance, a dwarfed variety of the larkspurmay at the same time bear white flowers, or even double white flowers, but the individuality of the single characters is not in the leastobscured by such combinations. 

The second point is the almost general occurrence of the same variety inextended series of species. White and double flowers, variegated leaves, dwarfs and many other instances may be cited. It is precisely thisuniversal repetition of the same character that strikes us as theessential feature of a variety. 

And again these two characteristics may now be considered separately. Let us begin with the sharpness of the varietal characters. In thisrespect varieties differ most obviously from elementary species. Theseare distinguished from their nearest allies in almost all organs. Thereis no prominent distinctive feature between the single forms of _Draba_[125] _Verna_, _Helianthemum_ or of _Taraxacum_; all characters arealmost equally concerned. The elementary species of _Draba_ arecharacterized, as we have seen, by the forms and the hairiness of theleaves, the number and height of the flower-stalks, the breadth andincision of the petals, the forms of the fruits, and so on. Every one ofthe two hundred forms included in this collective species has its owntype, which it is impossible to express by a single term. Their namesare chosen arbitrarily. Quite the contrary is the case with most of thevarieties, for which one word ordinarily suffices to express the wholedifference. 

White varieties of species with red or blue flowers are the most commoninstances. If the species has a compound color and if only one of theconstituents is lost, partially colored types arise as in _AgrostemmaCoronaria bicolor_. Or the spots may disappear and the color becomeuniform as in _Gentiana punctata concolor_ and the spotless Arum or_Arum maculatum immaculatum_. Absence of hairs produces forms as_Biscutella laevigata glabra_; lack of prickles gives the varietiesknown as _inermis, as for instance, _Ranunculus arvensis inermis_. _Cytisus prostratus_ has a variety _ciliata_, and _Solanum Dulcamara_, or the bitter-sweet, has a variety called _tomentosum_. The curiousmonophyllous [126] variety of the strawberry and many other forms willbe discussed later. 

To enlarge this list it would only be necessary to extract from a flora, or from a catalogue of horticultural plants, the names of the varietiesenumerated therein. In nearly every instance, where true varieties andnot elementary species are concerned, a single term expresses the wholecharacter. 

Such a list would also serve to illustrate the second point since thesame names would recur frequently. Long lists of varieties are calledalba, or inermis, or canescens or lutea, and many genera contain thesame appellations. In some instances the systematists use a diversity ofnames to convey exactly the same idea, as if to conceal the monotony ofthe character, as for instance in the case of the lack of hairs, whichis expressed by the varietal names of _Papaver dubium glabrum_, _Arabisciliata glabrata_, _Arabis hirsuta glaberrima_, _Veronica spicatanitens_, _Amygdalus persica laevis_, _Paeonia corallina Leiocarpa_, &c. 

On the contrary we find elementary species in different genera based onthe greatest possible diversity of features. The forms of _Taraxacum_ or_Helianthemum_ do not repeat those of _Draba_ or _Viola_. In roses andbrambles the distinguishing features are characteristic of the type, as[127] they are evidently derived from it and limited to it. And this isso true that nobody claims the grade of elementary species for whiteroses or white brambles, but everyone recognizes that forms divergingfrom the nearest species by a single character only, are to be regardedas varieties. 

This general conviction is the basis on which we may build up a moresharply defined distinction between elementary species and varieties. Itis an old rule in systematic botany, that no form is to be constituted aspecies upon the basis of a single character. All authors agree on thispoint; specific differences are derived from the totality of theattributes, not from one organ or one quality. This rule is intimatelyconnected with the idea that varieties are derived from species. Thespecies is the typical, really existing form from which the variety hasoriginated by a definite change. In enumerating the different forms thespecies is distinguished by the term of genuine or typical, often onlyindicated as _a_ or the first; then follow the varieties sometimes inorder of their degree of difference, sometimes simply in alphabeticalorder. In the case of elementary species there is no real type; no oneof them predominates because all are considered to be equal in rank, andthe systematic species to which they [128] are referred is not a reallyexisting form, but is the abstraction of the common type of all, just asit is in the case of a genus or of a family. 

Summarizing the main points of this discussion, we find that elementaryspecies are of equal rank and together build up the collective orsystematic ideal species. Varieties on the other hand are derived from areal and commonly, still existing type. 

I hope that I have succeeded in showing that the difference betweenelementary species, or, as they are often called, smaller or subspecies, on the one hand and varieties on the other, is quite a marked one. However, in order to recognize this principle it is necessary to limitthe term variety, to those propagating themselves by seed and are ofpure and not of hybrid origin. 

But the principle as stated here, does not involve an absolute contrastbetween two groups of characters. It is more a difference in ourknowledge and appreciation of them than a difference in the thingsthemselves. The characters of elementary species are, as a rule, new tous, while those of varieties are old and familiar. It seems to me thatthis is the essential point. 

And what is it that makes us familiar with them? Obviously thecontinuous recurrence of the same changes, because by a constantrepetition they must of course lose their novelty. 

[129] Presently we shall look into these characters more in detail andthen we shall find that they are not so simple as might be supposed atfirst sight; but precisely because we are so familiar with them, wereadily see that their different features really belong to a singlecharacter; while in elementary species everything is so new that it isimpossible for us to discern the unities of the new attributes. 

If we bear in mind all these difficulties we cannot wonder at theconfusion on this question that seems to prevail everywhere. Someauthors following Linnaeus simply call all the subdivisions of species, varieties; others follow Jordan and avoid the difficulty by designatingall smaller forms directly as species. The ablest systematists prefer toconsider the ordinary species as collective groups, calling theirconstituents "The elements of the species, " as was done by A. P. DeCandolle, Alph. De Candolle and Lindley. 

By this method they clearly point out the difference between thesubdivisions of wild species as they ordinarily occur, and the varietiesin our gardens, which would be very rare, were they not singled out andpreserved. 

Our familiarity with a character and our grounds for calling it an oldacquaintance may result from two causes, which in judging a new [130]variety are essentially different. The character in question may bepresent in the given species or it may be lacking, but present in theother group. In the first case a variety can only be formed by the lossof the character, in the second case it arises by the addition of a newone. 

The first mode may be called a negative process, while the second isthen to be designated as positive. And as it is more easy to lose whatone has than to obtain something new, negative varieties are much morecommon than are positive ones. 

Let us now take an instance of a character that is apt to vary in bothways, for this is obviously the best way of making clear what is meantby a negative and a positive change. 

In the family of the composites we find a group of genera with two formsof florets on each flower-head. The hermaphrodite ones are tubular with5, or rarely 4, equal teeth, and occupy the center of the head. Theseare often called the flosculous florets or disk-florets. Those of thecircumference are ligulate and ordinarily unisexual, without stamens. Inmany cases they are sterile, having only an imperfect ovary. They arelarge and brightly colored and are generally designated as ray-florets. As instances we may cite the camomile (_Anthemis nobilis_), the wildcamomile (_Matricaria Chamomilla_), [131] the yarrow (_AchilleaMillefolium_), the daisies, the Dahlia and many others. Species occur inthis group of plants from time to time that lack the ray-florets, as inthe tansy (_Tanacetum vulgare_) and some _artemisias_. And the genus ofthe marigolds or _Bidens_ is noted for containing both of these types. The smaller and the three-toothed marigold (_B. Cernua_ and _B. Tripartita_) are very common plants of wet soil and swamps, ordinarilylacking the ray-florets, and in some countries they are very abundantand wholly constant in this respect, never forming radiate flower-heads. On the other hand the white-flowered and the purple marigold (_B. Leucantha_ and _B. Atropurpurea_) are cultivated species of our gardens, prized for their showy flower-heads with large white or deeply colored, nearly black-purple florets. 

Here we have opportunity to observe positive and negative varieties ofthe same character. The smaller, and the three-toothed marigold occurfrom time to time, provided with ray florets, showing a positivevariation. And the white marigold has produced in our gardens a varietywithout rays. Such varieties are quite constant, never returning to theold species. Positive and negative varieties of this kind are by nomeans rare among the compositae. 

[132] In systematic works the positive ones are as a rule called"radiate, " and the negative ones "discoid. " Discoid forms of theordinary camomile, of the daisy, of some asters (_Aster Tripolium_), andof some centauries have been described. Radiate forms have been observedin the tansy (_Tanacetum vulgare_), the common horse-weed or Canadafleabane (_Erigeron canadensis_) and the common groundsel (_Seneciovulgaris_). Taken broadly the negative varieties seem to be somewhatmore numerous than the positive ones, but it is very difficult to cometo a definite conclusion on this point. 

Quite the contrary is the case with regard to the color-varieties of redand blue flowers. Here the loss of color is so common that every onecould give long lists of examples of it. Linnaeus himself supposed thatno blue or red-colored wild species would be without a white variety. Itis well known that he founded his often criticized prescript never totrust to color in recognizing or describing a species, on this belief. 

On the other hand there are some red varieties of white-floweredspecies. But they are very rare, and little is known about theircharacters or constancy. Blue varieties of white species are not found. The yarrow (_Achillea Millefolium_) has a red-flowered form, whichoccurs [133] from time to time in sunny and sandy localities. I haveisolated it and cultivated it during a series of years and during manygenerations. It is quite true to its character, but the degree of itscoloring fluctuates between pink and white and is extremely variable. Perhaps it can be considered as an inconstant variety. A redfloweredform of the common _Begonia semperflorens_ is cultivated under the nameof "Vernon, " the white hawthorn (_Crataegus Oxyacantha_) is often seenwith red flowers, and a pink-flowered variety of the "Silverchain" or"Bastard acacia" (_Robinia Pseud-Acacia_) is not rarely cultivated. The"Crown" variety of the yellow wall-flower and the black varieties, arealso to be considered as positive color variations, the black being duein the latter cases to a very great amount of the red pigment. 

Among fruits there are also some positive red varieties of greenish oryellowish species, as for instance the red gooseberry (_RibesGrossularia_) and the red oranges. The red hue is far more common inleaves, as seen among herbs, in cultivated varieties of _Coleus_ and inthe brown leaved form of the ordinary white clover, among trees andshrubs in the hazelnut (_Corylus_), the beech (_Fagus_), the birch(_Betula_), the barberry (_Berberis_) and many others. But though mostof these forms are very ornamental and abundant [134] in parks andgardens, little is as yet known concerning the origin of their varietalattributes and their constancy, when propagated by seeds. Besides theray-florets and the colors, there are of course a great many othercharacters in which varieties may differ from their species. In most ofthe cases it is easy to discern whether the new character is a positiveor a negative one. And it is not at all necessary to scrutinize verynarrowly the list of forms to become convinced that the negative form isthe one which prevails nearly everywhere, and that positive aberrationsare in a general sense so rare that they might even be taken forexceptions to the rule. 

Many organs and many qualities may be lost in the origination of avariety. In some instances the petals may disappear, as in _Nigella_, orthe stamens, as in the Guelder-rose (_Viburnum Opulus_) and the_Hortensia_ and in some bulbs even the whole flowers may be wanting, asin the beautiful "Plumosa" form of the cultivated grape-hyacinth or_Muscari comosum_. Fruits of the pineapples and bananas without seedsare on record as well as some varieties of apples and pears, of raisinsand oranges. And some years ago Mr. Riviere of Algeria described a dategrowing in his garden that forms fruit without pits. The stoneless plumof Mr. [135] Burbank of Santa Rosa, California, is also a very curiousvariety, the kernel of which is fully developed but naked, no hardsubstance intervening between it and the pulp. 

More curious still are the unbranched varieties consisting of a singlestem, as may be seen sometimes in the corn or maize and in the fir. Fir-trees of some three or four meters in height without a singlebranch, wholly naked and bearing leaves only on the shoots of the lastyear's growth at the apex of the tree, may be seen. Of course theycannot bear seed, and so it is with the sterile maize, which neverproduces any seed-spikes or staminate flowers. Other seedless varietiescan be propagated by buds; their origin is in most cases unknown, and weare not sure as to whether they should be classified with the constantor with the inconstant varieties. 

A very curious loss is that of starch in the grains of the sugar-cornand the sugar-peas. It is replaced by sugar or some allied substance(dextrine). Equally remarkable is the loss of the runners in theso-called "Gaillon" strawberries. 

Among trees the pendulous or weeping, and the broomlike or fastigiateforms are very marked varieties, which occur in species belonging toquite different orders. The ash, the beach, some willows, many othertrees and some [136] finer species of garden-plants, as _Sophorajaponica_, have given rise to weeping varieties, and the yew-tree or_Taxus_ has a fastigiate form which is much valued because of itsascending branches and pyramidal habit. So it is with the pyramidalvarieties of oaks, elms, the bastard-acacia and some others. 

It is generally acknowledged that these forms are to be considered asvarieties on the ground of their occurrence in so wide a range ofspecies, and because they always bear the same attributes. The pendulousforms owe their peculiarity to a lengthening of the branches and a lossof their habit of growing upwards; they are too weak to retain avertical position and the response to gravity, which is ordinarily thecause of the upright growth, is lacking in them. As far as we know, thecause of this weeping habit is the same in all instances. The fastigiatetrees and shrubs are a counterpart of the weeping forms. Here thetendency to grow in a horizontal direction is lacking, and with it thebilateral and symmetric structure of the branches has disappeared. Inthe ordinary yew-tree the upright stem bears its needles equallydistributed around its circumference, but on the branches the needlesare inserted in two rows, one to the left and one to the right. All theneedles turn their upper surfaces upwards, [137] and their lowersurfaces downwards, and all of them are by this means placed in a singlehorizontal plane, and branching takes place in the same plane. Evidentlythis general arrangement is another response to gravity, and it is thefailure of this reaction which induces the branches to grow upwards andto behave like stems. 

Both weeping and fastigiate characters are therefore to be regarded assteps in a negative direction, and it is highly important that even suchmarked departures occur without transitions or intermediate forms. Ifthese should occur, though ever so rarely, they would probably have beenbrought to notice, on account of the great prospect the numerousinstances would offer. The fact that they are lacking, proves that thesteps, though apparently great, are in reality to be considered ascovering single units, that cannot be divided into smaller parts. Unfortunately we are still in the dark as to the question of theinheritance of these forms, since in most cases it is difficult toobtain pure seed. 

We now consider the cases of the loss of superficial organs, of whichthe nectarines are example. These are smooth peaches, lacking the softhairy down, that is a marked peculiarity of the true peaches. They occurin different [138] races of the peach. As early as the beginning of thepast century, Gallesio described no less than eight subvarieties ofnectarines, each related to a definite race of peach. Most of themreproduce themselves truly from seed, as is well known in this countryconcerning the clingstones, freestones and some other types. Nectarineshave often varied, giving rise to new sorts, as in the case of the whitenectarine and many others differing greatly in appearance and flavor. Onthe other hand it is to be remarked, that the trees do not differ inother respects and cannot be distinguished while young, the varietalmark being limited to the loss of the down on the fruit. Peaches havebeen known to produce nectarines, and nectarines to yield true peaches. Here we have another instance of positive and negative steps withreference to the same character, but I cannot withhold an expression ofsome doubt as to the possibility of crossing and subsequently splittingup of the hybrids as a more probable explanation of at least some of thecases quoted by various writers. 

Smooth or glabrous varieties often occur, and some of them have alreadybeen cited as instances of the multiplication of varietal names. Positive aberrations are rather rare, and are mostly restricted to agreater density of the [139] pubescence in some hairy species, as in_Galeopsis Ladanum canescens_, _Lotus corniculatus hirsutus_ and so on. But _Veronica scutellata_ is smooth and has a pubescent variety, andCytisus prostratus and _C. Spinescens_ are each recorded to have aciliate form. 

Comparable with the occurrence and the lack of hairs, is the existenceor deficiency of the glaucous effect in leaves, as is well known in thecommon _Ricinus_. Here the glaucous appearance is due to wax distributedin fine particles over the surface of the leaves, and in the greenvariety this wax is lacking. Other instances could be given as in thegreen varieties of _Papaver alpinum_ and _Rumex scutatus_. No positiveinstances are recorded in this case. 

Spines and prickles may often disappear and give rise to unarmed anddefenceless types. Of the thorn-apples both species, the whiteflowered_Datura Stramonium_ and the purple _D. Tatula_ have such varieties. Spinach has a variety called the "Dutch, " which lacks the prickles ofthe fruit; it is a very old form and absolutely constant, as are alsothe thornless thorn-apples. Last year a very curious instance of apartial loss of prickles was discovered by Mr. Cockerell of East LasVegas in New Mexico. It is a variety of the American cocklebur, oftencalled sea-burdock, or the [140] hedgehog-burweed, a stout and commonweed of the western states. Its Latin name is _Xanthium canadense_ or_X. Commune_ and the form referred to is named by Mr. Cockerell, _X. Wootoni_, in honor of Professor E. O. Wooton who described the firstcollected specimens. 

The burs of the common species are densely covered with long prickles, which are slightly hooked at the apex. In the new form, which is similarin all other respects to the common cocklebur, the burs are more slenderand the prickles much less numerous, about 25 to the bur and mostlystouter at the base. It occurs abundantly in New Mexico, always growingwith the common species, and seems to be quite constant from seed. Mr. Cockerell kindly sent me some burs of both forms, and from these Iraised in my garden last year a nice lot of the common, as well as ofthe _Wootoni_ plants. 

Spineless varieties are recorded for the bastard-acacia, the holly andthe garden gooseberry (_Ribes Grossularia_, or _R. Uva-crispa_). Aspineless sport of the prickly Broom (_Ulex europaeus_) has been seenfrom time to time, but it has not been propagated. 

Summarizing the foregoing facts, we have excellent evidence of varietiesbeing produced either by the loss of some marked peculiarity or by theacquisition of others that are already [141] present in allied species. There are a great many cases however, in which the morphologic cause ofthe dissimilarity is not so easily discerned. But there is no reason todoubt that most of them will be found to conform to the rule on closerinvestigation. Therefore we can consider the following as the principaldifference between elementary species and varieties; that the firstarise by the acquisition of entirely new characters, and the latter bythe loss of existing qualities or by the gain of such peculiarities asmay already be seen in other allied species. 

If we suppose elementary species and varieties originated by suddenleaps or mutations, then the elementary species have mutated in the lineof progression, some varieties have mutated in the line ofretrogression, while others have diverged from their parental types in aline of depression, or in the way of repetition. This conception agreesquite well with the current idea that in the building up of thevegetable kingdom according to the theory of descent, it is species thatform the links of the chain from the lower forms to the more highlyorganized later derivatives. Otherwise expressed, the system is built upof species, and varieties are only local and lateral, but never of realimportance for the whole structure. 

[142] Heretofore we have generally assumed, that varieties differ fromthe parent-species in a single character only, or at least that only oneneed be considered. We now come to the study of those varieties, whichdiffer in more than one character. Of these there are two types. In thefirst the points of dissimilarity are intimately connected with oneanother, in the second they are more or less independent. 

The mutually related peculiarities may be termed correlative, and wetherefore speak, in such cases, of correlative variability. Thisphenomenon is of the highest importance and is of general occurrence. But before describing some examples, it is as well to note that in thelecture on fluctuating variability, cases of a totally different naturewill be dealt with, which unfortunately are designated by the same term. Such merely fluctuating variations are therefore to be left out of thepresent discussion. 

The purple thorn-apple, which is considered by some writers as a varietyof the white-flowered species or _Datura Stramonium_, and by others as aseparate species, _D. Tatula_, will serve as an illustration. But as itsdistinguishing attributes, as far as we are concerned with them here, are of the nature described above as characteristic of varietalpeculiarities no objection [143] can be made to our using them as a caseof correlative variability. 

The essential character of the purple thornapple lies in the color ofthe flowers, which are of a very beautiful pale blue. But this color isnot limited to the corolla. It is also to be seen in the stems and inthe stalks and veins of the leaves, which are stained with a deeppurple, the blue color being added to the original green. Even on thesurface of the leaves it may spread into a purplish hue. On the stems itis to be met with everywhere, and even the young seedlings show it. Thisis of some importance, as the young plants when unfolding theircotyledons and primary leaves, may be distinguished by this means fromthe seedlings of the white flowered species. 

In crossing experiments it is therefore possible to distinguish thewhites and the blues, even in young seedlings, and experience shows thatthe correlation is quite constant. The color can always be relied upon;if lacking in the seedlings, it will be lacking in the stems and flowersalso; but if the axis of the young plant is ever so slightly tinged, thecolor will show itself in its beauty in the later stages of the life ofthe plant. 

This is what we term correlation. The colors of the different organs arealways in agreement. It is true that they require the concurrence of[144] light for development, and that in the dark or in a faint lightthe seedlings are apt to remain green when they should become purple, but aside from such consideration all organs always come true to theircolor, whether pure green and white, or whether these are combined withthe blue tinge. This constancy is so absolute that the colors of thedifferent organs convey the suggestion, that they are only separatemarks of a single character. 

It is on this suggestion that we must work, as it indicates the cause ofthe correlation. Once present, the faculty of producing the anthocyan, the color in question, will come into activity wherever and wheneveropportunity presents itself. It is the cell-sap of the ordinary celltissue or parenchyma, which is colored by the anthocyan, and for thisreason all organs possessing this tissue, may exhibit the color inquestion. 

Thus the color is not a character belonging to any single organ or cell, nor is it bound to a morphologic unit; it is a free, physiologicquality. It is not localized, but belongs to the entire plant. If wewish to assume for its basis material representative particles, theseparticles must be supposed to be diffused throughout the whole body ofthe plant. 

This conception of a physiologic unit as the [145] cause of colors andother qualities is evidently opposed to the current idea of the cellsand tissues as the morphologic units of the plants. But I do not doubt, that in the long run it will recommend itself as much to the scientistas to the breeder. For the breeder, when desiring to keep his varietiesup to their standard, or when breeding to a definite idea, obviouslykeeps his standard and his ideal for the whole plant, even if he breedsonly for flowers or for fruit. 

I have chosen the color of the purple thornapple as a first example, butthe colors of other plants show so many diverging aspects, all pointingso clearly to the same conclusion, that it would be well to take a moreextensive view of this interesting subject. 

First we must consider the correlation in the colors of flowers andfruits. If both are colored in the species, whether red or brown orpurple or nearly black, and a variety lacking this hue is known, it willbe lacking in both organs. If the color is pure, the flowers and berrieswill become white, but such cases are rare. Ordinarily a yellowish orgreenish tinge underlies the ornamental color, and if this latterdisappears, the yellowish ground will become manifest. So for instancein the Belladonna, a beautiful perennial herb with great shiny black, but very poisonous, fruits. Its flowers are brown, but in [146] somewoods a variety with greenish flowers and bright yellow berries occurs, which is also frequently seen in botanic gardens. The anthocyan dye islacking in both organs, and the same is the case with the stems and theleaves. The lady's laurel or _Daphne Mezereum_ has red corollas, purpleleaves and red fruits; its white flowered variety may be distinguishedby lack of the red hue in the stems and leaves, and by their beautifulyellow berries. Many other instances could be given, since the loss ofcolor in berries is a very common occurrence, so common that forinstance, in the heath-family or Ericaceae, with only a few exceptions, all berry-bearing species have white-fruited varieties. 

The same correlation is observed in the seeds. The white-flowered flaxmay be seen to yield yellow and not brown seeds as in the blue species. Many varieties of flowers may be recognized by the color of their seeds, as in the poppies, stocks and others. Other white-flowered varieties maybe distinguished when germinating, their young axes being of a pureinstead of a purplish green. It is a test ordinarily used by gardeners, to purify their flower beds long before the blooming time, when thinningor weeding them. Even in wild plants, as in _Erodium_, _Calluna_, _Brunella_ and others, a botanist may recognize the rare white-flowered[147] variety by the pure green color of the leaves, at times when it isnot in flower. Some sorts of peas bear colored flowers and a red mark onthe stipules of their leaves. Among bulbous plants many varieties may berecognized even in the dry bulbs by the different tinges of the outerscales. 

Leaving the colors, we come now to another instance of correlation, which is still more astonishing. For it is as rare, as color-varietiesare common. It is afforded by some plants the leaves of which, insteadof being entire or only divided into large parts, are cleft to a greaterextent by repeated fissures of the marginal lobes. Such foliarvariations are often seen in gardens, where they are cultivated fortheir beauty or singularity, as the laciniated alders, fern-leaved, beeches and limes, oakleaved laburnums, etc. Many of them are describedunder the varietal name of _laciniata_. In some cases this fissureextends to the petals of the flowers, and changes them in a way quiteanalogous to the aberrancy of the leaves. 

This is known to occur with a variety of brambles, and is often seen inbotanic gardens in one of the oldest and most interesting of allanomalies, the laciniated variety of the greater celandine or_Chelidonium majus_. Many other instances could be given. Most of thembelong to the [148] group of negative variations, as we have definedthem. But the same thing occurs also with positive varieties, though ofcourse, such cases are very rare. The best known instance is that of theever-flowering begonia, _Begonia semperflorens_, which has green leavesand white flowers, but which has produced garden varieties with a brownfoliage and pink flowers. Here also the new quality manifests itself indifferent organs. 

Enough has now been said on correlative changes, to convince us thatthey are as a rule to be considered as the expression of some generalinternal or physiologic quality, which is not limited to a single organ, but affects all parts of the organism, provided they are capable ofundergoing the change. Such characters are therefore to be considered asunits, and should be referred to the group of single characters. 

Opposed to these are the true compound characters, which consist ofdifferent units. These may be segregated by the production of varieties, and thereby betray the separate factors of the complex group. 

The most beautiful instances of such complex characters are offered bythe colors of some of the most prized garden-flowers. Rarely these areof a single hue, often two or three shades contribute to the effect, andin some cases special [149] spots or lines or tracings are to be seen ona white or on a colored background. That such spots and lines areseparate units is obvious and is demonstrated by the fact that sometimesspotless varieties occur, which in all other respects have kept thecolors of the species. The complexity of the color is equally evident, whenever it is built up of constituents of the anthocyan and of theyellow group. The anthocyan dye is limited to the sap-cavity of thecells, while the yellow and pure orange colors are fixed in specialorgans of the protoplasm. The observation under the microscope shows atonce the different units, which though lying in the same cell and inalmost immediate vicinity of each other are always wholly separated fromone another by the wall of the vacuole or sapfilled cell-cavity. 

The combination of red and yellow gives a brown tinge, as in thecultivated wall-flower, or those bright hues of a dark orange-red, whichare so much sought in tulips. By putting such flowers for a short timein boiling water, the cells die and release the red pigment, whichbecomes diffused in the surrounding fluids and the petals are leftbehind with their yellow tinge. In this way it is easy to separate theconstituents, and demonstrate the compound nature of the originalcolors. 

[150] But the diversity of the color patterns is far from beingexhausted with these simple instances. Apart from them, or joined tothem, other complications are frequently seen, which it is impossible toanalyze in such an artificial way. Here we have to return to our formerprinciple, the comparison of different varieties. Assuming that singleunits may be lost, irrespective of the others, we may expect to findthem segregated by variation, wherever a sufficiently wide range ofcolor-varieties is in cultivation. In fact, in most cases a high degreeof dissimilarity may be reached in the simplest way by such a separationof the components, and by their combination into most diverse smallergroups. A very nice instance of such an analysis of flower-colors isafforded by the ordinary snapdragon. The beautiful brown red color ofthis common garden-plant is composed on one side of yellow elements, onthe other of red units. Of the yellow there are two, one staining thewhole corolla with a light hue, as is to be seen in the pure yellowvariety called _luteum. This form has been produced by the loss of thewhole group of the red constituents. If the yellow tinge is also lost, there arises a white variety, but this is not absolutely colorless, butshows the other yellow constituent. This last stains only some smallparts [151] of the lips of the flower around the throat, brightening, asit seems, the entrance for the visiting insects. In many of the red orreddish varieties this one yellow patch remains, while the generalyellow hue fails. In the variety called "Brilliant" the yellow groundmakes the red color more shiny, and if it is absent the pure carminetinge predominates. 

It is readily seen, that in the ordinary form the lips are of a darkerred than the tube. This evident dissimilarity indicates some complexity. And in fact we have two varieties which exhibit the two causes of thisattribute separately. One of them is called "Delila, " and has the redcolor limited to the lips, whilst the tube is pure white. The other iscalled "Fleshy, " and is of a pale pink throughout the whole corolla. Adding these two units to one another, we get the original dark red ofthe wild type, and it may be briefly stated here, that the way ofeffecting such an addition is given us in the crossing of the "Fleshy"and the "Delila" variety, the hybrid showing the two colors andreturning thereby to the old prototype. 

Other cases of compound flower colors or of color patterns might begiven as in the _Mimulus_ and the poppy, and in most of these cases somevarieties are to be seen in our gardens which show only the singleconstituents of the group. 

[152] Many dark flowers have an intermediate bright hued form besidesthe white variety, as in the case of roses, asters, _Nicandra_ and soon. 

Intermediate forms with respect to stature may also be seen. Theopium-poppy, the snapdragon, peas, the _Nicandra_, and many othergarden-plants have not only dwarf varieties, but also some ofintermediate height. These, though they are intermediate between thetall and dwarf types, cannot be considered as transitions, as betweenthem and the extremes, intermediates are, as a rule wholly lacking. Instances of the same occurrence of three types may be seen in the seedsof maize ("Cuzco, " "Horse-dent" and "Gracillima") of beans and someother plants. The _Xanthium Wootoni_, above referred to, with only partof the prickles of Xanthium commune is also a very curious instance ofthe demonstration of the compound nature of a character. 

Summarizing the conclusions that may be drawn from the evidence given inthis lecture, we have seen that varieties differ from elementary speciesin that they do not possess anything really new. They originate for thegreater part in a negative way, by the apparent loss of some quality, and rarely in a positive manner by acquiring a character, already seenin allied species. These characters are not of the nature of [153]morphologic entities, but are to be considered as physiologic units, present in all parts of the organisms, and manifesting themselves whereever occasion is afforded. They are units in the sense that they mayappear and disappear singly. But very often they are combined to yieldcompound characters, which are capable of analysis. Opportunities forsuch an analysis are afforded by these groups of cultivated varieties, of which some members show a single distinguishing quality, or a numberof them. 

[154]

LECTURE VI

STABILITY AND REAL ATAVISM

It is generally believed that varieties are principally distinguishedfrom species by their inconstancy. This conception is derived from somespecial cases and transferred to others, and in its common form thisbelief must have originated from the confusion which exists as to themeaning of the term variety. It is true that vegetative varieties as arule run back, when propagated by seeds; they are an obvious instance ofinconstancy. In the second place we have considered the group ofinconstant or sporting varieties, which of course we must exclude whenstudying the stability of other types. However, even these sportingvarieties are unstable only to a certain degree, and in a broader sensewill prove to be as true to their character as the most constant types. 

Having separated these two groups, which include also the wide range ofhybrid forms, we may next consider only those varieties of pure origin, and ordinarily propagated by seeds, [155] which have been discussed informer chapters. Their general character lies in their fidelity to type, and in the fact that this is single, and not double, as in the sportingvarieties. 

But the current belief is, that they are only true to theirpeculiarities to a certain degree, and that from time to time, and notrarely, they revert to the type from which they have arisen. Suchreversion is supposed to prove that they are mere varieties, and at thesame time to indicate empirically the species from which they havesprung. 

In the next lecture we shall examine critically the evidence on whichthis assumption rests. Before doing so however, it will be necessary tocollate the cases in which there is no reversion at all, or in which thereversion is absent at least in experimental and pure sowings. 

In the present state of our knowledge it is very difficult to decide, whether or not true reversion occurs in constant varieties. If it doesoccur, it surely does so very rarely and only under unusualcircumstances, or in particular individuals. However when suchindividuals are multiplied by buds and especially when they are the onlyrepresentatives of their type, the reversion, though theoretically rare, will be shown by nearly every specimen of the variety. Examples of thiswill be given below. 

[156] They are generally called atavists or reversionists, but eventhese terms are sometimes used in a different sense. 

Lastly it is to be said that the empirical and experimental evidence asto the question of constancy is not as extensive as it should be. Theexperimental conditions are seldom described, and it is only recentlythat an interest in the matter has been awakened. Much remains to bedone. Among other things the innumerable varieties of trees, shrubs andperennial herbs should be tested as to their constancy when grown frompurely fertilized seeds. Many of them may be included among the numberthat sport constantly. 

Leaving aside the doubtful or insufficiently studied cases, we may nowturn our attention to the facts that prove the absolute stability of alarge number of varieties, at least as far as such completeness can beattained by experiment or observation. 

The best proof is afforded by the varieties which grow wild inlocalities where they are quite isolated from the species, and where forthis reason, no possibility of crossing disturbs the significance of theproof. As one instance the rayless form of the wild camomile, or the_Matricaria Chamomilla discoidea_ may be mentioned. Many systematistshave been so strongly [157] impressed with its absolute constancy andits behavior as an ordinary species, that they have elevated it, as itis called, to the rank of a species. As such it is described under thename of _Matricaria discoidea_ DC. It is remarkable for its rapid andwidespread distribution, as of late years it has become naturalized indifferent parts of America and of Europe, where it is to be seenespecially in France and in Norway. Experimentally I raised insucceeding years between 1000 and 2000 seedlings, but observed no traceof reversion, either in the strongest or in the numerous very small andweak individuals which appeared in the cultures. 

The tansy-ragwort or _Senecio Jacobaea_ may be chosen as a secondinstance. It is a perennial herb with short rootstocks and stout stemsbearing numerous short-peduncled heads in large compact corymb; itmultiplies itself abundantly by seeds and is very common on the sanddunes of Holland. It has two forms, differing only in the occurrence orthe lack of the ray florets. But these two varieties occupy differentlocalities and are even limited to different provinces. As far as I havebeen able to ascertain on numerous excursions during a series of years, they never sport, and are only intermingled on the outskirts of theirhabitats. The rayless form is generally considered as the [158] varietybut it is quite as stable as the radiate species. 

The radiate varieties of marigold, quoted in a former lecture, seem tobe equally constant, when growing far away from their prototypes. Isowed the seeds of a single plant of the radiate form of _Bidenscernua_, and found all of the seedlings came true, and in the next yearI had from their seed between 2, 000 and 3, 000 flowering individuals, allequally radiate. Many species of composites have been tried, and theyare all constant. On the other hand rare sports of this kind have beenobserved by Murr and other authors. 

Many kinds of vegetables and of fruits give instances of stability. White strawberries, green grapes, white currants, crisped lettuce, crisped parsley and some other crisped forms may be cited. The spinagewithout prickles is a widely known instance. White-flowered flax neverreverts to the blue prototype, if kept pure. Sugar-peas and sugar-cornafford further instances. Strawberries without runners have come truefrom seed ever since their first appearance, over a hundred years ago. 

Many garden-varieties, the stability of which under ordinarycircumstances is doubtful, because of their being sown too close toother varieties of the same species, have been tested in [159] respectto their stability by different writers and at different times. In doingthis it is plain that it is very essential to be sure of the purity ofthe seed. Specimens must be grown in positions isolated from theirallies, and if possible be pollinated artificially with the exclusion ofthe visits of insects. This may be done in different ways. If it is arare species, not cultivated in the neighborhood, it is often sufficientto make sure of this fact. Pollen may be conveyed by bees from distancesof some ten or twenty meters, or in rare cases from some hundred metersand more, but a greater distance is ordinarily sufficient for isolation. If the flowers fertilize themselves, as is more often the case than isgenerally supposed, or if it is easy to pollinate them artificially, with their own pollen or in small groups of similar individuals, thebest way is to isolate them by means of close coverings. When flowering, the plants are as a rule too large to be put under bell-glasses, andmoreover such coverings would keep the air moist, and cause theflower-buds to be thrown off. The best coverings are of netting, or ofcanvas of sufficiently wide mesh, although after a long experience Igreatly prefer cages of fine iron-wire, which are put around and overthe whole plant or group of plants, and fastened securely and tightly tothe ground. 

[160] Paper bags also may be made use of. They are slipped over theflowering branches, and bound together around the twigs, thus enclosingthe flowers. It is necessary to use prepared papers, in order that theymay resist rain and wind. The best sort, and the one that I use almostexclusively in my fertilization-experiments, is made of parchment-paper. This is a wood-pulp preparation, freed artificially from the so-calledwood-substance or lignin. Having covered the flowers with care, andhaving gathered the seeds free from intermixtures and if possibleseparately for each single individual, it only remains to sow them inquantities that will yield the greatest possible number of individuals. Reversions are supposed to be rare and small groups of seedlings ofcourse would not suffice to bring them to light. Only sowings of manyhundreds or thousands of individuals are decisive. Such sowings can bemade in one year, or can be extended over a series of years and ofgenerations. Hildebrand and Hoffman have preferred the last method, andso did Hofmeister and many others. Hildebrand sowed the white hyacinth, and the white varieties of the larkspur, the stock and the sweet pea. Hoffman cultivated the white flax and many other varieties andHofmeister extended his sowings [161] over thirty years with the whitevariety of the yellow foxglove (_Digitalis parviflora_). White-floweredvarieties of perennial garden plants were used in my own experiments. Ibought the plants, flowered them under isolation in the way describedabove, gathered the seeds from each individual separately and sowed themin isolated groups, keeping many hundreds and in some cases above athousand plants up to the time of flowering. Among them I found only oneinconstant variety, the white form of the yellow columbine, _Aquilegiachrysantha_. It evidently belonged to the group of sporting varietiesalready referred to. All others came absolutely true to type without anyexception. The species experimented with, were _Campanula persicifolia_, _Hyssopus officinalis_, _Lobelia syphilitica_, _Lychnis chalcedonica_, _Polemonium dissectum_, _Salvia sylvestris_ and some others. Tested inthe same way I found the white varieties of the following annual plantsalso quite true: _Chrysanthemum coronarium_, _Godetia amoena_, _Linumusitatissimum_, _Phlox drummondi_, and _Silene Armeria_. To these may beadded the white hemlock stork's-bill (_Erodium cicutarium album_) whichgrows very abundantly in some parts of my fatherland, and is easilyrecognizable by its pure green leaves and stems, even when notflowering. I cultivated it, in large numbers [162] during fivesucceeding generations, but was never able to find even the slightestindication of a reversion to the red prototype. The scarlet pimpernel or_Anagallis arvensis_ has a blue variety which is absolutely constant. Even in Britton and Brown's "Flora, " which rarely enumerates varieties, it is mentioned as being probably a distinct species. Eight hundredblooming seedlings were obtained from isolated parents, all of the sameblue color. The New Zealand spinage (_Tetragonia expansa_) has agreenish and a brownish variety, the red color extending over the wholefoliage, including the stems and the branches. I have tried both of themduring several years, and they never sported into each other. I raisedmore than 5, 000 seedlings, from the different seeds of one lot of thegreen variety in succeeding years, but neither those germinating in thefirst year, nor the others coming into activity after two, three or fouryears of repose gave any sign of the red color of the original species. 

It is an old custom to designate intermediate forms as hybrids, especially when both the types are widely known and the intermediatesrare. Many persons believe that in doing so, they are giving anexplanation of the rarer forms. But since the laws of hybridism arecoming to be known we shall have to break with [163] all such usages. Sofor instance there are numerous flowers which are of a dark red or adark blue color, and which, besides a white variety, have a pink or apale blue form. Such pale varieties are of exactly the same value asothers, and on testing they are found to be equally stable. So forinstance the pink variety of the Sweet William (_Silene Armeria rosea_), the _Clarkia pulchella carnea_ and the pale variety of the corn-cockle, called usually _Agrostemma Githago nicaeensis_ or even simply _A. Nicaeensis_. The latter variety I found pure during ten succeedinggenerations. Another notable stable intermediate form is the poppybearing the Danish flag (_Papaver somniferum Danebrog_). It is an oldvariety, and absolutely pure when cultivated separately. A long list ofother instances might easily be given. 

Many garden-varieties, that are still universally prized and cultivatedare very old. It is curious to note how often such forms have beenintroduced as novelties. The common foxglove is one of the bestexamples. It has a monstrous variety, which is very showy because itbears on the summit of its raceme and branches, large erect cup-shapedflowers, which have quite a different aspect from the normalthimbleshaped side-blossoms. These flowers are ordinarily described asbelonging to the anomaly [164] known as "peloria, " or regular form of anormally symmetric type; they are large and irregular on the stems andthe vigorous branches but slender and quinate on the weaker twigs. Theirbeauty and highly interesting anomalous character has been the cause oftheir being described many times, and nearly always as a novelty; theyhave been recently re-introduced into horticulture as such, though theywere already cultivated before the middle of the last century. Aboutthat time very good descriptions with plates were published in thejournal "Flora" by Vrolik, but afterwards they seem to have beenforgotten. The peloric variety of the foxglove always comes true fromseed, though in the strict sense of the word which we have chosen forour discussion, it does not seem to be a constant and pure variety. 

It is very interesting to compare old botanical books, or even olddrawings and engravings containing figures of anomalous plants. Thecelebrated Pinacothec of Munich contains an old picture by Holbein(1495-1543) representing St. Sebastian in a flower-garden. Of the plantsmany are clearly recognizable, and among others there is one of the"one-leaved" variety of the strawberry, which may still be met with inbotanical gardens. In the year 1671 a Dutch botanist, Abraham Muntingpublished [165] a large volume on garden-plants, containing a greatnumber of very good engravings. Most of them of course show normalplants, but intermixed with these are varieties, that are still incultivation and therefore must be at least two centuries old. Others, though not figured, are easily recognized by their names anddescriptions. The cockscomb is the most widely known, but many white ordouble flowered varieties were already cultivated at that time. Thestriped Jalappa, the crested Sedum, the fasciated crown-imperial, whitestrawberries, red gooseberries and many others were known to Munting. 

Some varieties are as old as culture itself, and it is generally knownthat the Romans cultivated the white form of the opium-poppy and usedthe foliage of the red variety of the sugarbeet as a vegetable. 

In our time flowers and fruits are changing nearly as rapidly as thefancies and tastes of men. Every year new forms are introduced and usurpthe place of older ones. Many are soon forgotten. But if we look at oldcountry gardens, a goodly number of fine and valued old sorts are stillto be found. It would be worth while to make special collections ofliving plants of old varieties, which surely would be a good andinteresting work and bring about a conviction [166] of the stability ofpure strains. Coming now to the other side of the question, we mayconsider those cases of reversion which have been recorded from time totime, and which always have been considered as direct proofs of thevarietal character of the reverting form. Reversion means the fallingback or returning to another type, and the word itself expresses theidea that this latter type is the form from which the variety hasarisen. 

Some instances of atavism of this kind are well known, as they are oftenrepeated by individuals that are multiplied by buds or by grafting. Before looking attentively into the different features of the many casesof rare reversions it will be advisable to quote a few examples. 

The flowering-currant of the Pacific Coast or North American scarletribes (_Ribes sanguineum_), a very popular ornamental shrub, will serveas a good example. It is prized because of its brilliant red racemes offlowers which blossom early in the spring, before the appearance of theleaves. From this species a white form has arisen, which is an old andwidely cultivated one, but not so highly prized because of its paleflowers. These are not of a pure white, but have retained a faintreddish hue. The young twigs and the stalks of the [167] leaves affordan instance of correlated variability since in the species the red colorshows itself clearly mixed with the green, while in the variety thistinge is wholly wanting. 

Occasionally this white-flowered currant reverts back to the originalred type and the reversion takes place in the bud. One or two buds on ashrub bearing perhaps a thousand bunches of white flowers produce twigsand leaves in which the red pigment is noticeable and the flowers ofwhich become brightly colored. If such a twig is left on the shrub, itmay grow further, ramify and evolve into a larger group of branches. Allof them keep true to the old type. Once reverted, the branches remainforever atavistic. It is a very curious sight, these small groups of redbranches among the many white ones. And for this reason attention isoften called to it, and more than once I myself have had the opportunityof noting its peculiarities. It seems quite certain that by plantingsuch shrubs in a garden, we may rely upon seeing sooner or later somenew buds reverting to the prototype. 

Very little attention seems hitherto to have been given to this curiousphenomenon, though in many respects it deserves a closer investigation. The variety is said to have originated from seed in Scotland, many yearsago, and [168] seems to be propagated only by cuttings or by grafting. If this is true, all specimens must be considered as constitutingtogether only one individual, notwithstanding their wide distribution inthe gardens and parks of so many countries. This induces me to suppose, that the tendency to reversion is not a character of the variety assuch, but rather a peculiarity of this one individual. In other words itseems probable that when the whitish variety arises a second time fromthe red species, it is not at all necessary that it should exhibit thissame tendency to revert. Or to put it still in another way, I think thatwe may suppose that a variety, which might be produced repeatedly fromthe same original stock, would only in rare individuals have a tendencyto revert, and in most cases would be as absolutely constant as thespecies itself. 

Such a conception would give us a distinct insight into the cause of therarity of these reversions. Many varieties of shrubs and trees haveoriginated but once or twice. Most of them must therefore, if oursupposition is correct, be expected to be stable and only a few may beexpected to be liable to reversions. 

Among the conifers many very good cases of reversions by buds are to befound in gardens and glasshouses. They behave exactly like the whitishcurrant. But as the varietal characters [169] are chiefly found in thefoliage and in the branches, these aberrations are to be seen on theplants during the whole year. Moreover they are in some cases much morenumerous than in the first instance. The _Cryptomeria_ of Japan has avariety with twigs resembling ropes. This is not caused by a twisting, but only by a curvature of the needles in such a way that they seem togrow in spiral lines around the twigs. This variety often reverts to thetype with widely spread, straight needles. And on many a specimen four, five, or more reverted branches may be seen on different parts of thesame shrub. Still more widely cultivated is the shrub called_Cephalotaxus pedunculata fastigiata_, and more commonly known under itsold name of _Podocarpus koraiana_. It is the broomlike variety of aspecies, nearly allied to the common American and European species ofyew, (_Taxus minor_ and _T. Baccata_). It is a low shrub, with broadlylinear leaves of a clear green. In the species the leaves are arrangedin two rows, one to the left and one to the right of the horizontallygrowing and widely spreading branches. In the variety the branches areerect and the leaves inserted on all sides. When sporting, it returns tothe bilateral prototype and flat wings of fan-shaped twigs are producedlaterally on its dense broom-like tufts. 

[170] Wherever this variety is cultivated the same reversion may beseen; it is produced abundantly, and even under seemingly normalcircumstances. But as in the case of the _Ribes_ all the specimens arederived by buds from a single original plant. The variety was introducedfrom Japan about the year 1860, but is probably much older. Nothing isknown as to its real origin. It never bears flowers or fruits. It iscurious to note that the analogous variety of the European yew, _Taxusbaccata fastigiata_, though much more commonly cultivated than the_Cephalotaxus_, never reverts, at least as far as I have been able toascertain. This clearly corroborates the explanation given above. 

After considering these rare instances of more widely known reversions, we may now examine the question of atavism from a broader point of view. But in doing so it should once more be remembered, that all cases ofhybridism and also all varieties sporting annually or frequently, are tobe wholly excluded. Only the very rare occurrence of instances ofatavism in varieties that are for the rest known to be absolutelyconstant, is to be considered. 

Atavism or reversion is the falling back to a prototype. But what is aprototype? We may take the word in a physiologic or in a systematicsense. Physiologically the signification is a [171] very narrowlyrestricted one; and includes only those ancestors from which a form isknown to have been derived. But such evidence is of course historic. Ifa variety has been observed to spring from a definite species, and ifthe circumstances have been sufficiently ascertained not to leave theslightest doubt as to its pure origin, and if moreover all the evidencehas been duly recorded, we may say that the origin of the variety ishistorically known. In most cases we must be content with the testimony, given somewhat later, and recorded after the new variety had theopportunity of showing its greater merits. 

If it now happens that such a variety of recorded origin shouldoccasionally revert to its parent-species, we have all we can wish for, in the way of a thoroughly proved case of atavism. But such instancesare very rare, as the birth of most varieties has only been veryimperfectly controlled. 

Next to this comes the systematic relation of a variety to its species. The historic origin of the variety may be obscure, or may simply beforgotten. But the distinguishing marks are of the order described inour last lecture, either in the positive or in the negative direction, and on this ground the rarer form is considered to be a variety of themore wide-spread one. If [172] now the presumed variety sports and runsover to the presumed type, the probability of the supposed relation isevidently enhanced. But it is manifest that the explanation rests uponthe results of comparative studies, and not upon direct observations ofthe phenomena themselves. 

The nearer the relations between the two types in question, the lessexposed to doubt and criticism are the conclusions. But the domain ofatavism is not restricted to the cases described. Quite on the contrarythe facts that strike us most forcibly as being reversions are thosethat are apt to give us an insight into the systematic affinity of ahigher degree. We are disposed to make use of them in our attempts toperfect the natural system and to remould it in such a way as to becomea pedigree of the related groups. Such cases of atavism no doubt occur, but the anomalies referred to them must be interpreted merely on theground of our assumptions as to the relative places in the system to beassigned to the different forms. 

Though such instances cannot be considered as belonging strictly to thesubject we are dealing with, I think it may be as well to give anexample, especially as it affords an occasion for referring to thehighly important researches of Heinricher on the variability andatavistic [173] tendencies of the pale blue flag or _Iris pallida_. Theflowers of the blue flags have a perianth of six segments united belowinto a tube. The three outer parts are dilated and spreading, orreflexed, while the three inner usually stand erect, but in most speciesare broad and colored like the outer ones. Corresponding to the outer, perianth-segments are the three stamens and the three, petal-likedivisions of the style, each bearing a transverse stigma immediatelyabove the anther. They are pollinated by bumble-bees, and in someinstances by flies of the genus _Rhingia_, which search for the honey, brush the pollen out of the anthers and afterwards deposit it on thestigma. According to systematic views of the monocotyledons the originalprototype of the genus _Iris_ must have had a whorl of six equal, ornearly equal perianth-segments and six stamens, such as are now seen inthe more primitive types of the family of the lilies, as for instance inthe lilies themselves, the tulips, hyacinths and others. As to theperianth this view is supported by the existence of one species, the_Iris falcifolia_, the perianth of which consists of six equal parts. But species with six stamens are wholly lacking. Heinricher however, incultivating some anomalous forms of _Iris pallida_, succeeded in fillingout this gap and in producing [174] flowers with a uniform perianth andsix stamens, recalling thereby the supposed ancestral type. The way inwhich he got these was as follows: he started from some slightdeviations observed in the flowers of the pale species, sowed the seedsin large numbers and selected from the seedlings only those whichclearly showed anomalies in the expected atavistic direction. Byrepeating this during several generations he at last reached his goaland was able to give reality to the prototype, which formerly was only ahypothetical one. The _Iris kaempferi_, a large-flowered Japanesespecies much cultivated in gardens, is very variable in the number ofthe different parts of its flowers, and may in some instances be seeneven with six stamens. If studied in the same way as Heinricher's iris, it no doubt will yield highly interesting and confirmatory results. 

Many other instances of such systematic atavism could be given, andevery botanist can easily add some from memory. Many anomalies, occurring spontaneously, are evidently due to the same principle, but itwould take too long to describe them. 

Reversion may occur either by buds or by seeds. It is highly probablethat it occurs more readily by sexual than by asexual propagation. Butif we restrict the discussion to the limits [175] hitherto observed, seed-reversions must be said to be extremely rare. Or rather cases whichare sufficiently certain to be relied upon, are very rare, and perhapswholly lacking. Most of the instances, recorded by various writers, areopen to question. Doubts exist as to the purity of the seeds and thepossibility of some unobserved cross disturbing the results. 

In the next lecture we shall deal in general with the ordinary causesand results of such crosses. We shall then see that they are so commonand occur so regularly under ordinary circumstances that we can neverrely on the absolute purity of any seeds, if the impossibility of anoccasional cross has not been wholly excluded, either by thecircumstances themselves, or by experimental precautions taken duringthe flowering period. 

For these reasons cases of atavism given without recording thecircumstances, or the precautions that guarantee the purity of thefertilization, should always be disregarded. And moreover another proofshould always be demanded. The parent which yielded the seeds might beitself a hybrid and liable to reversions by the ordinary laws of thesplitting up of hybrids. Such cases should likewise be discarded, sincethey bring in confusing elements. If we review the long list of recordedcases by these [176] strict methods of criticism very few instances willbe found that satisfy legitimate demands. On this ground it is by farsafer in the present state of our knowledge, to accept bud-variationsonly as direct proofs of true atavism. And even these may not always berelied on, as some hybrids are liable to split up in a vegetative way, and in doing so to give rise to bud-variations that are in many respectsapparently similar to cases of atavism. But fortunately such instancesare as yet very rare. 

After this discussion it would be bold indeed to give instances ofseed-atavism, and I believe that it will be better to refrain whollyfrom doing so. 

Many instances of so-called atavism are of purely morphologic nature. The most interesting cases are those furnished by the forms which someplants bear only while young, and which evidently connect them withallied species, in which the same features may be seen in the adultstate. Some species of the genus _Acacia_ bear bipinnate leaves, whileothers have no leaves at all, but bear broadened and flattened petiolesinstead. The second type is presumed to be descended from the first bythe loss of the leaflets and the modification of the stalks into flatand simple phyllodes. But many of them are liable to recall thisprimitive form [177] when very young, in the first two or three, orsometimes in eight or ten primary leaves. These leaves are small becauseof the weakness of the young plant and therefore often more or lessreduced in structure. But they are usually strictly bipinnate andthereby give testimony as to their descent from species which bear suchleaves throughout their life. 

Other similar cases could be given, but this will suffice. They oncemore show how necessary it is to separate the different cases, throwntogether until now, under this general name of atavism. It would be farbetter to give them all special names, and as long as these are notavailable we must be cautious not to be misguided by the name, andespecially not to confuse different phenomena with one another, becauseat the present time they bear the same names. 

Taking into consideration the relatively numerous restrictions resultingfrom this discussion, we will now make a hasty survey of some of themore notable and generally acknowledged cases of atavism bybud-propagation. But it should be repeated once more that most of thehighly cultivated plants, grown as vegetables, or for their fruit orflowers, have so many crosses in their ancestry, that it seems better toexclude them from all considerations, in which purity of [178] descentis a requisite. By so doing, we exclude most of the facts which wereuntil now generally relied upon. For the roses, the hyacinths, thetulips, the chrysanthemums always have furnished the largestcontributions to the demonstrations of bud-variation. But they have beencrossed so often, that doubt as to the purity of the descent of anysingle form may recur, and may destroy the usefulness of their manyrecorded cases of bud-variation for the demonstration of real atavism. The same assertion holds good in many other cases, as with _Azalea_ and_Camellia_. And the striped varieties of these genera belong to thegroup of ever-sporting forms, and therefore will be considered later on. So it is with carnations and pinks, which occasionally vary by layering, and of which some kinds are so uncertain in character that they arecalled by floriculturists "catch-flowers. " On the other hand there is alarger group of cases of reversion by buds, which is probably not ofhybrid nature, nor due to innate inconstancy of the variety, but must beconsidered as pure atavism. I refer to the bud-variations of so many ofour cultivated varieties of shrubs and trees. Many of them arecultivated because of their foliage. They are propagated by grafting, and in most cases it is probable that all the numerous specimens [179]of the same variety have been derived in this way from one primitive, aberrant individual. We may disregard variegated leaves, spotted ormarked with white or yellow, because they are too inconstant types. 

We may next turn our attention to the varieties of trees with cutleaves, as the oakleaved _Laburnum_, the parsley-leaved vine and thefern-leaved birch. Here the margin of the leaves is deeply cut anddivided by many incisions, which sometimes change only the outer partsof the blade, but in other cases may go farther and reach, or nearlyreach, the midvein, and change the simple leaf into a seemingly compoundstructure. The anomaly may even lead to the almost complete loss of allthe chorophyll-tissue and the greater part of the lateral veins, as inthe case of the cut-leaved beech or _Fagus sylvatica pectinata_. 

Such varieties are often apt to revert by buds to the common forms. Thecut-leaved beech sometimes reverts partially only, and the branchesoften display the different forms of cut-leaved, fern-like, oak-leavedand other variously shaped leaves on the same twigs. But this is merelydue to the wide variability of the degree of fissure and is to beconsidered only as a fluctuation between somewhat widely distantextremes, which may even apparently include [180] the form of the commonbeech-leaves. It is not a bud-variation at all, and it is to be met withquite commonly while the true reversions by buds are very rare and areof the nature of sports appearing suddenly and remaining constant on thesame twig. Analogous phenomena of wide variability with true reversionmay be seen in the variety of the European hornbeam called _CarpinusBetulus heterophylla_. The leaves of this tree generally show thegreatest diversity in form. Some other cases have been brought togetherby Darwin. In the first place a subvariety of the weeping-willow withleaves rolled up into a spiral coil. A tree of this kind kept true fortwenty-five years and then threw out a single upright shoot bearing flatleaves. The barberry (_Berberis_) offers another case; it has a wellknown variety with seedless fruit, which can be propagated by cuttingsor layers, but its runners are said always to revert to the common form, and to produce ordinary berries with seeds. Most of the cases referredto by Darwin, however, seem to be doubtful and cannot be considered astrue proofs of atavism until more is known about the circumstances underwhich they were produced. 

Red or brown-leaved varieties of trees and shrubs also occasionallyproduce green-leaved branches, and in this way revert to the type [181]from which they must evidently have arisen. Instances are on record ofthe hazel, _Corylus Avellana_, of the allied _Corylus tubulosa_, of thered beech, the brown birch and of some other purple varieties. Even thered bananas, which bear fruits without seeds and therefore have no otherway of being propagated than by buds, have produced a green variety withyellow fruits. The _Hortensia_ of our gardens is another instance of asterile form which has been observed to throw out a branch with cymesbearing in their center the usual small staminate and pistillate flowersinstead of the large radiate and neutral corollas of the variety, thereby returning to the original wild type. Crisped weeping-willows, crisped parsley and others have reverted in a similar manner. 

All such cases are badly in need of a closer investigation. And as theyoccur only occasionally, or as it is commonly stated, by accident, thestudent of nature should be prepared to examine carefully any case whichmight present itself to him. Many phases of this difficult problem couldno doubt be solved in this way. First of all the question arises as towhether the case is one of real atavism, or is only seemingly so, beingdue to hybrid or otherwise impure descent of the varying individual, andsecondly whether it may be only an instance of the regularly [182]occurring so-called atavism of the sporting varieties with which weshall deal in a later lecture. If it proves to be real atavism and rare, the case should be accurately described and figured, or photographed ifpossible; and the exact position of the reverting bud should beascertained. Very likely the so-called dormant or resting buds are moreliable to reversions than the primary ones in the arils of the leaves ofyoung twigs. Then the characters of the atavistic branches should beminutely compared with those of the presumed ancestor; they may be quiteidentical with them or slightly divergent, as has been asserted in someinstances. The atavism may be complete in one case, but more or lessincomplete in others. By far the most interesting point is the question, as to what is to be expected from the seeds of such an atavistic branch. Will they keep true to the reverted character, or return to thecharacters of the plant which bears the retrograde branch? Will all ofthem do so, or only part of them, and how large a part? It is veryastonishing that this question should still be unsolved where so manyindividual trees bear atavistic branches that remain on them throughlong series of years. But then many such branches do not flower at all, or if they flower and bear seed, no care is taken to prevent [183]cross-fertilization with the other flowers of the same plant, and theresults have no scientific value. For anyone who cares to work with theprecautions prescribed by science, a wide field is here open forinvestigation, because old reverted branches may be met with much lessrarely than new ones. 

Finally the possibility is always to be considered that the tendency tobud-reversions may be a special feature of some individuals, and may notbe met with in others of the same variety. I have spoken of this before. For the practical student it indicates that a specimen, once observed toproduce atavistic buds, may be expected to do the same thing again. Andthen there is a very good chance that by combining this view with theidea that dormant buds are more apt to revert than young ones, we mayget at a method for further investigation, if we recur to the practiceof pruning. By cutting away the young twigs in the vicinity of dormantbuds, we may incite these to action. Evidently we are not to expect thatin so doing they will all become atavistic. For this result is not atall assured; on the contrary, all that we might hope to attain would bethe possibility of some of them being induced to sport in the desireddirection. 

Many questions in scientific research can only [184] be answered by longand arduous work in well equipped laboratories; they are not to beattempted by every one. But there are other problems which the mostcomplete of institutions are not able to study if opportunity is notoffered them, and such opportunities are apt to occur more often infields, gardens, parks, woods and plains, than in the relatively smallexperimental gardens of even the largest institution. Therefore, whosoever has the good fortune to find such sports, should never allowthe occasion to pass without making an investigation that may bringresults of very great importance to science. 

[185]

LECTURE VII

FALSE ATAVISM OR VICINISM

About the middle of the last century Louis de Vilmorin showed that itwas possible to subject plants to the methods of amelioration of racesthen in use for domestic animals, and since that time atavism has playeda large part in all breeding-processes. It was considered to be thegreatest enemy of the breeder, and was generally spoken of as a definiteforce, working against and protracting the endeavors of thehorticulturist. 

No clear conception as to its true nature had been formulated, and eventhe propriety of designating the observed phenomena by the term atavismseemed doubtful. Duchesne used this word some decades ago to designatethose cases in which species or varieties revert spontaneously, or fromunknown internal causes, to some long-lost characters of theirancestors. Duchesne's definition was evidently a sharp and useful one, since it developed for the first time the idea of latent or dormantqualities, [186] formerly active, and awaiting probably throughcenturies an occasion to awaken, and to display the lost characters. 

Cases of apparent reversion were often seen in nurseries, especially inflower culture, which under ordinary circumstances are rarely whollypure, but always sport more or less into the colors and forms of alliedvarieties. Such sporting individuals have to be extirpated regularly, otherwise the whole variety would soon lose its type and its uniformityand run over to some other form in cultivation in the vicinity. For thisreason atavism in nurseries causes much care and labor, and consequentlyis to be dealt with as a very important factor. 

From time to time the idea has suggested itself to some of the bestauthorities on the amelioration of plants, that this atavism was not dueto an innate tendency, but, in many cases at least, was produced bycrosses between neighboring varieties. It is especially owing to Verlotthat this side of the question was brought forward. But breeders as arule have not attached much importance to this supposition, chieflybecause of the great practical difficulties attending any attempt toguard the species of the larger cultures against intermixture with othervarieties. Bees and humble-bees fly from bud to bud, and carry thepollen from one [187 ] sort to another, and separation by greatdistances would be required to avoid this source of impurity. Unfortunately the arrangements and necessities of large cultures make itimpossible to isolate the allied varieties from each other. 

From a theoretical point of view the origin of these impurities is ahighly important question. If the breeders' atavism is due to crosses, and only to this cause, it has no bearing at all on the question of theconstancy of varieties. And the general belief, that varieties aredistinguished from true species by their repeated reversion and thateven such reversibility is the real distinction of a variety, would nothold. 

For this reason I have taken much trouble in ascertaining thecircumstances which attend this form of atavism. I have visited a numberof the leading nurseries of Europe, tested their products in variousways, and made some experiments on the unavoidable conditions ofhybridizing and on their effect on the ensuing generations. Theseinvestigations have led me to the conclusion, that atavism, as it isgenerally described, always or nearly always is due to hybridization, and therefore it is to be considered as untrue or false atavism. 

True atavism, or reversion caused by an innate latent tendency, seemsto be very rare, [188] and limited to such cases as we have spoken ofunder our last heading. And since the definition, given to this term byits author, Duchesne, is generally accepted in scientific works, itseems better not to use it in another sense, but rather to replace it insuch cases by another term. For this purpose I propose the wordvicinism, derived from the Latin vicinus or neighbor, as indicating thesporting of a variety under the influence of others in its vicinity. Used in this way, this term has the same bearing as the word atavism ofthe breeders, but it has the advantage of indicating the true causethereof. 

It is well known that the term variability is commonly employed in thebroadest possible sense. No single phenomenon can be designated by thisname, unless some primary restriction be given. Atavism and vicinism areboth cases of variability, but in wholly different sense. For thisreason it may be as well, to insert here a short survey of the generalmeanings to be conveyed by the term variation. It implies in the firstplace the occurrence of a wide range of forms and types, irrespective oftheir origin, and in the second place the process of the change in suchforms. In the first signification it is nearly identical withpolymorphy, or richness of types, especially so when these [189] typesare themselves quite stable, or when it is not at all intended to raisethe question of their stability. In scientific works it is commonly usedto designate the occurrence of subspecies or varieties, and the same isthe case in the ordinary use of the term when dealing with cultivatedplants. A species may consist of larger or smaller groups of such units, and they may be absolutely constant, never sporting if hybridization isprecluded, and nevertheless it may be called highly variable. Theopium-poppy affords a good instance. It "varies" in height, in color offoliage and flowers; the last are often double or laciniated; it mayhave white or bluish seeds, the capsules may open themselves or remainclosed and so on. But every single variety is absolutely constant, andnever runs into another, when the flowers are artificially pollinatedand the visits of insects excluded. So it is with many other species. They are at the same time wholly stable and very variable. 

The terms variation and variety are used frequently when speaking ofhybrids. By crossing forms, which are already variable in the sense justmentioned, it is easy to multiply the number of the types, and even incrossing pure forms the different characters may be combined indifferent ways, the resulting combinations [190] yielding new, and veryoften, valuable varieties. But it is manifest that this form ofvariation is of quite another nature from the variations of pure races. Many hybrid varieties are quite constant, and remain true to their typeif no further crosses are made; many others are artificially propagatedonly in a vegetative way, and for this reason are always found true. Hybrid varieties as a rule were formerly confused with pure varieties, and in many instances our knowledge as to their origin is quiteinsufficient for sharp distinctions. To every student of nature it isobvious, that crossing and pure variability are wholly distinct groupsof phenomena, which should never be treated under the same head, orunder the same name. 

Leaving aside polymorphy, we may now discuss those cases of variability, in which the changes themselves, and not only their final results play apart. Of such changes two types exist. First, the ever-recurringvariability, never absent in any large group of individuals, anddetermining the differences which are always to be seen between parentsand their children, or between the children themselves. This type iscommonly called "individual variability" and since this term also hasstill other meanings, it has of late become customary to use instead theterm "fluctuating variability. " [191] And to avoid the repetition of thelatter word it is called "fluctuation. " In contrast to thesefluctuations are the so-called sports or single varieties, not rarelydenominated spontaneous variations, and for which I propose to use theterm "mutations. " They are of very rare occurrence and are to beconsidered as sudden and definite steps. 

Lastly, we have to consider those varieties, which vary in a much widerrange than the ordinary ones, and seem to fluctuate between two oppositeextremes, as for instance variegated leaves, cultivated varieties withvariegated or striped flowers, double flowers and some other anomalies. They are eversporting and ever-returning from one type to the other. Ifhowever, we take the group of these extremes and their intermediates asa whole, this group remains constant during the succeeding generations. Here we find once more an instance of the seemingly contradictorycombination of high variability and absolute constancy. It means thatthe range of variability has quite definite limits, which in the commoncourse of things, are never transgressed. 

We may infer therefore that the word variability has such a wide rangeof meanings that it ought never be used without explanation. [192]Nothing indeed, is more variable than the signification of the termvariable itself. 

For this reason, we will furthermore designate all variations under theinfluence of neighbors with the new and special term "vicinism. " Italways indicates the result of crossing. 

Leaving this somewhat lengthy terminological discussion, we now come tothe description of the phenomenon itself. In visiting the plantations ofthe seedsmen in summer and examining the large fields of garden-flowersfrom which seed is to be gathered, it is very rare to find a plot quitepure. On the contrary, occasional impurities are the rule. Every plotshows anomalous individuals, red or white flowers among a field of blue, normal among laciniated, single among double and so on. The most curiousinstance is afforded by dwarf varieties, where in the midst of hundredsand thousands of small individuals of the same height, some specimensshow twice their size. So for instance, among the dwarfs of thelarkspur, _Delphinium Ajacis_. 

Everywhere gardeners are occupied in destroying these "atavists, " asthey call them. When in full bloom the plants are pulled up and thrownaside. Sometimes the degree of impurity is so high, that great piles ofdiscarded plants of the same species lie about the [193] paths, as Ihave seen at Erfurt in the ease of numerous varieties of the Indiancress or _Tropaeolum_. 

Each variety is purified at the time when it shows its characters mostclearly. With vegetables, this is done long before flowering, but withflowers only when in full bloom, and with fruits, usually afterfertilization has been accomplished. It needs no demonstration to showthat this difference in method must result in very diverging degrees ofpurity. 

We will confine ourselves to a consideration of the flowers, and askwhat degree of purity may be expected as the result of the eliminationof the anomalous plants during the period of blooming. 

Now it is evident that the colors and forms of the flowers can only beclearly distinguished, when they are fully displayed. Furthermore it isimpossible to destroy every single aberrant specimen as soon as it isseen. On the contrary, the gardener must wait until all or nearly allthe individuals of the same variety have displayed their characters, asonly in this way can all diverging specimens be eliminated by a singleinspection. Unfortunately the insects do not wait for this selection. They fertilize the flowers from the beginning, and the damage will havebeen done [194] long before the day of inspection comes around. Crossesare unavoidable and hybrid seeds will unavoidably come into the harvest. Their number may be limited by an early eradication of the vicinists, orby the elimination of the first ripe seeds before the beginning of theregular harvest, or by other devices. But some degree of impurity willremain under ordinary circumstances. 

It seems quite superfluous to give more details. In any case in whichthe selection is not done before the blooming period, some impuritiesmust result. Even if it is done before that time, errors may occur, andamong hundreds and thousands of individuals a single anomalous one mayescape observation. 

The conclusion is, that flower seeds as they are offered in commerce, are seldom found absolutely pure. Every gardener knows that he will haveto weed out aberrant plants in order to be sure of the purity of hisbeds. I tested a large number of samples of seeds for purity, boughtdirectly from the best seed growers. Most of them were found to containadmixtures and wholly pure samples were very rare. 

I will now give some illustrative examples. From seeds of a yellowsnapdragon, I got one red-flowered specimen among half a hundred [195]yellow ones, and from the variety "Delila" of the same species two redones, a single white and two belonging to another variety called"Firefly. " _Calliopsis tinctoria_ has three varieties, the ordinarytype, a brown-flowered one and one with tubular rays. Seeds of each ofthese three sorts ordinarily contain a few belonging to the others. _Iberis umbellata rosea_ often gives some white and violet examples. The"Swan" variety of the opium-poppy, a dwarfish double-flowered form of apure white, contained some single-flowered and some red-flowered plants, when sown from commercial seed are said to be pure. But these were onlyoccasional admixtures, since after artificial fertilization of thetypical specimens the strain at once became absolutely pure, andremained so for a series of generations, as long as the experiment wascontinued. Seeds of trees often contain large quantities of impurities, and the laciniated varieties of birch, elder and walnut have often beenobserved to come true only in a small number of seedlings. 

In the case of new or young varieties, seed merchants often warn theircustomers as to the probable degree of purity of the seeds offered, inorder to avoid complaints. For example the snow-white variety of thedouble daisy, _Bellis perennis plena_, was offered at the start ascontaining [196] as much as 20% of red-flowered specimens. 

Many fine varieties are recorded to come true from seed, as in the caseof the holly with yellow fruits, tested by Darwin. Others have beenfound untrue to a relatively high degree, as is notorious in the case ofthe purple beech. Seeds of the laciniated beech gave only 10% oflaciniated plants in experiments made by Strasburger; seeds of themonophyllous acacia, _Robinia Pseud-Acacia monophylla_, were found to betrue in only 30% of the seedlings. Weeping ashes often revert to theupright type, red May-thorns (_Crataegus_) sometimes revert nearlyentirely to the white species and the yellow cornel berry is recorded tohave reverted in the same way to the red berries of the _Cornus Mas_. 

Varieties have to be freed by selection from all such impurities, sinceisolation is a means which is quite impracticable under ordinarycircumstances. Isolation is a scientific requirement that should neverbe neglected in experiments, indeed it may be said to be the first andmost important requisite for all exact research in questions ofvariability and inheritance. But in cultivating large fields of alliedvarieties for commercial purposes, it is impossible to grow them at suchdistances from each other [197] as to prevent cross-pollination by thevisits of bees. 

This purification must be done in nearly every generation. The oldestvarieties are to be subjected to it as well as the latest. There is noregular amelioration, no slow progression in the direction of becomingfree from these admixtures. Continuous selection is indispensable tomaintain the races in the degree of purity which is required incommerce, but it does not lead to any improvement. Nor does it go so faras to become unnecessary in the future. This shows that there must be acontinuous source of impurities, which in itself is not neutralized byselection, but of which selection can only eliminate the deterioratingelements. 

The same selection is usually applied to new varieties, when theyoccasionally arise. In this case it is called "fixing, " as gardenersgenerally believe that through selection the varieties are brought tothe required degree of purity. This belief seems to rest mainly onobservations made in practice, where, as we have seen, isolation is ofvery rare application. Most varieties would no doubt be absolutely purefrom the first moment of their existence, if it were only possible tohave them purely fertilized. But in practice this is seldom to beobtained. Ordinarily the breeder is content with such slow [198]improvement as may be obtained with a minimum of cost, and this mostlyimplies a culture in the same part of the nursery with older varietiesof the same species. Three, four or five years are required to purifythe novelty, and as this same length of time is also required to producesufficient quantities of seed for commercial purposes, there is nostrong desire to shorten the period of selection and fixation. I hadoccasion to see this process going on with sundry novelties at Erfurt inGermany. Among them a chamois-colored variety of the common stock, abluish _Clarkia elegans_ and a curiously colored opium-poppy may bementioned. In some cases the crossfertilization is so overwhelming, thatin the next generation the novelty seems entirely to have disappeared. 

The examples given may suffice to convey a general idea of thephenomenon, ordinarily called atavism by gardeners, and consideredmostly to be the effect of some innate tendency to revert to theancestral form. It is on this conception that the almost universalbelief rests, that varieties are distinguished, as such, from species bytheir inconstancy. Now I do not deny the phenomenon itself. The impurityof seeds and cultures is so general and so manifest, and may so easilybe tested by every one [199] that it cannot reasonably be subjected toany doubt. It must be conceded to be a fact, that varieties as a rulerevert to their species under the ordinary circumstances of commercialculture. And I cannot see any reason why this fact should not beconsidered as stating a principal difference between varieties andspecies, since true species never sport into one another. 

My objection only refers to the explanation of the observed facts. According to my view nearly all these ordinary reversions are due tocrosses, and it is for this reason that I proposed to call them by aseparate name, that of "vicinists. " Varieties then, by means of suchspontaneous intercrossing sport into one another, while species eitherdo not cross, or when crossing produce hybrids that are otherwiseconstituted and do not give the impression of atavistic reversion. 

I must not be content with proposing this new conception, but must givethe facts on which this assumption rests. These facts are the results ofsimple experiments, which nevertheless are by no means easy to carryout, as they require the utmost care to secure the absolute purity ofthe seeds that are employed. This can only be guaranteed by previouscultures of isolated plants or groups of plants, or by artificialpollination. 

[200] Once sure of this preliminary condition, the experiment simplyconsists in growing a variety at a given distance from its species andallowing the insects to transfer the pollen. After harvesting the seedthus subjected to the presumed cause of the impurities, it must be sownin quantities, large enough to bring to light any slight anomaly, and tobe examined during the period of blooming. 

The wild seashore aster, _Aster Tripolium_, will serve as an example. Ithas pale violet or bluish rays, but has given rise to a white variety, which on testing, I have found pure from seed. Four specimens of thiswhite variety were cultivated at a distance of nearly 100 meters from alarge lot of plants of the bluish species. I left fertilization to thebees, harvested the seeds of the four whites separately and had fromthem the following year more than a thousand flowering plants. All ofthem were of the purest white, with only one exception, which was aplant with the bluish rays of the species, wholly reverting to itsgeneral type. As the variety does not give such reversions whencultivated in isolation, this sport was obviously due to some cross inthe former year. In the same way I tried the white Jacob's ladder, _Polemonium coeruleum_ album in the neighborhood of the blue-floweredspecies, the distance [202] in this case being only 40 meters. Of twohundred seeds one became a blue atavist, or rather vicinist, while allothers remained true to the white type. The same was observed in thewhite creeping thyme, or _Thymus Serpyllum album_, and the whiteself-heal, _Brunella vulgaris alba_, gave even so much as 28% seedlingswith purple corollas out of some 400 specimens, after being cultivatedin close proximity to its parent-species. I have tried many otherspecies, but always with the same result. Such atavists only arise bycultivation in the proximity of allied varieties, never in isolation. They are not real atavists, but only vicinists. 

In order to show this yet more clearly, I made another experiment withthe white selfheal. I had a lot of the pinnate-leaved variety withpurple flowers and somewhat stouter stems, and cultivated single plantsof the whiteflowering sort at distances that varied from 2-16 meters. The seeds of each plant were collected and sown separately, those of thenearest gave up to 5 or 6 hybrids from the seeds of one parent, whilethose of the farthest gave only one purple-flowered plant for eachparent. Evidently the chance of the pollen being carried by bees is muchgreater on short than on longer distances. 

True hybrids between species may arise in quite the same way, and sinceit is obviously impossible to attribute them to an innate tendency toreversion, they afford an absolutely irrefutable proof of the assertionthat pollen is often brought by insects from one lot of plants toanother. In this way I obtained a hybrid between the common Jacob'sladder and the allied species _Polemonium dissectum_. With a distance of100 meters between them I had two hybrid seeds among a hundred of pureones. At a similar distance pollen was carried over from the wildradish, _Raphanus Raphanistrum_, to the allied _Raphanus caudatus_, andI observed the following year some very nice hybrids among my seedlings. A hybrid-bean between _Phaseolus nanus_ and _P. Multiflorus_, and somehybrids between the yellow daisy, _Chrysanthemum segetum_ and the allied_Chrysanthemum coronarium_ or ox-eye daisy which also arosespontaneously in my garden between parents cultivated at recordeddistances, might further be noted. Further details of these experimentsneed not be given. Suffice to say, that occasional crosses betweenspecies do occur, and not even rarely, that they are easily recognizedas such and cannot be confused with cases of atavism, and that thereforethey give proof to the assumption that in the same way crossesordinarily occur also between varieties [203] of the same species, ifcultivated at small distances apart, say 40-50 meters or even more. Vicinism therefore, may play a part in all such cultures, enough toaccount for all the impurities observed in the nurseries or incommercial seed-samples. 

Of course this whole discussion is limited to such species as are notonly as a rule visited by insects, but are dependent on these visits fortheir fertilization. Most of our garden-flowers are included in thiscategory. If not then we may expect to find the cultures and seeds pure, irrespective of the distances between allied varieties, as for instancewith peas, which are known to be self-fertilizing. Another instance isgiven by the barley. One of the most curious anomalous varieties of thiscereal, is the "Nepaul-barley, " with its small adventitious flowers onthe palets or inner scales. It is a very old, widely cultivated sort, which always comes true from seed, and which has been tested in repeatedexperiments in my garden. The spikelets of this curious plant areoneflowered and provided with two linear glumes or outer scales. Of theinner scales or palets, the outer one is three-lobed at the summit, hence the varietal name of _Hordeum vulgare trifurcatum_. The centrallobe is oblong and hollow, covering a small supernumerary floretinserted [204] at its base. The two lateral lobes are narrower, sometimes linear, and are often prolonged into an awn, which isgenerally turned away from the center of the spike. The central lobesometimes bears two florets at its base, although but one is usuallypresent and it may be incomplete. 

I might give one more instance from my own experience. A variety of theevening-primrose with small linear petals was once found by one of mysons growing wild near Amsterdam. It was represented by only oneindividual, flowering among a great many of the ordinary type with broadpetals. But the evening-primroses open their anthers in the morning, fertilize themselves during the day, and only display their beautifulflowers in the evening, after the pollination has been accomplished. They then allure evening moths, such as _Agrotis_ and _Plusia_, by theirbright color, their sweet honeysmell and their nectar. Since thefertilization is accomplished many hours before opening, crosses areeffected only in rare instances, and the seeds commonly remain true tothe parent type. The seeds of this one plant, when sown separately in mygarden, produced exclusively flowers with the small linear petals oftheir parent. Although I had a hundred individuals bearing manythousands of flowers, there was not an instance of reversion. And suchwould [205] immediately have been observed, had it occurred, because thehybrids between the cruciate and the normal flowers are notintermediate, but bear the broad petals of the _O. Biennis_. 

We may now take up another phase of the question, that of the runningout of new varieties, shortly after their introduction into a newcountry, or later. 

The most widely known instance of this is that of the American corn inBaden, recorded by Metzger and quoted by Darwin as a remarkable instanceof the direct and prompt action of climate on a plant. It has since beenconsidered as a reversion to the old type. Such reversions invariablyoccur, according to Wallace, in cases of new varieties, which have beenproduced quickly. But as we now know, such reversions are due tospontaneous crosses with the old form, and to the rule, that the hybridsof such origin are not intermediate, but assume the features of theolder of the two parents. In the light of this experience, Metzger'sobservation becomes a typical instance of vicinism. It relates to the"Tuscarora" corn of St. Louis, a variety with broad and flat whiteseeds. 

About the year 1840, this corn was introduced into Baden in Germany, andcultivated by Metzger. In the first year it came true to type, and [206]attained a height of 12 feet, but the season did not allow its seeds toripen normally. Only a few kernels were developed before the winter. From this seed plants of a wholly different type came the next year, ofsmaller stature, and with more brownish and rounded kernels. They alsoflowered earlier and ripened a large number of seeds. The depression onthe outer side of the seed had almost disappeared, and the originalwhite had become darker. Some of the seeds had even become yellow and intheir rounded form they approached the common European maize. Obviouslythey were hybrids, assuming the character of their pollen-parent, whichevidently was the ordinary corn, cultivated all around. The observationof the next year showed this clearly, for in the third generation nearlyall resemblance to the original and very distinct American species waslost. If we assume that only those seeds ripened which reverted to theearly-ripening European type, and that those that remained true to thevery late American variety could not reach maturity, the case seems tobe wholly comprehensible, without supposing any other factors to havebeen at work than those of vicinism, which though unknown at the periodof Metzger's and Darwin's writings, seems now to be fully understood. Noinnate tendency to run out and no changing influence of the climate arerequired for an adequate explanation of the facts. 

In the observation quoted, what astonishes us most, is the greatrapidity of the change, and the short time necessary for the offspringof the accidental crosses to completely supplant the introduced type. Inthe lecture on the selection of elementary species, closely analogouscases were described. One of them was the wild oat or _Avena fatua_which rapidly supplants the cultivated oats in bad years in parts of thefields. Other instances were the experiments of Risler with the"Galland" wheat and the observation of Rimpau on "Rivett's bearded"wheat. 

Before leaving the question of vicinism and its bearing on the generalbelief of the instability of varieties, which when tested with due care, prove to be quite stable, it may be as well to consider the phenomenafrom another point of view. Our present knowledge of the effects ofcrosses between varieties enables us to formulate some general rules, which may be used to calculate, and in some way to predict, the natureof the impurities which necessarily attend the cultivation of alliedspecies in close vicinity. And this mode of cultivation being in almostuniversal use in the larger nurseries, [208] we may, by this discussion, arrive at a more scientific estimation of the phenomena of vicinism, hitherto described. 

The simplest case that may be given, is when an ordinary retrogradevariety is cultivated with the species to which it belongs. Forinstance, if dwarfs are cultivated next to the taller type, or a whitevariety next to the red or blue-flowering species, or thornless forms inneighboring beds with the armed species. Bees and Bumble-bees, butterflies and moths are seen flying from flower to flower, collectingthe honey and carrying pollen. I frequently saw them cross the limits ofthe neighboring beds. Loaded with the pollen of the variety they visitthe flowers of the different species and impregnate the stigma with it. And returning to the variety they bring about similar crosses in theflowers of the latter. Hybrid seeds will develop in both cases andbecome mixed with the crop. We now have to ask the question, what sortof plants will arise from these hybrid seeds. As a general rule we maystate, first, that the hybrids of either form of cross are practicallythe same, secondly that they are not intermediate, but that thecharacter of one parent prevails to the almost absolute exclusion of theother and in the third place that the older character dominates theyounger. 

[209] The hybrid offspring will therefore, in the main, have thecharacter of the species and be indistinguishable from it, or show onlysuch differences as escape ordinary observation. When occurring in theseeds of the variety they betray themselves as soon as the differentialcharacters are displayed. Between the thousands of flowering plants of awhite variety the hybrids will instantly catch the eye by their red orblue corollas. Quite the contrary effect results from the admixture ofhybrids with the seeds of the species itself. Here no difference willshow itself, even in the fullest bloom. The effect of the spontaneouscrosses will pass unobserved. The strain, if pure in the first year, will seem to be still in the same condition. Or in other terms, theunavoidable spontaneous crosses will disturb the purity of the varietyin the second year, while they do not seem to interfere at all with theuniformity of the species. The direct effect of the visits of theinsects is evident in the first case, but passes unobserved in thelatter. 

From this it would seem, that spontaneous crosses are hurtful tovarieties, but are innocuous to true species. Certainly this would beso, were there no selection. But it is easily seen, that through thisoperation the effect becomes quite the opposite. For when the fields[210] are inspected at the time of the fullest display of the varietalcharacters, the obvious hybrids will be eliminated, but the hidden oneswill of necessity be spared, as they are concealed among the species bythe similarity of their type. Hence, the harvest of the variety may berendered pure or nearly so, while the harvest of the species will retainthe seeds of the hybrids. Moreover it will contain seeds originated bythe spontaneous but numerous crosses of the true plants with thesparsely intermingled hybrids. 

This brings us to the question, as to what will be the visibleconsequences of the occurrence of such invisible hybrids in thefollowing generation. In opposition to the direct effects justdescribed, we may call them indirect. To judge of their influence, wemust know how hybrid seeds of the first generation behave. 

In one of our lectures we will deal with the laws that show thenumerical relations known as the laws of Mendel. But for our presentpurpose, these numerical relations are only of subordinate importance. What interests us here is the fact that hybrids of varieties do notremain constant in the second generation but usually split as it issaid, remaining hybrid only in part of their offspring, the otherportion returning to the parental types. This however, will show itselfonly in those individuals [211] which reassume the character of thevarietal parent, all the others apparently remaining true to the type ofthe species. Now it is easy to foresee what must happen in the secondgeneration if the first generation after the cross is supposed to bekept free from new vicinistic influences, or from crosses withneighboring varieties. 

We may limit ourselves in the first place to the seeds of the unobservedhybrids. For the greater part they will repeat the character of theirparents and still remain concealed. But a small number will display thevarietal marks, as for example showing white flowers in a field of blueones. Hence, the indirect consequence of the spontaneous crosses will bethe same in the species, as was the direct effect in the variety, onlythat it appears a year later. It will then be eliminated in the processof selection. 

Obviously, this elimination conduces only to a partial purification. Theconspicuous plants will be destroyed, but a greater number of hybridswill remain, still concealed by their resemblance to the general typeand will be spared to repeat the same process next year. So while thevariety may be freed every year from the impurities brought into it inthe preceeding summer, the admixtures of the species [212] will continueduring a number of years, and it may not be possible to get rid of themat all. 

It is an often recurring assertion that white varieties of coloredspecies are the most stable of all horticultural races. They are oftensaid to be at least as constant as the species itself, and even tosurpass it in this quality. With our present state of knowledge, theexplanation of this general experience is easily given. For selectionremoves the effect of spontaneous crosses from the variety in each year, and renders it practically pure, while it is wholly inadequate toproduce the same effects on the species, because of the concealedhybrids. 

The explanation given in this simple instance may be applied to the caseof different varieties of the same species, when growing together andcrossed naturally by insects. 

It would take too long to go into all the details that presentthemselves here to the student of nature and of gardens. I will onlystate, that since varieties differ principally from their species by thelack of some sharp character, one variety may be characterized by thelack of color of the flowers, another by the lack of pubescence, a thirdby being dwarfed, and so on. Every character must be studied separatelyin its effects on the offspring [213] of the crosses. And it istherefore easily seen, that the hybrids of two varieties may resembleneither of them, but revert to the species itself. This is necessarilyand commonly the case, since it is always the older or positivecharacters that prevail in the hybrids and the younger or negative thatlie hidden. So for instance, a blue dwarf larkspur, crossed with a tallwhite variety, must give a tall blue hybrid, reassuming in bothcharacters the essentials of the species. 

Keeping this rule in view, it will be easy to calculate what may beexpected from spontaneous crosses for a wide range of occurrences, andthus to find an explanation of innumerable cases of apparent variabilityand reversion in the principle of vicinism. Students have only torecollect that specific characters prevail over varietal ones, and thatevery character competes only with its own antagonist. Or to give asharper distinction: whiteness of flowers cannot be expected to beinterchanged with pubescence of leaves. 

In concluding I will point out another danger which in the principle ofvicinism may be avoided. If you see a plant in a garden with all thecharacteristics of its species, how can you be sure that it is truly arepresentative of the species, and not a hybrid? The prevailing [214]characters are in either case the same. Perhaps on close inspection youmay find in some cases a slight difference, some character being not asfully developed in the hybrid as in the species. But when such is notthe case, or where the opportunity for such a closer examination iswanting, a hybrid may easily be taken for a specimen of the pure race. Now take the seeds of your plant and sow them. If you had not supposedit to be hybrid you will be astonished at finding among its progeny someof a wholly different type. You will be led to conclude that you areobserving a sudden change in structure such as is usually called asport. 

Or in other words you may think that you are assisting at theorigination of a new variety. If you are familiar with the principle ofvicinism, you will refrain from such an inference and consider thesupposition of a hybrid origin. But in former times, when this principlewas still unknown and not even guessed at, it is evident that manymistakes must have been made, and that many an instance, which until nowhas been considered reliable proof of a so-called single variation, isin fact only a case of vicinism. In reading the sparse literature onsports, numerous cases will be found, which cannot stand this test. Inmany instances crossing must be looked to as an explanation, [215] andin other cases the evidence relied upon does not suffice to exclude thisassumption. Many an old argument has of late lost its force by thistest. 

Returning to our starting point we may now state that regular reversionsto a specific type characterize a form as a variety of that species. These reversions, however, are not due to an innate tendency, but tounobserved spontaneous crosses. 

[217]

LECTURE VIII

LATENT CHARACTERS

No organism exhibits all of its qualities at any one time. Many of themare generally dormant and await a period of activity. For some of themthis period comes regularly, while in others the awakening depends uponexternal influences, and consequently occurs very irregularly. Those ofthe first group correspond to the differences in age; the secondconstitute the responses of the plant to stimuli includingwound-injuries. 

Some illustrative examples may be quoted in order to give a precise ideaof this general conception of dormant or latent characters. Seed leavesare only developed in the seed and the seedling; afterwards, during theentire lifetime of the plant, the faculty of producing them is not madeuse of. Every new generation of seeds however, bears the same kind ofseed leaves, and hence it is manifest that it is the same quality, whichshows itself from time to time. 

The primary leaves, following the seed-leaves, are different in manyspecies, from the later ones, and the difference is extremely pronouncedin some cases of reduction. Often, when leaves are lacking in the adultplant, being replaced by flattened stalks as in the case of the acacias, or by thorns, or green stems and twigs as in the prickly broom or _Ulexeuropaeus_, the first leaves of the young plant may be more highlydifferentiated, being pinnate in the first case and bearing threeleaflets in the second instance. This curious behavior which is verycommon, brings the plants, when young, nearer to their allies than inthe adult state, and manifestly implies that the more perfect state ofthe leaves is latent throughout the life of the plant, with theexception of the early juvenile period. 

_Eucalyptus Globulus_, the Australian gum tree, has opposite and broadlysessile leaves during the first years of its life. Later these disappearand are replaced by long sickle-shaped foliage organs, which seem to bescattered irregularly along the branches. The juvenile charactersmanifestly lie dormant during the adult period, and that this is so, maybe shown artificially by cutting off the whole crown of the tree, whenthe stem responds by producing numerous new branches, which assume the[218] shape proper to the young trees, bearing sessile and oppositeleaves. 

It seems quite unnecessary to give further instances. They are familiarto every student. It is almost safe to say that every character has itsperiods of activity and of inactivity, and numbers of flowers and fruitscan be mentioned as illustrations. One fact may be added to show thatnearly every part of the plant must have the power of producing all ornearly all the characters of the individual to which it belongs. Thisproof is given by the formation of adventitious buds. These, when onceformed, may grow out into twigs, with leaves and flowers and roots. Theymay even be separated from the plants and used as cuttings to reproducethe whole. Hence we may conclude that all tissues, which possess thepower of producing adventitious buds, must conceal in a latent state, all the numerous characters required for the full development of thewhole individual. 

Adventitious buds may proceed from specialized cells, as on the marginof the leaves of _Bryophyllum calycinum_; or from the cells of specialtissues, as in the epidermis of the begonias; or they may be provoked bywounds in nearly every part of the plant, provided it be able to healthe wound by swelling tissues or [219] callus. The best instance isafforded by elms and by the horse-chestnut. If the whole tree is hewndown the trunk tries to repair the injury by producing smallgranulations of tissue between the wood and the bark, which graduallycoalesce while becoming larger. From this new ring of living matterinnumerable buds arise, that expand into leafy branches, showing clearlythat the old trunk possesses, in a latent state, all the qualities ofthe whole crown. Indeed, such injured stumps may be used for theproduction of copses and hedges. 

All the hitherto recorded cases of latency have this in common, thatthey may become active during the life-time of any given individualonce, or oftener. This may be called the ordinary type of latency. 

Besides this there is another form of latent characters, in which thisawakening power is extremely limited, or wholly absent. It is thesystematic latency, which may be said to belong to species and varietiesin the same way as the ordinary latency belongs to individuals. As thisindividual latency may show itself from time to time during the life ofa given plant, the first may only become active from time to time duringthe whole existence of the variety or the species. It has no regularperiod of activity, nor may it be incited by artificial stimulation. 

[220] It emerges from concealment only very rarely and only on its owninitiative. Such instances of atavism have been described in previouslectures, and their existence has been proved beyond doubt. 

Systematic latency explains the innumerable instances in which speciesare seen to lack definite characteristics which ordinarily do not fail, either in plants at large, or in the group or family to which the plantbelongs. If we take for instance the broom-rape or _Orobanche_, or someother pale parasite, we explain their occurrence in families of plantswith green leaves, by the loss of the leaves and of the green color. Butevidently this loss is not a true one, but only the latency of thosecharacters. And even this latency is not a complete one, as littlescales remind us of the leaves, and traces of chlorophyll still exist inthe tissues. Numerous other cases will present themselves to everypractical botanist. 

Taking for granted that characters, having once been acquired, maybecome latent, and that this process is of universal occurrencethroughout the whole vegetable and animal kingdom, we may now come to amore precise and clear conception of the existing differences betweenspecies and varieties. 

For this purpose we must take a somewhat [221] broader view of the wholeevolution of the vegetable kingdom. It is manifest that highly developedplants have a larger number of characters than the lower groups. Thesemust have been acquired in some way, during preceding times. Suchevolution must evidently be called a process of improvement, or aprogressive evolution. Contrasted to this is the loss, or the latency ofcharacters, and this may be designated retrogressive or retrogradeevolution. But there is still a third possibility. For a latentcharacter may reassume its activity, return to the active state, andbecome once more an important part of the whole organization. Thisprocess may be designated as degressive evolution; it obviouslycompletes the series of the general types of evolution. 

Advancement in general in living nature depends on progressiveevolution. In different parts of the vegetable kingdom, and even indifferent families this progression takes place on different lines. Bythis means it results in an ever increasing divergency between theseveral groups. Every step is an advance, and many a step must have beentaken to produce flowering plants from the simplest unicellular algae. 

But related to, and very intimately connected with this advancement isthe retrogressive [222] evolution. It is equally universal, perhapsnever failing. No great changes have been attained, without acquiringnew qualities on one side, and reducing others to latency. Everywheresuch retrogressions may be seen. The polypetalous genera _Pyrola_, _Ledum_, and _Monotropa_ among the sympetalous heaths, are a remarkableinstance of this. The whole evolution of the monocotyledons from thelowest orders of dicotyledons implies the seeming loss of cambial growthand many other qualities. In the order of aroids, from the calamus-rootor sweet flag, with its small but complete flowers, up to the reducedduckweeds (_Lemna_), almost an unbroken line of intermediate steps maybe traced showing everywhere the concurrence of progressive andretrogressive evolution. 

Degressive evolution is not so common by far, and is not so easy torecognize, but no doubt it occurs very frequently. It is generallycalled atavism, or better, systematic atavism, and the clearest casesare those in which a quality which is latent in the greater part of afamily or group, becomes manifest in one of its members. Bracts in theinflorescence of crucifers are ordinarily wanting, but may be seen insome genera, _Erucastrum pollichii_ being perhaps the [223] most widelyknown instance, although other cases might easily be cited. 

For our special purpose we may take up only the more simple cases thatmay be available for experimental work. The great lines of evolution ofwhole families and even of genera and of many larger species obviouslylie outside the limits of experimental observation. They are the outcomeof the history of the ancestors of the present types, and a repetitionof their history is far beyond human powers. We must limit ourselves tothe most recent steps, to the consideration of the smallest differences. But it is obvious that these may be included under the same heads as thelarger and older ones. For the larger movements are manifestly to beconsidered only as groups of smaller steps, going in the same direction. 

Hence we conclude, that even the smallest steps in the evolution ofplants which we are able to observe, may be divided into progressive, retrogressive and degressive ones. The acquisition of a single newquality is the most simple step in the progressive line, the becominglatent and the reactivating of this same quality are the prototypes ofthe two other classes. 

Having taken this theoretical point of view, it remains to inquire, howit concurs with the [224] various facts, given in former lectures andhow it may be of use in our further discussions. 

It is obvious that the differences between elementary species andvarieties on the one hand, and between the positive and negativevarieties as distinguished above, are quite comparable with ourtheoretical views. For we have seen that varieties can always beconsidered as having originated by an apparent loss of some quality ofthe species, or by the resumption of a quality which in allied speciesis present and visible. In our exposition of the facts we have of courselimited ourselves to the observable features of the phenomena withoutsearching for a further explanation. For a more competent inquiryhowever, and for an understanding of wider ranges of facts, it isnecessary to penetrate deeper into the true nature of the impliedcauses. 

Therefore we must try to show that elementary species are distinguishedfrom each other by the acquisition of new qualities, and that varietiesare derived from their species either by the reduction of one or morecharacteristics to the latent state, or by the energizing of dormantcharacters. 

Here we meet with a great difficulty. Hitherto varieties and subspecieshave never been clearly defined, or when they have been, it was [225]not by physiological, but only by morphological research. And the claimsof these two great lines of inquiry are obviously very diverging. Morphological or comparative studies need a material standard, by whichit may be readily decided whether certain groups of animals and plantsare to be described or de-nominated as species, as subspecies or asvarieties. To get at the inner nature of the differences is in mostcases impossible, but a decision must be made. The physiological line ofinquiry has more time at its disposal; it calls for no haste. Itsexperiments ordinarily cover years, and a conclusion is only to bereached after long and often weary trials. There is no making a decisionon any matter until all doubtful points have been cleared up. Of course, large groups of facts remain uncertain, awaiting a closer inquiry, andthe teacher is constrained to rely on the few known instances ofthoroughly investigated cases. These alone are safe guides, and it seemsfar better to trust to them and to make use of them for the constructionof sharp conceptions, which may help us to point out the lines ofinquiry which are still open. 

Leaving aside all such divisions and definitions, as were stamped withthe name of provisional species and varieties by the great systematist, [226] Alphonse De Candolle, we may now try to give the proofs of ourassertion, by using only those instances that have been thoroughlytested in every way. 

We may at once proceed to the retrogressive or negative varieties. Thearguments for the assumption that elementary species owe their origin tothe acquisition of new qualities may well be left for later lectureswhen we shall deal with the experimental proofs in this matter. 

There are three larger groups of facts, on which the assumption oflatent characters in ordinary varieties rests. These are true atavism, incomplete loss of characters, and systematic affinity. Before dealingwith each of these separately, it may be as well to recall once morethat in former lectures we have treated the apparent losses only asmodifications in a negative way, without contemplating the underlyingcauses. 

Let us recall the cases of bud-atavism given by the whitish variety ofthe scarlet _Ribes_, by peaches and nectarines, and by conifers, including _Cephalotaxus_ and _Cryptomeria_. These and many otheranalogous facts go to prove the relation of the variety to the species. Two assumptions are allowable. In one the variety differs from thespecies by the total loss of the [227] distinctive character. In theother this character is simply reduced to an inactive or dormant state. The fact of its recurrence from time to time, accompanied by secondarycharacters previously exhibited, is a manifest proof of the existence ofsome relation between the lost and the resumed peculiarity. Evidentlythis relation cannot be accounted for on the assumption of an absolutedisappearance; something must remain from which the old features may berestored. 

This lengthy discussion may be closed by the citation of the cases, inwhich plants not only show developmental features of a former state, butalso reproduce the special features they formerly had, but seeminglyhave lost. Two good illustrative examples may be given. One is affordedby the wheat-ear carnation, the other by the green dahlias, and bothhave occurred of late in my own cultures. 

A very curious anomaly may from time to time be observed in large bedsof carnations. It bears no flowers, but instead of them small greenears, which recall the ears of wheat. Thence the name of "Wheat-ear"carnation. On closer inspection it is easily seen how they originate. The normal flowers of the carnations are preceded by a small group ofbracts, [228] which are arranged in opposite pairs and thereforeconstitute four rows. 

In this variety the flower is suppressed and this loss is attended by acorresponding increase of the number of the pairs of bracts. Thismalformation results in square spikes or somewhat elongated headsconsisting only of the greenish bracts. As there are no flowers, thevariety is quite sterile, and as it is not regarded by horticulturistsas an improvement on the ordinary bright carnations, it is seldommultiplied by layering. Notwithstanding this, it appears from time totime and has been seen in different countries and at different periods, and, what is of great importance for us, in different strains ofcarnations. Though sterile, and obviously dying out as often as itsprings into existence, it is nearly two centuries old. It was describedin the beginning of the 18th century by Volckamer, and afterwards byJaeger, De Candolle, Weber, Masters, Magnus and many other botanists. Ihave had it twice, at different times and from different growers. 

So far as I have been able to ascertain reversions of this curiouscarnation to normal flowers have not yet been recorded. Such amodification occurred last summer in my garden on a plant which had notbeen divided or layered, but on which the slender branches had [229]been left on the stem. Some of them remained true to the varietal typeand bore only green spikes. Others reverted wholly or partially to theproduction of normal flowers. Some branches bore these only, others hadspikes and flowers on neighboring twigs, and in still other instanceslittle spikes had been modified in such manner that a more or less welldeveloped flower was preceded by some part of an ear. 

The proof that this retrograde modification was due to the existence ofa character in the latent state was given by the color of the flowers. If the reverted bud had only lost the power of producing spikes, theywould evidently simply have returned to the characteristics of theordinary species, and their color would have been a pale pink. Insteadof this, all flowers displayed corollas of a deep brown. They obviouslyreverted to their special progenitor, the chance variety from which theyhad sprung, and not to the common prototype of the species. Of course itwas not possible to ascertain from which variety the plant had reallyoriginated, but the reproduction of any one clearly defined varietalmark is in itself proof enough of their origin, and of the latency ofthe dark brown flower-color in this special case. 

A still better proof is afforded by a new type of green dahlia. Theordinary green dahlia [230] has large tufts of green bracts instead offlowering heads, the scales of the receptacle having assumed the textureand venation of leaves, and being in some measure as fleshy. But thegreen heads retain the form of the ordinary flower-heads, and as theyhave no real florets that may fade away, they remain unchanged on theplants, and increase in number through the whole summer. The new typesof green dahlia however, with which I have now to deal, aredistinguished by the elongation of the axis of the head, which isthereby changed into a long leafy stalk, attaining a length of severalinches. These stalks continue growing for a very long time, and for themost part die without producing anything else than green fleshy scales. 

This long-headed green dahlia originated at Haarlem some years ago, inthe nursery of Messrs. Zocher & Co. It was seen to arise twice, fromdifferent varieties. Both of these were double-flowered, one a deepcarmine with white tips on the rays, the other of a pale orange tint, known by the name of "Surprise. " As they did not bear any florets orseeds, they were quite sterile. The strain arising from the carminevariety was kindly given to me by Messrs. Zocher & Co. , and waspropagated in my garden, while the other was kept in the nursery. In theearlier cultures both remained true to [231] their types, neverproducing true florets. No mark of the original difference was to beseen between them. But last summer (1903) both reverted to theirprototypes, bearing relatively large numbers of ordinary doubleflowerheads among the great mass of green stalks. Some intermediateforms also occurred consisting of green-scaled stalks ending in smallheads with colored florets. 

Thus far we have an ordinary case of reversion. But the important sideof the phenomenon was, that each plant exactly "recollected" from whichparent it had sprung. All of those in my garden reverted to the carmineflorets with white tips, and all of those in the nursery to the paleorange color and the other characteristics of the "Surprise" variety. 

It seems absolutely evident, that no simple loss can account for thisdifference. Something of the character of the parent-varieties must haveremained in the plant. And whatever conception we may formulate of thesevestigial characters it is clear that the simplest and most obvious ideais their preservation in a dormant or latent state. Assuming that thedistinguishing marks have only become inactive by virescence, it ismanifest that on returning each will show its own peculiarities, asrecorded above. Our second point was the incomplete loss of [232] thedistinguishing quality in some varieties. It is of general occurrence, though often overlooked. Many white varieties of colored flowers givestriking instances, among them many of the most stable and most prizedgarden-flowers. If you look at them separately or in little bouquetsthey seem to be of irreproachable purity. But if you examine large bedsa pale hue will become visible. In many cases this tinge is so slight asto be only noticeable in a certain illumination, or by looking in anoblique direction across the bed; in others it is at once evident assoon as it has been pointed out. It always reminds the observer of thecolor of the species to which the variety belongs, being bluish inviolets and harebells, reddish in godetias and phloxes, in _SileneArmeria_ and many others. It proves that the original color quality ofthe species has not wholly, but only partly disappeared. It is dormant, but not entirely obliterated; latent, but not totally concealed;inactive, but only partially so. Our terminology is an awkward one; itpractically assumes, as it so often does in other cases, a conventionalunderstanding, not exactly corresponding to the simple meaning of thewords. But it would be cumbrous to speak always of partial inactivity, incomplete latency or half awakening qualities. Even such words assub-latent, [233] which would about express the real state of things, would have little chance of coming into general use. 

Such sub-latent colors are often seen on special parts in whitevarieties of flowers. In many cases it is the outer side of the petalswhich recalls the specific color, as in some white roses. In violets itis often on the spur that the remains of the original pigment are to beseen. In many instances it is on the tips of the petals or of thesegments of the corolla, and a large number of white or yellow flowersbetray their affinity to colored species by becoming red or bluish atthe edges or on the outer side. 

The reality of such very slight hues, and their relation to the originalpigment of the species may in some cases be proved by direct experiment. If it is granted that latency is not an absolute quality, then it willbe readily accepted, that even latency must be subjected to the laws ofgradual variation or fluctuating variability. We will deal with theselaws in a later lecture but every one knows that greater deviations thanthe ordinary may be attained by sowing very large numbers and byselecting from among them the extreme individuals and sowing anew fromtheir seed. In this way the slightest tinge of any latent color may be[234] strengthened, not indeed to the restoration of the tinge of thespecies, but at least so far as to leave no doubt as to the identity ofthe visible color of the species and the latent or sublatent one of thevariety. 

I made such an experiment with the peach leaved harebell or _Campanulapersicifolia_. The white variety of this species, which is often metwith in our gardens, shows a very pale bluish hue when cultivated inlarge quantities, which however is subject to individual variations. Iselected some plants with a decided tinge, flowered them separately, sowed their seeds, and repeated this during two generations. The resultwas an increase of the color on the tips of the segments of the corollain a few individuals, most of them remaining as purely white as theoriginal strain. But in those few plants the color was very manifest, individually variable in degree, but always of the same blue as in thespecies itself. 

Many other instances could be given. Smooth varieties are seldomabsolutely so, and if scattering hairs are found on the leaves or onlyon some more or less concealed parts, they correspond in their characterto those of the species. So it is with prickles, and even the thornlessthorn-apple has fruits with surfaces far from smooth. The thornlesshorse-chestnut [235] has in some instances such evident protuberances onthe valves of its fruits, that it may seem doubtful whether it is a pureand stable variety. 

Systematic latency may betray itself in different ways, either by normalsystematic marks, or by atavism. With the latter I shall deal at lengthon another occasion, and therefore I will give here only one very clearand beautiful example. It is afforded by the common red clover. Obviously the clovers, with their three leaflets in each leaf, stand inthe midst of the great family of papilionaceous plants, the leaves ofwhich are generally pinnate. Systematic affinity suggests that the"three leaved" forms must have been derived from pinnate ancestors, evidently by the reduction of the number of the leaflets. In somespecies of clover the middle of the three is more or less stalked, as isordinarily the case in pinnate leaves; in others it is as sessile as areits neighbors. In a subsequent chapter I will describe a very finevariety, which sometimes occurs in the wild state and may easily beisolated and cultivated. It is an ordinary red clover with five leafletsinstead of three, and with this number varying between three and seven, instead of being nearly wholly stable as in the common form. It producesfrom time to time pinnate leaves, [236] very few indeed, and onlyrarely, but then often two or three or even more on the same individual. Intermediate stages are not wanting, but are of no consequence here. Thepinnate leaves obviously constitute a reversion to some prototype, tosome ancestor with ordinary papilionaceous leaves. They give proof ofthe presence of the common character of the family, concealed here in alatent state. Any other explanation of this curious anomaly wouldevidently be artificial. On the other hand nothing is really known aboutthe ancestors of clover, and the whole conception rests only on theprevailing views of the systematic relationships in this family. But, asI have already said, further proof must be left for a subsequentoccasion. 

Many instances, noted in our former lectures, could be quoted here. Thesystematic distribution of rayed and rayless species and varieties amongthe daisy-group of the composites affords a long series of examples. Accidental variations in both directions occur. The Canada fleabane or_Erigeron canadensis_, the tansy or _Tanacetum vulgare_ and some othersmay at times be seen with ray-florets, and according to Murr, they maysometimes be wanting in _Aster Tripolium_, _Bellis perennis_, somespecies of _Anthemis_, _Arnica montana_ and in a number [237] of otherwell-known rayed species. Another instance may be quoted; it has beenpointed out by Grant Allen, and refers to the dead-nettle or Lamiumalbum. Systematically placed in a genus with red-flowering species, wemay regard its white color as due to the latency of the general redpigment. 

But if the flower of this plant is carefully examined, it will be foundin most cases not to be purely white, but to have some dusky lines andmarkings on its lower lip. Similar devices are observed on the lip ofthe allied _Lamium maculatum_, and in a less degree on the somewhatdistant _Lamium purpureum_. With _Lamium maculatum_ or spotteddead-nettle, the affinity is so close that even Bentham united the twoin a single species, considering the ordinary dead-nettle only as avariety of the dappled purple type. For the support of this conceptionof a specific or varietal retrograde change many other facts areafforded by the distribution of the characteristic color and of theseveral patterns of the lips of other labiates, and our generalunderstanding of the relationships of the species and genera in thisfamily may in a broad sense be based on the comparison of theseseemingly subordinate characteristics. 

The same holds good in many other cases, and systematists have oftenbecome uncertain [238] as to the true value of some form, by itsrelationship to the allied types in the way of retrogressivemodification. Color-differences are so showy, that they easilyovershadow other characters. The white and the blue thorn-apple, thewhite and the red campion (_Lychnis vespertina_ and _diurna_) and manyother illustrative cases could be given, in which two forms arespecifically separated by some authors, but combined by others on theground of the retrograde nature of some differentiating mark. 

Hitherto we have dealt with negative characters and tried to prove thatthe conception of latency of the opposite positive characteristics is amore natural explanation of the phenomenon than the idea of a completeloss. We have now to consider the positive varieties, and to show thatit is quite improbable that here the species have struck out forthemselves a wholly new character. In some instances such may have beenthe case, but then I should prefer to treat these rather as elementaryspecies. But in the main we will have to assume the latency of thecharacter in the species and its reassumption by the variety whenoriginating, as the most probable explanation. 

Great stress is laid upon this conception by the fact, that positivevarieties are so excessively rare when compared with the commonoccurrence [239] of negative ones. Indeed, if we put aside the radiateand the color-varieties of flowers and foliage, hardly any cases can becited. We have dealt with this question in a former lecture, and may nowlimit ourselves to the positive color-varieties. 

The latency of the faculty of producing the red pigment in leaves mustobviously be accepted for nearly the whole vegetable kingdom. Oaks andelms, the beautiful climbing species of Ampelopsis, many conifers, asfor instance _Cryptomeria japonica_, some brambles, the Guelder-rose(_Viburnum Opulus_) and many other trees and shrubs assume a more orless bright red color in the fall. During summer this tendency must havebeen dormant, and that this is so, is shown by the young leaves of oaksand others, which, when unfolding in the spring show a similar but palerhue. Moreover, there is a way of awakening the concealed powers at anytime. We have only to inflict small wounds on the leaves, or to cutthrough the nerves or to injure them by a slight bruising, and theleaves frequently respond with an intense reddening of the livingtissues around and especially above the wounds. _Azolla caroliniana_, aminute mosslike floating plant allied to the ferns, responds to lightand cold with a reddish tinge, and to shade or warmth with a pure green. The foliage [240] of many other plants behaves likewise, as also doapples and peaches on the insolated sides of the fruits. It is quiteimpossible to state these groups of facts in a more simple way than bythe statement that the tendency to become red is almost generallypresent, though latent in leaves and stems, and that it comes intoactivity whenever a stimulus provokes it. 

Now it must be granted that the energizing of such a propensity underordinary circumstances is quite another thing from the origination of apositive variety by the evolution of the same character. In the varietythe activity has become independent of outer influences or dependentupon them in a far lesser degree. The power of producing the redpigments is shown to be latent by the facts given above, and we see thatin the variety it is no longer latent but is in perfect and lastingactivity throughout the whole life of the plant. 

Red varieties of white flowers are much more rare. Here the latency ofthe red pigment may be deduced partly from general arguments like thosejust given, partly from the special systematic relations in the givencases. Hildebrand has clearly worked out this mode of proof. He showedby the critical examination of a large number of instances that theoccurrence of the red-flowered varieties is contingent upon the [241]existence of red species in the same genus, or in some rare cases, innearly allied genera. Colors that are not systematically present in thegroup to which a white species belongs are only produced in itsvarieties in extremely rare cases. 

We may quote some special rules, indicated by Hildebrand. Blue speciesare n the main very rare, and so are blue varieties of white speciesalso. Carnations, Asiatic or cultivated buttercups (_Ranunculusasiaticus_), _Mirabilis_, poppies, _Gladiolus_, _Dahlia_, and some otherhighly cultivated or very old garden-plants have not been able toproduce true blue flowers. But the garden-anemone (_Anemone coronaria_)has allies with very fine blue flowers. The common stock has bluishvarieties and is allied to _Aubretia_ and _Hesperis_, and gooseberrieshave a red form, recalling the ordinary currant. In nearly all otherinstances of blue or red varieties every botanist will be able to pointout some allied red or blue species, as an indication of the probablesource of the varietal character. 

Dark spots on the lower parts of the petals of some plants affordanother instance, as in poppies and in the allied _Glaucium_, where theysometimes occur as varietal and in other cases as specific marks. 

The yellow fails in many highly developed [242] flowers, which are notliable to produce yellow variations, as in _Salvia_, _Aster_, _Centaurea_, _Vinca_, _Polygala_ and many others. Even the rare paleyellowish species of some of these genera have no tendency in thisdirection. The hyacinths are the most remarkable, if not the sole knowninstance of a species having red and blue and white and yellowvarieties, but here the yellow is not the bright golden color of thebuttercups. 

The existence of varietal colors in allied species obviously points to acommon cause, and this cause can be no other than the latency of thepigment in the species that do not show it. 

The conception of latency of characters as the common source of theorigination of varieties, either in the positive or in the negative way, leads to some rules on variability, which are known under the namesgiven to them by Darwin. They are the rules of repeated, homologous, parallel and analogous variability. Each of them is quite general, andmay be recognized in instances from the most widely distant families. Each of them is quite evident and easily understood on the principle oflatency. 

By the term of repeated variability is meant the well-known phenomenon, that the same variety has sprung at different times and in different[243] countries from the same species. The repetition obviouslyindicates a common internal cause. The white varieties of blue- andred-flowered plants occur in the wild state so often, and in most of theinstances in so few individuals that a common pedigree is absolutelyimprobable. In horticulture this tendency is widely and vexatiouslyknown, since the repetition of an old variety does not bring anyadvantage to the breeder. The old name of "conquests, " given by thebreeders of hyacinths, tulips and other flower-bulbs to any novelty, indisregard of the common occurrence of repetitions, is an indication ofthe same experience in the repeated appearance of certain varieties. 

The rule of parallel variations demands that the same characteroccasionally makes its appearance in the several varieties or races, descended from the same species, and even in widely distinct species. This is a rule, which is very important for the general conception ofthe meaning of the term variety as contrasted with elementary species. For the recurrence of the same deviation always impresses us as avarietal mark. Laciniated leaves are perhaps the most beautifulinstance, since they occur in so many trees and shrubs, as the walnuttree, the beech, the birch, the hazelnut, and even in [244] brambles andsome garden-varieties of the turnip (_Brassica_). 

In such cases of parallel variations the single instances obviouslyfollow the same rules and are therefore to be designated as analogous. Pitchers or ascidia, formed by the union of the margins of a leaf, areperhaps the best proof. They were classified by Morren under two heads, according to their formation from one or more leaves. Monophyllouspitchers obey the same law, viz. : that the upper side of the leaf hasbecome the inner side of the pitcher. Only one exception to this rule isknown to me. It is afforded by the pitchers of the banyan or holyfig-tree, _Ficus religiosus_, but it does not seem to belong to the sameclass as other pitchers, since as far as it has been possible toascertain the facts, these pitchers are not formed by a few leaves as inall other cases, but by all the leaves of the tree. 

In some cases pitchers are only built up of part of the leaf-blade. Suchpartial malformations obey a rule, that is common to them and to otherfoliar enations, viz. : that the side of the leaf from which they emerge, is always their outer side. The inner surface of these enationscorresponds to the opposite side of the leaf, both in color and inanatomical structure. The last of the four rules above mentioned is[245] that of the homologous variability. It asserts that the samedeviation may occur in different, but homologous parts of the sameplant. We have already dealt with some instances, as the occurrence ofthe same pigment in the flowers and foliage, in the fruits and seeds ofthe same plant, as also illustrated by the loss of the red or blue tingeby flowers and berries. Other instances are afforded by the curious factthat the division of the leaves into numerous and small segments isrepeated by the petals, as in the common celandine and some sorts ofbrambles. 

It would take too long to make a closer examination of the numerouscases which afford proof of these statements. Suffice it to say thateverywhere the results of close inspection point to the general rule, that the failure of definite qualities both in species and in varietiesmust, in a great number of cases, be considered as only apparent. Hiddenfrom view, occasionally reappearing, or only imperfectly concealed, thesame character must be assumed to be present though latent. 

In the case of negative or retrogressive varieties it is the transitionfrom the active into a dormant state to which is due the origin of thevariety. Positive varieties on the contrary owe their origin to thepresence of some character [246] in the species in the latent state, andto the occasional re-energizing thereof. 

Specific or varietal latency is not the same thing as the ordinarylatency of characters that only await their period of activity, or theexternal influence which will awake them. They are permanently latent, and could well be designated by the word perlatent. They spring intoactivity only by some sudden leap, and then at once become independentof ordinary external stimulation. 

[247]

LECTURE IX

CROSSES OF SPECIES AND VARIETIES

In the foregoing lectures I have tried to show that there is a realdifference between elementary species and varieties. The first are ofequal rank, and together constitute the collective or systematicspecies. The latter are usually derived from real and still existingtypes. Elementary species are in a sense independent of each other, while varieties are of a derivative nature. 

Furthermore I have tried to show that the ways in which elementary orminor species must have originated from their common ancestor must bequite different from the mode of origin of the varieties. We haveassumed that the first come into existence by the production ofsomething new, by the acquirement of a character hitherto unnoticed inthe line of their ancestors. On the contrary, varieties, in most cases, evidently owe their origin to the loss of an already existing character, or in other less frequent cases, to the re-assumption of a quality [248]formerly lost. Some may originate in a negative, others in a positivemanner, but in both cases nothing really new is acquired. 

This distinction holds good for all cases in which the relationshipbetween the forms in question is well known. It seems entirelyjustifiable therefore to apply it also to cases in which the systematicaffinity is doubtful, as well as to instances in which it is impossibleto arrive at any taxonomic conclusions. The extreme application of theprinciple would no doubt disturb the limits between many species andvarieties as now recognized. It is not to be forgotten however that alltaxonomic distinctions, which have not been confirmed by physiologictests are only provisional, a view acknowledged by the bestsystematists. Of course the description of newly discovered forms cannot await the results of physiologic inquiries; but it is absolutelyimpossible to reach definite conclusions on purely morphologic evidence. This is well illustrated by the numerous discords of opinion ofdifferent authors on the systematic worth of many forms. 

Assuming the above mentioned principle as established, and disregardingdoubtful cases as indicated, the term progressive evolution is used todesignate the method in which elementary species must have originated. It is the [249] manner in which all advance in the animal and vegetablekingdoms must have taken place, continuously adding new characters tothe already existing number. Contrasted with this method of growingdifferentiation, are the retrogressive modifications, which simplyretrace a step, and the degressive changes in which a backward step isretraced and old characters revived. No doubt both of these methods havebeen operative on a large scale, but they are evidently not in the lineof general advancement. 

In all of these directions we see that the differentiating marks showmore or less clearly that they are built up of units. Allied forms areseparated from each other without intermediates. Transitions are whollywanting, although fallaciously apparent in some instances owing to thewide range of fluctuating variability of the forms concerned, or to theoccurrence of hybrids and subvarieties. 

These physiologic units, which in the end must be the basis for thedistinction of the systematic units, may best be designated by the termof "unit-characters. " Their internal nature is as yet unknown to us, andwe will not now look into the theories, which have been propounded as tothe probable material basis underlying them. For our present purpose theempirical evidence of the general occurrence of [250] sharp limitsbetween nearly related characters must suffice. As Bateson has put it, species are discontinuous, and we must assume that their characters arediscontinuous also. 

Moreover there is as yet no reason for trying to make a completeanalysis of all the characters of a plant. No doubt, if attained, suchan analysis would give us a deep insight into the real internalconstruction of the intricate properties of organisms in general. Buttaxonomic studies in this direction are only in their infancy and do notgive us the material required for such an analysis. Quite on thecontrary, they compel us to confine our study to the most recentlyacquired, or youngest characters, which constitute the differentiatingmarks between nearly allied forms. 

Obviously this is especially the case in the realm of the hybrids, sinceonly nearly related forms are able to give hybrid offspring. In dealingwith this subject we must leave aside all questions concerning moreremote relationships. 

It is not my purpose to treat of the doctrine of hybridization at anylength. Experience is so rapidly increasing both in a practical and in apurely scientific direction that it would take an entire volume to giveonly a brief survey of the facts and of all the proposed theories. 

[251] For our present purposes we are to deal with hybrids only in sofar as they afford the means of a still better distinction betweenelementary species and varieties. I will try to show that these twocontrasting groups behave in quite a different manner, when subjected tocrossing experiments, and that the hope is justified that some daycrosses may become the means of deciding in any given instance, what isto be called a species, and what a variety, on physiologic grounds. Itis readily granted that the labor required for such experiments, isperhaps too great for the results to be attained, but then it may bepossible to deduce rules from a small series of experiments, which maylead us to a decision in wider ranges of cases. 

To reach such a point of view it is necessary to compare the evidencegiven by hybrids, with the conclusions already attained by thecomparison of the differentiating characteristics of allied forms. 

On this ground we first have to inquire what may be expected respectingthe internal nature and the outcome of the process of crossing in thevarious cases cited in our former discussion. 

We must always distinguish the qualities, which are the same in bothparents, from those that constitute the differentiating marks in everysingle cross. In respect to the first [252] group the cross is not atall distinguished from a normal fertilization, and ordinarily thesecharacters are simply left out of consideration. But it should never beforgotten that they constitute the enormous majority, amounting tohundreds and thousands, whereas the differentiating marks in each caseare only one or two or a few at most. The whole discussion is to belimited to these last-named exceptions. We must consider first whatwould be the nature of a cross when species are symmetrically combined, and what must be the case when varieties are subjected to the sametreatment. In so doing, I intend to limit the discussion to the mosttypical cases. We may take the crosses between elementary species of thesame or of very narrowly allied systematic species on the one side, andon the other, limit treatment to the crossing of varieties with thespecies, from which they are supposed to have sprung by a retrogrademodification. Crosses of different varieties of the same species withone another obviously constitute a derivative case, and should only bediscussed secondarily. And crosses of varieties with positive ordepressive characters have as yet so rarely been made that we may welldisregard them. 

Elementary species differ from their nearest allies by progressivechanges, that is by the acquirement [253] of some new character. Thederivative species has one unit more than the parent. All otherqualities are the same as in the parent. Whenever such a derivative iscombined with its parent the result for these qualities will be exactlyas in a normal fertilization. In such ordinary cases it is obvious thateach character of the pollen-parent is combined with the same characterof the pistil-parent. There may be slight individual differences, buteach unit character will become opposed to, and united with, the sameunit-character in the other parent. In the offspring the units will thusbe paired, each pair consisting of two equivalent units. As to theircharacter the units of each single pair are the same, only they mayexhibit slight differences as to the degree of development of thischaracter. 

Now we may apply this conception to the sexual combination of twodifferent elementary species, assuming one to be the derivative of theother. The differentiating mark is only present in one of the parentsand wanting in the other. While all other units are paired in thehybrid, this one is not. It meets with no mate, and must thereforeremain unpaired. The hybrid of two such elementary species is in someway incomplete and unnatural. In the ordinary course of things allindividuals derive [254] their qualities from both parents; for eachsingle mark they possess at least two units. Practically but notabsolutely equal, these two opponents always work together and give tothe offspring a likeness to both parents. No unpaired qualities occur innormal offspring; these constitute the essential features of the hybridsof species and are at the same time the cause of their wide deviationsfrom the ordinary rules. 

Turning now to the varieties, we likewise need discuss theirdifferentiating marks only. In the negative types, these consist of theapparent loss of some quality which was active in the species. But itwas pointed out in our last lecture that such a change is an apparentloss. On a closer inquiry we are led to the assumption of a latent ordormant state. The presumably lost characters have not absolutely, or atleast not permanently disappeared. They show their presence by someslight indication of the quality they represent, or by occasionalreversions. They are not wanting, but only latent. 

Basing our discussion concerning the process of crossing on thisconception, and still limiting the discussion to one differentiatingmark, we come to the inference, that this mark is present and active inthe species, and present but dormant in the variety. Thus it is presentin both, and as all other characters not differentiating [255] findtheir mates in the cross, so these two will also meet one another. Theywill unite just as well as though they were both active or both dormant. For essentially they are the same, only differing in their degree ofactivity. From this we can infer, that in the crossing of varieties, nounpaired remainder is left, all units combining in pairs exactly as inordinary fertilization. 

Setting aside the contrast between activity and latency in this singlepair, the procedure in the inter-crossing of varieties is the same as inordinary normal fertilization. 

Summarizing this discussion we may conclude that in normal fertilizationand in the inter-crossing of varieties all characters are paired, whilein crosses between elementary species the differentiating marks are notmated. 

In order to distinguish these two great types of fertilization we willuse the term bisexual for the one and unisexual for the other. The termbalanced crosses then conveys the idea of complete bisexuality, allunit-characters combining in pairs. Unbalanced crosses are those inwhich one or more units do not find their mates and therefore remainunpaired. This distinction was proposed by Macfarlane when studying theminute structure of plant-hybrids in comparison with that of theirparents (1892). 

[256] In the first place it shows that a species hybrid may inherit thedistinguishing marks of both parents. In this way it may becomeintermediate between them, having some characters in common with thepollen-parent and others with the pistil-parent. As far as thesecharacters do not interfere with each other, they may be fully developedside by side, and in the main this is the way in which hybrid charactersare evolved. But in most cases our existing knowledge of the units isfar too slender to give a complete analysis, even of thesedistinguishing marks alone. We recognize the parental marks more or lessclearly, but are not prepared for exact delimitations. Leaving thesetheoretical considerations, we will pass to the description of someillustrative examples. 

In the first place I will describe a hybrid between two species of_Oenothera_, which I made some years ago. The parents were the commonevening-primrose or _Oenothera biennis_ and of its small-floweredcongener, _Oenothera muricata_. These two forms were distinguished byLinnaeus as different species, but have been considered by subsequentwriters as elementary species or so-called systematic varieties of onespecies designated with the name of the presumably older type, the _O. Biennis_. Varietal differences in a physiologic sense they [257] do notpossess, and for this reason afford a pure instance of unbalanced union, though differing in more than one point. 

I have made reciprocal crosses, taking at one time the small-floweredand at the other the common species as pistillate parent. These crossesdo not lead to the same hybrid as is ordinarily observed in analogouscases; quite on the contrary, the two types are different in mostfeatures, both resembling the pollen-parent far more than thepistil-parent. The same curious result was reached in sundry otherreciprocal crosses between species of this genus. But I will limitmyself here to one of the two hybrids. 

In the summer of 1895 I castrated some flowers of _O. Muricata_, andpollinated them with _O. Biennis_, surrounding the flowers with paperbags so as to exclude the visits of insects. I sowed the seeds in 1896and the hybrids were biennial and flowered abundantly the next year andwere artificially fertilized with their own pollen, but gave only a verysmall harvest. Many capsules failed, and the remaining contained onlysome few ripe seeds. 

From these I had in the following year the second hybrid generation, andin the same way I cultivated also the third and fourth. These were asimperfectly fertile as the first, and in [258] some years did not giveany seed at all, so that the operation had to be repeated in order tocontinue the experiment. Last summer (1903) I had a nice lot of some 25biennial specimens blooming abundantly. All in all I have grown some 500hybrids, and of these about 150 specimens flowered. 

These plants were all of the same type, resembling in most points thepollen-parent, and in some others the pistil-parent of the originalcross. The most obvious characteristic marks are afforded by theflowers, which in _O. Muricata_ are not half so large as in _biennis_, though borne by a calyx-tube of the same length. In this respect thehybrid is like the _biennis_ bearing the larger flowers. These may attimes seem to deviate a little in the direction of the other parent, being somewhat smaller and of a slightly paler color. But it is verydifficult to distinguish between them, and if _biennis_ and hybridflowers were separated from the plants and thrown together, it is verydoubtful whether one would succeed in separating them. 

The next point is offered by the foliage. The leaves of _O. Biennis_ arebroad, those of _O. Muricata_ narrow. The hybrid has the broad leaves of_O. Biennis_ during most of its life and at the time of flowering. Yetsmall deviations in the [259] direction of the other parent are notwanting, and in winter the leaves of the hybrid rosettes are often muchnarrower than those of _O. Biennis_, and easily distinguishable fromboth parents. A third distinction consists in the density of the spike. The distance between the insertion of the flowers of _O. Biennis_ isgreat when compared with that of _O. Muricata_. Hence the flowers of thelatter species are more crowded and those of _O. Biennis_ moredispersed, the spikes of the first being densely crowned with flowersand flower-buds while those of _O. Biennis_ are more elongated andslender. As a further consequence the _O. Biennis_ opens on the sameevening only one, two or three flowers on the same spike, whereas _O. Muricata_ bears often eight or ten or more flowers at a time. In thisrespect the hybrid is similar to the pistil-parent, and the crowding ofthe broad flowers at the top of the spikes causes the hybrids to be muchmore showy than either of the parent types. 

Other distinguishing marks are not recorded by the systematists, or arenot so sharply separated as to allow of the corresponding qualities ofthe hybrids being compared with them. 

This hybrid remains true to the description given. In some years Icultivated two generations [260] so as to be able to compare them withone another, but did not find any difference. The most interesting pointhowever, is the likeness between the first generation, which obviouslymust combine in its internal structure the units of both parents, andthe second and later generations which are only of a derivative nature. Next to this stands the fact that in each generation all individuals arealike. No reversion to the parental forms either in the whole type or inthe single characteristics has ever been observed, though the leaves ofsome hundreds, and the spikes and flowers of some 150 individual plantshave been carefully examined. No segregation or splitting up takesplace. 

Here we have a clear, undoubted and relatively simple, case of a trueand pure species hybrid. No occurrence of possible varietalcharacteristics obscures the result, and in this respect this hybridstands out much more clearly than all those between garden-plants, wherevarietal marks nearly always play a most important part. 

From the breeder's point of view our hybrid _Oenothera_ would be adistinct gain, were it not for the difficulty of its propagation. But toenlarge the range of the varieties this simple and stable form wouldneed to be treated anew, by [261] crossing it with the parent-types. Such experiments however, have miscarried owing to the too stable natureof the unit-characters. 

This stability and this absence of the splitting shown by varietal marksin the offspring of hybrids is one of the best proofs of unisexualunions. It is often obscured by the accompanying varietal marks, oroverlooked for this reason. Only in rare cases it is to be met with in apure state and some examples are given of this below. 

Before doing so, I must call your attention to another feature of theunbalanced unions. This is the diminution of the fertility, a phenomenonuniversally known as occurring in hybridizations. It has two phases. First, the diminished chance of the crosses themselves of giving fullcrops of seed, as compared with the pure fertilization of either parent. And, secondly, the fertility of the hybrids themselves. Seemingly, allgrades of diminished fertility occur and the oldest authors on hybridshave pointed out that a very definite relation exists between thedifferences of the parents and the degree of sterility, both of thecross and of the hybrid offspring. In a broad sense these two factorsare proportionate to each other, the sterility being the greater, thelesser the affinity between the parents. Many writers have [262] triedto trace this rule in the single cases, but have met with nearlyunsurmountable difficulties, owing chiefly to our ignorance of the unitswhich form the differences between the parents in the observed cases. 

In the case of _Oenothera muricata x biennis_ the differentiating unitsreduce the fertility to a low degree, threatening the offspring withalmost complete infertility and extinction. But then we do not knowwhether these characters are really units, or perhaps only seemingly soand are in reality composed of smaller entities which as yet we are notable to segregate. And as long as we are devoid of empirical means ofdeciding such questions, it seems useless to go farther into the detailsof the question of the sterility. It should be stated here however, thatpure varietal crosses, when not accompanied by unbalanced characters, have never showed any tendency to diminished fertility. Hence there canbe little doubt that the unpaired units are the cause of this decreasein reproductive power. 

The genus _Oenothera_ is to a large degree devoid of varietalcharacteristics, especially in the subgenus _Onagra_, to which_biennis_, _muricata_, _lamarckiana_ and some others belong. On theother hand it seems to be rich in elementary species, but an adequatestudy of [263] them has as yet not been made. Unfortunately many of thebetter systematists are in the habit of throwing all these interestingforms together, and of omitting their descriptive study. I have made alarge number of crosses between such undescribed types and as a rule gotconstant hybrid races. Only one or two exceptions could be quoted, asfor instance the _Oenothera brevistylis_, which in its crosses alwaysbehaves as a pure retrogressive variety. Instead of giving an exhaustivesurvey of hybrids, I simply cite my crosses between _lamarckiana_ and_biennis_, as having nearly the aspect of the last named species, andremaining true to this in the second generation without any sign ofreversion or of splitting. I have crossed another elementary species, the _Oenothera hirtella_ with some of my new and with some older Linneanspecies, and got several constant hybrid races. Among these theoffspring of a cross between _muricata_ and _hirtella_ is still incultivation. The cross was made in the summer of 1897 and last year(1903) I grew the fourth generation of the hybrids. These had thecharacters of the _muricata_ in their narrow leaves, but the elongatedspikes and relatively large flowers of the _hirtella_ parent, andremained true to this type, showing only slight fluctuations and neverreverting or segregating [264] the mixed characters. Both parents bearlarge capsules with an abundance of seed, but in the hybrids thecapsules remain narrow and weak, ripening not more than one-tenth theusual quantity of seed. Both parents are easily cultivated in annualgenerations and the same holds good for the hybrid. But whereas thehybrid of muricata and biennis is a stout plant, this type is weak withbadly developed foliage, and very long strict spikes. Perhaps it was notable to withstand the bad weather of the last few years. 

A goodly number of constant hybrids are described in literature, orcultivated in fields and gardens. In such cases the essential questionis not whether they are now constant, but whether they have been so fromthe beginning, or whether they prove to be constant whenever theoriginal cross is repeated. For constant hybrids may also be the issueof incipient splittings, as we shall soon see. 

Among other examples we may begin with the hybrid alfalfa or hybridlucerne (_Medicago media_). It often originates spontaneously betweenthe common purple lucerne or alfalfa and its wild ally with yellowflowers and procumbent stems, the _Medicago falcata_. This hybrid iscultivated in some parts of Germany on a large scale, as it is moreproductive than [265] the ordinary lucerne. It always comes true fromseed and may be seen in a wild state in parks and on lawns. It is one ofthe oldest hybrids with a pure and known lineage. The original cross hasbeen repeated by Urban, who found the hybrid race to be constant fromthe beginning. 

Another very notorious constant hybrid race is the _Aegilopsspeltaeformis_. It has been cultivated in botanic gardens for more thanhalf a century, mostly in annual or biennial generations. It issufficiently fertile and always comes true. Numerous records have beenmade of it, since formerly it was believed by Fabre and others to be aspontaneous transition from some wild species of grass to the ordinarywheat, not a cross. Godron, however, showed that it can be producedartificially, and how it has probably sprung into existence wherever itis found wild. The hybrid between _Aegilops ovata_, a small weed, andthe common wheat is of itself sterile, producing no good pollen. But itmay be fertilized by the pollen of wheat and then gives rise to asecondary hybrid, which is no other than the _Aegilops speltaeformis_. This remained constant in Godron's experiments during a number ofgenerations, and has been constant up to the present time. 

[266] Constant hybrids have been raised by Millardet between severalspecies of strawberries. He combined the old cultivated forms with newlydiscovered types from American localities. They ordinarily showed onlythe characteristics of one of their parents and did not exhibit any newcombination of qualities, but they came true to this type in the secondand later generations. 

In the genus _Anemone_, Janczewski obtained the same results. Somecharacters of course may split, but others remain constant, and whenonly such are present, hybrid races result with new combinations ofcharacters, which are as constant as the best species of the same genus. The hybrids of Janczewski were quite fertile, and he points out thatthere is no good reason why they should not be considered as good newspecies. If they had not been produced artificially, but found in thewild state, their origin would have been unknown, and there can be nodoubt that they would have been described by the best systematists asspecies of the same value as their parents. Such is especially the casewith a hybrid between _Anemone magellanica_ and the common _Anemonesylvestris_. 

Starting from similar considerations Kerner von Marilaun pointed out thefact long ago that many so-called species, of rare occurrence, [267]standing between two allied types, may be considered to have originatedby a cross. Surely a wide field for abuse is opened by such anassertion, and it is quite a common habit to consider intermediate formsas hybrids, on the grounds afforded by their external characters alone, and without any exact knowledge of their real origin and often withoutknowing anything as to their constancy from seed. All such apparentexplanations are now slowly becoming antiquated and obsolete, but thecases adduced by Kerner seem to stand this test. 

Kerner designates a willow, _Salix ehrhartiana_ as a constant hybridbetween _Salix alba_ and _S. Pentandra_. _Rhododendron intermedium_ isan intermediate form between the hairy and the rusty species from theSwiss Alps, _R. Hirsutum_ and _R. Ferrugineum_, the former growing onchalky, and the other on silicious soils. Wherever both these types ofsoil occur in the same valley and these two species approach oneanother, the hybrid _R. Intermedium_ is produced, and is often seen tobe propagating itself abundantly. As is indicated by the name, itcombines the essential characters of both parents. 

_Linaria italica_ is a hybrid toad-flax between _L. Genistifolia_ and_L. Vulgaris_, a cross which I have repeated in my garden. _Droseraobovata_ [268] is a hybrid sundew between _D. Anglica_ and _D. Rotundifolia_. _Primula variabilis_ is a hybrid between the two commonprimroses, _P. Officinalis_ and _P. Grandiflora_. The willow-herb(_Epilobium_), the self-heal (_Brunella_) and the yellow pond-lilies(Nuphar) afford other instances of constant wild hybrids. 

Macfarlane has discovered a natural hybrid between two species of sundewin the swamps near Atco, N. J. The parents, _D. Intermedia_ and _D. Filiformis_, were growing abundantly all around, but of the hybrid onlya group of eleven plants was found. A detailed comparison of the hybridwith its parents demonstrated a minute blending of the anatomicalpeculiarities of the parental species. 

Luther Burbank of Santa Rosa, California, has produced a great manyhybrid brambles, the qualities of which in many respects surpass thoseof the wild species. Most of them are only propagated by cuttings andlayers, not being stable from seed. But some crosses between theblackberry and the raspberry (_R. Fruticosus_ and _R. Idaeus_) whichbear good fruit and have become quite popular, are so fixed in theirtype as to reproduce their composite characters from seed with as muchregularity as the species of _Rubus_ found in nature. Among them are the"Phenomenal" and the [269] "Primus. " The latter is a cross between theCalifornian dewberry and the Siberian raspberry and is certainly to beregarded as a good stable species, artificially produced. Bell Saltercrossed the willow-herbs _Epilobium tetragonum_ and _E. Montanum_, andsecured intermediate hybrids which remained true to their type duringfour successive generations. 

Other instances might be given. Many of them are to be found inhorticultural and botanical journals which describe their systematic andanatomical details. The question of stability is generally dealt with inan incidental manner, and in many cases it is difficult to reachconclusions from the facts given. Especially disturbing is thecircumstance that from a horticultural point of view it is quitesufficient that a new type should repeat itself in some of its offspringto be called stable, and that for this reason absolute constancy israrely proved. 

The range of constant hybrids would be larger by far were it not for twofacts. The first is the absolute sterility of so many beautiful hybrids, and the second is the common occurrence of retrogressive charactersamong cultivated plants. To describe the importance of both these groupsof facts would take too much [270] time, and therefore it seems best togive some illustrative examples instead. 

Among the species of _Ribes_ or currant, which are cultivated in ourgardens, the most beautiful are without doubt the Californian and theMissouri currant, or _Ribes sanguineum_ and _R. Aureum_. A third form, often met with, is "Gordon's currant, " which is considered to be ahybrid between the two. It has some peculiarities of both parents. Theleaves have the general form of the Californian parent, but are assmooth as the Missouri species. The racemes or flower-spikes are denselyflowered as in the red species, but the flowers themselves are of ayellow tinge, with only a flesh-red hue on the outer side of the calyx. It grows vigorously and is easily multiplied by cuttings, but it neverbears any fruit. Whether it would be constant, if fertile, is thereforeimpossible to decide. _Berberis ilicifolia_ is considered as a hybridbetween the European barberry (_B. Vulgaris_) and the cultivated shrub_Mahonia aquifolia_. The latter has pinnate leaves, the former undividedones. The hybrid has undivided leaves which are more spiny than those ofthe European parent, and which are not deciduous like them, but persistduring the winter, a peculiarity inherited from the _Mahonia_. As far asI [271] have been able to ascertain, this hybrid never produces seed. 

Another instance of an absolutely sterile hybrid is the often quoted_Cytisus adami_. It is a cross between the common laburnum (_CytisusLaburnum_) and another species of the same genus, _C. Purpureus_, andhas some traits of both. But since the number of differentiating marksis very great in this case, most of the organs have become intermediate. It is absolutely sterile. But it has the curious peculiarity ofsplitting in a vegetative way. It has been multiplied on a large scaleby grafting and was widely found in the parks and gardens of Europeduring the last century. Nearly all these specimens reverted from timeto time to the presumable parents. Not rarely a bud of Adam's laburnumassumed all the qualities of the common laburnum, its larger leaves, richer flowered racemes, large and brightly yellow flowers and itscomplete fertility. Other buds on the same tree reverted to the purpleparent, with its solitary small flowers, its dense shrublike branchesand very small leaves. These too are fertile, though not producing theirseeds as abundantly as the _C. Laburnum_ reversions. Many a botanist hassown the seeds of the latter and obtained only pure common _C. Laburnum_plants. I had a lot of nearly a hundred seedlings [272] myself, many ofwhich have already flowered, bearing the leaves and flowers of thecommon species. Seeds of the purple reversions have also been sown, andalso yielded the parental type only. 

Why this most curious hybrid sports so regularly and why others alwaysremain true to their type is as yet an open question. 

But recalling our former consideration of this subject the suppositionseems allowable that the tendency to revert is not connected with thetype of the hybrid, but is apt to occur in some rare individuals ofevery type. But since most of the sterile hybrids are only known to usin a single individual and its vegetative offspring, this surmise offersan explanation of the rare occurrence of sports. 

Finally, we must consider some of the so called hybrid races or strainsof garden-plants. _Dahlia_, _Gladiolus_, _Amaryllis_, _Fuchsia_, _Pelargonium_ and many other common flowers afford the best knowninstances. Immeasurable variability seems here to be the result ofcrossing. But on a closer inspection the range of characters is not sovery much wider in these hybrid races than in the groups of parentspecies which have contributed to the origin of the hybrids. Ourtuberous begonias owe their variability to at least seven originalparent species, [273] and to the almost incredible number ofcombinations which are possible between their characters. The first ofthese crosses was made in the nursery of Veitch and Sons near London bySeden, and the first hybrid is accordingly known as _Begonia sedeni_ andis still to be met with. It has been superseded by subsequent crossesbetween the _sedeni_ itself and the _Veitchi_ and _rosiflora_, the_davisii_, the _clarkii_ and others. Each of them contributed itsadvantageous qualities, such as round flowers, rosy color, erect flowerstalks, elevation of the flowers above the foliage and others. Newcrosses are being made continuously, partly between the already existinghybrids and partly with newly introduced wild species. Only rarely is itpossible to get pure seeds, and I have not yet been able to ascertainwhether the hybrids would come true from seed. Specific and varietalcharacters may occur together in many of the several forms, but nothingis as yet accurately known as to their behavior in pure fertilizations. Constancy and segregation are thrown together in such a manner thatextreme variability results, and numerous beautiful types may be had, and others may be expected from further crosses. For a scientificanalysis, however, the large range of recorded facts and the writtenhistory, which at first sight [274] seems to have no lacunae, are notsufficient. Most of the questions remain open and need investigation. Itwould be a capital idea to try to repeat the history of the begonias orany other hybrid race, making all the described crosses and thenrecording the results in a manner requisite for complete and carefulscientific investigations. 

Many large genera of hybrid garden-flowers owe their origin to speciesrich in varieties or in elementary subspecies. Such is the case with thegladiolus and the tulips. In other cases the original types have notbeen obtained from the wild state but from the cultures of othercountries. 

The dahlias were cultivated in Mexico when first discovered byEuropeans, and the chrysanthemums have been introduced from the oldgardens of Japan. Both of them consisted of various types, whichafterwards have been increased chiefly by repeated intercrossing. 

The history of many hybrid races is obscure, or recorded by differentauthorities in a different way. Some have derived their evidence fromone nursery, some from another, and the crosses evidently may have beendifferent in different places. The early history of the gladiolus is aninstance. The first crosses are recorded to have been made between_Gladiolus_ [275] _psittacinus_ and _G. Cardinalis_, and between theirhybrid, which is still known under the name of gandavensis_ and the_purpureo-auratus_. But other authors give other lines of descent. So itis with _Amaryllis_, which is said by De Graaff to owe its stripes to_A. Vittata_, its fine form to _A. Brasiliensis_, the large petals to_A. Psittacina_, the giant flowers to _A. Leopoldi_, and the piebaldpatterns to _A. Pardina_. But here, too, other authors give otherderivations. 

Summarizing the results of our inquiry we see in the first place howvery much remains to be done. Many old crosses must be repeated andstudied anew, taking care of the purity of the cross as well as of theharvesting of the seeds. Many supposed facts will be shown to be ofdoubtful validity. New facts have to be gathered, and in doing so thedistinction between specific and varietal marks must be taken strictlyinto account. The first have originated as progressive mutations; theygive unbalanced crosses with a constant offspring, as far as experiencenow goes. The second are chiefly due to retrograde modifications, andwill be the subject of the next lecture. 

[276]

LECTURE X

MENDEL'S LAW OF BALANCED CROSSES

In the scientific study of the result of crosses, the most essentialpoint is the distinction of the several characters of the parents intheir combination in the hybrids and their offspring. From a theoreticalpoint of view it would be best to choose parents which would differ onlyin a single point. The behavior of the differentiating character mightthen easily be seen. 

Unfortunately, such simple cases do not readily occur. Most species, andeven many elementary species are distinguished by more than one quality. Varieties deviating only in one unit-character from the species, aremore common. But a closer inspection often reveals some secondarycharacters which may be overlooked in comparative or descriptivestudies, but which reassume their importance in experimental crossings. 

In a former lecture we have dealt with the qualities which must beconsidered as being due to the acquisition of new characters. If we[277] compare the new form in this case with the type from which it hasoriginated, it may be seen that the new character does not find itsmate, or its opposite, and it will be unpaired in the hybrid. 

In the case of retrogressive changes the visible modification is due, atleast in the best known instances, to the reduction of an active qualityto a state of inactivity or latency. Now if we make a cross between aspecies and its variety, the differentiating character will be due tothe same internal unit, with no other difference than that it is activein the species and latent in the variety. In the hybrid these twocorresponding units will make a pair. But while all other pairs in thesame hybrid individuals consist of like antagonists, only this pairconsists of slightly unlike opponents. 

This conception of varietal crosses leads to three assertions, whichseem justifiable by actual experience. 

First, there is no reason for a diminution of the fertility, as allcharacters are paired in the hybrid, and no disturbance whatever ensuesin its internal structure. Secondly, it is quite indifferent, how thetwo types are combined, or which of them is chosen as pistillate andwhich as staminate parent. The deviating pair will have the sameconstitution in both cases, being [278] built up of one active and onedormant unit. Thirdly this deviating pair will exhibit the active unitwhich it contains, and the hybrid will show the aspect of the parent inwhich the character was active and not that of the parent in which itwas dormant. Now the active quality was that of the species, and itslatent state was found in the variety. Hence the inference that hybridsbetween a species and its retrograde variety will bear the aspect of thespecies. This attribute may be fully developed, and then the hybrid willnot be distinguishable from the pure species in its outer appearance. Orthe character may be incompletely evolved, owing to the failure ofcooperation of the dormant unit. In this case the hybrid will be in somesense intermediate between its parents, but these instances are morerare than the alternate ones, though presumably they may play animportant part in the variability of many hybrid garden-flowers. 

All of these three rules are supported by a large amount of evidence. The complete fertility of varietal hybrids is so universallyacknowledged that it is not worth while to give special instances. Withmany prominent systematists it has become a test between species andvarieties, and from our present point of view this assumption iscorrect. Only the test is of little use in practice, as fertility may bediminished [279] in unbalanced unions in all possible degrees, accordingto the amount of difference between the parents. If this amount isslight, if for instance, only one unit-character causes the difference, the injury to fertility may, be so small as to be practically nothing. Hence we see that this test would not enable us to judge of the doubtfulcases, although it is quite sufficient as a proof in cases of widerdifferences. 

Our second assertion related to the reciprocal crosses. This is the namegiven to two sexual combinations between the same parents, but withinterchanged places as to which furnishes the pollen. In unbalancedcrosses of the genus _Oenothera_ the hybrids of such reciprocal unionsare often different, as we have previously shown. Sometimes bothresemble the pollen parent more, in other instances the pistil-parent. In varietal crosses no such divergence is as yet known. It would bequite superfluous to adduce single cases as proofs for this rule, whichwas formerly conceived to hold good for hybrids in general. The work ofthe older hybridists, such as Koelreuter and Gaertner affords numerousinstances. 

Our third rule is of a wholly different nature. Formerly the distinctionbetween elementary species and varieties was not insisted upon, and theprinciple which stamps retrograde changes [280] as the true character ofvarieties is a new one. Therefore it is necessary to cite a considerableamount of evidence in order to prove the assertion that a hybrid bearsthe active character of its parent-species and not the inactivecharacter of the variety chosen for the cross. 

We may put this assertion in a briefer form, stating that the activecharacter prevails in the hybrid over its dormant antagonist. Or as itis equally often put, the one dominates and the other is recessive. Inthis terminology the character of the species is dominant in the hybridwhile that of the variety is recessive. Hence it follows that in thehybrid the latent or dormant unit is recessive, but it does not followthat these three terms have the same meaning, as we shall see presently. The term recessive only applies to the peculiar state into which thelatent character has come in the hybrid by its pairing with theantagonistic active unit. 

In the first place it is of the highest importance to consider crossesbetween varieties of recorded origin and the species from which theyhave sprung. When dealing with mutations of celandine we shall see thatthe laciniated form originated from the common celandine in a garden atHeidelberg about the year 1590. Among my _Oenotheras_ one of the eldestof the recent productions is the _O. Brevistylis_ or short [281] styledspecies which was seen for the first time in the year 1889. The thirdexample offered is a hairless variety of the evening campion, _Lychnisvespertina_, found the same year, which hitherto had not been observed. 

For these three cases I have made the crosses of the variety with theparent-species, and in each case the hybrid was like the species, andnot like the variety. Nor was it intermediate. Here it is proved thatthe older character dominates the younger one. 

In most cases of wild, and of garden-varieties, the relation betweenthem and the parent-species rests upon comparative evidence. Often thevariety is known to be younger, in other cases it may be only of localoccurrence, but ordinarily the historic facts about its origin havenever been known or have long since been forgotten. 

The easiest and most widely known varietal crosses are those betweenvarieties with white flowers and the red- or blue-flowered species. Herethe color prevails in the hybrid over the lack of pigment, and as a rulethe hybrid is as deeply tinted as the species itself, and cannot bedistinguished from it, without an investigation of its hereditaryqualities. Instances may be cited of the white varieties of thesnapdragon, of the red clover, the long-spurred violet (_Viola_ [282]_cornuta_) the sea-shore aster (_Aster Tripolium_), corn-rose(_Agrostemma Githago_), the Sweet William (_Silene Armeria_), and manygarden flowers, as for instance, the _Clarkia pulchella_, the_Polemonium coeruleum_, the _Veronica longifolia_, the gloxinias andothers. If the red hue is combined with a yellow ground-color in thespecies, the variety will be yellow and the hybrid will have the red andyellow mixture of the species as for instance, in the genus _Geum_. Thetoad-flax has an orange-colored palate, and a variety occurs in whichthe palate is of the same yellow tinge as the remaining parts of thecorolla. The hybrid between them is in all respects like theparent-species. 

Other instances could be given. In berries the same rule prevails. Theblack nightshade has a variety with yellow berries, and the black colorreturns in the hybrid. Even the foliage of some garden-plants may affordinstances, as for instance, the purplish amaranth (_Amaranthuscaudatus_). It has a green variety, but the hybrid between the two hasthe red foliage of the species. 

Special marks in leaves and in flowers follow the same rule. Somevarieties of the opium poppy have large black patches at the basal endof the petals, while in others this pattern is entirely white. Incrossing two such varieties, [283] for instance, the dark "Mephisto"with the white-hearted "Danebrog, " the hybrid shows the active characterof the dark pattern. 

Hairy species crossed with their smooth varieties produce hairy hybrids, as in some wheats, in the campion (_Lychnis_), in _Biscutella_ andothers. The same holds good for the crosses between spiny species andtheir unarmed derivatives, as in the thorn-apple, the corn-crowfoot(_Ranunculus arvensis_) and others. 

Lack of starch in seeds is observed in some varieties of corn and ofpeas. When such derivatives are crossed with ordinary starch-producingtypes, the starch prevails in the hybrid. 

It would take too much time to give further examples. But there is stillone point which should be insisted upon. It is not the systematicrelation of the two parents of a cross, that is decisive, but only theoccurrence of the same quality, in the one in an active, and in theother in an inactive condition. Hence, whenever this relation occursbetween the parents of a cross, the active quality prevails in thehybrid, even when the parents differ from each other in other respectsso as to be distinguished as systematic species. The white and redcampions give a red hybrid, the black and pale henbane (_Hyoscyamusniger_ and _H. Pallidus_) give a hybrid [284] with the purple veins andcenter in the corolla of the former, the white and blue thornappleproduce a blue hybrid, and so on. Instances of this sort are common incultivated plants. 

Having given this long list of examples of the rule of the dominancy ofthe active character over the opposite dormant unit, the questionnaturally arises as to how the antagonistic units are combined in thehybrid. This question is of paramount importance in the consideration ofthe offspring of the hybrids. But before taking it up it is as well tolearn the real signification of recessiveness in the hybrids themselves. 

Recessive characters are shown by those rare cases, in which hybridsrevert to the varietal parent in the vegetative way. In other words bybud-variations or sports, analogous to the splitting of Adam's laburnuminto its parents, by means of bud-variation already described. But herethe wide range of differentiating characters of the parents of this mostcurious hybrid fail. The illustrative examples are extremely simple, andare limited to the active and inactive condition of only one quality. 

An instance is given by the long-leaved veronica (_Veronicalongifolia_), which has bluish flowers in long spikes. The hybridbetween [285] this species and its white variety has a blue corolla. Butoccasionally it produces some purely white flowers, showing its power ofseparating the parental heritages, combined in its internal structures. This reversion is not common, but in thousands of flowering spikes onemay expect to find at least one of them. Sometimes it is a whole stemspringing from the underground system and bearing only white flowers onall its spikes. In other instances it is only a side branch whichreverts and forms white flowers on a stem, the other spikes of whichremain bluish. Sometimes a spike even differentiates longitudinally, bearing on one side blue and on the other white corollas, and the whitestripe running over the spike may be seen to be long and large, ornarrow and short in various degrees. In such cases it is evident thatthe heritages of the parents remain uninfluenced by each other duringthe whole life of the hybrid, working side by side, but the activeelement always prevails over its latent opponent which is ready to breakfree whenever an opportunity is offered. 

It is now generally assumed that this incomplete mixture of the parentalqualities in a hybrid, this uncertain and limited combination is thetrue cause of the many deviations, exhibited by varietal hybrids whencompared with their [286] parents. Partial departures are rare in thehybrids themselves, but in their offspring the divergence becomes therule. 

Segregation seems to be a very difficult process in the vegetative way, but it must be very easy in sexual reproduction, indeed so easy as toshow itself in nearly every single instance. 

Leaving this first generation, the original hybrids, we now come to adiscussion of their offspring. Hybrids should be fertilized either bytheir own pollen, or by that of other individuals born from the samecross. Only in this case can the offspring be considered as a means ofarriving at a decision as to the internal nature of the hybridsthemselves. Breeders generally prefer to fertilize hybrids with thepollen of their parents. But this operation is to be considered as a newcross, and consequently is wholly excluded from our present discussion. Hence it follows that a clear insight into the heredity of hybrids maybe expected only from scientific experiments. Furthermore some of thediversity observed as a result of ordinary crosses, may be due to theinstability of the parents themselves or at least of one of them, sincebreeders ordinarily choose for their crosses some already very variablestrain. Combining such a strain with the desirable qualities of somenewly imported species, a new strain may [287] result, having the newattribute in addition to all the variability of the old types. Inscientific experiments made for the purpose of investigating the generallaws of hybridity, such complex cases are therefore to be whollyexcluded. The hereditary purity of the parents must be considered as oneof the first conditions of success. 

Moreover the progeny must be numerous, since neither constancy, nor theexact proportions in the case of instability, can be determined with asmall lot of plants. 

Finally, and in order to come to a definite choice of research material, we should keep in mind that the chief object is to ascertain therelation of the offspring to their parents. Now in nearly all cases theseeds are separated from the fruits and from one another, before itbecomes possible to judge of their qualities. One may open a fruit andcount the seeds, but ordinarily nothing is noted as to their characters. In this respect no other plant equals the corn or maize, as the kernelsremain together on the spike, and as it has more than one varietycharacterized by the color, or constitution, or other qualities of thegrains. A corn-grain, however, is not a seed, but a fruit containing aseed. Hence the outer parts pertain to the parent plant and only theinnermost ones to the [288] seedling and therefore to the followinggeneration. Fruit-characters thus do not offer the qualities we need, only the qualities resulting from fertilizations are characteristic ofthe new generation. Such attributes are afforded in some cases by thecolor, in others by the chemical constitution. 

We will choose the latter, and take the sugarcorn in comparison with theordinary or starch producing forms for our starting point. Both sugar-and starch-corns have smooth fruits when ripening. No difference is tobe seen in the young ripe spikes. Only the taste, or a direct chemicalanalysis might reveal the dissimilarity. But as soon as the spikes aredried, a diversity is apparent. The starchy grains remain smooth, butthe sugary kernels lose so much water that they become wrinkled. Theformer becomes opaque, the latter more or less transparent. Every singlekernel may instantly be recognized as belonging to either of the typesin question, even if but a single grain of the opposite quality might bemet with on a spike. Kernels can be counted on the spike, and sinceordinary spikes may bear from 300-500 grains and often more, thenumerical relation of the different types may be deduced with greataccuracy. 

Coming now to our experiment, both starchy [289] and sugary varietiesare in this respect wholly constant, when cultivated separately. Nochange is to be seen in the spikes. Furthermore it is very easy to makethe crosses. The best way is to cultivate both types in alternate rowsand to cut off the staminate panicles a few days before they open theirfirst flowers. If this operation is done on all the individuals of onevariety, sparing all the panicles of the other, it is manifest that allthe plants will become fertilized by the latter, and hence that thecastrated plants will only bear hybrid seeds. 

The experiment may be made in two ways; by castrating the sugary or thestarchy variety. In both cases the hybrid kernels are the same. As totheir composition they repeat the active character of the starchyvariety. The sugar is only accumulated as a result of an incapacity ofchanging it into starch, and the lack of this capacity is to beconsidered as a retrogressive varietal mark. The starch-producing unitcharacter, which is active in the ordinary sorts of corns, is thereforelatent in sugar-corn. 

In order to obtain the second generation, the hybrid grains are sownunder ordinary conditions, but sufficiently distant from any othervariety of corn to insure pure fertilization. The several individualsmay be left to pollinate [290] each other, or they may be artificiallypollinated with their own pollen. 

The outcome of the experiments is shown by the spikes, as soon as theydry. Each spike bears two sorts of kernels irregularly dispersed overits surface. In this point all the spikes are alike. On each of them onemay see on the first inspection that the majority of the kernels arestarch-containing seeds, while a minor part becomes wrinkled andtransparent according to the rule for sugary seeds. This fact shows atonce that the hybrid race is not stable, but has differentiated theparental characters, bringing those of the varietal parent to perfectpurity and isolation. Whether the same holds good for the starchyparent, it is impossible to judge from the inspection of the spikes, since it has been seen in the first generation that the hybrid kernelsare not visibly distinguished from those of the pure starch-producinggrains. 

It is very easy to count the number of both sorts of grains in the spikeof such a hybrid. In doing so we find, that the proportion is nearly thesame on all the spikes, and only slight variations would be found inhundreds of them. One-fourth of the seeds are wrinkled and three-fourthsare always smooth. The number may vary in single instances and be alittle more or a little less than 25%, ranging, for [291] instance, from20 to 27%, but as a rule, the average is found nearly equal to 25%. 

The sugary kernels, when separated from the hybrid spikes and sownseparately, give rise to pure sugary race, in no degree inferior inpurity to the original variety. But the starchy kernels are of differenttypes, some of them being internally like the hybrids of the firstgeneration and others like the original parent. To decide between thesetwo possibilities, it is necessary to examine their progeny. 

For the study of this third hybrid generation we will now take anotherexample, the opium poppies. They usually have a dark center in theflowers, the inferior parts of the four petals being stained a deeppurple, or often nearly black. Many varieties exhibit this mark as alarge black cross in the center of the flower. In other varieties thepigment is wanting, the cross being of a pure white. Obviously it isonly reduced to a latent condition, as in so many other cases of loss ofcolor, since it reappears in a hybrid with the parent-species. 

For my crosses I have taken the dark-centered "Mephisto" and the"Danebrog, " or Danish flag, with a white cross on a red field. Thesecond year the hybrids were all true to the type of "Mephisto. " Fromthe seeds of each artificially self-fertilized capsule, one-fourth(22. 5%) [292] in each instance reverted to the varietal mark of thewhite cross, and three-fourths (77. 5%) retained the dark heart. Oncemore the flowers were self-pollinated and the visits of insectsexcluded. The recessives now gave only recessives, and hence we mayconclude that the varietal marks had returned to stability. The darkhearted or dominants behaved in two different ways. Some of themremained true to their type, all their offspring being dark-hearted. Evidently they had returned to the parent with the active mark, and hadreassumed this type as purely as the recessives had reached theirs. Butothers kept true to the hybrid character of the former generation, repeating in their progeny exactly the same mixture as their parents, the hybrids of the first generation, had given. 

This third generation therefore gives evidence, that the second thoughapparently showing only two types, really consists of three differentgroups. Two of them have reassumed the stability of their originalgrandparents, and the third has retained the instability of the hybridparents. 

The question now arises as to the numerical relation of these groups. Our experiments gave the following results: [293]

 Cross 1. Generation 2. Generation 3. Generation

 Mephisto 4- 100% Mephisto | / | / | 77. 5 % Dom. | / \ > --All Mephisto \ | \ 9- all hybrids with 83-68% | 22. 5 % Rec. Dominants and 17-32% | recessives. 100% Danebrog. Danebrog

Examining these figures we find one-fourth of constant recessives, ashas already been said, further one-fourth of constant dominants, and therest or one half as unstable hybrids. Both of the pure groups havetherefore reappeared [293] in the same numbers. Calling A the specimenswith the pure active mark, L those with the latent mark, and H thehybrids, these proportions may be expressed as follows:

 1A+2H+1L. 

This simple law for the constitution of the second generation ofvarietal hybrids with a single differentiating mark in their parents iscalled the law of Mendel. Mendel published it in 1865, but his paperremained nearly unknown to scientific hybridists. It is only of lateyears that it has assumed a high place in scientific literature, andattained the first rank as an investigation on fundamental questions ofheredity. [294] Read in the light of modern ideas on unit characters itis now one of the most important works on heredity and has alreadywidespread and abiding influence on the philosophy of hybridism ingeneral. 

But from its very nature and from the choice of the material made byMendel, it is restricted to balanced or varietal crosses. It assumespairs of characters and calls the active unit of the pair dominant, andthe latent recessive, without further investigations of the question oflatency. It was worked out by Mendel for a large group of varieties ofpeas, but it holds good, with only apparent exceptions, for a wide rangeof cases of crosses of varietal characters. Recently many instances havebeen tested, and even in many cases third and later generations havebeen counted, and whenever the evidence was complete enough to betrusted, Mendel's prophecy has been found to be right. 

According to this law of Mendel's the pairs of antagonistic charactersin the hybrid split up in their progeny, some individuals reverting tothe pure parental types, some crossing with each other anew, and sogiving rise to a new generation of hybrids. Mendel has given a verysuggestive and simple explanation of his formula. Putting this in theterminology of to-day, and limiting it to the occurrence of only [295]one differential unit in the parents, we may give it in the followingmanner. In fertilization, the characters of both parents are notuniformly mixed, but remain separated though most intimately combined inthe hybrid throughout life. They are so combined as to work togethernearly always, and to have nearly equal influence on all the processesof the whole individual evolution. But when the time arrives to produceprogeny, or rather to produce the sexual cells through the combinationof which the offspring arises, the two parental characters leave eachother, and enter separately into the sexual cells. From this it may beseen that one-half of the pollen-cells will have the quality of oneparent, and the other the quality of the other. And the same holds goodfor [296] the egg-cells. Obviously the qualities lie latent in thepollen and in the egg, but ready to be evolved after fertilization hastaken place. 

Granting these premises, we may now ask as to the results of thefertilization of hybrids, when this is brought about by their ownpollen. We assume that numerous pollen grains fertilize numerous eggcells. This assumption at once allows of applying the law ofprobability, and to infer that of each kind of pollen grains one-halfwill reach egg-cells with the same quality [297] and the other halfovules with the opposite character. 

Calling P pollen and O ovules, and representing the active mark by P andO, the latent qualities by P' and O', they would combine as follows:

 P + 0 giving uniform pairs with the active mark, P + 0' giving unequal pairs, P' + 0 giving unequal pairs, P' + 0' giving uniform pairs with the latent mark. 

In this combination the four groups are obviously of the same size, eachcontaining one-fourth of the offspring. Manifestly they correspondexactly to the direct results of the experiments, P + O representing theindividuals which reverted to the specific mark, P' + O' those whoreassumed the varietal quality and P + O' and P + O' those whohybridized [298] for the second time. These considerations lead us tothe following form of Mendel's, P + O = 1/4 Active or 1A, P + O' > = 1/2 Hybrid or 2 H, P' + O

 P' + O' = 1/4 Latent or 1 L, Which is evidently the same as Mendel's empirical law given above. 

To give the proof of these assumptions Mendel has devised a very simplecrossing experiment, [299] which he has effected with his varieties ofpeas. I have repeated it with the sugar-corn, which gives far bettermaterial for demonstration. It starts from the inference that ifdissimilarity among the pollen grains is excluded, the diversity of theovules must at once became manifest and vice versa. In other terms, if ahybrid of the first generation is not allowed to fertilize itself, butis pollinated by one of its parents, the result will be in accordancewith the Mendelian formula. 

In order to see an effect on the spikes produced in this way, it is ofcourse necessary to fertilize them with the pollen of the variety, andnot with that of the specific type. The latter would give partly purestarchy grains and partly hybrid kernels, but these would assume thesame type. But if we pollinate the hybrid with pollen of a puresugar-corn, we may predict the result as follows. 

If the spike of the hybrid contains dormant paternal marks in one-halfof its flowers and in the other half maternal latent qualities, thesugar-corn pollen will combine with one-half of the ovules to givehybrids, and with the other half so as to give pure sugar-grains. Hencewe see that it will be possible to count out directly the two groups ofovules on inspecting the ripe and dry spikes. Experience teaches us[298] that both are present, and in nearly equal numbers; one-half ofthe grains remaining smooth, and the other half becoming wrinkled. 

The corresponding experiment could be made with plants of a puresugar-race by pollination with hybrid pollen. The spikes would showexactly the same mixture as in the above case, but now this may beconsidered as conclusive proof that half the pollen-grains represent thequality of one parent and the other half the quality of the other. 

Another corollary of Mendel's law is the following. In each generationtwo groups return to purity, and one-half remains hybrid. These lastwill repeat the same phenomenon of splitting in their progeny, and it iseasily seen that the same rule will hold good for all succeedinggenerations. According to Mendel's principle, in each year there is anew hybridization, differing in no respect from the first and originalone. If the hybrids only are propagated, each year will show one-fourthof the offspring returning to the specific character, one-fourthassuming the type of the variety and one-half remaining hybrid. I havetested this with a hybrid between the ordinary nightshade with blackberries, and its variety, _Solanum nigrum chlorocarpum_, with paleyellow fruits. Eight generations of the hybrids were cultivated, [299]disregarding always the reverting offspring. At the end I counted theprogeny of the sixth and seventh generations and found figures for theirthree groups of descendants, which exactly correspond to Mendel'sformula. 

Until now we have limited ourselves to the consideration of singledifferentiating units. This discussion gives a clear insight into thefundamental phenomena of hybrid fertilization. It at once shows thecorrectness of the assumption of unit-characters, and of their pairingin the sexual combinations. 

But Mendel's law is not at all restricted to these simple cases. Quiteon the contrary, it explains the most intricate questions ofhybridization, providing they do not transgress the limits ofsymmetrical unions. But in this realm nearly all results may becalculated beforehand, on the ground of the principle of probability. Only one more assumption need be discussed. The several pairs ofantagonistic characters must be independent from, and uninfluenced by, one another. This premise seems to hold good in the vast majority ofcases, though rare exceptions seem to be not entirely wanting. Hence thenecessity of taking all predictions from Mendel's law only asprobabilities, which will prove true in most, but not necessarily in allcases. [300] But here we will limit ourselves to normal cases. 

The first example to be considered is obviously the assumption that theparents of a cross differ from each other in respect to two characters. A good illustrative example is afforded by the thorn-apple. I havecrossed the blue flowered thorny form, usually known as _Datura Tatula_, with the white thornless type, designated as _D. Stramonium inermis_. Thorns and blue pigment are obviously active qualities, as they aredominant in the hybrids. In the second generation both pairs ofcharacters are resolved into their constituents and paired anewaccording to Mendel's law. After isolating my hybrids during the periodof flowering, I counted among their progeny:

 128 individuals with blue flowers and thorns 47 individuals with blue flowers and without thorns 54 individuals with white flowers and thorns 21 individuals with white flowers and without thorns ---- 250

The significance of these numbers may easily be seen, when we calculatewhat was to be expected on the assumption that both characters followMendel's law, and that both are independent from each other. Then wewould have three-fourths blue offspring and one-fourth individuals withwhite flowers. Each of these [301] two groups would consist ofthorn-bearing and thornless plants, in the same numerical relation. Thus, we come to the four groups observed in our experiment, and areable to calculate their relative size in the following way:

 Proportion Blue with thorns 3/4 X 3/4 = 9/16 = 56. 25% 9 Blue, unarmed 3/4 X 1/4 = 3/16 = 18. 75% 3 White with thorns 1/4 X 3/4 = 3/16 = 18. 75% 3 White, unarmed 1/4 X 1/4 = 1/16 = 6. 25% 1

In order to compare this inference from Mendel's law and the assumptionof independency, with the results of our experiments, we must calculatethe figures of the latter in percentages. In this way we find:

 Found Calculated Blue with thorns 128=51% 56. 25% Blue unarmed 47=19% 18. 75% White with thorns 54=22% 18. 75% White unarmed 21= 8% 6. 25%

The agreement of the experimental and the theoretical figures is asclose as might be expected. 

This experiment is to be considered only as an illustrative example of arule of wide application. The rule obviously will hold good in all suchcases as comply with the two conditions already premised, viz. : thateach character agrees with Mendel's law, and that both are whollyindependent of each other. It is clear that our figures show thenumerical composition [302] of the hybrid offspring for any singleinstance, irrespective of the morphological nature of the qualitiesinvolved. 

Mendel has proved the correctness of these deductions by his experimentswith peas, and by combining their color (yellow or green) with thechemical composition (starch or sugar) and other pairs of characters. Iwill now give two further illustrations afforded by crosses of theordinary campion. I used the red-flowered or day-campion, which is aperennial herb, and a smooth variety of the white evening-campion, whichflowers as a rule in the first summer. The combination of flower-colorand pubescence gave the following composition for the second hybridgeneration:

 Number % Calculation Hairy and red 70 44 56. 25% Hairy and white 23 14 18. 75% Smooth and red 46 23 18. 75% Smooth and white 19 12 6. 25%

For the combination of pubescence and the capacity of flowering in thefirst year I found:

 Number % Calculated Hairy, flowering 286 52 56. 25% Hairy, without stem 128 23 18. 75% Smooth, flowering 96 17 18. 75% Smooth, without stem 42 8 6. 25%

Many other cases have been tested by different writers and the generalresult is the [303] applicability of Mendel's formula to all casescomplying with the given conditions. 

Intentionally I have chosen for the last example two pairs ofantagonisms, relating to the same pair of plants, and which may betested in one experiment and combined in one calculation. 

For the latter we need only assume the same conditions as mentionedbefore, but now for three different qualities. It is easily seen thatthe third quality would split each of our four groups into two smallerones in the proportion of 3/4 : 1/4. 

We would then get eight groups of the following composition:

 9/16 X 3/4 = 27/64 or 42. 2% 9/16 X 1/4 = 9/64 " 14. 1% 3/16 X 3/4 = 9/64 " 14. 1% 3/16 X 1/4 = 3/64 " 4. 7% 3/16 X 3/4 = 9/64 " 14. 1% 3/16 X 1/4 = 3/64 " 4. 7% 1/16 X 3/4 = 3/64 " 4. 7% 1/16 X 1/4 = 1/64 " 1. 6%

The characters chosen for our experiment include the absence of stem andflowers in the first year, and therefore would require a second year todetermine the flower-color on the perennial specimens. Instead of doingso I have taken another character, shown by the teeth of the capsuleswhen opening. These curve outwards [304] in the red campion, but lackthis capacity in the evening-campion, diverging only until an uprightposition is reached. The combination of hairs, colors and teeth giveseight groups, and the counting of their respective numbers ofindividuals gave the following:

 Teeth Hairs Flowers of capsules Number % Calculated

 Hairy red curved 91 47 42. 2% Hairy red straight 15 7. 5 14. 1% Hairy white curved 23 12 14. 1% Hairy white straight 17 8. 5 4. 7% Smooth red curved 23 12 14. 1% Smooth red straight 9 4. 5 4. 7% Smooth white curved 5 2. 5 4. 7% Smooth white straight 12 6 1. 6%

The agreement is as comprehensive as might be expected from anexperiment with about 200 plants, and there can be no doubt that arepetition on a larger scale would give still closer agreement. 

In the same way we might proceed to crosses with four or moredifferentiating characters. But each new character will double thenumber of the groups. Four characters will combine into 16 groups, fiveinto 32, six into 64, seven into 128, etc. Hence it is easily seen thatthe size of the experiments must be made larger and larger in the sameratio, if we intend to expect numbers equally trustworthy. For [305]seven differentiating marks 16, 384 individuals are required for acomplete series. And in this set the group with the seven attributes allin a latent condition would contain only a single individual. 

Unfortunately the practical value of these calculations is not verygreat. They indicate the size of the cultures required to get all thepossible combinations, and show that in ordinary cases many thousands ofindividuals have to be cultivated, in order to exhaust the whole rangeof possibilities. They further show that among all these thousands, onlyvery few are constant in all their characters; in fact, it may easily beseen that with seven differentiating points among the 16, 384 namedabove, only one individual will have all the seven qualities in pureactive, and only one will have them all in a purely dormant condition. Then there will be some with some attributes active and others latent, but their numbers will also be very small. All others will split up inthe succeeding generation in regard to one or more of their apparentlyactive marks. And since only in very rare cases the stable hybrids canbe distinguished by external characters from the unstable ones, thestability of each individual bearing a desired combination of characterswould have to be established by experiment [306] after purefertilization. Mendel's law teaches us to predict the difficulties, buthardly shows any way to avoid them. It lays great stress on the oldprescript of isolation and pure fertilization, but it will have to beworked out and applied to a large number of practical cases before itwill gain a preeminent influence in horticultural practice. 

Or, as Bailey states it, we are only beginning to find a pathway throughthe bewildering maze of hybridization. 

This pathway is to be laid out with regard to the followingconsiderations. We are not to cross species or varieties, or evenaccidental plants. We must cross unit-characters, and consider theplants only as the bearers of these units. We may assume that theseunits are represented in the hereditary substance of the cell-nucleus bydefinite bodies of too small a size to be seen, but constitutingtogether the chromosomes. We may call these innermost representatives ofthe unit-characters pangenes, in accordance with Darwin's hypothesis ofpangenesis, or give them any other name, or we may even wholly abstainfrom such theoretical discussion, and limit ourselves to the conceptionof the visible character-units. These units then may be present, orlacking and in the first case active, or latent. 

[307] True elementary species differ from each other in a number ofunit-characters, which do not contrast. They have arisen by progressivemutation. One species has one kind of unit, another species has anotherkind. On combining these, there can be no interchange. Mendelism assumessuch an interchange between units of the same character, but in adifferent condition. Activity and latency are such conditions, andtherefore Mendel's law obviously applies to them. They require pairs ofantagonistic qualities, and have no connection whatever with thosequalities, which do not find an opponent in the other parent. Now, onlypure varieties afford such pure conditions. When undergoing furthermodifications, some of them may be in the progressive line and others inthe retrogressive. Progressive modifications give new units, which arenot in contrast with any other, retrograde changes turn active unitsinto the latent condition and so give rise to pairs. Ordinary speciesgenerally originate in this way, and hence differ from each other partlyin specific, partly in varietal characters. As to the first, they givein their hybrids stable peculiarities, while as to the latter, theysplit up according to Mendel's law. 

Unpaired or unbalanced characters lie side by side with paired orbalanced qualities, and they [308] do so in nearly all the crosses madefor practical purposes, and in very many scientific experiments. EvenMendel's peas were not pure in this respect, much less do the campionsnoted above differ only in Mendelian characters. 

Comparative and systematic studies must be made to ascertain the truenature of every unit in every single plant, and crossing experimentsmust be based on these distinctions in order to decide what laws areapplicable in any case. 

[309]D. EVER-SPORTING VARIETIES

LECTURE XI

STRIPED FLOWERS

Terminology is an awkward thing. It is as disagreeable to be compelledto make new names, as to be constrained to use the old faulty ones. Different readers may associate different ideas with the same terms, andunfortunately this is the case with much of the terminology of thescience of heredity and variability. What are species and what arevarieties? How many different conceptions are conveyed by the termsconstancy and variability? We are compelled to use them, but we are notat all sure that we are rightly understood when we do so. 

Gradually new terms arise and make their way. They have a more limitedapplicability than the old ones, and are more narrowly circumscribed. They are not to supplant the older terms, but permit their use in a moregeneral way. 

[310] One of these doubtful terms is the word _sport_. It often meansbud-variation, while in other cases it conveys the same idea as the oldbotanical term of mutation. But then all sorts of seemingly suddenvariations are occasionally designated by the same term by one writer oranother, and even accidental anomalies, such as teratological ascidia, are often said to arise by sports. 

If we compare all these different conceptions, we will find that theirmost general feature is the suddenness and the rarity of the phenomenon. They convey the idea of something unexpected, something not always ornot regularly occurring. But even this demarcation is not universal, andthere are processes that are regularly repeated and nevertheless arecalled sports. These at least should be designated by another name. 

In order to avoid confusion as far as possible, with the least change inexisting terminology, I shall use the term "ever-sporting varieties" forsuch forms as are regularly propagated by seed, and of pure and nothybrid origin, but which sport in nearly every generation. The term is anew one, but the facts are for the most part new, and require to beconsidered in a new light. Its meaning will become clearer at once whenthe illustrations afforded by [311] striped flowers are introduced. Inthe following discussion it will be found most convenient to give asummary of what is known concerning them, and follow this by aconsideration of the detailed evidence obtained experimentally, whichsupports the usage cited. 

The striped variety of the larkspur of our gardens is known to producemonochromatic flowers, in addition to striped ones. They may be borne bythe same racemes, or on different branches, or some seedlings from thesame parent-plant may bear monochromatic flowers while others may bestriped. Such deviations are usually called sports. But they occuryearly and regularly and may be observed invariably when the culturesare large enough. Such a variety I shall call "ever-sporting. "

The striped larkspur is one of the oldest garden varieties. It has keptits capacity of sporting through centuries, and therefore may in somesense be said to be quite stable. Its changes are limited to a rathernarrow circle, and this circle is as constant as the peculiarities ofany other constant species or variety. But within this circle it isalways changing from small stripes to broad streaks, and from them topure colors. Here the variability is a thing of absolute constancy, while the constancy consists in eternal changes. Such apparent [312]contradictions are unavoidable, when we apply the old term to suchunusual though not at all new cases. Combining the stability and thequalities of sports in one word, we may evidently best express it by thenew term of eversporting variety. 

We will now discuss the exact nature of such varieties, and of the lawsof heredity which govern them. But before doing so, I might point out, that this new type is a very common one. It embraces most of theso-called variable types in horticulture, and besides these a wide rangeof anomalies. 

Every ever-sporting variety has at least two different types, around andbetween which it varies in numerous grades, but to which it isabsolutely limited. Variegated leaves fluctuate between green and white, or green and yellow, and display these colors in nearly all possiblepatterns. But there variability ends, and even the patterns areordinarily narrowly prescribed in the single varieties. Double flowersafford a similar instance. On one side the single type, on the other thenearly wholly double model are the extreme limits, between which thevariability is confined. So it is also with monstrosities. The raceconsists of anomalous and normal individuals, and displays between themall possible combinations of normal and monstrous [313] parts. But itsvariability is restricted to this group. And large as the group may seemon first inspection, it is in reality very narrow. Many monstrosities, such as fasciated branches, pitchers, split leaves, peloric flowers, andothers constitute such ever-sporting varieties, repeating theiranomalies year by year and generation after generation, changing as muchas possible, but remaining absolutely true within their limits as longas the variety exists. 

It must be a very curious combination of the unit-characters whichcauses such a state of continuous variability. The pure quality of thespecies must be combined with the peculiarity of the variety in such away, that the one excludes the other, or modifies it to some extent, although both never fully display themselves in the same part of thesame plant. A corolla cannot be at once monochromatic and striped, norcan the same part of a stem be twisted and straight. But neighboringorgans may show the opposite attributes side by side. 

In order to look closer into the real mechanism of this form ofvariability, and of this constant tendency to occasional reversions, itwill be best to limit ourselves first to a single case, and to try togather all the evidence, which can be obtained by an examination of thehereditary relations of its sundry constituents. 

[314] This may best be done by determining the degree of inheritance forthe various constituents of the race during a series of years. It isonly necessary to apply the two precautions of excluding allcross-fertilization, and of gathering the seeds of each individualseparately. We do not need to ascertain whether the variety as such ispermanent; this is already clear from the simple fact of its antiquityin so many cases. We wish to learn what part each individual, or eachgroup of individuals with similar characters, play in the common line ofinheritance. In other words, we must build up a genealogical tree, embracing several generations and a complete set of the single casesoccurring within the variety, in order to allow of its being consideredas a part of the genealogy of the whole. It should convey to us an ideaof the hereditary relations during the life-time of the variety. 

It is manifest that the construction of such a genealogical treerequires a number of separate experiments. These should be extended overa series of years. Each should include a number of individuals largeenough to allow the determination of the proportion of the differenttypes among the offspring of a single plant. A species which is easilyfertilized by its own pollen, and which bears capsules with [315] largequantities of seeds, obviously affords the best opportunities. As such, I have chosen the common snapdragon of the gardens, _Antirrhinum majus_. It has many striped varieties, some tall, others of middle height, or ofdwarfed stature. In some the ground-color of the flowers is yellow, inothers it is white, the yellow disappearing, with the exception of alarge mark in the throat. On these ground-colors the red pigment is seenlying in streaks of pure carmine, with white intervals where the yellowfails, but combined with yellow to make a fiery red, and with yellowintervals when that color is present. This yellow color is quiteconstant and does not vary in any marked degree, notwithstanding thefact that it seems to make narrower and broader stripes, according tothe parts of the corolla left free by the red pigment. But it is easilyseen that this appearance is only a fallacious one. 

The variety of snapdragon chosen was of medium height and with theyellow ground-color, and is known by horticulturists as _A. Majus luteumrubro-striatum_. As the yellow tinge showed itself to be invariable; Imay limit my description to the red stripes. 

Some flowers of this race are striped, others are not. On a hasty surveythere seem to be three types, pure yellow, pure red, and stripes [316]with all their intermediate links of narrower and broader, fewer andmore numerous streaks. But on a close inspection one does not succeed infinding pure yellow racemes. Little lines of red may be found on nearlyevery flower. They are the extreme type on this side of the range ofvariability. From them an almost endless range of patterns passes overto the broadest stripes and even to whole sections of a pure red. Butthen, between these and the wholly red flowers we observe a gap, whichmay be narrower by the choice of numerous broad striped individuals, butwhich is never wholly filled up. Hence we see that the red flowers are aseparate type within the striped variety. 

This red type springs yearly from the striped form, and yearly revertsto it. This is what in the usual descriptions of this snapdragon, iscalled its sporting. The breadth of the streaks is considered to be anordinary case of variability, but the red flowers appear suddenly, without the expected links. Therefore they are to be considered assports. Similarly the red forms may suddenly produce striped ones, andthis too is to be taken as a sport, according to the usual conception ofthe word. 

Such sports may occur in different ways. Either by seeds, or by buds, oreven within the single spikes. Both opposite reversions, [317] fromstriped to red and from red to stripes, occur by seed, even by thestrictest exclusion of cross-fertilization. As far as my experiments go, they are the rule, and parent-plants that do not give such reversions, at least in some of their offspring, are very rare, if not whollywanting. Bud-variations and variations within the spike I have as yetonly observed on the striped individuals, and never on the red ones, though I am confident that they might appear in larger series ofexperiments. Both cases are more common on individuals with broadstripes than on plants bearing only the narrower red lines, as might beexpected, but even on the almost purely yellow individuals they may beseen from time to time. Bud-variations produce branches with spikes ofuniform red flowers. Every bud of the plant seems to have equal chancesto be transformed in this way. Some striped racemes bear a few redflowers, which ordinarily are inserted on one side of the spike only. Asthey often cover a sharply defined section of the raceme, thiscircumstance has given rise to the term of sectional variability tocover such cases. Sometimes the section is demarcated on the axis of theflower-spike by a brownish or reddish color, sharply contrasting withthe green hue of the remaining parts. Sectional variation may be lookedat as a [318] special type of bud-variation, and from this point of viewwe may simplify our inquiry and limit ourselves to the inheritance ofthree types, the striped plants, the red plants and the red asexualvariants of the striped individuals. In each case the heredity should beobserved not only for one, but at least for two successive generations. 

Leaving these introductory remarks I now come at once to thegenealogical tree, as it may be deduced from my experiments:

 Year 1896 95% Striped 84% Red | | 1895 Striped Individual Red Indiv. \ / 1895 98% Striped 71% Red | | 1894 Striped branches. Red branches. \ / 1894 98% Striped 76% Red | | 1893 90% Striped Indiv. 10% Red Indiv. \ / 1892 Striped Individual

This experiment was begun in the year 1892 with one individual out of alarge lot of striped plants grown from seeds which I had purchased froma firm in Erfurt. The capsules were gathered separately from thisindividual and about 40 flowering plants were obtained from the seeds inthe following year. Most of them had neatly striped flowers, somedisplayed broader stripes and spare flowers were seen with one [319]half wholly red. Four individuals were found with only uniform redflowers. These were isolated and artificially pollinated, and the samewas done with some of the best striped individuals. The seeds from everyparent were sown separately, so as to allow the determination of theproportion of uniform red individuals in the progeny. 

Neither group was constant in its offspring. But as might be expected, the type of the parent plant prevailed in both groups, and more stronglyso in the instances with the striped, than with the red ones. Or, inother words seed-reversions were more numerous among the alreadyreverted reds than among the striped type itself. I counted 2% reversionin the latter case, but 24% from the red parents. 

Among the striped plants from the striped parents, I found some thatproduced bud variations. I succeeded in isolating these red floweringbranches in paper bags and in pollinating them with their own pollen, and subjected the striped spikes of the same individuals to a similartreatment. Three individuals gave a sufficient harvest from both types, and these six lots of seeds were sown separately. The striped flowersrepeated their character in 98% of their offspring, the red twigs inonly 71%, the [320] remaining individuals sporting into the oppositegroup. 

In the following year I continued the experiment with the seeds of theoffspring of the red bud-variations. The striped individuals gave 95%, but in the red ones only 84% of the progeny remained true to the parenttype. 

From these figures it is manifest that the red and striped types differfrom one another not only in their visible attributes, but also in thedegree of their heredity. The striped individuals repeat theirpeculiarity in 90-98% of their progeny, 2-10% sporting into the uniformred color. On the other hand the red individuals are constant in 71-84%of their offspring, while 16-29% go over to the striped type. Or, briefly, both types are inherited to a high degree, but the striped typeis more strictly inherited than the red one. 

Moreover the figures show that the degree of inheritance is notcontingent upon the question as to how the sport may have arisen. Bud-sports show the same degree of inheritance as seed-sports. Sexualand asexual variability therefore seem to be one and the same process inthis instance. But the deeper meaning of this and other special featuresof our genealogical tree are still awaiting further investigation. Itseems that much important evidence might [321] come from an extension ofthis line of work. Perhaps it might even throw some light on theintimate nature of the bud-variations of ever-sporting varieties ingeneral. Sectional variations remain to be tested as to the degree ofinheritance exhibited, and the different occurrences as to the breadthof the streaks require similar treatment. 

In ordinary horticultural practice it is desirable to give someguarantee as to what may be expected to come from the seeds of brightlystriped flowers. Neither the pure red type, nor the nearly yellowracemes are the object of the culture, as both of them may be had purefrom their, own separate varieties. In order to insure proper striping, both extremes are usually rejected and should be rooted out as soon asthe flowering period begins. Similarly the broad-striped ones should berejected, as they give a too large amount of uniform red flowers. Clearly, but not broadly striped individuals always yield the mostreliable seed. 

Summing up once more the results of our pedigree-experiment, we mayassert that the striped variety of the snapdragon is wholly permanent, including the two opposite types of uniform color and of stripes. Itmust have been so since it first originated from the invariable uniform[322] varieties, about the middle of the last century, in the nursery ofMessrs. Vilmorin, and probably it will remain so as long as populartaste supports its cultivation. It has never been observed to transgressits limits or to sport into varieties without reversions or sports. Itfluctuates from one extreme to the other yearly, always recurring in thefollowing year, or even in the same summer by single buds. Highlyvariable within its limits, it is absolutely constant or permanent, whenconsidered as a definite group. 

Similar cases occur not rarely among cultivated plants. In the wildstate they seem to be wholly wanting. Neither are they met with asoccasional anomalies nor as distinct varieties. On the contrary, manygarden-flowers that are colored in the species, and besides this have awhite or yellow variety, have also striped sorts. The oldest instance isprobably the marvel of Peru, _Mirabilis Jalappa_, which already had morethan one striped variety at the time of its introduction from Peru intothe European gardens, about the beginning of the seventeenth century. Stocks, liver-leaf (_Hepatica_), dame's violet (_Hesperis_), SweetWilliam (_Dianthus barbatus_), and periwinkles (_Vinca minor_) seem tobe in the same condition, as their striped varieties were already quoted[323] by the writers of the same century. Tulips, hyacinths, _Cyclamen_, _Azalea_, _Camellia_, and even such types of garden-plants as the meadowcrane's-bill (_Geranium pratensev) have striped varieties. It is alwaysthe red or blue color which occurs in stripes, the underlying groundbeing white or yellow, according to the presence or absence of theyellow in the original color mixture. 

All these varieties are known to be permanent, coming true during longseries of successive generations. But very little is known concerningthe more minute details of their hereditary qualities. They come fromseed, when this is taken from striped individuals, and thence revertfrom time to time to the corresponding monochromatic type. But whetherthey would do so when self-fertilized, and whether the reversionaryindividuals are always bound to return towards the center of the groupor towards the opposite limit, remains to be investigated. Presumablythere is nowhere a real transgression of the limits, and never or onlyvery rarely and at long intervals of time a true production of anotherrace with other hereditary qualities. 

In order to satisfy myself on these points, I made somepedigree-cultures with the striped forms of dame's violet (_Hesperismatronalis_) [324] and of _Clarkia pulchella_. Both of them areever-sporting varieties. The experiments were conducted during fivegenerations with the violet, and during four with the striped Clarkia, including the progeny of the striped and of the monochromatic redoffspring of a primitive striped plant. I need not give the figures herefor the numerical relations between the different types of each group, and shall limit myself to the statement that they behaved in exactly thesame manner as the snapdragon. 

It is worth while to dwell a moment on the capacity of the individualswith red flowers to reproduce the striped type among their offspring. For it is manifest that this latter quality must have lain dormant inthem during their whole life. Darwin has already pointed out that when acharacter of a grandparent, which is wanting in the progeny, reappearsin the second generation, this quality must always be assumed to havebeen present though latent in the intermediate generation. To the manyinstances given by him of such alternative inheritance, themonochromatic reversionists of the striped varieties are to be added asa new type. It is moreover, a very suggestive type, since the latency ismanifestly of quite another character than for instance in the case ofMendelian hybrids, and probably more allied to those instances, [325]where secondary sexual marks, which are as a rule only evolved by onesex, are transferred to the offspring through the other. 

Stripes are by no means limited to flowers. They may affect the wholefoliage, or the fruits and the seeds, and even the roots. But all suchcases occur much more rarely than the striped flowers. An interestinginstance of striped roots is afforded by radishes. White and redvarieties of different shapes are cultivated. Besides them sometimes acurious motley sort may be seen in the markets, which is white with redspots, which are few and narrow in some samples, and more numerous andbroader in others. But what is very peculiar and striking is thecircumstance, that these stripes do not extend in a longitudinal, but ina transverse direction. Obviously this must be the effect of the verynotable growth in thickness. Assuming that the colored regions weresmall in the beginning, they must have been drawn out during the processof thickening of the root, and changed into transverse lines. Rarely astreak may have had its greatest extension in a transverse directionfrom the beginning, in which case it would only be broadened and notdefinitely changed in its direction. 

This variety being a very fine one, and more agreeable to the eye thanthe uniform colors, is [326] being more largely cultivated in somecountries. It has one great drawback: it never comes wholly true fromseed. It may be grown in full isolation, and carefully selected, all redor nearly monochromatic samples being rooted out long before blooming, but nevertheless the seed will always produce some red roots. The mostcareful selection, pursued through a number of years, has not beensufficient to get rid of this regular occurrence of reversionaryindividuals. Seed-growers receive many complaints from their clients onthis account, but they are not able to remove the difficulty. Thisexperience is in full agreement with the experimental evidence given bythe snapdragon, and it would certainly be very interesting to make acomplete pedigree-culture with the radishes to test definitely theircompliance with the rules observed for striped flowers. 

Horticulturists in such cases are in the habit of limiting themselves tothe sale of so-called mixed seeds. From these no client expects purity, and the normal and hereditary diversity of types is here in some senseconcealed under the impurities included in the mixture from lack ofselection. Such cases invite scrutiny, and would, no doubt, with themethods of isolation, artificial pollination, and the sowing of theseeds separately from each parent, yield [327] results of greatscientific value. Any one who has a garden, and sufficient perseveranceto make pure cultures during a series of years might make importantcontributions to scientific knowledge in this way. 

Choice might be made from among a wide range of different types. Avariety of corn called "Harlequin" shows stripes on its kernels, and oneear may offer nearly white and nearly red seeds and all the possibleintermediate steps between them. From these seeds the next generationwill repeat the motley ears, but some specimens will produce ears ofuniform kernels of a dark purple, showing thus the ordinary way ofreversion. Some varieties of beans have spotted seeds, and among a lotof them one may be sure to find some purely red ones. It remains to beinvestigated what will be their offspring, and whether they are due topartial or to individual variation. 

The cockscomb (_Celosia cristata_) has varieties of nearly all colorsfrom white and yellow to red and orange, and besides them some stripedvarieties occur in our gardens, with the stripes going from the lowerparts of the stem up to the very crest of the comb. They are on sale asconstant varieties, but nothing has as yet been recorded concerningtheir peculiar behavior in the inheritance of the stripes. [328] Stripedgrapes, apples and other fruits might be mentioned in this connection. 

Before leaving the striped varieties, attention is called to aninteresting deduction, which probably gives an explanation of one of themost widely known instances of ever-sporting garden plants. Stripedraces always include two types. Both of them are fertile, and each ofthem reproduces in its offspring both its own and the alternate type. Itis like a game of ball, in which the opposing parties always return theball. But now suppose that only one of the types were fertile and theother for some reason wholly sterile, and assume the reversionary, orprimitive monochromatic individuals to be fertile, and the derivativestriped specimens to bloom without seed. If this were the case, knowledge concerning the hereditary qualities would be greatly limited. In fact the whole pedigree would be reduced to a monochromatic strain, which would in each generation sport in some individuals into thestriped variety. But, being sterile, they would not be able to propagatethemselves. 

Such seems to be the case with the double flowered stocks. Their doubleflowers produce neither stamens nor pistils, and as each individual iseither double or single in all its flowers, the doubles are whollydestitute of seed. [329] Nevertheless, they are only reproduced by seedfrom single flowers, being an annual or biennial species. 

Stocks are a large family, and include a wonderful variety of colors, ranging from white and yellow to purple and red, and with somevariations toward blue. They exhibit also diversity in the habit ofgrowth. Some are annuals, including the ten-week and pyramidal forms;others are intermediates and are suitable for pot-culture; and thebiennial sorts include the well-known "Brompton" and "Queen" varieties. Some are large and others are small or dwarf. For their brightness, durability and fragrance, they are deservedly popular. There are evensome striped varieties. Horticulturists and amateurs generally know thatseed can be obtained from single stocks only, and that the doubleflowers never produce any. It is not difficult to choose single plantsthat will produce a large percentage of double blossoms in the followinggeneration. But only a percentage, for the experiments of the mostskilled growers have never enabled them to save seed, which would resultentirely in double flowering plants. Each generation in its turn is amotley assembly of singles and doubles. 

Before looking closer into the hereditary peculiarities of this old andinteresting ever-sporting [330] variety, it may be as well to give ashort description of the plants with double flowers. Generally speakingthere are two principal types of doubles. One is by the conversion ofstamens into petals, and the other is an anomaly, known under the nameof _petalomany_. 

The change of stamens into petals is a gradual modification. Allintermediate steps are easily to be found. In some flowers all stamensmay be enlarged, in others only part of them. Often the broadenedfilaments bear one or two fertile anthers. The fertility is no doubtdiminished, but not wholly destroyed. Individual specimens may occur, which cannot produce any seed, but then others of the same lot may be asfertile as can be desired. As a whole, such double varieties areregularly propagated by seed. 

Petalomany is the tendency of the axis of some flowers never to make anystamens or pistils, not even in altered or rudimentary form. Instead ofthese, they simply continue producing petals, going on with thisproduction without any other limit than the supply of available food. Numerous petals fill the entire space within the outer rays, and in theheart of the flower innumerable young ones are developed half-way, notobtaining food enough to attain [331] full size. Absolute sterility isthe natural consequence of this state of things. 

Hence it is impossible to have races of petalomanous types. If theabnormality happens to show itself in a species, which normallypropagates itself in an asexual way, the type may become a vegetativevariety, and be multiplied by bulbs, buds or cuttings, etc. Somecultivated anemones and crowfoots (_Ranunculus_) are of this character, and even the marsh-marigold (_Caltha palustris_) has a petalomanousvariety. I once found in a meadow such a form of the meadow-buttercup(_Ranunculus_ acris_), and succeeded in keeping it in my garden forseveral years, but it did not make seeds and finally died. Camellias areknown to have both types of double flowers. The petalomanous type ishighly regular in structure, so much so as to be too uniform in all itsparts to be pleasing, while the conversion of stamens into petals in thealternative varieties gives to these flowers a more lively diversity ofstructure. Lilies have a variety called _Lilium candidum flore pleno_, in which the flowers seem to be converted into a long spike of bright, white narrow bracts, crowded on an axis which never seems to cease theirproduction. 

It is manifestly impossible to decide how all such sterile doubleflowers have originated. [332] Perhaps each of them originally had acongruent single-flowered form, from which it was produced by seed inthe same way as the double stocks now are yearly. If this assumption isright, the corresponding fertile line is now lost; it has perhaps diedout, or been masked. But it is not absolutely impossible that suchstrains might one day be discovered for one or another of these nowsterile varieties. 

Returning to the stocks we are led to the conception that some varietiesare absolutely single, while others consist of both single-flowered anddouble-flowered individuals. The single varieties are in respect to thischaracter true to the original wild type. They never give seed whichresults in doubles, providing all intercrossing is excluded. The othervarieties are ever-sporting, in the sense of this term previouslyassumed, but with the restriction that the sports are exclusivelyone-sided, and never return, owing to their absolute sterility. 

The oldest double varieties of stocks have attained an age of a centuryand more. During all this time they have had a continuous pedigree offertile and single-flowered individuals, throwing off in each generationa definite number of doubles. This ratio is not at all dependent onchance or accident, nor is it even variable to a remarkable degree. Quite on the contrary [333] it is always the same, or nearly the same, and it is to be considered as an inherent quality of the race. If leftto themselves, the single individuals always produce singles and doublesin the same quantity; if cultivated after some special method, theproportion may be slightly changed, bringing the proportion of doublesup to 60% or even more. 

Ordinarily the single and double members of such a race are quite equalin the remainder of their attributes, especially in the color of theirflowers. But this is not always the case. The colors of such a race mayrepeat for themselves the peculiarities of the ever-sporting characters. It often happens that one color is more or less strictly allied to thedoubles, and another to the singles. This sometimes makes it difficultto keep the various colors true. There are certain sorts, whichinvariably exhibit a difference in color between the single and thedouble flowers. The sulphur-yellow varieties may be adduced asillustrative examples, because in them the single flowers always comewhite. Hence in saving seed, it is impossible so to select the plant, that an occasional white does not also appear among the double flowers, agreeing in this deviation with the general rule of the eversportingvarieties. 

I commend all the above instances to those [334] who wish to makepedigree-cultures. The cooperation of many is needed to bring about anynotable advancement, since the best way to secure isolation is torestrict one's self to the culture of one strain, so as to avoid theintermixture of others. So many facts remain doubtful and open toinvestigation, that almost any lot of purchased seed may become thestarting point for interesting researches. Among these thesulphur-yellow varieties should be considered in the first place. 

In respect to the great questions of heredity, the stocks offer manypoints of interest. Some of these features I will now try to describe, in order to show what still remains to be done, and in what manner thestocks may clear the way for the study of the ever-sporting varieties. 

The first point, is the question, which seeds become double-flowered andwhich single-flowered plants? Beyond all doubt, the determination hastaken place before the ripening of the seed. But though the color of theseed is often indicative of the color of the flowers, as in some red orpurple varieties, and though in balsams and some other instances themost "highly doubled" flowers are to be obtained from the biggest andplumpest seeds, no such rule seems to exist respecting the doublestocks. Now if one half of the seeds gives doubles, and [335] the otherhalf singles, the question arises, where are the singles and the doublesto be found on the parent-plant?

The answer is partly given by the following experiment. Starting fromthe general rule of the great influence of nutrition on variability, itmay be assumed that those seeds will give most doubles, that are bestfed. Now it is manifest that the stem and larger branches are, in abetter condition than the smaller twigs, and that likewise the firstfruits have better chances than the ones formed later. Even in the samepod the uppermost seeds will be in a comparatively disadvantageousposition. This conception leads to an experiment which is the basis of apractical method much used in France in order to get a higher percentageof seeds of double-flowering plants. 

This method consists in cutting off, in the first place the upper partsof all the larger spikes, in the second place, the upper third part ofeach pod, and lastly all the small and weak twigs. In doing so thepercentage is claimed to go up to 67-70%, and in some instances evenhigher. This operation is to be performed as soon as the required numberof flowers have ceased blossoming. All the nutrient materials, destinedfor the seeds, are now forced to flow into these relatively few embryos, and it is clear that [336] they will be far better nourished than if nooperation were made. 

In order to control this experiment some breeders have made theoperation on the fruits when ripe, instead of on the young pods, andhave saved the seeds from the upper parts separately. This seed, produced in abundance, was found to be very poor in double flowers, containing only some 20-30%. On the contrary the percentage of doublesin the seed of the lower parts was somewhat augmented, and the averageof both would have given the normal proportion of 50%. 

Opposed to the French method is the German practice of cultivatingstocks, as I have seen it used on a very large scale at Erfurt and atother places. The stocks are grown in pots on small scaffolds, and notput on or into the earth. The obvious aim of this practice is to keepthe earth in the pots dry, and accordingly they are only scantilywatered. In consequence they cannot develop as fully as they would havedone when planted directly in the beds, and they produce only smallracemes and no weak twigs, eliminating thereby without further operationthe weaker seeds as by the French method. The effect is increased byplanting from 6-10 separate plants in each pot. 

It would be very interesting to make comparative [337] trials of bothmethods, in order to discover the true relation between the practice andthe results reached. Bath should also be compared with cultures on openplots, which are said to give only 50% of doubles. This last method ofculture is practiced wherever it is desired to produce great quantitiesof seeds at a low cost. Such trials would no doubt give an insight intothe relations of hereditary characters to the distribution of the foodwithin the plant. 

A second point is the proportional increase of the double-floweringseeds with age. If seed is kept for two or three years, the greater partof the grains will gradually die, and among the remainder there is foundon sowing, a higher percentage of double ones. Hence we may infer thatthe single-flowered seeds are shorter lived than the doubles, and thisobviously points to a greater weakness of the first. It is quite evidentthat there is some common cause for these facts and for the above citedexperience, that the first and best pods give more doubles. Much, however, remains to be investigated before a satisfactory answer can bemade to these questions. 

A third point is the curious practice, called by the French "esimpler, "and which consists in pulling out the singles when very young. It seemsto be done at an age when the flower-buds [338] are not yet visible, orat least are not far enough developed to show the real distinctivemarks. Children may be employed to choose and destroy the singles. Thereare some slight differences in the fullness and roundness of the budsand the pubescence of the young leaves. Moreover the buds of the doublesare said to be sweeter to the taste than those of the singles. But asyet I have not been able to ascertain, whether any scientificinvestigation of this process has ever been made, though according tosome communications made to me by the late Mr. Cornu, the practice seemsto be very general in the environs of Paris. In summer large fields maybe seen, bearing exclusively double flowers, owing to the weeding out ofthe singles long before flowering. 

Bud-variation is the last point to be taken up. It seems to be very rarewith stocks, but some instances have been recorded in literature. Darwinmentions a double stock with a branch bearing single flowers, and othercases are known to have occurred. But in no instance does the seed ofsuch a bud-variant seem to have been saved. Occasionally otherreversions also occur. From time to time specimens appear with moreluxurious growth and with divergent instead of erect pods. They arecalled, in Erfurt, "generals" on account [339] of their stiff and erectappearance, and they are marked by more divergent horns crowning thepods. They are said to produce only a relatively small number of doublesfrom their seeds, and even this small number might be due tofertilization with pollen of their neighbors. I saw some of thesereversionary types; when inspecting the nurseries of Erfurt, but as theyare, as a rule, thrown out before ripening their seed, nothing isexactly known about their real hereditary qualities. 

Much remains to be cleared up, but it seems that one of the best meansto find a way through the bewildering maze of the phenomena ofinheritance, is to make groups of related forms and to draw conclusionsfrom a comparison of the members of such groups. Such comparisons mustobviously give rise to questions, which in their turn will directly leadto experimental investigation. 

[340]

LECTURE XII

FIVE-LEAVED CLOVER

Every one knows the "four-leaved" clover. It is occasionally found onlawns, in pastures and by the roadsides. Specimens with five leafletsmay be found now and then in the same place, or on the same plant, butthese are rarer. I have often seen isolated plants with quaternateleaves, but only rarely have I observed individuals with more than onesuch leaf. 

The two cases are essentially dissimilar. They may appear to differ butlittle morphologically, but from the point of view of heredity they arequite different. Isolated quaternate leaves are of but little interest, while the occurrence of many on the same individual indicates a distinctvariety. In making experiments upon this point it is necessary totransplant the divergent individuals to a garden in order to furnishthem proper cultural conditions and to keep them under constantobservation. When a plant bearing a quaternate leaf is thus transplantedhowever, it rarely repeats the [341] anomaly. But when plants with twoor more quaternate leaves on the same individual are chosen it indicatesthat it belongs to a definite race, which under suitable conditions mayprove to become very rich in the anomalies in question. 

Obviously it is not always easy to decide definitely whether a givenindividual belongs to such a race or not. Many trials may be necessaryto secure the special race. I had the good fortune to find two plants ofclover, bearing one quinate and several quaternate leaves, on anexcursion in the neighborhood of Loosdrecht in Holland. Aftertransplanting them into my garden, I cultivated them during three yearsand observed a slowly increasing number of anomalous leaves. This numberin one summer amounted to 46 quaternate and 16 quinate leaves, and itwas evident that I had secured an instance of the rare "five-leaved"race which I am about to describe. 

Before doing so it seems desirable to look somewhat closer into themorphological features of the problem. Pinnate and palmate leaves oftenvary in the number of their parts. This variability is generally of thenature of a common fluctuation, the deviations grouping themselvesaround an average type in the ordinary way. Ash leaves bear five pairs, and [342] the mountain-ash (_Sorbus Aucuparia_) has six pairs ofleaflets in addition to the terminal one. But this number variesslightly, the weaker leaves having less, the stronger more pairs thanthe average. Such however, is not the case, with ternate leaves, whichseem to be quite constant. Four leaflets occur so very rarely that oneseems justified in regarding them rather as an anomaly than, as afluctuation. And this is confirmed by the almost universal absence oftwo-bladed clover-leaves. 

Considering the deviation as an anomaly, we may look into its nature. Such an inquiry shows that the supernumerary leaflets owe their originto a splitting of one or more of the normal ones. This splitting is notterminal, as is often the case with other species, and as it may be seensometimes in the clover. It is for the most part lateral. One of thelateral nerves grows out becoming a median nerve of the new leaflet. Intermediate steps are not wanting, though rare, and they show a gradualseparation of some lateral part of a leaflet, until this divisionreaches the base and divides the leaflet into two almost equal parts. Ifthis splitting occurs in one leaflet we get the "four-leaved" Clover, ifit occurs in two there will be five leaflets. And if, besides this, theterminal leaflet produces a derivative on one or both of its sides, [343] we obtain a crown of six or seven leaflets on one stalk. Such wereoften met with in the race I had under cultivation, but as a rule it didnot exceed this limit. 

The same phenomenon of a lateral doubling of leaflets may of course bemet with in other instances. The common laburnum has a variety whichoften produces quaternate and quinate leaves, and in strawberries I havealso seen instances of this abnormality. It occurs also in pinnateleaves, and complete sets of all the intermediate links may often befound on the false or bastard-acacia (_Robinia Pseud_Acacia_). 

Opposed to this increase of the number of leaflets, and still more rareand more curious is the occurrence of "single-leaved" varieties amongtrees and herbs with pinnate or ternate leaves. Only very few instanceshave been described, and are cultivated in gardens. The ashes and thebastard-acacia may be quoted among trees, and the "one-leaved"strawberry among herbs. Here it seems that several leaflets have beencombined into one, since this one is, as a rule, much larger than theterminal leaflet of an ordinary leaf of the same species. Thesemonophyllous varieties are interesting also on account of theircontinuous but often incomplete reversion to the normal type. 

[344] Pinnate and palmate leaves are no doubt derivative types. Theymust have originated from the ordinary simple leaf. The monophylly maytherefore be considered as a reversion to a more primitive state and themonophyllous varieties may be called atavistic. 

On the other hand we have seen that these atavistic varieties may revertto their nearest progenitors, and this leads to the curious conceptionof positive and negative atavism. For if the change of compound leavesinto single ones is a retrograde or negative step, the conversion ofsingle or ternate leaves into pinnate and palmate ones must evidently beconsidered in this case as positive atavism. 

This discussion seems to throw some light on the increase of leaflets inthe clover. The pea family, or the group of papilionaceous plants, haspinnate leaves ordinarily, which, according to our premises, must beconsidered as a derivative type. In the clovers and their allies thistype reverts halfway to the single form, producing only three leafletson each stalk. If now the clover increases its number of leaflets, thismay be considered as a reversion to its nearest progenitors, thepapilionaceous plants with pinnate leaves. Hence a halfway returning andtherefore positive atavism. And as I have already mentioned in a formerlecture, pinnate [345] leaves are also sometimes produced by my new raceof clover. 

Returning to the original plants of this race, it is evidentlyimpossible to decide whether they were really the beginning of a newstrain, and had originated themselves by some sudden change from thecommon type, or whether they belonged to an old variety, which hadpropagated itself perhaps during centuries, unobserved by man. But thesame difficulty generally arises when new varieties are discovered. Eventhe behavior of the plants themselves or of their progeny does notafford any means of deciding the question. The simplest way of statingthe matter therefore, is to say that I accidentally found twoindividuals of the "five-leaved" race. By transplanting them into mygarden, I have isolated them and kept them free from cross-fertilizationwith the ordinary type. Moreover, I have brought them under suchconditions as are necessary for the full development of theircharacters. And last but not least, I have tried to improve thischaracter as far as possible by a very rigid and careful selection. 

The result of all this effort has been a rapid improvement of my strain. I saved the seed of the original plants in 1889 and cultivated thesecond generation in the following year. It [346] showed some increaseof the anomaly, but not to a very remarkable degree. In the floweringperiod I selected four plants with the largest number of quaternate andquinate leaves and destroyed all the others. I counted in the average 25anomalous organs on each of them. From their seed I raised the thirdgeneration of my culture in the year 1891. 

This generation included some 300 plants, on which above 8, 000 leaveswere counted. More than 1, 000 were quaternate or quinate, the ternateleaves being still in the majority. But the experiment clearly showedthat "four-leaved" clovers may be produced in any desired quantity, provided that the seed of the variety is available. In the summer onlythree, four and five leaflets on one stalk were seen, but towards thefall, and after the selection of the best individuals, this numberincreased and came up to six and seven in some rare instances. 

The selection in this year was by no means easy. Nearly all theindividuals produced at least some quaternate leaves, and thereby showedthe variety to be quite pure. I counted the abnormal organs on a largegroup of the best plants, and selected 20 excellent specimens from them, with more than one-third of all their leaves changed in the desiredmanner. Having brought my race up to this point, I [347] was able tointroduce a new and far more easy mark, afforded by the seedlings, formy selections. This mark has since remained constant, and has broughtabout a rapid continuance of the improvement, without necessitating suchlarge cultures. 

This seedling in the various species of clover usually begins with afirst leaf above the cotyledons of a different structure from those thatfollow. It has only one blade instead of three. But in my variety theincrease of the number of the leaflets may extend to these primaryorgans, and make them binate or even ternate. Now it is obvious that anindividual, which begins with a divided primary leaf, will have agreater tendency to produce a large number of supernumerary leafletsthan a plant which commences in the ordinary way. Or in other words, theprimary leaves afford a sure criterion for the selection, and thisselection may be made in the seed-pans. In consequence, no youngindividual with an undivided primary leaf was planted out. Choosing the20 or 30 best specimens in the seed-pan, no further selection wasrequired, and the whole lot could be left to cross-fertilization byinsects. 

The observation of this distinguishing mark in the young seedlings hasled to the discovery of another quality as a starting-paint for further[348] selection. According to the general rule of pedigree-culture, theseeds of each individual plant are always saved and sowed separately. This is done even with such species as the clover, which are infertilewhen self-pollinated, and which are incapable of artificial pollinationon the required scale, since each flower produces only one seed. Myclover was always left free to be pollinated by insects. Obviously thismust have led to a diminution of the differentiating characters of theindividual plants. But this does not go far enough to obliterate thedifferences, and the selection made among the seedlings will alwaysthrow out at least a large part of those that have suffered from thecross. 

Leaving this discussion, we may inquire closer into the nature of thenew criterion afforded by the seedlings. Two methods present themselves. First, the choice of the best seedlings. In the second place it becomespossible to compare the parent-plants by counting the number ofdeviating seedlings. This leads to the establishment of a percentage forevery single parent, and gives data for comparisons. Two or threehundreds of seeds from a parent may easily be grown in one pan, and inthis way a sufficiently high degree of accuracy may be reached. Onlythose parents that give [349] the highest percentage are chosen, andamong their progeny only the seedlings with trifoliolate primary leavesare planted out. The whole procedure of the selection is by this meansconfined to the glasshouse during the spring, and the beds need not belarge, nor do they require any special care during the summer. 

By this method I brought my strain within two years up to an average ofnearly 90% of the seedlings with a divided primary leaf. Around thisaverage the real numbers fluctuated between the maximum of 99% and theminimum of 70% or thereabouts. This condition was reached by the sixthgeneration in the year 1894, and has since proved to be the limit, thegroup of figures remaining practically the same during all thesucceeding generations. 

Such selected plants are very rich in leaves with four, five and sixblades. Excluding the small leaves at the tops of the branches, andthose on the numerous weaker side-branches, these three groups includethe large majority of all the stronger leaves. In summer the range iswider, and besides many trifoliolate leaves the curiously shapedseven-bladed ones are not at all rare. In the fall and in the winter therange of variability is narrowed, and at first sight the plants oftenseem to bear only quinquefoliolate leaves. 

[350] I have cultivated a new generation of this race nearly every yearsince 1894, using always the strictest selection. This has led to auniform type, but has not been adequate to produce any furtherimprovement. Obviously the extreme limit, under the conditions ofclimate and soil, has been reached. This extreme type is alwaysdependent upon repeated selection. No constant variety, in the oldersense, has been obtained, nor was any indication afforded that such atype might ever be produced. On the contrary, it is manifest that thenew form belongs to the group of ever-sporting varieties. It is neverquite free from the old atavistic type of the trifoliolate leaves, andinvariably, when external conditions become less favorable, thisatavistic form is apt to gain dominion over the more refined varietalcharacter. Reversions always occur, both partial and individual. 

Some instances of these reversions may now be given. They are not ofsuch a striking character as those of the snapdragon. Intermediate stepsare always occurring, both in the leaves themselves, and in thepercentages of deviating seedlings of the several parent plants. 

On normal plants of my variety the quinquefoliolate leaves usuallycompose the majority, when there are no weak lateral branches, or whenthey are left out of consideration. Next [351] to these come the foursand the sixes, while the trifoliolate and seven-bladed types are nearlyequal in number. But out of a lot of plants, grown from seed of the sameparent, it is often possible to choose some in which one extremeprevails, and others with a preponderating number of leaves with theother extreme number of leaflets. If seed from these extremes are savedseparately, one strain, that with numerous seven-bladed leaves willremain true to the type, but the other will diverge more or less, producing leaves with a varying number of subdivisions. 

Very few generations of such opposite selection are required to reducethe race to an utterly poor one. In three years I was able to nearlyobliterate the type of my variety. I chose the seedlings with anundivided primary leaf, cultivated them and counted their offspringseparately after the sowing. I found some parents with only 2-3% ofseedlings with divided primary leaves. And by a repeated selection inthis retrograde direction I succeeded in getting a great number ofplants, which during the whole summer made only very few leaves withmore than three blades. But an absolute reversion could no more bereached in this direction than in the normal one. Any sowing withoutselection would be [352] liable to reduce the strain to an averagecondition. 

The production of varietal and of atavistic leaves is dependent to ahigh degree on external conditions. It agrees with the general rule, that favorable circumstances strengthen the varietal peculiarities, while unfavorable conditions increase the number of the parts with theatavistic attribute. These influences may be seen to have their effecton the single individuals, as well as on the generations growing fromtheir seed. I cannot cite here all the experimental material, but asingle illustrative example may be given. I divided a strong individualinto two parts, planted one in rich soil and the other in poor sand, andhad both pollinated by bees with the pollen of some normal individualsof my variety growing between them. The seeds of both were saved andsown separately, and the two lots of offspring cultivated close to eachother under the same external conditions. In the beginning no differencewas seen, but as soon as the young plants had unfolded three or fourleaves, the progeny of the better nourished half of the parent plantshowed a manifest advance. This difference increased rapidly and waseasily seen in the beds, even before the flowering period. 

This experience probably gives an explanation [353] why thequinquefoliolate variety is so seldom met with in the wild state. Foreven if it did occur more often, the plants would hardly findcircumstances favorable enough for the full development of theirvarietal character. They must often be so poor in anomalous leaves as tobe overlooked, or to be taken for instances of the commonly occurringquadrifoliolate leaves and therefore as not indicating the true variety. 

In the beginning of my discussion I have asserted the existence of twodifferent races of "four-leaved" clovers, a poor one and a rich one, andhave insisted on a sharp distinction between them. This distinctionpartly depends on experiments with clover, but in great part on testswith other plants. The previously mentioned circumstance, that clovercannot be pollinated on a sufficiently large scale otherwise than byinsects, prevents trials in more than one direction at the same time andin the same garden. For this reason I have chosen another species ofclover to be able to give proof or disproof of the assertion quoted. 

This species is the Italian, or crimson clover, which is sometimes alsocalled scarlet clover (_Trifolium incarnatum_). It is commonly used inEurope as a crop on less fertile soils than are required by the redclover. It is annual [354] and erect and more or less hairy, and hasstouter leaves than other kinds of clover. It has oblong or cylindricalheads with bright crimson flowers, and may be considered as one of themost showy types. As an annual it has some manifest advantages over theperennial species, especially in giving its harvest of hay at otherseasons of the year. 

I found some stray quaternate leaves of this plant some years ago, andtried to win from them, through culture and selection, a race that wouldbe as rich in these anomalies as the red clover. But the utmost care andthe most rigid selection, and all the attention I could afford, failedto produce any result. It is now ten years since I commenced thisexperiment, and more than once I have been willing to give it up. Lastyear (1903) I cultivated some hundreds of selected plants, but thoughthey yielded a few more instances of the desired anomaly than in thebeginning, no trace of a truly rich race could be discovered. Theexperimental evidence of this failure shows at least that stray"four-leaves" may occur, which do not indicate the existence of a true"four-" or "five-leaved" variety. 

This conception seems destined to become of great value in theappreciation of anomalies, as they are usually found, either in the wildstate [355] or in gardens. And before describing the details of myunsuccessful pedigree-culture, it may be as well to give some moreinstances of what occurs in nature. 

Stray anomalies are of course rare, but not so rare that they might notbe found in large numbers when perseveringly sought for. Pitcher-likeleaves may be found on many trees and shrubs and herbs, but ordinarilyone or only two of them are seen in the course of many years on the sameplant, or in the same strain. In some few instances they occur annuallyor nearly so, as in some individuals of the European lime-tree (_Tiliaparvifolia_) and of the common magnolia (_Magnolia obovata_). Many ofour older cultivated plants are very rich in anomalies of all kinds, and_Cyclamen_, _Fuchsia_, _Pelargonium_ and some others are notorioussources of teratologic phenomena. Deviations in flowers may often beseen, consisting of changes in the normal number of the several organs, or alterations in their shape and color. Leaves may have two tips, instead of one, the mid-vein being split near the apex, and the fissureextending more or less towards the base. Rays of the umbels ofumbelliferous plants may grow together and become united in groups oftwo or more, and in the same way the fruits of [356] the composites maybe united into groups. Many other instances could easily be given. 

If we select some of these anomalies for breeding-experiments, ourresults will not agree throughout, but will tend to group themselvesunder two heads. In some cases the isolation of the deviatingindividuals will at once show the existence of a distinct variety, whichis capable of producing the anomaly in any desired number of instances;only dependent on a favorable treatment and a judicious selection. Inother cases no treatment and no selection are adequate to give a similarresult, and the anomaly remains refractory despite all our endeavors tobreed it. The cockscomb and the peloric fox-glove are widely knowninstances of permanent anomalies, and others will be dealt with infuture lectures. On the other hand I have often tried in vain to win ananomalous race from an accidental deviation, or to isolate a teratologicvariety out of more common aberrations. Two illustrative examples may bequoted. In our next lecture we shall deal with a curious phenomenon inpoppies, consisting in the change of the stamens into pistils and givingrise to a bright crown of secondary capsules around the central one. Similar anomalies may be occasionally met with in other species of thesame genus. But they are rare, and may show [357] the conversion of onlya single stamen in the described manner. I observed this anomaly in apoppy called _Papaver commutatum_, and subjected it during several yearsto a rigid selection of the richest individuals. No amelioration was tobe gained and the culture had to be given up. In the same way I found onthe bulbous buttercup (_Ranunculus bulbosus_) a strain varying largelyin the number of the petals, amounting often to 6-8, and in some flowerseven yet to higher figures. During five succeeding years I cultivatedfive generations, often in large numbers, selecting always those whichhad the highest number of petals, throwing out the remainder and savingthe seed only from the very best plants. I got a strain of selectedplants with an average number of nine petals in every flower, and foundamong 4, 000 flowers four having 20 petals or more, coming up even to 31in one instance. But such rare instances had no influence whatever onthe selection, since they were not indicative of individual qualities, but occurred quite accidentally on flowers of plants having only theaverage number of petals. Now double flowers are widely known to occurin other species of the buttercups, both in the cultivated varieties andin some wild forms. For this reason it might be expected that through acontinuous selection of [358] the individuals with the largest numbers atendency to become double would be evolved. Such, however, was not thecase. No propensity to vary in any definite direction could be observed. Quite on the contrary, an average condition was quickly reached, andthen remained constant, strongly counteracting all selection. 

Such experiences clearly show that the same anomaly may occur indifferent species, and no doubt in strains of the same species fromdifferent localities, according to at least two different standards. Theone is to be called the poor, and the other the rich variety. The firstalways produces relatively few instances of the deviation, the last isapt to give as many of them as desired. The first is only half-way avariety, and therefore would deserve the name of a half-race; the secondis not yet a full constant variety, but always fluctuates to and frobetween the varietal and the specific mark, ever-sporting in bothdirections. It holds a middle position between a half-race and avariety, and therefore might be called a "middle-race. " But the termever-sporting variety seems more adequate to convey a right idea of thenature of this curious type of inheritance. 

From this discussion it will be seen that the behavior of the crimsonclover is not to be considered [359] as an exception, but as a widelyoccurring type of phenomenon, occurring perhaps in all sorts ofteratologic deviations, and in wide ranges of species and genera. Henceit may be considered worth while to give some more details of thisextended experiment. 

Ten years ago (1894-5) I bought and sowed about a pound of seed of thecrimson clover. Among many thousands of normal seedlings I found twowith three and one with four cotyledons. Trusting to the empirical rulesof correlation, I transplanted these three individuals in order toisolate them in the flowering period. 

One of them produced during the ensuing summer one four-bladed and onefive-bladed leaf. The seeds were saved separately and sown the followingspring and the expected result could soon be seen. Among some 250individual plants I counted 22 with one or two deviations, and 10 withfrom three to nine four- or five-bladed leaves. Proportions nearlysimilar have been observed repeatedly. Better nourished individuals haveproduced more deviating leaves on one plant, partly owing to the largernumber of stems and branches, and poor or average specimens have mostlybeen without any aberration or with only one or two abnormal leaves. Nofurther improvement could be attained. Quadrifoliolate leaves werealways rare, never [360] attaining a number that would put its stamp ona whole bed. I have endeavored to get some six- and seven-bladed crimsonclover leaves, but in vain; selection, culture of many hundreds ofindividuals, manure, and the best possible treatment has not beenadequate to produce them. Of course I am quite convinced that arepetition of my experiment on a far larger scale would yield thedesired types, but then only in such rare instances that they would haveno influence whatever on the average, or on the improvement of the race. The eighth generation in the year 1903 has not been noticeably betterthan the second and third generations after the first selection. 

In comparing this statement with the results gained in the experimentwith the red clover, the difference is at once striking. In one case arich variety was isolated, and, by better treatment and sharp methods ofselection, was brought up in a few years to its highest pitch ofdevelopment. In the other case a very weak race was shown to exist, andno amount of work and perseverance was adequate to improve it to anynoticeable degree. 

I wish to point out that the decision of what is to be expected fromdeviating specimens may become manifest within one or two generations. Even the generation grown from the seeds of [361] the first observedaberrant-individuals, if gathered after sufficient isolation during theperiod of blossoming, may show which type of inheritance is present, whether it is an unpromising half-race, or a richly endowed sportingvariety. I have kept such strains repeatedly after the first isolation, and a special case, that of cotyledoneous aberrations, will be dealtwith later. The first generation always gave a final decision, providedthat a suitable method of cultivation for the species under observationwas found at the beginning. This however, is a condition, which it isnot at all easy to comply with, when new sorts are introduced into agarden. Especially so when they had been collected in the wild state. Often one or two years, sometimes more, are necessary to find the propermethod of sowing, manuring, transplanting and, other cultural methodssatisfactory to the plants. Many wild species require more care and moremanure in gardens than the finest garden flowers. And a large number areknown to be dependent on very particular conditions of soil. 

One of the most curious features of anomalies, which has been learnedfrom accumulated instances, is the fact that they obey definite laws asto their occurrence on the different parts of the plant. Obviously suchlaws are [362] not apparent as long as each plant produces only one ortwo, or, at most, a few instances of the same deviation. On thecontrary, any existing regularity must betray itself, as soon as alarger number of instances is produced. A rule of periodicity becomesmost clearly manifest in such cases. 

This rule is shown by no other race in a more undoubted and evidentmanner than by the "five-leaved" clover. Evidently the several degreesof deviation, going from three to seven leaflets, may be regarded asresponses to different degrees of variation, and their distribution overthe stems and branches, or over the whole plant, may be considered asthe manifestation of the ever-changing internal tendency to vary. 

Considered from this point of view, my plants always showed a definiteperiodicity in this distribution, which is the same for the whole plant. Each of them, and each of the larger branches, begin with atavisticleaves or with slight deviations. These are succeeded by greaterdeviations, but only the strongest axes show as many as seven leafletson a stalk. This ordinarily does not occur before the height ofdevelopment is reached, and often only towards its close. Then thedeviation diminishes rapidly, returning often to atavistic leaves at thesummit of the stem or branch. I give the numbers of the [363] leaves ofa branch, in their order from the base to the top. They were as follows:

 3. 4. 5. 6. 7. 5. 5. 4. 

But this is a selected case, and such regular examples of the expectedperiodicity are rarely found. Often one or more of the various steps arelacking, or even leaves with smaller numbers may be interspersed amongthose with larger numbers of leaflets. But while the regularity of theperiodicity is in some degree diminished by such occurrences, yet therule always holds good, when taken broadly. It may be expressed bystating that the bases and apices have on the average fewer leaflets oneach leaf than the middle parts of the stem and branches, and that thenumber of leaflets gradually increases from the base toward a maximum, which is reached in organs on the middle or upper part of the axis, andthen diminishes from this toward the apex. 

This periodicity is not limited to the stems and branches, consideredsingly, but also holds good in a comparison made between the branches ofa single stem, in regard to their relative places on that stem. So it isalso for the whole plant. The first stems, produced by the subterraneanaxis, ordinarily show only a low maximum deviation: the next succeedingbeing [364] more divergent and the last ones returning to lessdifferentiated forms. 

It is evident that on a given stem the group of deviating leaves will beextended upward and downward, with the increase of the number of theseorgans. This shows that a stem, or even a plant, promises a higherdegree of differentiation if it commences with its aberration earlier. Hence it becomes possible to discern the most promising individuals inearly youth, and this conclusion leads to a very easy and reliablemethod of selection, which may be expressed simply as follows: theseedlings which commence earliest with the production of four- andfive-foliolate leaves are the best and should be selected for thecontinuance of the race. And it is easily seen that this rule agreeswith that given above, and which was followed in my pedigree-culture. 

Furthermore it is seen that there is a complete agreement between thelaw of periodicity and the responses of the deviations to nourishmentand other conditions of life. Weak plants only produce low degrees ofdeviation, the stronger the individual becomes, the higher it reaches inthe scale of differentiation, and the more often it develops leaves withfive or more blades. Whether weakness or strength are derived from outercauses, or from the internal [365] succession of the periods of life, isevidently of no consequence, and in this way the law of periodicity maybe regarded as a special instance of the more general law of response toexternal conditions. 

The validity of this law of periodicity is of course not limited to our"five-leaved" clover. Quite on the contrary it is universal ineversporting varieties. Moreover it may be ascertained and studied inconnection with the most widely different morphologic abnormalities, andtherefore affords easily accessible material for statistical inquiry. Iwill now give some further instances, but wish to insist first upon thenecessity of an inquiry on a far larger scale, as the evidence as yet isvery scanty. 

The great celandine (_Chelidonium majus_) has a very curious doublevariety. Its flowers are simpler and much more variable than in ordinarygarden-varieties. The process of doubling consists mainly in a change ofstamens into petals. This change is dependent on the season. On eachstem the earliest flowers are single. These are succeeded by blossomswith one or two converted stamens, and towards the summer this numberincreases gradually, attaining 10-11 and in some instances even morealtered filaments. Each year the same succession may be seen repeatingitself on the stems of [366] the old roots. Double tuberous begonias areordinarily absolutely sterile throughout the summer, but towards autumnthe new flowers become less and less altered, producing some normalstamens and pistils among the majority of metamorphosed organs. Fromthese flowers the seeds are saved. Sometimes similar flowers occur atthe beginning of the flowering-period. Double garden-camomiles(_Chrysanthemum inodorum plenissimum_) and many other double varietiesof garden-plants among the great family of the composites are verysensitive to external agencies, and their flower-heads are fuller themore favorable the external conditions. Towards the autumn many of themproduce fewer and fewer converted heads and often only these are fertileand yield seeds. 

Ascidia afford another instance of this periodicity, though ordinarilythey are by far too rare to show any regularity in their distribution. However, it is easy to observe that on lime-trees they prefer the lowerparts of each twig, while on magnolias the terminal leaves of thebranches are often pitcher-bearing. Ascidia of the white clover havebeen found in numbers, in my own experiment-garden, but always in thespringtime. The thickleaved saxifrage (_Saxifraga crassifolia_) is oftenvery productive of ascidia, especially in [367] the latter part of theseason, and as these organs may be developed to very different degrees, they afford fine material for the study of the law of periodicity. On agarden-cytisus (_Cytisus candicans attleyanus_) I once had the goodfortune to observe a branch with ascidia, which ordinarily are very rarein this species. It had produced seven ascidia in all, each formed bythe conversion of one leaflet on the trifoliolate leaves. The first sixleaves were destitute of this malformation and were quite normal. Thenfollowed a group of five leaves, constituting the maximum of the period. The first bore one small pitcher-like blade, the second and third, eachone highly modified organ, the fourth, two ascidia, and the last, oneleaflet with slightly connate margins. The whole upper part of thebranch was normal, with the exception of the seventeenth leaf, whichshowed a slight change in the same direction. All in all, the tendencyto produce ascidia increased from the beginning to the tenth leaf, anddecreased from this upward. 

The European Venus' looking-glass was observed in my garden to producesome quaternate and some quinate flowers on the same specimens. Thequinate were placed at the end of the branches, those with four petalsand sepals lower down. The peloric fox-glove shows the [368] highestdegree of metamorphy in the terminal flowers of the stem itself, theweaker branches having but little tendency towards the formation of theanomaly. The European pine or _Pinus sylvestris_ ordinarily has twoneedles in each sheath, but trifoliolate sheaths occur on the stems andstronger branches, where they prefer, as a rule, the upper parts of thesingle annual shoots. _Camellia japonica_ is often striped in the falland during the winter, but when flowering in the spring it returns tothe monochromatic type. 

Peloric flowers are terminal in some cases, but occur in the lower partsof the flower-spikes in others. Some varieties of gladiolus commence oneach spike with more or less double flowers, which, higher up, arereplaced by single ones. A wide range of bulbs and perennialgarden-plants develop their varietal characters only partly when grownfrom seed and flowering for the first time. The annualgarden-forget-me-not of the Azores (_Myosotis azorica_) has a varietywith curiously enlarged flowers, often producing 20 or morecorolla-segments in one flower. But this number gradually diminishes asthe season advances. It would be quite superfluous to give further proofof the general validity of the law of periodicity in ever-sportingvarieties. 

[369]

LECTURE XIII

PISTILLODY IN POPPIES

One of the most curious anomalies that may be met with in ornamentalgarden-plants is the conversion of stamens into pistils. It is neithercommon nor rare, but in most cases the change is so slight comparativelythat it is ordinarily overlooked. In the opium-poppy, on the contrary, it is very showy, and heightens the ornamental effect of the youngfruits after the fading of the flowers. Here the central capsule issurrounded by a large crown of metamorphosed stamens. 

This peculiarity has attracted the attention both of horticulturists andof botanists. As a rule not all the stamens are changed in this way butonly those of the innermost rows. The outer stamens remain normal andfertile, and the flowers, when pollinated with their own pollen, bear asrich a harvest of seeds as other opium-poppies. The change affects boththe filament and the anther, the former of which is dilated into asheath. Within this sheath perfect [370] and more or less numerousovules may be produced. The anthers become rudimentary and in theirplace broad leafy flaps are developed, which protrude laterally from thetip and constitute the stigmas. Ordinarily these altered organs aresterile, but in some instances a very small quantity of seed isproduced, and when testing their viability I succeeded in raising a fewplants from them. 

The same anomaly occurs in other plants. The common wall-flower(_Cheiranthus Cheiri_) and the houseleek (_Sempervivum tectorum_) arethe best known instances. Both have repeatedly been described by variousinvestigators. In compiling the literature of this subject it is veryinteresting to observe the two contrasting views respecting the natureof this anomaly. Some writers, and among them Masters in his "VegetableTeratology" consider the deviations to be merely accidental. Accordingto them some species are more subject to this anomaly than others, andthe houseleek is said to be very prone to this change. Goeppert, Hofmeister and others occasionally found the pistilloid poppies infields or gardens, and sowed their seeds in order to ascertain whetherthe accidental peculiarity was inheritable or not. On the other hand DeCandolle in his "Prodromus" mentions the pistilloid wall-flowers as adistinct [371] variety, under the name of _Cheiranthus Cheirigynantherus_, and the analogous form of the opium-poppy is not at all anaccidental anomaly, but an old true horticultural variety, which can bebought everywhere under the names of _Papaver somniferum monstruosum_ or_polycephalum_. Since it is an annual plant, only the seeds are forsale, and this at once gives a sufficient proof of its heredity. In allcases, where it was met with accidentally by botanists, it is to beassumed that stray seeds had been casually mixed with those of othervarieties, or that the habit had been transmitted by a spontaneouscross. 

Wherever opportunity led to experiments on heredity, distinct races werefound to be in possession of this quality, while others were not. It isof no use to cultivate large numbers of wall-flowers in the hope of oneday seeing the anomaly arise; the only means is to secure the strainfrom those who have got it. With poppies the various varieties are sooften intercrossed by bees, that the appearance of an accidental changemay sometimes be produced, and in the houseleek the pistilloid warilyseems to be the ordinary one, the normal strain being very rare orperhaps wholly wanting. 

Our three illustrative examples are good and permanent races, producingtheir peculiar qualities [372] regularly and abundantly. In this respectthey are however very variable and dependent on external circumstances. Such a regularity is not met with in other instances. Oftenpedigree-experiments lead to poor races, betraying their tendency todeviate only from time to time and in rare cases. Such instancesconstitute what we have called in a former lecture, "half races, " andtheir occurrence indicates that the casual observation of an anomaly isnot in itself adequate to give an opinion as to the chance of repetitionin sowing experiments. A large number of species seem to belong to thiscase, and their names may be found in the above mentioned work byMasters and elsewhere. But no effort has yet been made to separatethoroughly the pistilloid half-races from the correspondingever-sporting varieties. Some plants are recorded as being more liableto this peculiarity than others. 

Stamens are sometimes replaced by open carpels with naked ovules arisingfrom their edges and even from their whole inner surfaces. This may beseen in distinct strains of the cultivated bulbous Begonia, and morerarely in primroses. Here the apex of the carpellary leaf is sometimesdrawn out into a long style, terminated by a flattened spatulate stigma. 

The pistillody of the stamens is frequently [373] combined with anotherdeviation in the poppies. This is the growing together of some of thealtered stamens so as to constitute smaller or larger connate groups. Often two are united, sometimes three, four or more. Flowers withnumerous altered stamens are seldom wholly free from this mostundesirable secondary anomaly. I call it undesirable with respect toexperiments on the variability of the character. For it may easily beseen that while it is feasible to count the stamens even when convertedinto pistils, it is not possible when groups of them are more or lessintimately united into single bodies. This combination makes allenumeration difficult and inaccurate and often wholly unreliable. Insuch cases the observation is limited to a computation of the degree ofthe change, rather than to a strict numerical inquiry. Happily theresponses to the experimental influences are so marked and distinct thateven this method of describing them has proved to be wholly sufficient. 

In extreme instances I have seen all the changed stamens of a flower ofthe opium-poppy united into a single body, so as to form a close sheathall around the central ovary. Lesser sheaths, surrounding one-half orone-third of the capsule are of course less rarely met with. Leavingthis description of the outer appearance [374] of our anomaly, we maynow consider it from the double point of view of inheritance andvariability. 

The fact of inheritance is shown by the experience of many authors, andby the circumstance already quoted, that the variety has been propagatedfrom seed for more than half a century, and may be obtained from variousseed merchants. In respect to the variability, the variety belongs tothe ever-sporting group, constituting a type which is more closelyrelated to the "five-leaved" clover than to the striped flowers or eventhe double stocks. 

It fluctuates around an average type with half filled crowns, going asfar as possible in both directions, but never transgressing eitherlimit. It is even doubtful whether the presumable limits are, underordinary circumstances, ever reached. Obviously one extreme would be theconversion of all the stamens, and the other the absolute deficiency ofany marked tendency to such a change. Both may occur, and will probablybe met with from time to time. But they must be extremely rare, since inmy own extensive experiments, which were strictly controlled, I neverwas able to find a single instance of either of them. Some of the outerstamens have always remained unchanged, yielding enough pollen for theartificial pollination of [375] the central ovary, and on the other handsome rudiments of hardened filaments were always left, even if they werereduced to small protuberances on the thalamus of the flower. Betweenthese extremes all grades occur. From single, partially or whollychanged stamens upwards to 150 and over, all steps may be seen. It is atrue fluctuating variability. There is an average of between 50 and 100, constituting a nearly filled crown around the central capsule. Aroundthis average the smaller deviations are most numerous and the largerones more rare. The inspection of any bed of the variety suffices toshow that, taken broadly, the ordinary laws of fluctuating variabilityare applicable. No counting of the single individuals is required todispel all doubts on this point. 

Moreover all intermediate steps respecting the conversion of the singlestamens may nearly always be seen. Rarely all are changed into normalsecondary ovaries with a stigma and with a cavity filled with ovules. Often the stigma is incomplete or even almost wanting, in otherinstances the ovules are lacking or the cavity itself is only partiallydeveloped. Not rarely some stamens are reduced and converted into thinhard stalks, without any appearance of an ovary at their tip. But thenthe demarcation [376] between them and the thalamus fails, so that theycannot be thrown off when the flower fades away, but remain as smallstumps around the base of the more fully converted filaments. This factwould frequently render the enumeration of the altered organs quiteunreliable. 

For these reasons I have chosen a group of arbitrary stages in order toexpress the degree of deviation for a given lot of plants. The limitswere chosen so as to be sufficiently trustworthy and easy to ascertain. In each group the members could be counted, and a series of figures wasreached by this means which allowed of a further comparison of thecompeting sets of plants. 

It should be stated that in such experiments and especially in the caseof such a showy criterion as the pistilloid heads afford after the timeof flowering is over, the inspection of the controlling beds at onceindicates the result of the experiment. Even a hasty survey is in mostcases sufficient to get a definite conclusion. Where this is not thecase, the counting of the individuals of the various groups often doesnot add to the evidence, and the result remains uncertain. On the otherhand, the impression made by the groups of plants on the experimenterand on his casual visitors, cannot well be conveyed to the readers ofhis account by [377] other means than by figures. For this reason theresult of the experiments is expressed in this way. 

I made six groups. The first includes the cases where the whole circleis reduced to small rudiments. The second shows 1-10 secondary capsules. The two following constitute half a crown around the central fruit, thethird going up to this limit, the fourth going from this limit to anearly filled circle. Wholly filled circles of secondary capsuleswithout gaps give the two last degrees, the fifth requiring onlycontinuity of the circle, the sixth displaying a large and bright crownall around the central head. The fifth group ordinarily includes from90-100 altered stamens, while the sixth has from 100-150 of thesedeviating parts. 

In ordinary cultures the third and fourth group, with their interruptedcrowns, predominate. Large crowns are rare and flowers which at firstsight seem to be wholly normal, occur only under circumstancesdefinitely known to be unfavorable to growth, and to the development ofthe anomaly. 

Having reached by this means a very simple and easy method of statingthe facts shown by equal lots under contrasting influences, we will nowmake use of it to inquire into the relation [378] of this exceptionallyhigh degree of variability to the inner and outer conditions of life. 

As a rule, all experiments show the existence of such a relation. Unfavorable conditions reduce the numbers of altered stamens, favorablecircumstances raise it to its highest point. This holds true for lotsincluding hundreds of specimens, but also for the sundry heads of onebed, and often for one single plant. 

We may compare the terminal flower with those of the lateral branches ona plant, and when no special influences disturb the experiment, theterminal head ordinarily bears the richest crown. If the first has morethan 100 metamorphosed parts, the latter have often less than 50 on thesame plant. In poor soil, terminal heads are often reduced to 10-20monstrous organs, and in such cases I found the lateral flowers of thesame plants ordinarily with less than 10 altered stamens. In some casesI allowed the branches of the third and fourth degree, in other words, the side twigs of the first branches of my selected plants to grow outand produce flowers in the fall. They were ordinarily weak, sometimesvery small, having only 5-9 stigmas on their central fruit. Secondarycapsules were not seen on such flowers, even when the experiment wasrepeated on a [379] somewhat larger scale and during a series of years. 

Among the same lot of plants individual differences almost always occur. They are partly due to inequalities already existing in the seeds, andpartly to the diversity of the various parts of the same bed. Some ofthe plants become stout and have large terminal heads. Others remainvery weak, with a slender stem, small leaves and undersized flowers. Theheight and thickness of the stem, the growth of the foliage and of theaxillary buds are the most obvious measures of the individual strengthof the plant. The development of the terminal flower and the size of itsovary manifestly depends largely on this individual strength, as may beseen at once by the inspection of any bed of opium-poppies. Now thissize of the head can easily be measured, either by its height orcircumference, or by its weight. Moreover we can arrange them into aseries according to their size. If we do this with the polycephalousvariety, the relation between individual strength and degree ofmetamorphosis at once becomes manifest. The largest heads have thebrightest crowns, and the number of supernumerary carpels diminishes innearly exact proportion to the size of the fruits. Fruits with less than50 altered stamens weighed on an average 5 grams, [380] those with50-100 such organs 7 grams and those with a bright crown 10 grams, theappendices being removed before the weighing. Corresponding results havebeen reached by the comparison of the height of the capsules with theirabnormal surroundings. The degree of development of the monstrosity isshown by this observation to be directly dependent on, and in a senseproportionate to the individual strength of the plant. 

The differences between the specimens grown from a single lot of seeds, for instance from the seeds of one self-fertilized capsule are, as Ihave said, partly due to the divergences which are always present in abed, even if the utmost care has been taken to make it as uniform aspossible. These local differences are ordinarily underrated andoverlooked, and it is often considered to be sufficient to cultivatesmall lots of plants under apparently similar conditions on neighboringbeds, to be justified in imputing all the observed deviations of theplants to hereditary inequalities. This of course is true for largelots, whenever the averages only are compared. In smaller experimentsthe external conditions of the single individuals should always beconsidered carefully. Lots of one or two square meters suffice for suchcomparisons, but smaller lots are always subject to chances and [381]possibilities, which should never be left out of consideration. 

Therefore I will now point out some circumstances, which are ordinarilydifferent on various parts of one and the same bed. 

In the first place comes the inequality of the seeds themselves. Some ofthem will germinate earlier and others later. Those that display theircotyledons on a sunny day will be able to begin at once with theproduction of organic food. Others appear in bad weather, and will thusbe retarded in their development. These effects are of a cumulativenature as the young plants must profit by every hour of sunshine, according to the size of the cotyledons. Any inequality between twoyoung seedlings is apt to be increased by this cumulative effect. 

The same holds good for the soil of the bed. It is simply impossible tomix the manure so equally that all individuals receive the same amountof it from the very beginning. I am in the habit of using manures in adry and pulverized condition, of giving definite quantities to eachsquare meter, and of taking the utmost care to get equal distributionand mixture with the soil, always being present myself during this mostimportant operation. Nevertheless it is impossible to make thenourishment exactly equal for all the plants of even a small bed. 

[382] Any inequality from this cause will increase the difference in thesize of the young leaves, augment the inequality of their production oforganic matter and for this reason go on in an ever increasing rate. 

Rain and spraying, or on the other hand dryness of the soil, have stillgreater consequences. The slightest unevenness of the surface will causesome spots to dry rapidly and others to retain moisture during hours andeven sometimes during days. 

Seeds, germinating in such little moist depressions grow regularly andrapidly, while those on the dryer elevations may be retarded for hoursand days, before fully unfurling their seed-leaves. After heavy rainsthese differences may be observed to increase continually, and in someinstances I found that plants were produced only on the wet spots, whilethe dry places remained perfectly bare. From this the wet spots seem tobe the most favorable, but on the other hand, seeds may come togerminate there too numerously and so closely that the young plants willbe crowded together and find neither space nor light enough, for a freeand perfect development. The advantage may change to disadvantage inthis way unless the superfluous individuals are weeded out in due time. 

[383] From all these and other reasons some plants will be favored bythe external conditions from the beginning, while others will beretarded, and the effects will gradually increase until at last theybecome sufficient to account for a considerable amount of individualvariability. There is no doubt that the difference in the strength ofthe plant and in the size of the capsules, going from 5-10 grams for asingle fruit, are for the most part due to these unavoidablecircumstances. I have tried all conceivable means to find remedies forthese difficulties, but only by sowing my seeds in pans in a glass-househave I been able to reach more constant and equal conditions. Butunfortunately such a method requires the planting out of the youngseedlings in the beginning of the summer, and this operation is notwithout danger for opium-poppies, and especially not without importantinfluence on the monstrosity of the pistilloid variety. Consequently mysowings of this plant have nearly always been made in the beds. 

In order to show how great the influence of all these little things maybecome, we only have to make two sowings on neighboring beds and underconditions which have carefully been made as equal as possible. If weuse for these controlling experiments seeds from one and the samecapsule, it will soon become evident that [384] no exact similaritybetween the two lots may be expected. Such differences as may be seen inthese cases are therefore never to be considered of value when comparingtwo lots of seeds of different origin, or under varying conditions. Noamount of accuracy in the estimation of the results of a trial, or inthe counting out of the several degrees of the anomaly, is adequate toovercome the inaccuracy resulting from these differences. 

It is certainly of great importance to have a correct conception inregard to the influence of the surrounding conditions on the growth of aplant and on the development of the attribute we are to deal with. Noless important is the question of the sensibility of the plants to thesefactors. Obviously this sensibility must not be expected to remain thesame during the entire life-period, and periods of stronger and ofweaker responses may be discerned. 

In the first place it is evident that external or inner influences areable to change the direction of the development of an organ only so longas this development is not yet fully finished. In the young flower-budof the pistilloid poppy there must evidently be some moment in which itis definitely decided whether the young stamens will grow out normallyor become metamorphosed into secondary pistils. From this [385] momentno further change of external conditions is able to produce acorresponding change in the degree of the anomaly. The individualstrength of the whole plant may still be affected in a more or lessmanifest degree, but the number of converted stamens of the flower hasbeen definitely fixed. The sensitive period has terminated. 

In order to determine the exact moment of this termination of the periodof sensibility, I have followed the development of the flower budsduring the first weeks of the life of the young plants. The terminalflower may already be seen in young plants only seven weeks old, with astem not exceeding 5-6 cm. In height and a flower-bud with a diameter ofnearly 1 mm. , in which the stamens and secondary pistils are alreadydiscernible, but still in the condition of small rounded protuberanceson the thalamus. Though it is not possible at that time to observe anydifference between the future normal and converted stamens, it does notseem doubtful that the development is so far advanced, that in the innertissues the decision has already definitely been taken. In the next fewdays this decision rapidly becomes visible, and the different parts ofthe normal stamens and the metamorphosed carpels soon become apparent. From this observation it [386] can be inferred that the sensitive periodof the anomaly is limited for the terminal flower-head, to the first fewweeks of the life of the young plants. The secondary heads manifestlyleave this period at a somewhat later stage. 

In order to prove the accuracy of this conclusion I have tried to injurethe anomalies after the expiration of the first six or seven weeks. Ideprived them of their leaves, and damaged them in different ways. Isucceeded in making them very weak and slender, without being able todiminish the number of the supernumerary carpels. The proportionality ofthe size of the central fruit and the development of the surroundingcrown can often be modified or even destroyed by this means, and theapparent exceptions from this rule, which are often observed, may findtheir explanation in this way. 

In the second place I have tried to change the development of theanomaly during the period of sensibility, and even in the last part ofit. This experiment succeeded fully when carried out within the fifth orsixth week after the beginning of the germination. As means of injury Itransplanted the young plants. To this end I sowed my seeds in pans inunmanured soil, planted them out in little pots with richly preparedearth, grew them in these during a few weeks and afterwards transferredthem to the [387] beds, taking care that the pats were removed, but theballs of earth not broken. 

In consequence of this treatment the plants became very large andstrong, with luxuriant foliage and relatively numerous large flowers andfruits. But almost without exception they were poor in anomalousstamens, at least so on the terminal heads. On a lot of some 70 plantsmore than 50 had less than half a crown of secondary capsules, whilefrom the same packet of seed the control-plants gave in an equal numbermore than half of filled crowns on all plants with the exception of fiveweak specimens. 

It is curious to compare such artificially injured plants with theordinary cultures. Strong stems and heavy fruits, which otherwise arealways indicative of showy crowns, now bear fruits wholly or nearlydestitute of any anomalous change. The commonly prevailing rule seems tobe reversed, showing thereby the possibility of abolishing thecorrelation between individual strength and anomaly by an artificialencroachment upon the normal conditions. 

Aside from these considerations the experiments clearly give proof ofthe existence of a period of sensibility limited to the first weeks ofthe life of the plant for the terminal flower. This knowledge enables usto explain many apparent [388] parent abnormalities, which may occur inthe experiments. 

We now may take a broader view of the period of sensibility. Evidentlythe response to external influences will be greater the younger theorgan. Sensibility will gradually diminish, and the phenomena observedin the last part of this period may be considered as the last remainderof a reaction which previously must have been much stronger and muchreadier, providing that it would be possible to isolate them from, andcontrast them with, the other responses of the same plant. 

With the light thus cast upon the question, we may conclude that thesensitive period commences not only at the beginning of the germination, but must also be considered to include the life of the seed itself. Fromthe moment of fertilization and the formation of the young embryo thedevelopment must be subjected to the influence of external agencieswhich determine the direction it will take and the degree of developmentit will finally be able to acquire. Probably the time of growth of theembryo and of the ripening of the seed correspond exactly to the periodof highest sensibility. This period is only interrupted during theresting stage of the seed, to be repeated in germination. Afterwards thesensibility [389] slowly and gradually decreases, to end with thedefinite decision of all further growth sometime before the outer formof the organ becomes visible under the microscope. The last period oflife includes only an expansion of the tissues, which may still havesome influence on their final size, but not on their form. This has beendefinitely arrested before the end of the sensitive period, andordinarily before the commencement of that rapid development, which isusually designated by the name of growth, as contrasted with evolution. 

Within the seed the evolution of the young plant manifestly depends uponthe qualities and life-conditions of the parent-plant. The stronger thisis, and the more favorable circumstances it is placed under, the morefood will be available for the seed, and the healthier will be thedevelopment of the embryo. Only well-nourished plants givewell-nourished seeds, and the qualities of each plant are for thisreason at least, partly dependent on the properties of its parents andeven of its grandparents. 

From these considerations the inference is forced upon us that theapparently hereditary differences, which are observed to exist among theseeds of a species or a variety and even of a single strain or a singleparent-plant, may for a large part, and perhaps wholly, be the result[390] of the life-conditions of their parents and grandparents. Withinthe race all ssvariability would in this way be reduced to the effectsof external circumstances. Among these nourishment is no doubt the mostmomentous, and this to such a degree that older writers designated theexternal conditions by the term nourishment. According to Knightnutrition reigns supreme in the whole realm of variability, the kind offood and the method of nourishment coming into consideration only in asecondary way. The amount of useful nutrition is the all-importantfactor. 

If this is so, and if nutrition decides the degree of deviation of anygiven character, the widest deviating individuals are the best nourishedones. The best nourished not only during the period of sensibility ofthe attribute under consideration, but also in the broadest sense of theword. 

This discussion casts a curious light upon the whole question ofselection. Not of course upon the choice of elementary species orvarieties out of the original motley assembly which nature and oldcultures offer us, but upon the selection of the best individuals forisolation and for the improvement of the race. These are, according tomy views, only the best nourished ones. Their external conditions havebeen the [391] most favorable, not only from the beginning of their ownlife in the field, but also during their embryonic stages, and evenduring the preparation of these latter in the life of their parents andperhaps even their grandparents. Selection then, would only be thechoice of the best nourished individuals. 

In connection with the foregoing arguments I have tried to separate thechoicest of the poppies with the largest crown of pistilloid stamens, from the most vigorous individuals. As we have already seen, these twoattributes are as a rule proportional to one another. Exceptions occur, but they may be explained by some later changes in the externalcircumstances, as I have also pointed out. As a rule, these exceptionsare large fruits with comparatively too few converted stamens; they areexactly the contrary from what is required for a selection. Or plants, which from the beginning were robust, may have become crowded togetherby further growth, and for these reasons become weaker than theircongeners, though retaining the full development of the staminodalcrown, which was fixed during the sensitive period and before thecrowding. I have searched my beds yearly for several years in vain tofind individuals which might recommend themselves for selection withouthaving the stamp of permanent, [392] or at least temporarily better, nourishment. No starting-point for such an independent selection hasever been met with. 

Summing up the consequences of this somewhat extended discussion, we maystate it as a rule that a general proportion between the individualstrength and the degree of development of the anomaly exists. And fromthis point of view it is easy to see that all external causes which areknown to affect the one, must be expected to influence the other also. 

It will therefore hardly be necessary to give a full description of allmy experiments on the relations of the monstrosity to externalconditions. A hasty survey will suffice. 

This survey is not only intended to convey an idea of the relations ofpistilloid poppies to their environment, but may serve as an example ofthe principle involved. According to my experience with a large range ofother anomalies, the same rule prevails everywhere. And this rule is sosimple that exact knowledge of one instance may be considered assufficient to enable us to calculate from analogy what is to be expectedfrom a given treatment of any other anomaly. Our appreciation ofobserved facts and the conditions to be chosen for intended cultures arelargely dependent on such calculations. What I am now going to describe[393] is to be considered therefore as an experimental basis for suchexpectations. 

First of all comes the question how many individuals are to be grown ina given place. When sowing plants for experimental purposes it is alwaysbest to sow in rows, and to give as few seeds to each row as possible, so as to insure all necessary space to the young plants. On the otherhand the seeds do not all germinate, and after sowing too thinly, gapsmay appear in the rows. This would cause not only a loss of space, butan inequality between the plants in later life, as those nearest thegaps would have more space and more light, and a larger area for theirroots than those growing in the unbroken rows. Hence the necessity ofusing large quantities of seed and of weeding out a majority of youngplants on the spots where the greatest numbers germinate. 

Crowded cultures as a rule, will give weak plants with thin stems, mostly unbranched and bearing only small capsules. According to therule, these will produce imperfect crowns of secondary pistils. Theresult of any culture will thus be dependent to a high degree on thenumber of individuals per square meter. I have sown two similar andneighboring beds with the thoroughly mixed seeds of parent-plants of thesame strain and culture, using as much [394] as 2. 5 cu. Cm. Per squaremeter. On one of the beds I left all the germinating plants untouchedand nearly 500 of them flowered, but among them 360 were almost withoutpistillody, and only 10 had full crowns. In the other bed I weeded awaymore than half of the young plants, leaving only some 150 individualsand got 32 with a full crown, nearly 100 with half crowns and only 25apparently without monstrosity. 

These figures are very striking. From the same quantity of seed, inequal spaces, by similar exposure and treatment I got 10 fully developedinstances in one and 32 in the other case. The weeding out ofsupernumerary individuals had not only increased the percentage ofbright crowns, but also their absolute number per square meter. So thegreatest number of anomalies upon a given space may be obtained bytaking care that not too many plants are grown upon it: any increase ofthe number beyond a certain limit will diminish the probability ofobtaining these structures. The most successful cultures may be madeafter the maximum number of individuals per unit of area has beendetermined. A control-experiment was made under the same conditions andwith the same seed, but allowing much less for the same space. I sowedonly 1 cu. Cm. On my bed of 2 square meters, and thereby avoided [395]nearly all weeding out. I got 120 plants, and among them 30 with fullcrowns of converted stamens, practically the same number as after theweeding out in the first experiment. This shows that smaller quantitiesof seed give an equal chance for a greater number of large crowns, andshould therefore always be preferred, as it saves both seed and labor. 

Weeding out is a somewhat dangerous operation in a comparative trial. Any one who has done it often, knows that there is a strong propensityto root out the weaker plants and to spare the stronger ones. Obviouslythis is the best way for ordinary purposes, but for comparisonsevidently one should not discriminate. This rule is very difficult inpractice, and for this reason one should never sow more than isabsolutely required to meet all requirements. 

Our second point is the manuring of the soil. This is always of thehighest importance, both for normal and for anomalous attributes. Theconversion of the stamens into pistils is in a large measure dependentupon the conditions of the soil. I made a trial with some 800 floweringplants, using one sample of seed, but sowing one-third on richly manuredsoil, one-third on an unprepared bed of my garden, and one-third onnearly pure sand. In all other respects the three groups were treated inthe same way. Of [396] the manured plants one-half gave full crowns, ofthe non-manured only one-fifth, and on the sandy soil a still smallerproportion. Other trials led to the same results. I have often made useof steamed and ground horn, which is a manure very rich in nitrogenoussubstances. One-eighth of a kilo per square meter is an ample amount. And its effect was to increase the number of full crowns to anexceptional degree. 

In the controlling trial and under ordinary circumstances this figurereached some 50%, but with ground horn it came up as high as 90%. We maystate this result by the very striking assertion that the number oflarge crowns in a given culture may be nearly doubled by rich manure. 

All other external conditions act in a similar manner. The besttreatment is required to attain the best result. A sunny exposure is oneof the most essential requisites, and in some attempts to cultivate mypoppies in the shade, I found the pistillody strongly reduced, not asingle full crown being found in the whole lot. Often the weather may behurtful, especially during the earlier stages of the plants. I protectedmy beds during several trials by covering them with glass for a fewweeks, until the young plants reached the glass covering. I got a normalnumber of full crowns, some 55%, at a time [397] when the weather was sobad as to reduce the number in the control experiments to 10%. 

It would be quite superfluous to give more details or to describeadditional experiments. Suffice to say, that the results all point inthe same direction, and that pistillody of the poppies always clearlyresponds to the treatment, especially to external conditions during thefirst few weeks, that is, during the period of sensitiveness. Thehealthier and the stronger the plants the more fully they will developtheir anomaly. 

In conclusion something is to be said about the choice of the seed. Obviously it is possible to compare seeds of different origin by sowingand treating them in the same way, giving attention to all the pointsabove mentioned. In doing so the first question will be, whether thereis a difference between the seeds of strong plants with a bright crownaround the head and those of weaker individuals with lesser developmentof the anomaly. It is evident that such a difference must be expected, since the nutrition of the seed takes place during the period of thegreatest sensitiveness. 

But the experiments will show whether this effect holds good against theinfluences which tend to change the direction of the development of theanomaly during the time of germination. [398] The result of my attempthas shown that the choice of the seeds has a manifest influence upon theultimate development of the monstrosity, but that this influence is notstrong enough to overwhelm all other factors. 

The choice of the fullest or smallest crowns may be repeated duringsucceeding generations, and each time compared with a culture underaverage conditions. By this means we come to true selection-experiments, and these result in a notable and rapid change of the whole strain. Byselecting the brightest crowns I have come up in three years from 40 to90 and ultimately to 120 converted stamens in the best flower of myculture, and in selecting the smallest crowns I was able in three yearsto exclude nearly all good crowns, and to make cultures in which headswith less than half-filled crowns constituted the majority. But suchselected strains always remain very sensitive to treatment, and bychanging the conditions the effect may be wholly lost in a single year, or even turned in the contrary direction. In other words, the anomaly ismore dependent on external conditions during the germinating period thanon the choice of the seeds, providing these belong to the pistilloidvariety and have not deteriorated by some crossing with other sorts. 

At the beginning of this lecture I stated that [399] no selection isadequate to produce either a pure strain of brightly crownedflower-heads without atavism, or to conduce to an absolute and permanentloss of the anomaly. During a series of years I have tested my plants inboth directions, but without the least effect. Limits are soon reachedon both sides, and to transgress these seems quite impossible. 

Taking these limits as the marks of the variety, and considering allfluctuations between them as responses to external influences workingduring the life of the individual or governing the ripening of theseeds, we get a clear picture of a permanent ever-sporting type. Thelimits are absolutely permanent during the whole existence of thisalready old variety. They never change. But they include so wide a rangeof variability, that the extremes may be said to sport into one another, so much the more so as one of the extremes is to be consideredmorphologically as the type of the variation, while the other extremecan hardly be distinguished from the normal form of the species. 

[400]

LECTURE XIV

MONSTROSITIES

I have previously dealt with the question of the hereditary tendenciesthat cause monstrosities. These tendencies are not always identical forthe same anomaly. Two different types may generally, be distinguished. One of them constitutes a poor variety, the other a rich one. But thislatter is abundant and the first one is poor in instances of exactly thesame conformation. Therefore the difference only lies in the frequencyof the anomaly, and not in its visible features. In discovering aninstance of any anomaly it is therefore impossible to tell whether itbelongs to a poor or to a rich race. This important question can only beanswered by direct sowing-experiments to determine the degree ofheredity. 

Monstrosities are often considered as accidents, and rightfully so, atleast as long as they are considered from a morphological point of view. Physiology of course excludes all accidentality. And in our present easeit shows [401] that some internal hereditary quality is present, thoughoften latent, and that the observed anomalies are to be regarded asresponses of this innate tendency to external conditions. Our two typesdiffer in the frequency of these responses. Rare in the poor race, theyare numerous in the rich variety. The external conditions being the samefor both, the hereditary factor must be different. The tendency is weakin the one and strong in the other. In both cases, according to myexperience, it may be weakened or strengthened by selection and bytreatment. Often to a very remarkable degree, but not so far as totransgress the limits between the two races. Such transgression mayapparently be met with from time to time, but then the next generationgenerally shows the fallacy of the conclusion, as it returns more orless directly to the type from which the strain had been derived. Monstrosities should always be studied by physiologists from this pointof view. Poor and rich strains of the same anomaly seem at first sightto be so nearly allied that it might be thought to be very easy tochange the one into the other. Nevertheless such changes are not onrecord, and although I have made several attempts in this line, I neversucceeded in passing the limit. I am quite convinced that sometime [402]a method will be discovered of arbitrarily producing such conversions, and perhaps the easiest way to attain artificial mutations may lieconcealed here. But as yet not the slightest indication of thispossibility is to be found, save the fallacious conclusions drawn fromtoo superficial observations. 

Unfortunately the poor strains are not very interesting. Their chance ofproducing beautiful instances of the anomaly for which they arecultivated is too small. Exceptions to this rule are only afforded bythose curious and rare anomalies, which command general attention, andof which, therefore, instances are always welcome. In such cases theyare searched for with perseverance, and the fact of their rarityimpresses itself strongly on our mind. 

Twisted stems are selected as a first example. This monstrosity, called_biastrepsis_, consists of strongly marked torsions as are seen in manyspecies with decussate leaves, though as a rule it is very rare. Twoinstances are the most generally known, those of the wild valerian(_Valeriana officinalis_) and those of cultivated and wild sorts ofteasels (_Dipsacus fullonum_, _D. Sylvestris_, and others). Both ofthese I have cultivated during upwards of fifteen years, but withcontradictory results. The valerian is a perennial herb, multiplyingitself yearly by [403] slender rootstocks or runners producing at theirtips new rosettes of leaves and in the center of these the floweringstem. My original plant has since been propagated in this manner, andduring several years I preserved large beds with hundreds of stems, inothers I was compelled to keep my culture within more restricted limits. This plant has produced twisted stems of the curious shape, with anearly straight flag of leaves on one side, described by De Candolle andother observers, nearly every year. But only one or two instances ofabnormal stems occurred in each year, and no treatment has been foundthat proved adequate to increase this number in any appreciable manner. I have sown the seeds of this plant repeatedly, either from normal orfrom twisted stems, but without better results. It was highly desirableto be able to offer instances of this rare and interesting peculiarityto other universities and museums, but no improvement of the race couldbe reached and I have been constrained to give it up. My twistedvalerian is a poor race, and hardly anything can be done with it. Perhaps, in other countries the corresponding rich race may be hiddensomewhere, but I have never had the good fortune of finding it. 

This good fortune however, I did have with the wild teasel or _Dipsacussylvestris_. [404] Stems of this and of allied species are often metwith and have been described by several writers, but they were alwaysconsidered as accidents and nobody had ever tried to cultivate them. Inthe summer of 1885 I saw among a lot of normal wild teasels, two nicelytwisted stems in the botanical garden of Amsterdam. I at once proposedto ascertain whether they would yield a hereditary race and had all thenormal individuals thrown away before the flowering time. My two plantsflowered in this isolated condition and were richly pollinated byinsects. Of course, at that time, I knew nothing of the dependency ofmonstrosities on external conditions, and made the mistake of sowing theseeds and cultivating the next generation in too great numbers on asmall space. But nevertheless the anomaly was repeated, and the aberrantindividuals were once more isolated before flowering. The thirdgeneration repeated the second, but produced sixty twisted stems on some1, 600 individuals. The result was very striking and quite sufficient forall further researches, but the normal condition of the race was notreached. This was the case only after I had discovered the bad effectsof growing too many plants in a limited space. In the fourth generationI restricted my whole culture to about 100 individuals, and by thissimple [405] means at once got up to 34% of twisted stems. Thisproportion has since remained practically the same. I have selected andisolated my plants during five succeeding generations, but without anyfurther result, the percentage of twisted stems fluctuating between 30and about 45 according to the size of the cultures and the favorablenessor unfavorableness of the weather. 

It is very interesting to note that all depends on the question whetherone has the good fortune of finding a rich race or not, as thispedigree-culture shows. Afterwards everything depends on treatment andvery little on selection. As soon as the treatment becomes adequate, thefull strength of the race at once displays itself, but afterwards noselection is able to improve it to any appreciable amount. Of course, inthe long run, the responses will be the same as those of the pistilloidpoppies on the average, and some influence of selection will show itselfon closer scrutiny. 

Compared with the polycephalous poppies my race of twisted teasels ismuch richer in atavists. They are never absent, and always constitute alarge part of each generation and each bed, comprising somewhat morethan half of the individuals. Intermediate stages between them and thewholly twisted stems are not wanting, [406] and a whole series of stepsmay easily be observed from sufficiently large cultures. But they arealways relatively rare, and any lot of plants conveys the idea of adimorphous race, the small twisted stems contrasting strongly with thetall straight ones. 

A sharper contrast between good representatives of a race and theiratavists is perhaps to be seen in no other instance. All the detailscontribute to the differentiation in appearance. The whole stature ofthe plants is affected by the varietal mark. The atavists are not, as inthe case of the poppies, obviously allied with the type by a full rangeof intermediate steps, but quite distant from it by their rarity. Thereseems to be a gap in the same way as between the striped flowers of thesnapdragon and their uniform red atavists, while with the poppies theatavists may be viewed as being only the extremes of a series ofvariations fluctuating around some average type. 

From this reason it is as interesting to appreciate the hereditaryposition of the atavists of twisted varieties as it was for thered-flowered descendants of the striped flowers. In order to ascertainthis relation it is only necessary to isolate some of them during theblooming-period. I made this experiment in the summer of 1900 with theeighth generation of my race, and contrived [407] to isolate threegroups of plants by the use of parchment bags, covering themalternately, so the flowers of only one group were accessible toinsects, at a time. I made three groups, because the atavists show twodifferent types. Some specimens have decussate stems, others bear alltheir leaves in whorls of three, but in respect to the hereditarytendency of the twisting character this difference does not seem to beof any importance. 

In this way I got three lots of seeds and sowed enough of them to havethree groups of plants each containing about 150-200 well developedstems. Among these I counted the twisted individuals, and found nearlythe same numbers for all three. The twisted parents gave as many as 41%twisted children, but the decussate atavists gave even somewhat more, viz. , 44%, while the ternate specimens gave 37%. Obviously thedivergences between these figures are too slight to be dwelt upon, butthe fact that the atavists are as true or nearly as true inheritors ofthe twisted race as the best selected individuals is clearly proved bythis experience. 

It is evident that here we have a double race, including two types, which may be combined in different degrees. These combinations determinea wide range of changes in the stature of the plants, and it seemshardly right to use the [408] same term for such changes as for commonvariations. It is more a contention of opposite characters than a truephenomenon of simple variability. Or perhaps we might say that it is theeffect of the cooperation of a very variable mark, the twisting, with ascarcely varying attribute of the normal structure of the stem. Betweenthe two types an endless diversity prevails, but outwardly there arelimits which are never transgressed. The double race is as permanent, and in this sense as constant, as any ordinary simple variety, both inexternal form, and in its intimate hereditary qualities. 

I have succeeded in discovering some other rich races of twisted plants. One of them is the Sweet William (_Dianthus barbatus_), which yielded, after isolation, in the second generation, 25% of individuals withtwisted stems, and as each individual produces often 10 and more stems, I had a harvest of more than half a thousand of instances of thiscurious, and ordinarily very rare anomaly. My other race is a twistedvariety of _Viscaria oculata_, which is still in cultivation, as it hasthe very consistent quality of being an annual. It yielded last summer(1903) as high a percentage as 65 of twisted individuals, many of themrepeating the monstrosity on several branches. After some occasionalobservations _Gypsophila paniculata_ [409] seems to promise similarresults. On the other hand I have sowed in vain the seeds of twistedspecimens of the soapwort and the cleavewort (_Saponaria officinalis_and _Galium Aparine_). These and some others seem to belong to the samegroup as the valerian and to constitute only poor or so-calledhalf-races. 

Next to the torsions come the fasciated stems. This is one of the mostcommon of all malformations, and consists, in its ordinary form, of aflat ribbon-like expansion of the stems or branches. Below they arecylindrical, but they gradually lose this form and assume a flattenedcondition. Sometimes the rate of growth is unequal on different portionsor on the opposite sides of the ribbon, and curvatures are produced andthese often give to the fasciation a form that might be compared with ashepherd's crook. It is a common thing for fasciated branches and stemsto divide at the summit into a number of subdivisions, and ordinarilythis splitting occurs in the lower part, sometimes dividing the entirefasciated portion. In biennial species the rosette of the root-leaves ofthe first year may become changed by the monstrosity, the heartstretching in a transverse direction so as to become linear. In the nextyear this line becomes the base from which the stem grows. In such casesthe fasciated stems [410] are broadened and flattened from the verybeginning, and often retain the incipient breadth throughout theirfurther development. Species of primroses (_Primula japonica_ andothers), of buttercups (_Ranunculus bulbosus_), the rough hawksbeard(_Crepis biennis_), the Aster _Tripolium_, and many others could begiven as instances. 

Some of these are so rare as to be considered as poor races, and incultural trials do not produce the anomaly except in a very fewinstances. Heads of rye are found in a cleft condition from time totime, single at their base and double at the top, but this anomaly isonly exceptionally repeated from seed. Flattened stems of _Rubiatinctorum_ are not unfrequently met with on the fields, but they seem tohave as little hereditary tendency as the split rye (_Secale Cereale_). Many other instances could be given. Both in the native localities andin pedigree-cultures such ribboned stems are only seen from time totime, in successive years, in annual and biennial as well as inperennial species. The purple pedicularis (_Pedicularis palustris_) inthe wild state, and the sunflower among cultivated plants, may be citedinstead of giving a long list of analogous instances. 

On the other hand rich races of flattened stems are not entirelylacking. They easily betray [411] themselves by the frequency of theanomaly, and therefore may be found, and tried in the garden. Underadequate cultivation they are here as rich in aberrant individuals asthe twisted races quoted above, producing in good years from 30-40% andoften more instances. I have cultivated such rich races of the dandelion(_Taraxacum officinale_), of _Thrincia hirta_, of the dame's violet(_Hesperis matronalis_), of the hawkweed (_Picris hieracioides_), of therough hawksbeard (_Crepis biennis_), and others. 

Respecting the hereditary tendencies these rich varieties with flattenedstems may be put in the same category with the twisted races. Two pointshowever, seem to be of especial interest and to deserve a separatetreatment. 

The common cockscomb or _Celosia cristata_, one of the oldest and mostwidely cultivated fasciated varieties may be used to illustrate thefirst point. In beds it is often to be seen in quite uniform lots oflarge and beautiful crests, but this uniformity is only secured bycareful culture and selection of the best individuals. In experimentaltrials such selection must be avoided, and in doing so a wide range ofvariability at once shows itself. Tall, branched stems with fan-shapedtops arise, constituting a series of steps towards complete atavism. This last [412] however, is not to be reached easily. It often requiresseveral successive generations grown from seed collected from the mostatavistic specimens. And even such selected strains are always revertingto the crested type. There is no transgression, no springing over into apurely atavistic form, such as may be supposed to have once been theancestor of the present cockscomb. The variety includes crests andatavists, and may be perpetuated from both. Obviously every gardenerwould select the seeds of the brightest crests, but with care the fullcrests may be recovered, even from the worst reversionists in two orthree generations. It is a double race of quite the same constitution asthe twisted teasels. 

My second point is a direct proof of this assertion, but made with afasciated variety of a wild species. I took for my experiment the roughhawksbeard. In the summer of 1895 I isolated some atavists of the fifthgeneration of my race, which, by ordinary selection, gave in the averagefrom 20-40% of fasciated stems. My isolated atavists bore abundantfruit, and from these I had the next year a set of some 350 plants, outof which about 20% had broadened and linear rosettes. This proportioncorresponds with the degree of inheritance which is shown in many yearsby the largest and strongest [413] fasciated stems. It strengthens ourconclusion as to the innermost constitution of the double races orever-sporting varieties. 

Twisted stems and fasciations are very striking monstrosities. But theyare not very good for further investigation. They require too much spaceand too much care. The calculation of a single percentage requires thecounting of some hundreds of individuals, taking many square meters fortheir cultivation, and this, as my best races are biennial, during twoyears. For this reason the countings must always be very limited, andselection is restrained to the most perfect specimens. 

Now the question arises, whether this mark is the best upon which tofound selection. This seems to be quite doubtful. In the experiments onthe heredity of the atavists, we have seen that they are, at leastoften, in no manner inferior to even the best inheritors of the race. This suggests the idea that it is not at all certain that the visiblecharacters of a given individual are a trustworthy measure of its valueas to the transmission of the same character to the offspring. In otherwords, we are confronted with the existence of two widely differentgroups of characters in estimating the hereditary tendency. One is thevisible quality of the individuals and the other is the directobservation [414] of the degree in which the attribute is transmitted. These are by no means parallel, and seem in some sense to be nearlyindependent of each other. The fact that the worst atavists may have thehighest percentage of varietal units seems to leave no room for anotherexplanation. 

Developing this line of thought, we gradually arrive at the conclusionthat the visible attribute of a varying individual is perhaps the mostuntrustworthy and the most unreliable character for selection, even ifit seems in many cases practically to be the only available one. Thedirect determination of the degree of heredity itself is obviouslypreferable by far. This degree is expressed by the proportion of itsinheritors among the offspring, and this figure therefore should beelevated to the highest rank, as a measure of the hereditary qualities. Henceforward we will designate it by the name of hereditary percentage. 

In scientific experiments this figure must be determined for every plantof a pedigree-culture singly, and the selection should be foundedexclusively or at least mainly on it. It is easily seen that this methodrequires large numbers of individuals to be grown and counted. Some twoor three hundred progeny of one plant are needed to give the decisivefigure for this one [415] individual, and selection requires thecomparison of at least fifty or more individuals. This brings the totalamount of specimens to be counted up to some tens of thousands. Inpractice, where important interests depend upon the experiments, suchnumbers are usually employed and often exceeded, but for the culture ofmonstrosities, other methods are to be sought in order to avoid thesedifficulties. 

The idea suggests itself here that the younger the plants are, whenshowing their distinguishing marks, the more of them may be grown on asmall space. Hence the best way is to choose such attributes, as mayalready be seen in the young seedlings, in the very first few weeks oftheir lives. Fortunately the seed-leaves themselves afford suchdistinctive marks, and by this means the plants may be counted in thepans, requiring no culture at all in the garden. Only the selectedindividuals need be grown to ripen their seeds, and the whole selectionmay be made in the spring, in the glasshouse. Instead of being verytroublesome, the determination of the hereditary percentages becomes adefinite reduction of the size of the experiments. Moreover it mayeasily be effected by any one who cares for experimental studies, buthas not the means required for cultures on a larger scale. And lastly, there are [416] a number of questions about heredity, periodicity, dependency on nourishment and other life conditions, and even abouthybridizing, which may be answered by this new method. 

Seed-leaves show many deviations from the ordinary shape, especially indicotyledonous plants. A very common aberration is the multiplication oftheir number, and three seed-leaves in a whorl are not rarely met with. The whorl may even consist of four, and in rare cases of five or morecotyledons. Cleft cotyledons are also to be met with, and the fissuremay extend varying distances from the tips. Often all these deviationsmay be seen among the seedlings of one lot, and then it is obvious thattogether they constitute a scale of cleavages, the ternate andquaternate whorls being only cases where the cleaving has reached itsgreatest development. All in all it is manifest that here we are met byone type of monstrosity, but that this type allows of a wide range offluctuating variability. For brevity's sake all these cleft and ternate, double cleft and quaternate cotyledons and even the higher grades arecombined under one common name and indicated as tricotyls. 

A second aberration of young seed-plants is exactly opposite to this. Itconsists of the union of the two seed-leaves into a single organ. Thisordinarily betrays its origin by [417] having two separate apices, butnot always. Such seedlings are called syncotyledonous or syncotyls. Other monstrosities have been observed from time to time, but need notbe mentioned here. 

It is evident that the determination of the hereditary percentage isvery easy in tricotylous or syncotylous cultures. The parent plants mustbe carefully isolated while blooming. Many species pollinate themselvesin the absence of bees; from these the insects are to be excluded. Others have the stamens and stigmas widely separated and have to bepollinated artificially. Still others do not lend themselves to suchoperations, but have to be left free to the visits of bees and ofhumble-bees, this being the only means of securing seed from everyplant. At the time of the harvest the seeds should be gatheredseparately from each plant, and this precaution should also be observedin studies of the hereditary percentage at large, and in all scientificpedigree-cultures. Every lot of seeds is to be sown in a separate pan, and care must be taken to sow such quantities the three to four hundredseedlings will arise from each. As soon as they display theircotyledons, they are counted, and the number is the criterion of theparent-plant. Only parent-plants with the highest percentages areselected, and out of [418] their seedlings some fifty or a hundred ofthe best ones are chosen to furnish the seeds for the next generation. 

This description of the method shows that the selection is a double one. The first feature is the hereditary percentage. But then not all theseedlings of the selected parents can be planted out, and a choice hasto be made. This second selection may favor the finest tricotyls, or thestrongest individuals, or rely on some other character, but isunavoidable. 

We now come to the description of the cultures. Starting points are thestray tricotyls which are occasionally found in ordinary sowings. Inorder to increase the chance of finding them, thousands of seeds of thesame species must be inspected, and the range of species must be widenedas much as possible. 

Material for beginning such experiments is easily obtained, and almostany large sample of seeds will be found suitable. Some tricotyls will befound among every thousand seedlings in many species, while in othersten or a hundred times, as many plants must be examined to secure them, but species with absolutely pure dicotylous seeds are very rare. 

The second phase of the experiment, however, is not so promising. Somespecies are rich, and others are poor in this anomaly. This difference[419] often indicates what can be expected from further culture. Straytricotyls point to poor species or half-races, while more frequentdeviations suggest rich or double-races. In both cases however, thetrial must be made, and this requires the isolation of the aberrantindividuals and the determination of their hereditary percentage. 

In some instances the degree of their inheritance is only a very smallone. The isolated tricotyls yield 1 or 2% of inheritors, in some caseseven less, or upwards up to 3 or 4%. If the experiment is repeated, noamelioration is observed, and this result remains the same during aseries of successive generations. In the case of _Polygonumconvolvulus_, the Black bindweed, I have tried as many as sixgenerations without ever obtaining more than 3%. With other species Ihave limited myself to four successive years with the same negativeresult, as with spinage, the Moldavian dragon-head, (_Dracocephalummoldavicum_), and two species of corn catch-fly (_Silene conica_ and _S. Conoidea_). 

Such poor races hardly afford a desirable material for furtherinquiries. Happily the rich races, though rare, may be discovered alsofrom time to time. They seem to be more common among cultivated plantsand horticultural as well as agricultural species may be used. Hemp[420] and mercury (_Mercurialis annua_) among the first, snapdragon, poppies, _Phacelia_, _Helichrysum_, and _Clarkia_ among garden-flowersmay be given as instances of species containing the rich tricotylousdouble races. 

It is very interesting to note how strong the difference is between suchcases and those which only yield poor races. The rich type at oncebetrays itself. No repeated selection is required. The stray tricotylsthemselves, that are sought out from among the original samples, givehereditary percentages of a much higher type after isolation than thosequoted above. They come up to 10-20% and in some cases even to 40%. Asmay be expected, individual differences occur, and it must even besupposed that some of the original tricotyls may not be pure, buthybrids between tricotylous and dicotylous parents. These are at onceeliminated by selection, and if only the tricotyls which have thehighest percentages are chosen for the continuance of the new race, thesecond generation comes up with equal numbers of dicotyls and tricotylsamong the seedlings. The figures have been observed to range from 51-58%in the majority of the cases, and average 55%, rarely diverging somewhatmore from this average. 

Here we have the true type of an ever-sporting variety. Every year itproduces in the [421] same way heirs and atavists. Every plant, iffertilized with its own pollen, gives rise to both types. The parentitself may be tricotylous or dicotylous, or show any amount ofmultiplication and cleavage in its seed-leaves, but it always gives theentire range among its progeny of the variation. One may even select theatavists, pollinate them purely and repeat this in a succeedinggeneration without any chance of changing the result. On an average theatavists may give lower hereditary figures, but the difference will beonly slight. 

Such tricotylous double races offer highly interesting material forinquiries into questions of heredity, as they have such a wide range ofvariability. There is little danger in asserting that they go upwards tonearly 100%, and downwards to 0%, diverging symmetrically on both sidesof their average (50-55%). These limits they obviously cannottransgress, and are not even able to reach them. Samples of seedconsisting only of tricotyls are very rare, and when they are met withthe presumption is that they are too few to betray the rare aberrantsthey might otherwise contain. Experimental evidence can only be reachedby the culture of a succeeding generation, and this always discloses thehidden qualities, showing that the double [422] type was onlytemporarily lost, but bound to return as soon as new trials are made. 

This wide range of variability between definite limits is coupled with ahigh degree of sensibility and adequateness to the most divergingexperiments. Our tricotylous double races are perhaps more sensitive toselection than any other variety, and equally dependent on outercircumstances. Here, however, I will limit myself to a discussion of theformer point. 

In the second generation after the isolation of stray tricotylousseedlings the average condition of the race is usually reached, but onlyby some of the strongest individuals, and if we continue the race, sowing or planting only from their offspring, the next generation willshow the ordinary type of variability, going upwards in some anddownwards in other instances. With the _Phacelia_ and the mercury andsome others I had the good luck in this one generation to reach as highas nearly 90% of tricotylous seedlings, a figure indicating that thenormal dicotylous type had already become rare in the race. In othercases 80% or nearly 80% was easily attained. Any further divergence fromthe average would have required very much larger sowings, the effect ofselection between a limited number of parents being only to retain thehigh degree once [423] reached; so for instance with the mercury, I hadthree succeeding generations of selection after reaching the average of55%, but their extremes gave no increasing advance, remaining at 86, 92and 91%. 

If we compare these results with the effects of selection in twisted andfasciated races, we observe a marked contrast. Here they reached theirheight at 30-40%, and no number of generations had the power of makingany further improvement. The tricotyls come up in two generations to aproportion of about 54%, which shows itself to correspond to the averagetype. And as soon as this is reached, only one generation is required toobtain a very considerable improvement, going up to 80 or even 90%. 

It is evident that the cause of this difference does not lie in thenature of the monstrosity, but is due to the criterion upon which theselection is made. Selection of the apparently best individuals is onemethod, and it gives admirable results. Selection on the ground of thehereditary percentages is another method and gives results which are farmore advantageous than the former. 

In the lecture on the pistillody of the poppies we limited ourselves tothe selection of the finest individuals and showed that there is alwaysa manifest correlation between the individual [424] strength of theplant and the degree of development of its anomaly. The same holds goodwith other monstrosities, and badly nourished specimens of rich raceswith twisted or fasciated stems always tend to reversion. Thisreversion, however, is not necessarily correlated with the hereditarypercentage and therefore does not always indicate a lessening of thedegree of inheritance. This shows that even in those cases animprovement may be expected, if only the means can be found to subjectthe twisted and the fasciated races to the same sharp test as thetricotylous varieties. 

Much remains to be done, and the principle of the selection of parentsaccording to the average constitution of their progeny seems to be oneof the most promising in the whole realm of variability. 

Besides tricotylous, the syncotylous seedlings may be used in the sameway. They are more rarely met with, and in most instances seem to belongonly to the unpromising half-races. The black bindweed (_PolygonumConvolvulus_), the jointed charlock (Raphanus Raphanistrum), theglaucous evening-primrose (_Oenothera glauca_) and many other plantsseem to contain such half-races. On the other hand I found a plant of_Centranthus macrosiphon_ yielding as much as 55% of syncotylouschildren [425] and thereby evidently betraying the nature of a rich ordouble race. Likewise the mercury was rich in such deviations. But thebest of all was the Russian sunflower, and this was chosen for closerexperiments. 

In the year of 1888 I had the good luck to isolate some syncotylousseedlings and of finding among them one with 19% of inheritors among itsseeds. The following generation at once surpassed the ordinary averageand came up in three individuals to 76, 81 and even 89%. My race was atonce isolated and ameliorated by selection. I have tried to improve itfurther and selected the parents with the highest percentages duringseven more generations; but without any remarkable result. I got figuresof 90% and above, coming even in one instance up to the apparent purityof 100%. These, however, always remained extremes, the averagesfluctuating yearly between 80-90% or thereabouts, and the other extremesgoing nearly every year downwards to 50%, the value which would beattained, if no selection were made. 

Contra-selection is as easily made as normal selection. According to ourpresent principle it means the choice of the parents with the smallesthereditary percentage. One might easily imagine that by this means thedicotylous seedlings could be rendered pure. This, however, [426] is notat all the case. It is easy to return from so highly selected figures asfor instance 95% to the average about of 50%, as regression tomediocrity is always an easy matter. But to transgress this average onthe lower side seems to be as difficult as it is on the upper side. Icontinued the experiment during four succeeding generations, but was notable to go lower than about 10%, and could not even exclude the highfigures from my strain. Parents with 65-75% of syncotylous seedlingsreturned in each generation, notwithstanding the most carefulcontra-selection. The attribute is inherent in the race, and is not tobe eliminated by so simple a means as selection, nor even by a selectionon the ground of hereditary percentages. 

We have dealt with torsions and fasciations and with seedling variationsat some length, in order to point out the phases needing investigationaccording to recent views. It would be quite superfluous to considerother anomalies in a similar manner, as they all obey the same laws. Ahasty survey may suffice to show what prospects they offer to thestudent of nature. 

First of all come the variegated leaves. They are perhaps the mostvariable of all variations. They are evidently dependent on externalcircumstances, and by adequate nutrition the leaves may even becomeabsolutely white or [427] yellowish, with only scarcely perceptibletraces of green along the veins. Some are very old cultivated varieties, as the wintercress, or _Barbarea vulgaris_. They continuously sport intogreen, or return from this normal color, both by seeds and by buds. Sports of this kind are very often seen on shrubs or low trees, and theymay remain there and develop during a long series of years. Bud-sportsof variegated holly, elms, chestnuts, beeches and others might be cited. One-sided variegation on leaves or twigs with the opposite side whollygreen are by no means rare. It is very curious to note that variegationis perhaps the most universally known anomaly, while its hereditarytendencies are least known. 

Cristate and plumose ferns are another instance. Half races or rareaccidental cleavages seem to be as common with ferns as cultivateddouble races, which are very rich in beautiful crests. But much dependson cultivation. It seems that the spores of crested leaves are more aptto reproduce the variety than those of normal leaves, or even of normalparts of the same leaves. But the experiments on which this assertion ismade are old and should be repeated. Other cases of cleft leaves shouldalso be tested. Ascidia are far more common than is usually believed. Rare instances point [428] to poor races, but the magnolias andlime-trees are often so productive of ascidia as to suggest the idea ofever-sporting varieties. I have seen many hundred ascidia on onelime-tree, and far above a hundred on the magnolia. They differ widelyin size and shape, including in some cases two leaves instead of one, orare composed of only half a leaf or of even still a smaller part of thesummit. Rich ascidia-bearing varieties seem to offer notableopportunities for scientific pedigree-cultures. 

Union of the neighboring fruits and flowers on flower-heads, of the raysof the umbellifers or of the successive flowers of the racemes ofcabbages and allied genera, seem to be rare. The same holds good for theadhesion of foliar to axial organs, of branches to stems and other casesof union. Many of these cases return regularly in each generation, ormay at least be seen from time to time in the same strains. Proliferation of the inflorescence is very common and changes in theposition of staminate and pistillate flowers are not rare. We findstarting points for new investigations in almost any teratologicalstructure. Half-races and double-races are to be distinguished andisolated in all cases, and their hereditary qualities, the periodicityof the recurrence of the anomaly, the dependency on externalcircumstances [429] and many other questions have to be answered. 

Here is a wide field for garden experiments easily made, which mightultimately yield much valuable information on many questions of heredityof universal interest. 

[430]

LECTURE XV

DOUBLE ADAPTATIONS

The chief object of all experimentation is to obtain explanations ofnatural phenomena. Experiments are a repetition of things occurring innature with the conditions so guarded and so closely followed that it ispossible to make a clear analysis of facts and their causes, it beingrightfully assumed that the laws are the same in both cases. 

Experiments on heredity and the experience of the breeder find theiranalogy in the succession of generations in the wild state. Thestability of elementary species and of retrograde varieties is quite thesame under both conditions. Progression and retrogression are narrowlylinked everywhere, and the same laws govern the abundance of forms incultivated and in wild plants. 

Elementary species and retrograde varieties are easily recognizable. Ever-sporting varieties on the contrary are far less obvious, and inmany cases their hereditary relations have [431] had to be studied anew. A clear analogy between them and corresponding types of wild plants hasyet to be pointed out. There can be no doubt that such analogy exists;the conception that they should be limited to cultivated plants is notprobable. Striped flowers and variegated leaves, changes of stamens intocarpels or into petals may be extremely rare in the wild state, but the"five-leaved" clover and a large number of monstrosities cannot be saidto be typical of the cultivated condition. These, however, are of rareoccurrence, and do not play any important part in the economy of nature. 

In order to attain a better solution of the problem we must take abroader view of the facts. The wide range of variability ofever-sporting varieties is due to the presence of two antagonisticcharacters which cannot be evolved at the same time and in the sameorgan, because they exclude one another. Whenever one is active, theother must be latent. But latency is not absolute inactivity and mayoften only operate to encumber the evolution of the antagonisticcharacter, and to produce large numbers of lesser grades of itsdevelopment. The antagonism however, is not such in the exact meaning ofthe word; it is rather a mutual exclusion, because one of the opponentssimply takes the place of the other when absent, or supplements [432] itto the extent that it may be only imperfectly developed. This completionordinarily occurs in all possible degrees and thus causes the wide rangeof the variability. Nevertheless it may be wanting, and in the case ofthe double stocks only the two extremes are present. 

It is rather difficult to get a clear conception of the substitution, and it seems necessary to designate the peculiar relationship betweenthe two characters forming such a pair by a simple name. They might betermed alternating, if only it were clearly understood that thealternation may be complete, or incomplete in all degrees. Completealternation would result in the extremes, the incomplete condition inthe intermediate states. In some cases as with the stocks, the firstprevails, while in other cases, as with the poppies, the very extremesare only rarely met with. 

Taking such an alternation as a real character of the ever-sportingvarieties, a wide range of analogous cases is at once revealed among thenormal qualities of wild plants. Alternation is here almost universal. It is the capacity of young organs to develop in two divergingdirections. The definitive choice must be made in extreme youth, oroften at a relatively late period of development. Once made, this [433]choice is final, and a further change does not occur in the normalcourse of things. 

The most curious and most suggestive instance of such an alternation isthe case of the water-persicaria or _Polygonum amphibium_. It is knownto occur in two forms, one aquatic and the other terrestrial. These arerecorded in systematic works as varieties, and are described under thenames of _P. Amphibium_ var. _natans_ Moench, and _P. Amphibium_ var. _terrestre_ Leers or _P. Amphibium_ var. _terrestris_ Moench. Suchauthorities as Koch in his German flora, and Grenier and Godron in theirFrench flora agree in the conception of the two forms as varieties. 

Notwithstanding this, the two varieties may often be observed to sportinto one another. They are only branches of the same plant, grown underdifferent conditions. The aquatic form has floating or submerged stemswith oblong or elliptic leaves, which are glabrous and have longpetioles. The terrestrial plants are erect, nearly simple, more or lesshispid throughout, with lanceolate leaves and short petioles, oftennearly sessile. The aquatic form flowers regularly, producing itspeduncle at right angles from the floating stems, but the terrestrialspecimens are ordinarily seen without flower-spikes, which are butrarely met with, at least as far as my own experience goes. Intermediate[434] forms are very rare, perhaps wholly wanting, though in swamps theterrestrial plants may often vary widely in the direction of thefloating type. 

That both types sport into each other has long been recognized infield-observations, and has been the ground for the specific name of_amphibium_, though in this respect herbarium material seems usually tobe scant. The matter has recently been subjected to critical andexperimental studies by the Belgian botanist Massart, who has shown thatby transplanting the forms into the alternate conditions, the change mayalways be brought about artificially. If floating plants are establishedon the shore they make ascending hairy stems, and if the terrestrialshoots are submerged, their buds grow into long and slack, aquaticstems. Even in such experiments, intermediates are rare, both typesagreeing completely with the corresponding models in the wild state. 

Among all the previously described cases of horticultural plants andmonstrosities there is no clearer case of an ever-sporting variety thanthis one of the water-persicaria. The var. _terrestris_ sports into thevar. _natans_, and as often as the changing life conditions may requireit. It is-true that ordinary sports occur without our discerning thecause and without [435] any relation to adaptation. This however ispartly due to our lack of knowledge, and partly to the general rule thatin nature only such sports as are useful are spared by naturalselection, and what is useful we ordinarily term adaptive. 

Another side of the question remains to be considered. The word variety, as is now becoming generally recognized, has no special meaningwhatever. But here it is assumed in the clearly defined sense of asystematic variety, which includes all subdivisions of species. Suchsubdivisions may be, from a biological point of view, elementary speciesand also be eversporting varieties. They may be retrograde varieties, and the two alternating types may be described as separate varieties. 

It is readily granted that many writers would not willingly accept thisconclusion. But it is simply impossible to avoid it. The two forms ofthe water-persicaria must remain varieties, though they are only typesof the different branches of a single plant. 

If not, hundreds and perhaps thousands of analogous cases are at onceexposed to doubt, and the whole conception of systematic varieties wouldhave to be thrown over. Biologists of course would have no objection tothis, but the student of the flora of any given country [436] or regionrequires the systematic subdivisions and should always use his utmostefforts to keep them as they are. There is no intrinsic difficulty inthe statement that different parts of the same plant should constitutedifferent varieties. 

In some cases different branches of the same plant have been describedas species. So for instance with the climbing forms of figs. Under thename of _Ficus repens_ a fine little plant is quite commonly cultivatedas a climber in flower baskets. It is never seen bearing figs. On theother hand a shrub of our hothouses called _Ficus stipulata_, iscultivated in pots and makes a small tree which produces quite large, though non-edible figs. Now these two species are simply branches of thesame plant. If the _repens_ is allowed to climb up high along the wallsof the hothouses, it will at last produce stipulate branches with thecorresponding fruits. _Ficus radicans_ is another climbing form, corresponding to the shrub _Ficus ulmifolia_ of our glasshouses. Andquite the same thing occurs with ivy, the climbing stems of which neverflower, but always first produce erect and free branches with rhombicleaves. These branches have often been used as cuttings and yield littleerect and richly flowering shrubs, which are known in [437] horticultureunder the varietal name of _Hedera Helix arborea_. 

Manifestly this classification is as nearly right as that of the twovarieties of the water-persicaria. Going one step further, we meet withthe very interesting case of alpine plants. The vegetation of the higherregions of mountains is commonly called alpine, and the plants show alarge number of common features, differentiating them from the flora oflower stations. The mountain plants have small and dense foliage, withlarge and brightly-colored flowers. The corresponding forms of thelowlands have longer and weaker stems, bearing their leaves at greaterdistances, the leaves themselves being more numerous. The alpine forms, if perennial, have thick, strongly developed and densely branchedrootstocks with heavy roots, in which a large amount of food material isstored up during the short summer, and is available during the longwinter months of the year. 

Some species are peculiar to such high altitudes, while many forms fromthe lowlands have no corresponding type on the mountains. But a largenumber of species are common to both regions, and here the difference ofcourse is most striking. _Lotus corniculatus_ and _Calamintha Acinos_, _Calluna vulgaris_ and _Campanula_ [438] _rotundifolia_ may be quoted asinstances, and every botanist who has visited alpine regions may addother examples. Even the edelweiss of the Swiss Alps, _GnaphaliumLeontopodium_, loses its alpine characters, if cultivated in lowlandgardens. Between such lowland and alpine forms intermediates regularlyoccur. They may be met with whenever the range of the species extendsfrom the plains upward to the limit of eternal snow. 

In this case the systematists formerly enumerated the alpine plants as_forma alpestris_, but whenever the intermediate is lacking the term_Varietas alpestris_ was often made use of. 

It is simply impossible to decide concerning the real relation betweenthe alpine and lowland types without experiments. About the middle ofthe last century it was quite a common thing to collect plants not onlyfor herbarium-material, but also for the purpose of planting them ingardens and thus to observe their behavior under new conditions. Thiswas done with the acknowledged purpose of investigating the systematicsignificance of observed divergencies. Whenever these held good in thegarden they were considered to be reliable, but if they disappeared theywere regarded as the results of climatic conditions, or of the influenceof soil or nourishment. Between [439] these two alternatives, manywriters have tried to decide, by transplanting their specimens aftersome time in the garden, into arid or sandy soil, in order to seewhether they would resume their alpine character. 

Among the systematists who tested plants in this way, Nageli especially, directed his attention to the hawkweeds or _Hieracium_. On the SwissAlps they are very small and exhibit all the characters of the purealpine type. Thousands of single plants were cultivated by him in thebotanical garden of Munich, partly from seed and partly from introducedrootstocks. Here they at once assumed the tall stature of lowland forms. The identical individual, which formerly bore small rosettes of basalleaves, with short and unbranched flower-stalks, became richly leavedand often produced quite a profusion of flower-heads on branched stems. If then they were transplanted to arid sand, though remaining in thesame garden and also under the same climatic conditions they resumedtheir alpine characters. This proved nutrition to be the cause of thechange and not the climate. 

The latest and most exact researches on this subject are due to Bonnier, who has gone into all the details of the morphologic as well as of thephysiologic side of the problem. [440] His purpose was the study ofpartial variability under the influence of climate and soil. In everyexperiment he started from a single individual, divided it into twoparts and planted one half on a mountain and the other half on theplain. The garden cultures were made chiefly at Paris and Fontainebleau, the alpine cultures partly in the Alps, partly in the Pyrenees. Fromtime to time the halved plants were compared with each other, and thecultures lasted, as a rule, during the lifetime of the individual, oftencovering many years. 

The common European frostweed or _Helianthemum vulgare_ will serve toillustrate his results. A large plant growing in the Pyrenees in analtitude of 2, 400 meters was divided. One half was replanted on the samespot, and the other near Cadeac, at the base of the mountain range (740M. ). In order to exclude the effect of a change of soil, a quantity ofthe earth from the original locality was brought into the garden and theplant put therein. Further control experiments were made at Paris. Assoon as the two halved individuals commenced to grow and produced newshoots, the influence of the different climates made itself felt. On themountain, the underground portions remained strong and dense, the leavesand internodes small and hairy, the flowering stems nearly [441]procumbent, the flowers being large and of a deep yellow. At Cadeac andat Paris the whole plant changed at once, the shoots becoming elongatedand loose, with broad and flattened, rather smooth leaves and numerouspale-hued flowers. The anatomical structure exhibited correspondingdifferences, the intercellular spaces being small in the alpine plantand large in the one grown in the lowlands, the wood-tissues strong inthe first and weak in the second case. 

The milfoil (_Achillea Millefolium_) served as a second example, and theexperiments were carried on in the same localities. The long and thickrootstocks of the alpine plant bearing short stems only with a few densecorymbs contrasted markedly with the slender stems, loose foliage andrich groups of flowerheads of the lowland plant. The same differences, in inner and outer structures were observed in numerous instances, showing that the alpine type in these cases is dependent on the climate, and that the capacity for assuming the antagonistic characters ispresent in every individual of the species. The external conditionsdecide which of them becomes active and which remains inactive, and thecase seems to be exactly parallel to that of the water-persicaria. 

In the experiments of Bonnier the influence of the soil was, as a rule, excluded by transplanting [442] part of the original earth with thetransplanted half of the plant. From this he concluded that the observedchanges were due to the inequality of the climate. This involved threemain factors, light, moisture and temperature. On the mountains thelight is more intense, the air drier and cooler. Control-experimentswere made on the mountains, depriving the plants of part of the light. In various ways they were more or less shaded, and as a rule respondedto this treatment in the same way as to transplantation to the plainbelow. Bonnier concluded that, though more than one factor takes part ininciting the morphologic changes, light is to be considered as the chiefagency. The response is to be considered as a useful one, as the wholestructure of the alpine varieties is fitted to produce a large amount oforganic material in a short time, which enables the plants to thriveduring the short summers and long winters of their elevated stations. 

In connection with these studies on the influences of alpine climates, Bonnier has investigated the internal structure of arctic plants, andmade a series of experiments on growth in continuous electric light. Thearctic climate is cold, but wet, and the structure of the leaves iscorrespondingly loose, though the plants become [443] as small as on theAlps. Continuous electric light had very curious effects; the plantsbecame etiolated, as if growing in darkness, with the exception thatthey assumed a deep green tinge. They showed more analogy with thearctic than with the alpine type. 

The influence of the soil often produces changes similar to that ofclimate. This was shown by the above cited experiments of Nageli withthe hawkweeds, and may easily be controlled in other cases. Theground-honeysuckle or _Lotus corniculatus_ grows in Holland partly onthe dry and sandy soil of the dunes, and occasionally in meadows. It issmall and dense in the first case, with orange and often very darklycolored petals, while it is loose and green in the meadows, withyellower flowers. Numerous analogous cases might be given. On mountainslopes in South Africa, and especially in Natal, a species of compositeis found, which has been introduced into culture and is used as ahanging plant. It is called _Othonna crassifolia_ and has fleshy, nearlycylindrical leaves, and exactly mimics some of the crassulaceousspecies. On dry soil the leaves become shorter and thicker and assume areddish tinge, the stems remain short and woody and bear their leaves indense rosettes. On moist and rich garden-soil this aspect becomes [444]changed at once, the stems grow longer and of a deeper green. Intermediates occur, but notwithstanding this the two extremesconstitute clearly antagonistic types. 

The flora of the deserts is known to exhibit a similar divergent type. Or rather two types, one adapted to paucity of water, and the other to astorage of fluid at one season in order to make use of it at othertimes, as is the case with the cactuses. Limiting ourselves to thealternate group, we observe a rich and dense branching, small andcompact leaves and extraordinarily long roots. Here the analogy with thealpine varieties is manifest, and the dryness of the soil evidentlyaffects the plants in a similar way, as do the conditions of life inalpine regions. The question at once comes up as to whether here too wehave only instances of partial variability, and whether many of thetypical desert-species would lose their peculiar character bycultivation under ordinary conditions. The varieties of _Monardellamacrantha_, described by Hall, from the San Jacinto Mountain, Cal. , aresuggestive of such an intimate analogy with the cases studied byBonnier, that it seems probable that they might yield similar results, if tested by the same method. 

Leaving now the description of these special [445] cases, we may resumeour theoretical discussion of the subject, and try to get a clearerinsight into the analogy of ever-sporting varieties and the wild speciesquoted. All of them may be characterized by the general term ofdimorphism. Two types are always present, though not in the sameindividual or in the same organ. They exclude one another, and duringtheir juvenile stage a decision is taken in one direction or in theother. Now, according to the theory of natural selection, wild speciescan only retain useful or at least innocuous qualities, since allmutations in a wrong direction must perish sooner or later. Cultivatedspecies on the other hand are known to be largely endowed withqualities, which would be detrimental in the wild condition. Monstrosities are equally injurious and could not hold their own if leftto themselves. 

These same principles may be applied to ever-sporting or antagonisticpairs of characters. According to the theory of mutations such pairs maybe either useful or useless. But only the useful will stand furthertest, and if they find suitable conditions will become specific orvarietal characters. On this conclusion it becomes at once clear, whynatural dimorphism is, as a rule, a very useful quality, while thecultivated dimorphous varieties [446] strike us as something unnatural. The relation between cause and effect, is in truth other than it mightseem to be at first view, but nevertheless it exists, and is of thehighest importance. 

From this same conclusion we may further deduce some explanation of thehereditary races characterized by monstrosities. It is quite evidentthat the twisted teasels are inadequate for the struggle with their tallcongeners, or with the surrounding plants. Hence the conclusion that apure and exclusively twisted race would soon die out. The fact that suchraces are not in existence finds its explanation in this circumstance, and therefore it does not prove the impossibility or even theimprobability that some time a pure twisted race might arise. If chanceshould put such an accidental race in the hands of an experimenter, itcould be protected and preserved, and having no straight atavisticbranches, but being twisted in all its organs, might yield the mostcurious conceivable monstrosity, surpassing even the celebrated dwarftwisted shrubs of Japanese horticulturists. 

Such varieties however, do not exist at present. The ordinary twistedraces on the other hand, are found in the wild state and have only to beisolated and cultivated to yield large numbers [447] of twistedindividuals. In nature they are able to maintain themselves during longcenturies, quite as well as normal species and varieties. But they owethis quality entirely to their dimorphous character. A twisted race ofteasels might consist of successive generations of tall atavisticindividuals, and produce yearly some twisted specimens, which might bedestroyed every time before ripening their seeds. Reasoning from theevidence available, and from analogous cases, the variety would, evenunder such extreme circumstances, be able to last as long as any othergood variety or elementary species. And it seems to me that thisexplanation makes clear how it is possible that varieties, which arepotentially rich in their peculiar monstrosity, are discovered from timeto time among plants when tested by experimental methods. 

Granting these conclusions, monstrosities on the one side, anddimorphous wild species on the other, constitute the most strikingexamples of the inheritance of latent characters. 

The bearing of the phenomena of dimorphism upon the principles ofevolution formulated by Lamarck, and modified by his followers toconstitute Neo-Lamarckianism, remains to be considered. Lamarck assumedthat the external conditions directly affected the organisms in [448]such a way as to make them better adapted to life, under prevailingcircumstances. Nageli gave to this conception the name "Theory of directcausation" (Theorie der directen Bewirkung), and it has received theapproval of Von Wettstein, Strasburger and other German investigators. According to this conception a plant, when migrating from lowlands intothe mountains would slowly be changed and gradually assume alpinehabits. Once acquired this habit would become fixed and attain the rankof specific characters. In testing this theory by field-observations andculture-experiments, the defenders of the Nagelian principle couldeasily produce evidence upon the first point. The change oflowland-plants into alpine varieties can be brought about in numerouscases, and corresponding changes under the influence of soil, orclimate, or life-conditions are on record for the most variouscharacters and qualities. 

The second point, however, is as difficult to prove as the first is ofeasy treatment. If after hundreds and thousands of years of exposure toalpine or other extreme conditions a fixed change is proved to havetaken place, the question remains unanswered, whether the change hasbeen a gradual or a sudden one. Darwin pointed out that long periods oflife afford a [449] chance for a sudden change in the desired direction, as well as for the slow accumulation of slight deviations. Any mutationsin a wrong direction would at once be destroyed, but an accidentalchange in a useful way would be preserved, and multiply itself. If inthe course of centuries this occurred, they would be nearly sure tobecome established, however rare at the outset. Hence the positiveassertion is scarcely capable of direct proof. 

On the other hand the negative assertion must be granted fullsignificance. If the alpine climate has done no more than produce atransitory change, it is clear that thousands of years do not, necessarily, cause constant and specific alterations. This requirementis one of the indispensable supports of the Lamarckian theory. Thematter is capable of disproof however, and such disproof seems to beafforded by the direct evidence of the present condition of the alpinevarieties at large, and by many other similar cases. 

Among these the observations of Holtermann on some desert-plants ofCeylon are of the highest value. Moreover they touch questions which areof wide importance for the study of the biology of American deserts. Forthis reason I may be allowed to introduce them here at some length. 

[450] The desert of Kaits, in Northern Ceylon, nourishes on its dry andtorrid sands some species, represented by a large number of individuals, together with some rarer plants. The commonest forms are _ErigeronAsteroides_, _Vernonia cinerea_, _Laurea pinnatifida_, _Vicoaauriculata_, _Heylandia latebrosa_ and _Chrysopogon montanus_. In directcontrast with the ordinary desert-types they have a thin epidermis, withexposed stomata, features that ordinarily were characteristic of speciesof moister regions. They are annuals, growing rapidly, blooming andripening their seeds before the height of the dry season. Evidently theyare to be considered as the remainder of the flora of a previous period, when the soil had not yet become arid. They might be called relics. Ofcourse they are small and dwarf-like, when compared with allied forms. 

These curious little desert-plants disprove the Nagelian views in twoimportant points. First, they show that extreme conditions do notnecessarily change the organisms subjected to them, in a desirabledirection. During the many centuries that these plants must have existedin the desert in annual generations, no single feature in the anatomicalstructure has become changed. Hence the conclusion that small leaves, abundant rootstocks and short [451] stems, a dense foliage, a stronglycuticularized epidermis, few and narrow air-cavities in the tissues andall the long range of characteristics of typical desert-plants are not asimple result of the influence of climate and soil. There is no directinfluence in this sense. 

The second point, in which Nageli's idea is broken down by Holtermann'sobservations, results from the behavior of the plants of the Kaitsdesert when grown or sown on garden soil. When treated in this way theyat once lose the only peculiarity which might be considered as aconsequence of the desert-life of their ancestors, their dwarf stature. They behave exactly like the alpine plants in Bonnier's experiments, andwith even more striking differences. In the desert they attain a heightof a few centimeters, but in the garden they attain half a meter andmore in height. Nothing in the way of stability has resulted from theaction of the dry soil, not even in such a minor point as the height ofthe stems. 

From the facts and discussions we may conclude that double adaptation isnot induced by external influences, at least not in any way in which itmight be of use to the plant. It may arise by some unknown cause, or maynot be incited at all. In the first case the plant becomes capable ofliving under the alternating [452] circumstances, and if growing nearthe limits of such regions it will overlap and get into the new area. All other species, which did not acquire the double habit, are of courseexcluded, with such curious exceptions as those of Kaits. The typicalvegetation under such extreme conditions however, finds explanationquite as well by the one as by the other view. 

Leaving these obvious cases of double adaptation, there still remainsone point to be considered. It is the dwarf stature of so many desertand alpine plants. Are these dwarfs only the extremes of the normalfluctuating variability, or is their stature to be regarded as theexpression of some peculiar adaptive but latent quality? It is as yetdifficult to decide this question, because statistical studies of thisform of variability are still wanting. The capacity of ripening the seedon individuals of dwarf stature however, is not at all a universalaccompaniment of a variable height. Hence it cannot be considered as anecessary consequence of it. On the other hand the dwarf varieties ofnumerous garden-plants, as for instance: of larkspurs, snapdragon, opium-poppies and others are quite stable and thence are obviously dueto peculiar characteristics. Such characteristics, if combined with tallstature into a pair of antagonists, would yield a double [453]adaptation, and on such a base a hypothetical explanation could no doubtbe rested. Instead of discussing this problem from the theoretical side, I prefer to compare those species which are capable of assuming a dwarfstature under less uncommon conditions than those of alpine anddesert-plants. Many weeds of our gardens and many wild species have thiscapacity. They become very tall, with large leaves, richly branchedstems and numerous flowers in moist and rich soil. On bad soil, or ifgerminating too late, when the season is drier, they remain very small, producing only a few leaves and often limiting themselves to oneflower-head. This is often seen with thorn-apples and amaranths, andeven with oats and rye, and is notoriously the case with buckwheat. Gauchery has observed that the extremes differ often as much from oneanother as 1:10. In the case of the Canadian horseweed or _Erigeroncanadensis_, which is widely naturalized in Europe, the tallestspecimens are often twenty-five times as tall as the smallest, thedifference increasing to greater extremes, if besides the main stem, thelength of the numerous branches of the tall plants are taken intoconsideration. Other instances studied by the French investigator are_Erythraea pulchella_ and _Calamintha Acinos_. 

[454] Dimorphism is of universal occurrence in the whole vegetablekingdom. In some cases it is typical, and may easily be discerned fromextreme fluctuating variability. In others the contrast is not at allobvious, and a closer investigation is needed to decide between the twopossibilities. Sometimes the adaptive quality is evident, in other casesit is not. A large number of plants bear two kinds of leaves linked withone another by intermediate forms. Often the first leaves of a shoot, orthose of accidentally strong shoots, exhibit deviating shapes, and theusefulness of such occurrences seems to be quite doubtful. Theelongation of stems and linear leaves, and the reduction of lateralorgans in darkness, is manifestly an adaptation. Many plants havestolons with double adaptations which enable them to retain theircharacter of underground stems with bracts or to exchange it for thecharacteristics of erect stems with green leaves according to the outercircumstances. In some shrubs and trees the capacity of a number of budsto produce either flowers or shoots with leaves seems to be in the samecondition. The capacity of producing spines is also a double adaptation, active on dry and arid soil and latent in a moist climate or undercultivation, as with the wild and cultivated apple, and in theexperiments of Lothelier [455] with _Berberis_, _Lycium_ and otherspecies, which lose their spines in damp air. 

In some conifers the evolution of horizontal branches may be modified bysimply turning the buds upside down. Or the lateral branches can beinduced to become erect stems by cutting off the normal summit of atree. Numerous organs and functions lie dormant until aroused byexternal agencies, and many other cases could be cited, showing the wideoccurrence of double adaptation. 

There are, however, two points, which should not be passed over withoutsome mention. One of them is the influence of sun and shade on leaves, and the other the atavistic forms, often exhibited during the juvenileperiod. 

The leaves of many plants, and especially those of some shrubs andtrees, have the capacity of adapting themselves either to intense or todiffuse light. On the circumference of the crown of a tree the light isstronger and the leaves a small and thick, with a dense tissue. In theinner parts of the crown the light is weak and the leaves are broader inorder to get as much of it as possible. They become larger but thinner, consisting often of a small number of cell layers. The definitiveformation is made in extreme youth, often even during the previoussummer, at the time of the [456] very first evolution of the youngorgans within the buds. _Iris_, and _Lactuca Scariola_ or the pricklylettuce, and many other plants afford similar instances. As thedefinitive decision must be made in these cases long before the directinfluence of the conditions which would make the change useful is felt, it is hardly conceivable how they could be ascribed to this cause. 

It is universally known that many plants show deviating features whenvery young, and that these often remind us of the characters of theirprobable ancestors. Many plants that must have been derived from theirnearest systematic relatives, chiefly by reductions, are constantlybetraying this relation by a repetition of the ancestral marks duringtheir youth. 

There can be hardly a doubt that the general law of natural selectionprevails in such cases as it does in others. Or stated otherwise, it isvery probable, that in most cases the atavistic characters have beenretained during youth because of their temporary usefulness. Unfortunately, our knowledge of utility of qualities is as yet, veryincomplete. Here we must assume that what is ordinarily spared bynatural selection is to be considered as useful, [457] until directexperimental investigations have been made. 

So it is for instance with the submerged leaves of water-plants. As arule they are linear, or if compound, are reduced to densely branchingfiliform threads. Hence we may conclude that this structure is of someuse to them. Now two European and some corresponding American species ofwater-parsnip, the _Sium latifolium_ and _Berula angustifolia_ withtheir allies, are umbellifers, which bear pinnate instead of bi- ortri-pinnate leaves. But the young plants and even the young shoots whendeveloping from the rootstocks under water comply with the above rule, producing very compound, finely and pectinately dissected leaves. From asystematic point of view these leaves indicate the origin of thewater-parsnips from ordinary umbellifers, which generally have bi- andtripinnate leaves. 

Similar cases of double adaptation, dependent on external conditions atdifferent periods of the evolution of the plant are very numerous. Theyare most marked among leguminous plants, as shown by the trifoliolateleaves of the thorn-broom and allies, which in the adult state havegreen twigs destitute of leaves. 

As an additional instance of dimorphism and probable double adaptationto unrecognized external [458] conditions I might point to the genus_Acacia_. As we have seen in a previous lecture some of the numerousspecies of this genus bear bi-pinnate leaves, while others have onlyflattened leaf-stalks. According to the prevailing systematicconceptions, the last must have been derived from the first by the lossof the blades and the corresponding increase of size and superficialextension of the stalk. In proof of this view they exhibit, as we havedescribed, the ancestral characters in the young plantlets, and thisproduction of bi-pinnate leaves has probably been retained at the periodof the corresponding negative mutations, because of some distinct, though still unknown use. 

Summarizing the results of this discussion, we may state that usefuldimorphism, or double adaptation, is a substitution of characters quiteanalogous to the useless dimorphism of cultivated ever-sportingvarieties and the stray occurrence of hereditary monstrosities. The samelaws and conditions prevail in both cases. 

[459]

E. MUTATIONS

LECTURE XVI

THE ORIGIN OF THE PELORIC TOAD-FLAX

I have tried to show previously that species, in the ordinary sense ofthe word, consist of distinct groups of units. In systematic works thesegroups are all designated by the name of varieties, but it is usuallygranted that the units of the system are not always of the same value. Hence we have distinguished between elementary species and varietiesproper. The first are combined into species whose common original typeis now lost or unknown, and from their characters is derived anhypothetical image of what the common ancestor is supposed to have been. The varieties proper are derived in most cases from still existingtypes, and therefore are subjoined to them. A closer investigation hasshown that this derivation is ordinarily produced by the loss of somedefinite attribute, or by the re-acquisition of an apparently [460] lostcharacter. The elementary species, on the other hand, must have arisenby the production of new qualities, each new acquisition constitutingthe origin of a new elementary form. 

Moreover we have seen, that such improvements and such losses constitutesharp limits between the single unit-forms. Every type, of course, varies around an average, and the extremes of one form may sometimesreach or even overlap those of the nearest allies, but the offspring ofthe extremes always return to the type. The transgression is onlytemporary and a real transition of one form to another does not comewithin ordinary features of fluctuating variability. Even in the casesof eversporting varieties, where two opposite types are united withinone race, and where the succeeding individuals are continually swingingfrom one extreme to the other, passing through a wide range ofintermediate steps, the limits of the variety are as sharply defined andas free from real transgression as in any other form. 

In a complete systematic enumeration of the real units of nature, theelementary species and varieties are thus observed to be discontinuousand separated by definite gaps. Every unit may have its youth, may leada long life in the adult state and may finally die. But through [461]the whole period of its existence it remains the same, at the end assharply defined from its nearest allies as in the beginning. Should someof the units die out, the gaps between the neighboring ones will becomewider, as must often have been the case. Such segregations, howeverimportant and useful for systematic distinctions, are evidently only ofsecondary value, when considering the real nature of the unitsthemselves. 

We may now take up the other side of the problem. The question arises asto how species and varieties have originated. According to the Darwiniantheory they have been produced from one another, the more highlydifferentiated ones from the simpler, in a graduated series from themost simple forms to the most complicated and most highly organizedexisting types. This evolution of course must have been regular andcontinuous, diverging from time to time into new directions, and linkingall organisms together into one common pedigree. All lacunae in ourpresent system are explained by Darwin as due to the extinction of theforms, which previously filled them. 

Since Lamarck first propounded the conception of a common origin for allliving beings, much has been done to clear up our ideas as to the realnature of this process. The broader [462] aspect of the subject, including the general pedigree of the animal and vegetable kingdom, maybe said to have been outlined by Darwin and his followers, but thisphase of the subject lies beyond the limits of our present discussion. 

The other phase of the problem is concerned with the manner in which thesingle elementary species and varieties have sprung from one another. There is no reason to suppose that the world is reaching the end of itsdevelopment, and so we are to infer that the production of new speciesand varieties is still going on. In reality, new forms are observed tooriginate from time to time, both wild and in cultivation, and suchfacts do not leave any doubt as to their origin from other allied types, and according to natural and general laws. 

In the wild state however, and even with cultivated plants of the fieldand garden, the conditions, though allowing of the immediate observationof the origination of new forms, are by no means favorable for a closerinquiry into the real nature of the process. Therefore I shall postponethe discussion of the facts till another lecture, as their bearing willbe more easily understood after having dealt with more complete cases. 

These can only be obtained by direct experimentation. Comparativestudies, of course, [463] are valuable for the elucidation of generalproblems and broad features of the whole pedigree, but the narrower andmore practical question as to the genetic relation of the single formsto one another must be studied in another way, by direct experiment. Theexact methods of the laboratory must be used, and in this case thegarden is the laboratory. The cultures must be guarded with thestrictest care and every precaution taken to exclude opportunities forerror. The parents and grandparents and their offspring must be keptpure and under control, and all facts bearing upon the birth or originof the new types should be carefully recorded. 

Two great difficulties have of late stood in the way of suchexperimental investigation. One of them is of a theoretical, the, otherof a practical nature. One is the general belief in the supposedslowness of the process, the other is the choice of adequate materialfor experimental purposes. Darwin's hypothesis of natural selection asthe means by which new types arise, is now being generally interpretedas stating the slow transformation of ordinary fluctuating divergenciesfrom the average type into specific differences. But in doing so it isoverlooked that Quetelet's law of fluctuating variability was not yetdiscovered at the time, when Darwin propounded his theory. So there[464] is no real and intimate connection between these two greatconceptions. Darwin frequently pointed out that a long period of timemight be needed for slow improvements, and was also a condition for theoccurrence of rare sports. In any case those writers have been in error, according to my opinion, who have refrained from experimental work onthe origin of species, on account of this narrow interpretation ofDarwin's views. The choice of the material is quite another question, and obviously all depends upon this choice. Promising instances must besought for, but as a rule the best way is to test as many plants aspossible. Many of them may show nothing of interest, but some might leadto the desired end. 

For to-day's lecture I have chosen an instance, in which the groundsupon which the choice was based are very evident. It is the origin ofthe peloric toad-flax (_Linaria vulgaris peloria_). 

The ground for this choice lies simply in the fact that the pelorictoad-flax is known to have originated from the ordinary type atdifferent times and in different countries, under more or less divergentconditions. It had arisen from time to time, and hence I presumed thatthere was a chance to see it arise again. If this should happen underexperimental circumstances [465] the desired evidence might easily begathered. Or, to put it in other words, we must try to arrange things soas to be present at the time when nature produces another of these rarechanges. 

There was still another reason for choosing this plant for observationalwork. The step from the ordinary toad-flax to the peloric form is short, and it appears as if it might be produced by slow conversion. Theordinary species produces from time to time stray peloric flowers. Theseoccur at the base of the raceme, or rarely in the midst of it. In otherspecies they are often seen at the summit. Terminal pelories are usuallyregular, having five equal spurs. Lateral pelories are generally ofzygomorphic structure, though of course in a less degree than the normalbilabiate flowers, but they have unequal spurs, the middle one being ofthe ordinary length, the two neighboring being shorter, and thosestanding next to the opposite side of the flower being the shortest ofall. This curious remainder of the original, symmetrical structure ofthe flower seems to have been overlooked hitherto by the investigatorsof peloric toad-flaxes. 

The peloric variety of this plant is characterized by its producing onlypeloric flowers. No single bilabiate or one-spurred flower remains. 

[466] I once had a lot of nearly a hundred specimens of this finevariety, and it was a most curious and beautiful sight to observe themany thousands of nearly regular flowers blooming at the same time. Somedegree of variability was of course present, even in a large measure. The number of the spurs varied between four and six, transgressing theselimits in some instances, but never so far as to produce reallyone-spurred flowers. Comparing this variety with the ordinary type, twoways of passing over from the one to the other might be imagined. Onewould entail a slow increase of the number of the peloric flowers oneach plant, combined with a decrease of the number of the normal ones, the other a sudden leap from one extreme to the other without anyintermediate steps. The latter might easily be overlooked in fieldobservations and their failure may not have the value of direct proof. They could never be overlooked, on the other hand, in experimentalculture. 

The first record of the peloric toad-flax is that of Zioberg, a studentof Linnaeus, who found it in the neighborhood of Upsala. This curiousdiscovery was described by Rudberg in his dissertation in the year 1744. Soon afterwards other localities were discovered by Link near Gottingenin Germany about 1791 and afterwards [467] in the vicinity of Berlin, asstated by Ratzeburg, 1825. Many other localities have since beenindicated for it in Europe, and in my own country some have been notedof late, as for instance near Zandvoort in 1874 and near Oldenzaal in1896. In both these last named cases the peloric form arosespontaneously in places which had often been visited by botanists beforethe recorded appearance, and therefore, without any doubt, they musthave been produced directly and independently by the ordinary specieswhich grows in the locality. The same holds good for other occurrencesof it. In many instances the variety has been recorded to disappearafter a certain lapse of time, the original specimens dying out and nonew ones being produced. _Linaria_ is a perennial herb, multiplyingitself easily by buds growing on the roots, but even with this means ofpropagation its duration seems to have definite limits. 

There is one other important point arguing strongly for the independentappearance of the peloric form in its several localities. It is thedifficulty of fertilization and the high degree of sterility, even ifartificially pollinated. Bees and bumble-bees are unable to crawl intothe narrow tubular flowers, and to bring the fertilizing pollen to thestigma. Ripe capsules with seeds [468] have never been seen in the wildstate. The only writer who succeeded in sowing seeds of the peloricvariety was Wildenow and he got only very few seedlings. But even inartificial pollination the result is the same, the anthers seeming to beseriously affected by the change. I tried both self-fertilization andcross-pollination, and only with utmost care did I succeed in savingbarely a hundred seeds. In order to obtain them I was compelled tooperate on more than a thousand flowers on about a dozen peloric plants. 

The variety being wholly barren in nature, the assumption that theplants in the different recorded localities might have a common originis at once excluded. There must have been at least nearly as manymutations as localities. This strengthens the hope of seeing such amutation happen in one's own garden. It should also be remembered thatpeloric flowers are known to have originated in quite a number ofdifferent species of _Linaria_, and also with many of the allied specieswithin the range of the Labiatiflorae. 

I will now give the description of my own experiment. Of course this didnot give the expected result in the first year. On the contrary, it wasonly after eight years' work that I had the good fortune of observingthe mutation. [469] But as the whole life-history of the precedinggenerations had been carefully observed and recorded, the exactinterpretation of the fact was readily made. 

My culture commenced in the year 1886. I chose some plants of the normaltype with one or two peloric flowers besides the bilabiate majoritywhich I found on a locality in the neighborhood of Hilversum in Holland. I planted the roots in my garden and from them had the first floweringgeneration in the following summer. From their seeds I grew the secondgeneration in three following years. They flowered profusely andproduced in 1889 only one, and in 1890 only two peloric structures. Isaved the seeds in 1889 and had in 1890-1891 the third generation. Theseplants likewise flowered only in the second year, and gave among somethousands of symmetrical blossoms, only one five-spurred flower. Ipollinated this flower myself, and it produced abundant fruit withenough seeds for the entire culture in 1892, and they only were sown. 

Until this year my generations required two years each, owing to theperennial habit of the plants. In this way the prospects of the culturebegan to decrease, and I proposed to try to heighten my chances byhaving a new generation yearly. With this intention I sowed the [470]selected seeds in a pan in the glasshouse of my laboratory and plantedthem out as soon as the young stems had reached a length of some fewcentimeters. Each seedling was put in a separate pot, in heavily manuredsoil. The pots were kept under glass until the beginning of June, andthe young plants produced during this period a number of secondary stemsfrom the curious hypocotylous buds which are so characteristic of thespecies. These stems grew rapidly and as soon as they were strongenough, the plants were put into the beds. They all, at least nearlyall, some twenty specimens, flowered in the following month. 

I observed only one peloric flower among the large number present. Itook the plant bearing this flower and one more for the culture of thefollowing year, and destroyed all others. These two plants grew on thesame spot, and were allowed to fertilize each other by the agency of thebees, but were kept isolated from any other congener. They floweredabundantly, but produced only one-spurred bilabiate flowers during thewhole summer. They matured more than 10 cu. Cm. Of seeds. 

It is from this pair of plants that my peloric race has sprung. And asthey are the ancestors of the first closely observed case of peloricmutation, [471] it seems worth while to give some details regardingtheir fertilization. 

Isolated plants of _Linaria vulgaris_ do not produce seed, even iffreely pollinated by bees. Pollen from other plants is required. Thisrequirement is not at all restricted to the genus _Linaria_, as manyinstances are known to occur in different families. It is generallyassumed that the pollen of any other individual of the same species iscapable of producing fertilization, although it is to be said that acritical examination has been made in but few instances. 

This, however, is not the case, at least not in the present instance. Ihave pollinated a number of plants, grown from seed of the same strainand combined them in pairs, and excluded the visits of insects, andpollen other than that of the plant itself and that of the specimen withwhich it was paired. The result was that some pairs were fertile andothers barren. Counting these two groups of pairs, I found them nearlyequal in number, indicating thereby that for any given individual thepollen of half of the others is potent, but that of the other halfimpotent. From these facts we may conclude the presence of a curiouscase of dimorphy, analogous to that proposed for the primroses, butwithout visible differentiating marks in the flowers. At least suchopposite characters [472] have as yet not been ascertained in the caseof our toad-flax. 

In order to save seed from isolated plants it is necessary, for thisreason, to have at least two individuals, and these must belong to thetwo physiologically different types. Now in the year 1892, as in otheryears, my plants, though separated at the outset by distances of about20 cm. From each other, threw out roots of far greater length, growingin such a way as to abolish the strict isolation of the individuals. Anyplot may produce several stems from such roots, and it is manifestlyimpossible to decide whether they all belong to one original plant or tothe mixed roots of several individuals. No other strains were grown onthe same bed with my plants however, and so I considered all the stemsof the little group as belonging to one plant. But their perfectfertility showed, according to the experience described, that there musthave been at least two specimens mingled together. 

Returning now to the seeds of this pair of plants, I had, of course, notthe least occasion to ascribe to it any higher value than the harvest offormer years. The consequence was that I had no reason to make largesowings, and grew only enough young plants to have about 50 in bloom inthe summer of 1894. Among [473] these, stray peloric flowers wereobserved in somewhat larger number than in the previous generations, 11plants bearing one or two, or even three such abnormalities. Thishowever, could not be considered as a real advance, since such plantsmay occur in varying, though ordinarily small numbers in everygeneration. 

Besides them a single plant was seen to bear only peloric flowers; itproduced racemes on several stems and their branches. All were peloricwithout exception. I kept it through the winter, taking care to preservea complete isolation of its roots. The other plants were whollydestroyed. Such annihilation must include both the stems and roots andthe latter of course requires considerable labor. The following year, however, gave proof of the success of the operation, since my plantbloomed luxuriously for the second time and remained true to the type ofthe first year, producing peloric flowers exclusively. 

Here we have the first experimental mutation of a normal into a peloricrace. Two facts were clear and simple. The ancestry was known for over aperiod of four generations, living under the ordinary care andconditions of an experimental garden, isolated from other toad-flaxes, but freely fertilized by bees or at times by myself. This ancestry wasquite constant as to [474] the peloric peculiarity, remaining true tothe wild type as it occurs everywhere in my country, and showing in norespect any tendency to the production of a new variety. 

The mutation took place at once. It was a sudden leap from the normalplants with very rare peloric flowers to a type exclusively peloric. Nointermediate steps were observed. The parents themselves had bornethousands of flowers during two summers, and these were inspected nearlyevery day, in the hope of finding some pelories and of saving their seedseparately. Only one such flower was seen. If there had been more, say afew in every hundred flowers, it might be allowable to consider them asprevious stages, showing a preparation of the impending change. Butnothing of this kind was observed. There was simply no visiblepreparation for the sudden leap. 

This leap, on the other hand, was full and complete. No reminiscence ofthe former condition remained. Not a single flower on the mutated plantreverted to the previous type. All were thoroughly affected by the newattribute, and showed the abnormally augmented number of spurs, thetubular structure of the corolla and the round and narrow entrance ofits throat. The whole plant departed absolutely from the old type of itsprogenitors. 

[475] Three ways were open to continue my experiment. The first wasindicated by the abundant harvest from the parent-plants of themutation. It seemed possible to compare the numerical proportion of themutated seeds with those of normal plants. In order to ascertain thisproportion I sowed the greatest part of my 10 cu. Cm. Of seed andplanted some 2, 000 young plants in little pots with well-manured soil. Igot some 1, 750 flowering plants and observed among them 16 whollypeloric individuals. The numerical proportion of the mutation wastherefore in this instance to be calculated equal to about 1% of thewhole crop. 

This figure is of some importance. For it shows that the chance offinding mutations requires the cultivation of large groups ofindividuals. One plant in each hundred may mutate, and cultures of lessthan a hundred specimens must therefore be entirely dependent on chancefor the appearance of new forms, even if such should accidentally havebeen produced and lay dormant in the seed. In other cases mutations maybe more numerous, or on the contrary, more rare. But the chance ofmutative changes in larger numbers is manifestly much reduced by thisexperiment, and they may be expected to form a very small proportion ofthe culture. 

[476] The second question which arose from the above result was this. Could the mutation be repeated? Was it to be ascribed to some latentcause which might be operative more than once? Was there some hiddentendency to mutation, which, ordinarily weak, was strengthened in mycultures by some unknown influence? Was the observed mutation to beexplained by a common cause with the other cases recorded byfield-observations? To answer this question I had only to continue myexperiment, excluding the mutated individuals from any intercrossingwith their brethren. To this end I saved the seeds from duly isolatedgroups in different years and sowed them at different times. For variouscauses I was not prepared to have large cultures from these seeds, butnotwithstanding this, the mutation repeated itself. In one instance Iobtained two, in another, one peloric plant with exclusivelymany-spurred flowers. As is easily understood, these were related as"nieces" to the first observed mutants. They originated in quite thesame way, by a sudden leap, without any preparation and without anyintermediate steps. 

Mutation is proved by this experience to be of an iterative nature. Itis the expression of some concealed condition, or as it is generally[477] called, of some hidden tendency. The real nature of this state ofthe hereditary qualities is as yet wholly unknown. It would not be safeto formulate further conclusions before the evidence offered by theevening-primroses is considered. 

Thirdly, the question arises, whether the mutation is complete, not onlyas to the morphologic character, but also as to the hereditaryconstitution of the mutated individuals. But here unfortunately the highdegree of sterility of the peloric plants, as previously noted, makesthe experimental evidence a thing of great difficulty. During the courseof several years I isolated and planted together the peloric individualsalready mentioned, all in all some twenty plants. Each individual wasnearly absolutely sterile when treated with its own pollen, and the aidof insects was of no avail. I intercrossed my plants artificially, andpollinated more than a thousand flowers. Not a single one gave a normalfruit, but some small and nearly rudimentary capsules were produced, bearing a few seeds. From these I had 119 flowering plants, out of which106 were peloric and 13 one-spurred. The great majority, some 90%, werethus shown to be true to their new type. Whether the 10% reverting oneswere truly atavists, or whether they were [478] only vicinists, causedby stray pollen grains from another culture, cannot of course be decidedwith sufficient certitude. 

Here I might refer to the observations concerning the invisibledimorphous state of the flowers of the normal toad-flax. Individuals ofthe same type, when fertilized with each other, are nearly, but notabsolutely, sterile. The yield of seeds of my peloric plants agreesfairly well with the harvest which I have obtained from some of thenearly sterile pairs of individuals in my former trial. Hence thesuggestion is forced upon us that perhaps, owing to some unknown cause, all the peloric individuals of my experiment belonged to one and thesame type, and were sterile for this reason only. If this is true, thenit is to be presumed that all previous investigators have met the samecondition, each having at hand only one of the two required types. Andthis discussion has the further advantage of showing the way, in whichperhaps a full and constant race of peloric toad-flaxes may be obtained. Two individuals of different type are required to start from. They seemas yet never to have arisen from one group of mutations. But if it werepossible to combine the products of two mutations obtained in differentcountries and under different conditions, there would be a chance [479]that they might belong to the supposed opposite types, and thus befertile with one another. My peloric plants are still available, and theoccurrence of this form elsewhere would give material for a successfulexperiment. The probability thereof is enhanced by the experience thatmy peloric plants bear large capsules and a rich harvest of seeds whenfertilized from plants of the normal one-spurred race, while they remainnearly wholly barren by artificial fertilization with others. I supposethat they are infertile with the normal toad-flaxes of their own sexualdisposition, but fertile with those of the opposite constitution. At allevents the fact that they may bear abundant seed when properlypollinated is an indication of successful experiments on the possibilityof gaining a hereditary race with exclusively peloric flowers. And sucha race would be a distinct gain for sundry physiologic inquiries, andperhaps not wholly destitute of value from an horticultural point ofview. 

Returning now to the often recorded occurrence of peloric toad-flaxes inthe wild state and recalling our discussion about the improbability of adispersion from one locality to another by seed, and the probability ofindependent origin for most of these cases, we are confronted with theconception that a latent [480] tendency to mutation must be universallypresent in the whole species. Another observation, although it is of anegative character, gains in importance from this point of view. I referto the total lack of intermediate steps between normal and peloricindividuals. If such links had ordinarily been produced previous to thepurely peloric state they would no doubt have been observed from time totime. This is so much the more probable as _Linaria_ is a perennialherb, and the ancestors of a mutation might still be in a floweringcondition together with their divergent offspring. But no suchintermediates are on record. The peloric toad-flaxes are, as a rule, found surrounded by the normal type, but without intergrading forms. This discontinuity has already been insisted upon by Hofmeister andothers, even at the time when the theory of descent was most underdiscussion, and any link would surely have been produced as a proof of aslow and continuous change. But no such proof has been found, and theconclusion seems admissible that the mutation of toad-flaxes ordinarily, if not universally, takes place by a sudden step. Our experiment maysimply be considered as a thoroughly controlled instance of an oftenrecurring phenomenon. It teaches us how, in the [481] main, the peloricmutations must be assumed to proceed. 

This conception may still be broadened. We may include in it all similaroccurrences, in allied and other species. There is hardly a limit to thepossibilities which are opened up by this experience. But it will bewell to refrain from hazardous theorizing, and consider only those caseswhich may be regarded as exact repetitions of the same phenomenon and ofwhich our culture is one of the most recent instances on record. We willlimit ourselves to the probable origin of peloric variations at large, of which little is known, but some evidence may be derived from therecorded facts. Only one case can be said to be directly analogous toour observations. 

This refers to the peloric race of the common snapdragon, or_Antirrhinum majus_ of our gardens. It is known to produce peloric racesfrom time to time in the same way as does the toadflax. But thesnapdragon is self-fertile and so is its peloric variety. Some cases arerelatively old, and some of them have been recorded and in part observedby Darwin. Whence they have sprung and in what manner they wereproduced, seems never to have been noted. Others are of later origin, and among these one or two varieties have been accidentally produced[482] in the nursery of Mr. Chr. Lorenz in Erfurt, and are now for sale, the seeds being guaranteed to yield a large proportion of peloricindividuals. The peloric form in this case appeared at once, but was notisolated, and was left free to visiting insects, which of course crossedit with the surrounding varieties. Without doubt the existence of twocolor-varieties of the peloric type, one of a very dark red, indicatingthe "Black prince" variety as the pollen-parent, and the other with awhite tube of the corolla, recalling the form known as "Delila, " is dueto these crossings. I had last year (1903) a large lot of plants, partlynormal and partly peloric, but evidently of hybrid origin, from seedsfrom this nursery, showing moreover all intermediate steps betweennearly wholly peloric individuals and apparently normal ones. I havesaved the seeds of the isolated types and before seeing the flowers oftheir offspring, nothing can be said about the purity and constancy ofthe type, when freed from hybrid admixtures. The peloric snapdragon hasfive small unequal spurs at the base of its long tube, and in thisrespect agrees with the peloric toad-flax. 

Other pelories are terminal and quite regular, and occur in some speciesof _Linaria_, where I observed them in _Linaria dalmatica_. The [483]terminal flowers of many branches were large and beautifully peloric, bearing five long and equal spurs. About their origin and inheritancenothing is known. 

A most curious terminal pelory is that of the common foxglove or_Digitalis purpurea_. As we have seen in a previous lecture, it is anold variety. It was described and figured for the first time by Vrolikof Amsterdam, and the original specimens of his plates are still to beseen in the collections of the botanic garden of that university. Sincehis time it has been propagated by seed as a commercial variety, and maybe easily obtained. The terminal flower of the central stem and those ofthe branches only are affected, all other flowers being wholly normal. Almost always it is accompanied by other deviations, among which amarked increase of the number of the parts of the corolla and otherwhorls is the most striking. Likewise supernumerary petals on the outerside of the corolla, and a production of a bud in the center of thecapsule may be often met with. This bud as a rule grows out after thefading away of the flower, bursting through the green carpels of theunripe fruit, and producing ordinarily a secondary raceme of flowers. This raceme is a weak but exact repetition of the first, bearingsymmetrical foxgloves all [484] along and terminating in a peloricstructure. On the branches these anomalies are more or less reduced, according to the strength of the branch, and conforming to the rule ofperiodicity, given in our lecture on the "five-leaved" clover. Throughall this diminution the peloric type remains unchanged and thereforebecomes so much the purer, the weaker the branches on which it stands. 

I am not sure whether such peloric flowers have ever been purelypollinated and their seed saved separately, but I have often observedthat the race comes pure from the seed of the zygomorphic flowers. It isas yet doubtful whether it is a half race or a double race, and whetherit might be purified and strengthened by artificial selection. Perhapsthe determination of the hereditary percentage described when dealingwith the tricotyls might give the clue to the acquisition of a higherspecialized race. The variety is old and widely disseminated, but mustbe subjected to quite a number of additional experiments before it canbe said to be sufficiently understood. 

The most widespread peloric variety is that of _Gloxinia_. It has erectinstead of drooping flowers; and with the changed position the structureis also changed. Like other pelories it has five equal stamens insteadof four unequal [485] ones, and a corolla with five equal segmentsinstead of an upper and a lower lip. It shows the peloric condition inall of its flowers and is often combined with a small increase of thenumber of the parts of the whorls. It is for sale under the name of_erecta_, and may be had in a wide range of color-types. It seems to bequite constant from seed. 

Many other instances of peloric flowers are on record. Indian cress or_Tropaeolum majus_ loses the spur in some double varieties and with itmost of its symmetrical structure; it seems to be considered justly as apeloric malformation. Other species produce such anomalies only fromtime to time and nothing is known about their hereditary tendency. Oneof the most curious instances is the terminal flower of the raceme ofthe common laburnum, which loses its whole papilionaceous character andbecomes as regularly quinate as a common buttercup. 

Some families are more liable to pelorism than others. Obviously all thegroups, the flowers of which are not symmetrical, are to be excluded. But then we find that labiates and their allies among the dicotyledonousplants, and orchids among the monocotyledonous ones are especiallysubjected to this alteration. In both groups many genera and a long listof species [486] could be quoted as proof. The family of the labiatesseems to be essentially rich in terminal pelories, as for instance inthe wild sage or _Salvia_ and the dead-nettle or _Lamium_. Here thepelories have long and straight corolla-tubes, which are terminated by awhorl of four or five segments. Such forms often occur in the wild stateand seem to have a geographic distribution as narrowly circumscribed asin the case of many small species. Those of the labiates chiefly belongto southern Europe and are unknown at least in some parts of the othercountries. On the contrary terminal pelories of _Scrophularia nodosa_are met with from time to time in Holland. Such facts clearly point to acommon origin, and as only the terminal flowers are affected by themalformation, the fertility of the whole plant is evidently notseriously infringed upon. 

Before leaving the labiates, we may cite a curious instance of pelorismin the toad-flax, which is quite different from the ordinary peloricvariety. This latter may be considered from a morphologic standpoint tobe owing to a five-fold repetition of the middle part of the underlip. This conception would at once explain the occurrence of five spurs andof the orange border all around the corolla-tube. We might readilyimagine that any other of the five [487] parts of the corolla could berepeated five-fold, in which case there would be no spur, and no orangehue on the upper corolla-ring. Such forms really occur, though they seemto be more rare than the five-spurred pelories. Very little is knownabout their frequency and hereditary qualities. 

Orchids include a large number of peloric monstrosities and moreover awild pelory which is systematically described not only as a separatespecies but even as a new genus. It bears the name of _Uropediumlindenii_, and is so closely related to _Cypripedium caudatum_ that manyauthors take it for the peloric variety of this plant. It occurs in thewild state in some parts of Mexico, where the _Cypripedium_ also grows. Its claims to be a separate genus are lessened by the somewhat monstrouscondition of the sexual organs, which are described as quite abnormal. But here also, intermediates are lacking, and this fact points to asudden origin. 

Many cases of pelorism afford promising material for further studies ofexperimental mutations. The peloric toad-flax is only the prototype ofwhat may be expected in other cases. No opportunity should be lost toincrease the as yet too scanty, evidence on this point. 

[488]

LECTURE XVII

THE PRODUCTION OF DOUBLE FLOWERS

Mutations occur as often among cultivated plants as among those in thewild state. Garden flowers are known to vary markedly. Much of theirvariability, however, is due to hybridism, and the combination ofcharacters previously separate has a value for the breeder nearly equalthe production of really new qualities. Nevertheless there is no doubtthat some new characters appear from time to time. 

In a previous lecture we have seen that varietal characters have manyfeatures in common. One of them is their frequent recurrence both in thesame and in other, often very distantly related, species. Thisrecurrence is an important factor in the choice of the material for anexperimental investigation of the nature of mutations. 

Some varieties are reputed to occur more often and more readily thanothers. White-colored varieties, though so very common, seem for themost part to be of ancient date, but only few [489] have a known origin, however. Without any doubt many of them have been found in a wild stateand were introduced into culture. On the other hand double flowers areexceedingly rare in the wild state, and even a slight indication of atendency towards doubling, the stray petaloid stamens, are only rarelyobserved growing wild. In cultivation, however, double flowers are offrequent occurrence; hence the conclusion that they have been producedin gardens and nurseries more frequently than perhaps any other type ofvariety. 

In the beginning of my experimental work I cherished the hope of beingable to produce a white variety. My experiments, however, have not beensuccessful, and so I have given them up temporarily. Much better chancesfor a new double variety seemed to exist, and my endeavors in thisdirection have finally been crowned with success. 

For this reason I propose to deal now with the production of doubleflowers, to inquire what is on record about them in horticulturalliterature, and to give a full description of the origin thereof in aninstance which it was my good fortune to observe in my garden. 

Of course the historical part is only a hasty survey of the question andwill only give such evidence as may enable us to get an idea of the[490] chances of success for the experimental worker. In the second halfof the seventeenth century (1671), my countryman, Abraham Munting, published a large book on garden plants with many beautiful figures. Itis called "Waare Oeffeninge der Planters, " or "True Exercises WithPlants. " The descriptions pertain to ordinary typical species in greaterpart, but garden varieties receive special attention. Among these a longlist of double flowers are to be seen. Double varieties of poppies, liverleaf (Hepatica), wallflowers (_Cheiranthus_), violets, _Caltha_, _Althaea_, _Colchicum_, and periwinkles (_Vinca_), and a great manyother common flowers were already in cultivation at that time. 

Other double forms have been since added. Many have been introduced fromJapan, especially the Japanese marigold, _Chrysanthemum indicum_. Othershave been derived from Mexico, as for instance the double zinnias. Thesingle dahlias only seem to have been originally known to theinhabitants of Mexico. They were introduced into Spain at about 1789, and the first double ones were produced in Louvain, Belgium, in 1814. The method of their origin has not been described, and probably escapedthe originators themselves. But in historical records we find thecurious statement that it took place after three years' work. Thisindicates [491] a distinct plan, and the possibility of carrying it to apractical conclusion within a few years' time. 

Something more is known about other cases. Garden anemones, _Anemonecoronaria_, are said to have become double in the first half of the lastcentury in an English nursery. The owner, Williamson, observing in hisbeds a flower with a single broadened stamen, saved its seedsseparately, and in the next generations procured beautifully filledflowers. These he afterwards had crossed by bees with a number ofcolored varieties, and in this way succeeded in producing many newdouble types of anemone. 

The first double petunia is known to have suddenly and accidentallyarisen from ordinary seed in a private garden at Lyons about 1855. Fromthis one plant all double races and-varieties have been derived bynatural and partly by artificial crosses. Carriere, who reported thisfact, added that likewise other species were known at that time toproduce new double varieties rapidly. The double fuchsias originatedabout the same time (1854) and ten years later the range of doublevarieties of this plant had become so large that Carriere found itimpossible to enumerate all of them. 

Double carnations seem to be relatively old, double corn-flowers anddouble blue-bells being [492] of a later period. A long list couldeasily be made, to show that during the whole history of horticulturedouble varieties have arisen from time to time. As far as we can judge, such appearances have been isolated and sudden. Sometimes they spranginto existence in the full display of their beauty, but most commonlythey showed themselves for the first time, exhibiting only sparesupernumerary petals. Whenever such sports were worked up, a few yearssufficed to reach the entire development of the new varietal attribute. 

From this superficial survey of historical facts, the inference isforced upon us that the chance of producing a new double variety is goodenough to justify the attempt. It has frequently succeeded for practicalpurposes, why should it not succeed as well for purely scientificinvestigation? At all events the type recommends itself to the studentof nature, both on account of its frequency, and of the apparentinsignificance of the first step, combined with the possibility ofrapidly working up from this small beginning of one superfluous petaltowards the highest degree of duplication. 

Compared with the tedious experimental production of the pelorictoad-flax, the attempt to produce a double flower has a distinctattraction. The peloric toad-flax is nothing new; the [493] experimentwas only a repetition of what presumably takes place often within thesame species. To attempt to produce a double variety we may choose anyspecies, and of course should select one which as yet has not been knownto produce double flowers. By doing so we will, if we succeed, producesomething new. Of course, it does not matter whether the new variety hasan horticultural interest or not, and it seems preferable to choose awild or little cultivated species, to be quite sure that the variety inquestion is not already in existence. Finally the prospect of successseems to be enhanced if a species is chosen, the nearest allies of whichare known to have produced double flowers. 

For these reasons and others I chose for my experiment thecorn-marigold, or _Chrysanthemum segetum_. It is also called the goldencornflower. In the wheat and rye fields of central Europe it associateswith the blue-bottle or blue corn-flower. It is sometimes cultivated andthe seeds are offered for sale by many nurserymen. It has a cultivatedvariety, called _grandiflorum_, which is esteemed for its brilliancy andlong succession of golden bloom. This variety has larger flower-heads, surrounded with a fuller border of ray-florets. The species belongs to agenus many species of which have produced [494] double varieties. One ofthem is the Japanese marigold, others are the _carinatum_ and the_imbricatum_ species. Nearly allied are quite a number of garden-plantswith double flower-heads, among which are the double camomiles. 

My attention was first drawn to the structure of the heads andespecially to the number of the ray-florets of the corn-marigold. Thespecies appertains to that group of composites which have a head ofsmall tubular florets surrounded by a broad border of rays. These rays, when counted, are observed to occur in definite numbers, which areconnected with each other by a formula, known as "the series" of Braunand Schimper. In this formula, which commences with 1 and 2, each numberis equal to the sum of the two foregoing figures. Thus 5, 8 and 13 arevery frequent occurrences, and the following number, 21, is a mostgeneral one for apparently full rays, such as in daisies, camomiles, _Arnica_ and many other wild and cultivated species. 

These numbers are not at all constant. They are only the averages, around which the real numbers fluctuate. There may even be anoverlapping of the extremes, since the fluctuation around 13 may even gobeyond 8 and 21, and so on. But such extremes are only found in strayflowers, occurring on the same [495] individuals with the lesser degreesof deviation. 

Now the marigold averages 13, and the _grandiflorum_ 21 rays. The wildspecies is pure in this respect, but the garden-variety is not. Theseeds which are offered for sale usually contain a mixture of both formsand their hybrids. So I had to isolate the pure types from this mixtureand to ascertain their constancy and mutual independency. To this end Iisolated from the mixture first the 13-rayed, and afterwards the21-rayed types. As the marigolds are not sufficiently self-fertile, andare not easily pollinated artificially, it seemed impossible to carry onthese two experiments at the same time and in the same garden. I devotedthe first three years to the lower form, isolated some individuals with12-13 rays out of the mixture of 1892 and counted the ray-florets on theterminal head of every plant of the ensuing generation next year. Icultivated and counted in this way above 150 individuals and found anaverage of exactly 13 with comparatively few individuals displaying 14or only 12 rays, and with the remainder of the plants groupedsymmetrically around this average. I continued the experiment for stillanother year and found the same group of figures. I was then satisfiedas to the purity of the isolated strain. Next year I sowed a new mixturein [496] order to isolate the reputed pure _grandiflorum_ type. Duringthe beginning of the flowering period I ruthlessly threw away all plantsdisplaying less than 21 rays in the first or terminal head. But thisselection was not to be considered as complete, because the 13-rayedrace may eventually transgress its boundary and come over to the 21 andmore. This made a second selection necessary. On the selected plants allthe secondary heads were inspected and their ray-florets counted. Someindividuals showed an average of about 13 and were destroyed. Othersgave doubtful figures and were likewise eliminated, and only 6 out of alot of nearly 300 flowering plants reached an average of 21 for all ofthe flowers. 

Our summer is a short one, compared with the long and beautiful summerof California, and it was too late to cut off the faded and the openflowers, and await new ones, which might be purely fertilized after thedestruction of all minor plants. So I had to gather the seed fromflowers, which might have been partially fertilized by the wrong pollen. This however, is not so great a drawback in selection experiments asmight be supposed at first sight. The selection of the following year issure to eliminate the offspring of such impure parentage. 

[497] A far more important principle is that of the hereditarypercentage, already discussed in our lecture on the selection ofmonstrosities. In our present case it had to be applied only to the sixselected plants of 1895. To this end the seeds of each of them were sownseparately, the ray-florets of the terminal heads of each of the newgeneration were counted, and curves and averages were made up for thesix groups. Five of them gave proof of still being mixtures and werewholly rejected. The children of the sixth parent, however, formed agroup of uniform constitution, all fluctuating around the desiredaverage of 21. All in all the terminal heads of over 1, 500 plants havebeen subjected to the somewhat tedious work of counting theirray-florets. And this not in the laboratory, but in the garden, withoutcutting them off. Otherwise it would obviously have been impossible torecognize the best plants for preservation. I chose only two plantswhich in addition recommended themselves by the average number of rayson their secondary heads, sowed their seeds next year separately andcompared the numerical constitution of their offspring. Both groupsaveraged 21 and were distributed very symmetrically around this mean. This result [498] showed that no further selection could be of anyavail, and that I had succeeded in purifying the 21-rayed _grandiflorum_variety. 

It is from this _grandiflorum_ that I have finally produced my doublevariety. In the year 1896 I selected from among the above quoted 1, 500plants, 500 with terminal heads bearing 21 or more rays. On these Icounted the rays of all the secondary heads about the middle of August(1896) and found that they had, as a rule, retrograded to lower figures. On many thousands of heads only two were found having 22 rays. Allothers had the average number of 21 or even less. I isolated theindividual which bore these two heads, allowed them to be fertilized byinsects with the pollen of some of the best plants of the same group, but destroyed the remainder. 

This single exceptional plant has been the starting point of my doublevariety. It was not remarkable for its terminal head, which exhibitedthe average number of rays of the 21-rayed race. Nor was itdistinguished by the average figure for all its heads. It was onlyselected because it was the one plant which had some secondary headswith one ray more than all the others. This indication was very slight, and could not have been detected save by the counting of the rays ofthousands of heads. 

[499] But the rarity of the anomaly was exactly the indication wanted, and the same deviation would have had no signification whatever, had itoccurred in a group fluctuating symmetrically around the average figure. On the other hand, the observed anomaly was only an indication, and noguarantee of future developments. 

Here it should be remarked that the indication alluded to was not theappearance of the expected character of doubling in ever so slight ameasure. It was only a guide to be followed in further work. The realcharacter of double flower-heads among composites lies in the productionof rays on the disk. No increase of the number of the outer rays canhave the same significance. A hasty inspection of double flower headsmay convey the idea that all rays are arranged around a little centralcluster of disk-florets, the remainder of the original disk-florets buta closer investigation will always reveal the fallacy of thisconclusion. Hidden between the inner rays, and covered by them, lie thelittle tubular and fertile florets everywhere on the disk. They may notbe easily seen, but if the supernumerary rays are pulled out, the diskmay be seen to bear numerous small florets at intervals. But theseintervals are not at all numerous, showing thereby that only arelatively small number of tubes has been [500] converted into rays. This conversion is obviously the true mark of the doubling, and beforetraces of it are found, no assertion whatever can be given as to theissue of the pedigree experiment. 

Three more years were required before this first, but decisive trace wasdiscovered. During these years I subjected my strain to the same sharpselection as has already been described. The chosen ancestor of the racehad flowered in 1896, and the next year I sowed its seeds only. Fromthis generation I chose the one plant with the largest number of rays inits terminal head, and repeated this in the following year. 

The consequence was that the average number of rays increased rapidly, and with it the absolute maximum of the whole strain. The average cameup from 21 to 34. Brighter and brighter crowns of the yellow raysimproved my race, until it became difficult and very time consuming tocount all the large rays of the borders. The largest numbers determinedin the succeeding generations increased by leaps from 21 to 34 in thefirst year, and thence to 48 and 66 in the two succeeding summers. Everyyear I was able to save enough seed from the very best plant and to useit only for the continuance of the race. Before the selected plants wereallowed to open the flowers from which the seed [501] was to begathered, nearly the whole remaining culture was exterminated, exceptingonly some of the best examples, in order to have the required materialfor cross-pollination by insects. Each new generation was thereby assharply selected as possible with regard to both parents. 

All flower-heads were of course closely inspected. Not the slightestindication of real doubling was discovered, even in the summer of 1899in the fourth generation of my selected race. But among the best the newcharacter suddenly made its appearance. It was at the commencement ofSeptember (1899), too late to admit of the seeds ripening before winter. An inspection of the younger heads was made, which revealed three headswith some few rays in the midst of the disk on one plant, the result ofthe efforts of four years. Had the germ of the mutation lain hiddenthrough all this time? Had it been present, though dormant in theoriginal sample of seed? Or had an entirely new creation taken placeduring my continuous endeavors? Perhaps as their more or less immediateresult? It is obviously impossible to answer these questions, beforefurther and similar experiments shall have been performed, bringing tolight other details that will enable us to reach a more definiteconclusion. 

[502] The fact that the origination of such forms is accessible todirect investigation is proven quite independently of all furtherconsiderations. The new variety came into existence at once. The leapmay have been made by the ancestor of the year 1895, or by the plant of1899, which showed the first central rays, or the sport may have beengradually built up during those four years. In each case there was aleap, contrasting with the view which claims a very long succession ofyears for the development of every new character. 

Having discovered this first trace of doubling, it was to be expectedthat the new variety would be at once as pure and as rich as otherdouble composites usually are. Some effect of the crossing with theother seed-bearing individuals might still disturb this uniformity inthe following year, but another year's work would eliminate even thissource of impurity. 

These two years have given the expected result. The average number ofthe rays, which had already arisen from 13 to 34 now at once came up to47 and 55, the last figure being the sum of 21 and 34 and therefore theprobable uttermost limit to be reached before absolute doubling. Themaximum numbers came as high as 100 in 1900, and reached even 200 in1901. Such heads are as completely double as are the [503] brightestheads of the most beautiful double commercial varieties of composites. Even the best white camomiles (_Chrysanthemum inodorum_) and thegold-flowers or garden-marigolds (_Calendula officinalis_) do not comenearer to purity since they always have scores of little tubular floretsbetween the rays on their disks. 

Real atavists or real reversionists were seen no more after the firstpurification of the race. I have continued my culture and secured lastsummer (1903) as many and as completely doubled heads as previously. Therace has at once become permanent and constant. It has of course a widerange of fluctuating variability, but the lower limit has been worked upto about 34 rays, a figure never reached by the _grandiflorum_ parent, from which my new variety is thus sharply separated. 

Unfortunately the best flowers and even the best individuals of my raceare wholly barren. Selection has reached its practical limit. Seeds mustbe saved from less dense heads, and no way has been found of avoidingit. The ray-florets are sterile, even in the wild species, and whengrowing in somewhat large numbers on the disk, they conceal the fertileflowers from the visiting insects, and cause them also to be sterile. The same is the case with the best cultivated forms. Their showiestindividuals are [504] barren, and incapable of the reproduction of therace. 

This last is therefore, of necessity, always continued by means ofindividuals whose deviation from the mean average is the least. But inmany cases the varieties are so highly differentiated that selection hasbecome quite superfluous for practical purposes. I have alreadydiscussed the question as to the actual moment, in which the change ofthe _grandiflorum_ variety into the new _plenum_ form must be assumed tohave taken place. In this respect some stress is to be laid on the factthat the improvement through selection has been gradual and continuous, though very rapid from the first moment. But with the appearance of thefirst stray rays within the disk, this continuity suddenly changed. Allthe children of this original mutated plant showed the new character, the rays within the disk, without exception. Not on all the heads, noreven on the majority of the heads on some individuals, but on some headsall gave clear proof of the possession of the new attribute. This waspresent in all the representatives of the new race, and had never beenseen in any of their parents and grandparents. Here there was evidentlya sudden leap, at least in the external form of the plants. And it seemsto me to be the most simple conception, [505] that this visible leapdirectly corresponded to that inner change, which brought about thecomplete inheritability of the new peculiarity. It is very interestingto observe how completely my experience agrees with the results of theobservations of breeders at large. No doubt a comparison is difficult, and the circumstances are not adequate to a close study. 

Isolation and selection have been applied commonly only so far as wasconsistent with the requirements of practical horticulture, and ofcourse a determination of the hereditary percentage was never made. Thedisregard of this feature made necessary a greater length of time and alarger number of generations to bring about the desired changes. Notwithstanding this, however, it has been seen that double varietiesare produced suddenly. This may have occurred unexpectedly or after afew years' effort toward the end desired. Whether this sudden appearanceis the consequence of a single internal differentiating step, or of therapid succession of lesser changes, cannot yet be made out. The extremevariability of double flowers and the chance of their appearance withonly slight indications of the previous petaloid alterations of a fewstamens may often result in their origin being overlooked, whilesubsequent generations may come in for full notice. [506] In the greaternumber of cases recorded it remains doubtful whether the work said to bedone to obtain a new double variety was done before the appearance ofthese preliminary indications or afterward. 

In the first case, it would correspond with our selection of largenumbers of florets in the outer rays, in the second however, with theordinary purification of new races from hybrid mixtures. 

In scientific selection-experiments such crosses are of course avoided, and the process of purification is unnecessary, even as in the_Chrysanthemum_ culture. The first generation succeeding the originalplant with disk-rays was in this respect wholly uniform and true to thenew type. 

In practice the work does not start from such slight indications, and isdone with no other purpose in view than to produce double flowers inspecies in which they did not already exist. Therefore it is of thehighest importance to know the methods used and the chances of success. Unfortunately the evidence is very scanty on both points. 

Lindley and other writers, on horticultural theory and practice assertthat a large amount of nourishment tends to produce double flowers, while a culture under normal conditions, [507] even if the plants arevery strong and healthy, has no such effect. But even here it remainsdoubtful whether it applies to the period before or after the internalmutation. On the other hand success is not at all to be relied upon, noris the work to be regarded as easy. The instances of double flowers saidto be obtainable at will, are too rare in comparison with the number ofcases, where the first indication of them was found accidentally. 

Leaving all these doubtful points, which will have to be cleared up byfurther scientific investigation, the high degree of variabilityrequires further discussion. It may be considered from three differentpoints of view according to the limit of the deviation from the average, to the dependency on external conditions and to periodicity. It seemsbest to take up the last two points first. 

On a visit to a nursery at Erfurt I once inspected an experiment with anew double variety of the common blue-bottle or blue corn-flower. Theplants were dependent on the weather to a high degree. Bad weatherincreased the number of poorly filled flower-heads, while warm and sunnydays were productive of beautiful double flowers. The heads that areborne by strong branches have a greater tendency to become double thanthose of the weaker ones, [508] and towards the autumn, when all thoseof the first group are faded away, and only a weak though large sectionof the heads is still flowering, the whole aspect of the varietygradually retrogrades. The same law of dependency and periodicity isprevalent everywhere. In my own cultures of the improved field-marigoldI have observed it frequently. The number of the ray-florets may beconsidered as a direct response to nourishment, both when this isdetermined by external circumstances, and when it depends on theparticular strength of the branch, which bears the head in question. Itis a case exactly similar to that of the supernumerary carpels of thepistilloid poppy, and the deductions arrived at with that variety may beapplied directly to double flowers. 

This dependency upon nourishment is of high practical importance incombination with the usual effect of the doubling which makes theflowers sterile. It is a general rule that the most perfect flowers donot produce seed. At the height of the flowering period the externalcircumstances are the most favorable, and the flowering branches stillconstitute the stronger axes of the plants. Hence we may infer thatsterility will prevail precisely in this period. Many varieties areknown to yield only seeds from the very last flowers, as for instancesome [509] double begonias. Others bear only seed on their weakerlateral branches, as the double camomile, or become fertile only towardsthe fall, as is often the case with the above quoted Erfurt variety ofthe blue-bottle. As far as I have been able to ascertain, such seeds arequite adequate for the reproduction and perpetuation of the doublevarieties, but the question whether there are differences between theseeds of the more or less double flowers of the same plants stillremains open. It is very probable, from a theoretical point of view, that such differences exist, but perhaps they are so slight, as to havepractically no bearing on the question. 

On the ground of their wide range of variability, the double varietiesmust be regarded as pertaining to the group of ever-sporting forms. Onone side they fluctuate in the direction towards such petalomanousflowers as are borne by the stocks and others, which we have previouslydiscussed. Here no trace of the fertile organs is left. But this extremeis never reached by petaloid double flowers. A gap remains which, oftenoverlooked, always exists, and which sharply separates the two types. Onthe other hand the alteration of the stamens gradually relapses toperfectly single flowers. Here the analogy with the pistillody of thepoppies and with the "five-leaved" clover is obvious. 

[510] This conception of the inner nature of double flowers explains thefact that the varietal mark is seldom seen to be complete throughoutlarger groups of individuals, providing these have not been alreadyselected by this character. _Tagetes africana_ is liable to produce somepoorly filled specimens, and some double varieties of carnations areoffered for sale with the note that the seed yields only 80% of doubles. With _Chrysanthemum coronarium_ and blue-bottles this figure is oftenannounced to be only about 50%. No doubt it is partly due to impurities, caused by vicinism, but it is obviously improbable that the effect ofthese impurities should be so large. 

Some cases of partial reversion may be interpreted in the same way. Among the garden anemones, _Anemone coronaria_, there is a varietycalled the "Bride, " on account of its pure white dowers. It is for salewith single and with double flowers, and these two forms are known tosport into one another, although they are multiplied in the vegetativeway. Such cases are known to be of quite ordinary occurrence. Of coursesuch sports must be considered as partial, and the same stem may bearboth types of flowers. It even happens that some particular flower ispartly double and partly single. Mr. Krelage, of Haarlem, had thekindness to [511] send me such a curious flower. One half of it wascompletely double, while the other half was entirely single, bearingnormal and fertile stamens in the ordinary number. 

The same halfway doubling is recorded to occur among compositessometimes, and from the same source I possess in my collection a head of_Pyrethrum roseum_, bearing on half of its disk elongated corolla tubes, and on the other half the small disk-florets of the typical species. 

It is a current belief, that varieties are improved by continuedculture. I have never been able to ascertain the grounds on which thisconviction rests. It may be referred either to the purity of the race orto the complete development of the varietal character. In the first caseit is a question of hybrid mixtures from which many young varieties mustbe freed before being placed on the market. But as we have already seenin a former lecture, this requires only three or four years, andafterwards the degree of purity is kept up to the point which proves tobe the most suitable for practical purposes. The complete development ofthe varietal character is a question restricted to ever-sportingvarieties, since in white flowers and other constant varieties thisdegree is variable in a very small and unimportant measure. [512] Hencethe double flowers seem to afford a very good example for thisdiscussion. 

It can be decided by two facts. First by a consideration of the oldestdouble varieties, and secondly by that of the very youngest. Are theolder ones now in a better condition than at the outset? Have theyreally been gradually improved during the centuries of their existence?Obviously this can only be answered by a comparison of the figures givenby older writers, with the varieties as they are now in culture. Munting's drawings and descriptions are now nearly two centuries and ahalf old, but I do not find any real difference between his doublevarieties and their present representatives. So it is in other cases inwhich improvements by crossing or the introduction of new forms does notvitiate the evidence. Double varieties, as a rule, are exactly the samenow, as they were at the time of their first introduction. 

If this were otherwise one would expect that young double varietiesshould in the main display only slight grades of the anomaly, and thatthey would require centuries to reach their full development. Nothing ofthe kind is on record. On the contrary the newest double sorts are saidto be not only equal to their predecessors, but to excel them. As a rulesuch claims may be exaggerated, but not to any great extent. [513] Thisis proven in the simplest way by the result of our own experiment. 

In the double field-marigold we have the very first generation of avariety of pure and not hybrid origin. It shows the new attribute in itsfull development. It has flower-heads nearly as completely filled as thebest double varieties of allied cultivated composites. In the secondgeneration it reached heads with 200 rays each, and much larger numberswill seldom be seen in older species on heads of equal size. I havecompared my novelty with the choicest double camomiles and others, butfailed to discover any real difference. Improvement of the varietydeveloped in the experiments carried on by myself seems to be excludedby the fact that it comes into conflict with the same difficulty thatconfronts the older cultivated species, viz. : the increasing sterilityof the race. 

It is perfectly evident that this double marigold is now quite constant. Continuously varying about a fixed average it may live throughcenturies, but the mean and the limits will always remain the same, asin the case of the ever-sporting varieties. 

Throughout this lecture I have spoken of double flowers and doubleflower-heads of composites as of one single group. They are as nearlyrelated from the hereditary point of [514] view, as they are divergentin other respects. It would be superfluous to dwell any longer upon thedifference between heads and flowers. But it is as well to point out, that the term double flowers indicates a motley assemblage of differentphenomena. The hen-and-chicken daisy, and the corresponding variety ofthe garden cineraria (_Cineraria cruenta_), are extremes on one side. The hen-and-chicken type occurs even in other families and is known toproduce most curious anomalies, as with _Scabiosa_, the supernumeraryheads of which may be produced on long stalks and become branchedthemselves in the same manner. 

Petalody of the stamens is well known to be the ordinary type ofdoubling. But it is often accompanied by a multiplication of the organs, both of the altered stamens and of the petals themselves. Thisproliferation may consist in median or in lateral cleavages, and in bothcases the process may be repeated one or more times. It would be quitesuperfluous to give more details, which may be gathered from anymorphologic treatise on double flowers. But from the physiologic pointof view all these cases are to be considered as one large group, complying with previously given definitions of the ever-sportingvarieties. They are very variable and wholly permanent. Obviously this[515] permanency agrees perfectly with the conception of their suddenorigin. 

[516]

LECTURE XVIII

NEW SPECIES OF _OENOTHERA_

In our experiments on the origin of peloric varieties and double flowerswe were guided in the choice of our material by a survey of the evidencealready at hand. We chose the types known to be most commonly producedanew, either in the wild state or under the conditions of cultivation. In both instances our novelty was a variety in the ordinary sense of theword. Our pedigree-culture was mainly an experimental demonstration ofthe validity of conclusions, which had previously been deduced from suchobservations as can be made after the accidental birth of new forms. 

From these facts, and even from these pedigree-experiments, it isscarcely allowable to draw conclusions as to the origin of real species. If we want to know how species originate, it is obviously necessary tohave recourse to direct observation. The question is of the highestimportance, both for the theory of descent, and for our conception ofthe real nature of [517] systematic affinities at large. Many authorshave tried to solve it on the ground of comparative studies and ofspeculations upon the biologic relations of plants and animals. But invain. Contradiction and doubt still reign supreme. All our hopes nowrest on the result of experiments. 

Unfortunately such experiments seemed simply impossible a few years ago. What is to guide us in the choice of the material? The answer may onlybe expected from a consideration of elementary species. For it isobvious that they only can be observed to originate, and that thesystematic species, because they are only artificial groups of lowerunities, can never become the subject of successful experimentalinquiry. 

In previous lectures we tried to clear up the differences existingbetween nearly related elementary species. We have seen that they affectall of the attributes of the plants, each of them changing in somemeasure all of the organs. Nevertheless they were due to distinctunities and of the lowest possible degree. Such unit-steps may thereforebe expected to become visible some time or other by artificial means. Onthe other hand, mutations as a rule make their appearance in groups, andthere are many systematic species which on close inspection [518] havebeen shown to be in reality composite assemblages. Roses and brambles, hawkweeds and willows are the best known examples. Violets and _Drabaverna_, dandelions and helianthemums and many other instances were dealtwith in previous lectures. Even wheat and barley and corn affordinstances of large groups of elementary species. Formerly mixed in thefields, they became separated during the last century, and nowconstitute constant races, which, for brevity's sake, are dealt withunder the name of varieties. 

In such groups of nearly allied forms the single members must evidentlybe of common origin. It is not necessary for them to have originated allin the same place or at the same time. In some cases, as with _Drabaverna_, the present geographic distribution points to a commonbirthplace, from whence the various forms may about the same period haveradiated in all directions. The violets on the other hand seem toinclude widely diffused original forms, from which branches have startedat different times and in different localities. 

The origin of such groups of allied forms must therefore be the objectof our research. Perhaps we might find a whole group, perhaps only partof it. In my opinion we have the right to assume that if _Draba_ andviolets and [519] others have formerly mutated in this way, otherspecies must at present be in the same changeable condition. And ifmutations in groups, or such periodic mutations should be the rule, itis to be premised that these periods recur from time to time, and thatmany species must even now be in mutating condition, while others arenot. 

It is readily granted that the constant condition of species is thenormal one, and that mutating periods must be the exception. This factdoes not tend to increase our prospect of discovering a species in astate of mutability. Many species will have to be tested before findingan instance. On the other hand, a direct trial seems to be the only wayto reach the goal. No such special guides as those that led us to thechoice of pelories and double flowers are available. The only indicationof value is the presumption that a condition of mutability might becombined with a general state of variability at large, and that groupsof plants of very uniform features might be supposed to be constant inthis respect too. On the contrary, anomalies and deviations if existentin the members of one strain, or found together in one native localityof a species, might be considered as an indication in the desireddirection. 

Few plants vary in the wild state in such a [520] measure as to givedistinct indications. All have to be given a trial in the garden underconditions as similar as possible to their natural environments. Cultivated plants are of course to be excluded. Practically they havealready undergone the experience in question and can not be expected tochange their habits soon enough. Moreover they are often of hybridorigin. The best way is to experiment with the native plants of one'sown country. 

I have made such experiments with some hundred species that grow wild inHolland. Some were very variable, as for instance, the jointed charlock(_Raphanus Raphanistrum_) and the narrow-leaved plantain (_Plantagolanceolata_). Others seemed more uniform, but many species, collectedwithout showing any malformation, subsequently produced them in mygarden, either on the introduced plants themselves or among theiroffspring. From this initial material I have procured a long series ofhereditary races, each with some peculiar anomaly for its specialcharacter. But this result was only a secondary gain, a meagerconsolation for the negative fact that no real mutability could bediscovered. 

My plants were mostly annuals or biennials, or such perennials as underadequate treatment might produce flowers and seeds during their [521]first summer. It would be of no special use to enumerate them. Thenegative result does not apply to the species as such, but only to theindividual strain, which I collected and cultivated. Many species, whichare quite constant with us, may be expected to be mutable in other partsof their range. 

Only one of all my tests met my expectations. This species proved to bein a state of mutation, producing new elementary forms continually, andit soon became the chief member of my experimental garden. It was one ofthe evening primroses. 

Several evening-primroses have at different times been introduced intoEuropean gardens from America. From thence they have spread into thevicinity, becoming common and exhibiting the behavior of indigenoustypes. _Oenothera biennis_ was introduced about 1614 from Virginia, ornearly three centuries ago. _O. Muricata_, with small corollas andnarrow leaves, was introduced in the year 1789 by John Hunneman, and _O. Suaveolens_, or sweet-scented primrose, a form very similar to the_biennis_, about the same time, in 1778, by John Fothergill. This formis met with in different parts of France, while the _biennis_ and_muricata_ are very common in the sandy regions of Holland, where I haveobserved them for [522] more than 40 years. They are very constant andhave proven so in my experiments. Besides these three species, thelarge-flowered evening-primrose, or _Oenothera lamarckiana_, is found insome localities in Holland and elsewhere. We know little concerning itsorigin. It is supposed to have come from America in the same way as itscongeners, but as yet I have not been able to ascertain on what groundsthis supposition rests. As far as I know, it has not been seen growingwild in this country, though it may have been overlooked. The fact thatthe species of this group are subject to many systematic controversiesand are combined by different writers into systematic species indifferent ways, being often considered as varieties of one or two types, easily accounts for it having been overlooked. However, it would be ofgreat interest to ascertain whether _O. Lamarckiana_ yet grows inAmerica, and whether it is in the same state of mutability here as it isin Holland. 

The large-flowered evening-primrose was also cultivated about thebeginning of the last century in the gardens of the Museum d'HistoireNaturelle, at Paris, where it was noticed by Lamarck, who at oncedistinguished it as an undescribed species. He wrote a completedescription [523] of it and his type specimens are still preserved inthe herbarium of the Museum, where I have compared them with the plantsof my own culture. Shortly afterwards it was renamed by Seringe, inhonor of its eminent discoverer, whose name it now bears. So Lamarckunconsciously discovered and described himself the plant, which after acentury, was to become the means of an empirical demonstration of hisfar-reaching views on the common origin of all living beings. 

_Oenothera lamarckiana_ is considered in Europe as a garden-plant, muchprized for parks and ornamental planting. It is cultivated byseed-merchants and offered for sale. It has escaped from gardens, andhaving abundant means for rapid multiplication, has become wild in manyplaces. As far as I know its known localities are small, and it is to bepresumed that in each of them the plant has escaped separately fromculture. It was in this state that I first met with this beautifulspecies. 

Lamarck's evening-primrose is a stately plant, with a stout stem, attaining often a height of 1. 6 meters and more. When not crowded themain stem is surrounded by a large circle of smaller branches, growingupwards from its base so as often to form a dense bush. These branchesin their turn have numerous lateral [524] branches. Most of them arecrowned with flowers in summer, which regularly succeed each other, leaving behind them long spikes of young fruits. The flowers are largeand of a bright yellow color, attracting immediate attention, even froma distance. They open towards evening, as the name indicates, and arepollinated by humble-bees and moths. On bright days their duration isconfined to one evening, but during cloudy weather they may still befound open on the following morning. Contrary to their congeners theyare dependent on visiting insects for pollination. _O. Biennis_ and _O. Muricata_ have their stigmas in immediate contact with the antherswithin the flower-buds, and as the anthers open in the morning precedingthe evening of the display of the petals, fecundation is usuallyaccomplished before the insects are let in. But in _O. Lamarckiana_ nosuch self-fertilization takes place. The stigmas are above the anthersin the bud, and as the style increases in length at the time of theopening of the corolla, they are elevated above the anthers and do notreceive the pollen. Ordinarily the flowers remained sterile if notvisited by insects or pollinated by myself, although rare instances ofself-fertilization were seen. 

In falling off, the flowers leave behind them a stout ovary with fourcells and a large number [525] of young seeds. The capsule when ripe, opens at its summit with four valves, and contains often from two tothree hundred seeds. A hundred capsules on the main stem is an averageestimate, and the lateral branches may ripen even still more fruits, bywhich a very rapid dissemination is ensured. 

This striking species was found in a locality near Hilvers, in thevicinity of Amsterdam, where it grew in some thousands of individuals. Ordinarily biennial, it produces rosettes in the first, and stems in thesecond year. Both the stems and the rosettes were at once seen to behighly variable, and soon distinct varieties could be distinguishedamong them. 

The first discovery of this locality was made in 1886. Afterwards Ivisited it many times, often weekly or even daily during the first fewyears, and always at least once a year up to the present time. Thisstately plant showed the long-sought peculiarity of producing a numberof new species every year. Some of them were observed directly on thefield, either as stems or as rosettes. The latter could be transplantedinto my garden for further observation, and the stems yielded seeds tobe sown under like control. Others were too weak to live a sufficientlylong time in the field. They were discovered by sowing seed fromindifferent plants [526] of the wild locality in the garden. A third andlast method of getting still more new species from the original strain, was the repetition of the sowing process, by saving and sowing the seedwhich ripened on the introduced plants. These various methods have ledto the discovery of over a dozen new types, never previously observed ordescribed. 

Leaving the physiologic side of the relations of these new forms for thenext lecture, it would be profitable to give a short description of theseveral novelties. To this end they may be combined under five differentheads, according to their systematic value. The first head includesthose which are evidently to be considered as varieties, in the narrowersense of the word, as previously given. The second and third headsindicate the real progressive elementary species, first those which areas strong as the parent-species, and secondly a group of weaker types, apparently not destined to be successful. Under the fourth head I shallinclude some inconstant forms, and under the last head those that areorganically incomplete. 

Of varieties with a negative attribute, or real retrograde varieties, Ihave found three, all of them in a flowering condition in the field. Ihave given them the names of _laevifolia_, _brevistylis_ and _nanella_. 

[527] The _laevifolia_, or smooth-leaved variety, was one of the veryfirst deviating types found in the original field. This was in thesummer of 1887, seventeen years ago. It formed a little group of plantsgrowing at some distance from the main body, in the same field. I foundsome rosettes and some flowering stems and sowed some seed in the fall. The variety has been quite constant in the field, neither increasing innumber of individual plants nor changing its place, though now closelysurrounded by other _Lamarckiana_s. In my garden it has proved to beconstant from seed, never reverting to the original _lamarckiana_, provided intercrossing was excluded. 

It is chiefly distinguished from Lamarck's evening-primrose by itssmooth leaves, as the name indicates. The leaves of the original formshow numerous sinuosities in their blades, not at the edge, but anywherebetween the veins. The blade shows numbers of convexities on eithersurface, the whole surface being undulated in this manner; it lacks alsothe brightness of the ordinary evening-primrose or _Oenothera biennis_. 

These undulations are lacking or at least very rare on the leaves of thenew _laevifolia_. Ordinarily they are wholly wanting, but at timessingle leaves with slight manifestations of this [528] character maymake their appearance. They warn us that the capacity for suchsinuosities is not wholly lost, but only lies dormant in the newvariety. It is reduced to a latent state, exactly as are the apparentlylost characters of so many ordinary horticultural varieties. 

Lacking the undulations, the _laevifolia_ leaves are smooth and bright. They are a little narrower and more slender than those of the_lamarckiana_. The convexities and concavities of leaves are said to beuseful in dry seasons, but during wet summers, such as those of the lastfew years, they must be considered as very harmful, as they retain someof the water which falls on the plants, prolonging the action of thewater on the leaves. This is considered by some writers to be of someutility after slight showers, but was observed to be a source ofweakness during wet weather in my garden, preventing the leaves fromdrying. Whether the _laevifolia_ would do better under suchcircumstances, remains to be tested. 

The flowers of the _laevifolia_ are also in a slight degree differentfrom those of _lamarckiana_. The yellow color is paler and the petalsare smoother. Later, in the fall, on the weaker side branches thesedifferences increase. The _laevifolia_ petals become smaller and areoften not emarginated at the apex, becoming ovate [529] instead ofobcordate. This shape is often the most easily recognized and moststriking mark of the variety. In respect to the reproductive organs, thefertility and abundance of good seed, the _laevifolia_ is by no meansinferior or superior to the original species. 

_O. Brevistylis_, or the short-styled evening primrose, is the mostcurious of all my new forms. It has very short styles, which bring thestigmas only up to the throat of the calyx tube, instead of upwards ofthe anthers. The stigmas themselves are of a different shape, moreflattened and not cylindrical. The pollen falls from the anthersabundantly on them, and germinates in the ordinary manner. 

The ovary which in _lamarckiana_ and in all other new forms is whollyunderneath the calyx-tube, is here only partially so. This tube isinserted at some distance under its summit. The insertion divides theovary into two parts: an upper and a lower one. The upper part is muchreduced in breadth and somewhat attenuated, simulating a prolongation ofthe base of the style. The lower part is also reduced, but in anothermanner. At the time of flowering it is like the ovary of _lamarckiana_, neither smaller nor larger. But it is reached by only a very fewpollen-tubes, and is therefore always incompletely fertilized. It does[530] not fall off after the fading away of the flower, as unfertilizedovaries usually do; neither does it grow out, nor assume the uprightposition of normal capsules. It is checked in its development, and atthe time of ripening it is nearly of the same length as in thebeginning. Many of them contain no good seeds at all; from others I havesucceeded in saving only a hundred seeds from thousands of capsules. 

These seeds, if purely pollinated, and with the exclusion of the visitsof insects, reproduce the variety, entirely and without any reversion tothe _lamarckiana_ type. 

Correlated with the detailed structures is the form of the flower-buds. They lack the high stigma placed above the anthers, which in the_lamarckiana_, by the vigorous growth of the style, extends the calyxand renders the flower bud thinner and more slender. Those of the_brevistylis_ are therefore broader and more swollen. It is quite easyto distinguish the individuals by this striking character alone, although it differs from the parent in other particulars. 

The leaves of the _O. Brevistylis_ are more rounded at the tip, but thedifference is only pronounced at times, slightly in the adult rosettes, but more clearly on the growing summits of the stems and branches. Bythis character, the plants [531] may be discerned among the others, someweeks before the flowers begin to show themselves. But the character bywhich the plants may be most easily recognized from a distance in thefield is the failure of the fruits. They were found there nearly everyyear in varying, but always small numbers. 

Leaving the short-styled primrose, we come now to the last of our groupof retrograde varieties. This is the _O. Nanella_, or the dwarf, and isa most attractive little plant. It is very short of stature, reachingoften a height of only 20-30 cm. , or less than one-fourth of that of theparent. It commences flowering at a height of 10-15 cm. , while theparent-form often measures nearly a meter at this stage of itsdevelopment. Being so very dwarfed the large flowers are all the morestriking. They are hardly inferior to those of the _lamarckiana_, andagree with them in structure. When they fade away the spike is rapidlylengthened, and often becomes much longer than the lower or vegetativepart of the stem. 

The dwarfs are one of the most common mutations in my garden, and wereobserved in the native locality and also grown from seeds saved there. Once produced they are absolutely constant. I have tried many thousandsof seeds from various dwarf mutants, and never observed [532] any traceof reversion to the _lamarckiana_ type. I have also cultivated them insuccessive generations with the same result. In a former lecture we haveseen that contrary to the general run of horticultural belief, varietiesare as constant as the best species, if kept free from hybridadmixtures. This is a general rule, and the exceptions, or cases ofatavism are extremely rare. In this respect it is of great interest toobserve that this constancy is not an acquired quality, but is to beconsidered as innate, because it is already fully developed at the verymoment when the original mutation takes place. 

From its first leaves to the rosette period, and through this to thelengthening of the stem, the dwarfs are easily distinguished from anyother of their congeners. The most remarkable feature is the shape ofthe leaves. They are broader and shorter, and especially at the basethey are broadened in such a way as to become apparently sessile. Thestalk is very brittle, and any rough treatment may cause the leaves tobreak off. The young seedlings are recognizable by the shape of thefirst two or three leaves, and when more of them are produced, therosettes become dense and strikingly different from others. Later leavesare more nearly like the parent-type, but the petioles remain short. Thebases of the blades are frequently [533] almost cordate, the laminaethemselves varying from oblong-ovate to ovate in outline. The stems areoften quite unbranched, or branched only at the base of the spike. Strong secondary stems are a striking attribute of the _lamarckiana_parent, but they are lacking, or almost so in the dwarfs. The stem isstraight and short, and this, combined with the large crown of brightflowers, makes the dwarfs eminently suitable for bed or border plants. Unfortunately they are very sensitive, especially to wet weather. 

_Oenothera gigas_ and _O. Rubrinervis_, or the giant, and the red-veinedevening-primroses, are the names given to two robust and stout species, which seem to be equal in vigor to the parent-plant, while divergingfrom it in striking characters. Both are true elementary species, differentiated from _lamarckiana_ in nearly all their organs andqualities, but not showing any preponderating character of a retrogradenature. Their differences may be compared with those of the elementaryspecies of other genera, as for instance, of _Draba_, or of violets, aswill be seen by their description. 

The giant evening-primrose, though not taller in stature than _O. Lamarckiana_, deserves its name because it is so much stouter in allrespects. [534] The stems are robust, often with twice the diameter of_lamarckiana_ throughout. The internodes are shorter, and the leavesmore numerous, covering the stems with a denser foliage. This shortnessof the internodes extends itself to the spike, and for this reason theflowers and fruits grow closer together than on the parent-plant. Hencethe crown of bright flowers, opening each evening, is more dense andmore strikingly brilliant, so much the more so as the individual flowersare markedly larger than those of the parents. In connection with thesecharacters, the flower-buds are seen to be much stouter than those of_lamarckiana_. The fruits attain only half the normal size, but arebroader and contain fewer, but larger seeds. 

The _rubrinervis_ is in many respects a counterpart to the _gigasv, butits stature is more slender. The spikes and flowers are those of the_lamarckianav, but the bracts are narrower. Red veins and red streaks onthe fruits afford a striking differentiating mark, though they are notabsolutely lacking in the parent-species. A red hue may be seen on thecalyx, and even the yellow color of the petals is somewhat deepened inthe same way. Young plants are often marked by the pale red tinge of themid-veins, but in adult rosettes, or from lack of sunshine, this hue isoften very faint. 

[535] The leaves are narrow, and a curious feature of this species isthe great brittleness of the leaves and stems, especially in annualindividuals, especially in those that make their stem and flowers in thefirst year. High turgidity and weak development of the mechanical andsupporting tissues are the anatomical cause of this deficiency, thebast-fibers showing thinner walls than those of the parent-type underthe microscope. Young stems of _rubrinervis_ may be broken off by asharp stroke, and show a smooth rupture across all the tissues, whilethose of _lamarckiana_ are very tough and strong. 

Both the giant and the red-veined species are easily recognized in therosette-stage. Even the very young seedlings of the latter are clearlydifferentiated from the _lamarckiana_, but often a dozen leaves arerequired, before the difference may be seen. Under such circumstancesthe young plants must reach an age of about two months before it ispossible to discern their characters, or at least before thesecharacters have become reliable enough to enable us to judge of eachindividual without doubt. But the divergencies rapidly become greater. The leaves of _O. Gigas_ are broader, of a deeper green, the blade moresharply set off against the stalk, all the rosettes [536] becoming stoutand crowded with leaves. Those of _O. Rubrinervis_ on the contrary arethin, of a paler green and with a silvery white surface; the blades areelliptic, often being only 2 cm. Or less in width. They are acute at theapex and gradually narrowed into the petiole. 

It is quite evident that such pale narrow leaves must produce smallerquantities of organic food than the darker green and broad organs of the_gigas_. Perhaps this fact is accountable partly, at least, for the morerobust growth of the giant in the second year. Perhaps also somerelation exists between this difference in chemical activity and thetendency to become annual or biennial. The _gigas_, as a rule, producesfar more, and the _rubrinervis_ far less biennial plants than the_lamarckiana_. Annual culture for the one is as unreliable as biennialculture for the other. _Rubrinervis_ may be annual in apparently allspecimens, in sunny seasons, but _gigas_ will ordinarily remain in thestate of rosettes during the entire first summer. It would be veryinteresting to obtain a fuller insight into the relation of the lengthof life to other qualities, but as yet the facts can only be detailed asthey stand. 

Both of these stout species have been found [537] quite constant fromthe very first moment of their appearance. I have cultivated them fromseed in large numbers, and they have never reverted to the_lamarckiana_. From this they have inherited the mutability or thecapacity of producing at their turn new mutants. But they seem to havedone so incompletely, changing in the direction of more absoluteconstancy. This was especially observed in the case of _rubrinervis_, which is not of such rare occurrence as _O. Gigas_, and which it hasbeen possible to study in large numbers of individuals. So for instance, the "red-veins" have never produced any dwarfs, notwithstanding they areproduced very often by the parent-type. And in crossing experiments alsothe red-veins gave proof of the absence of a mutative capacity for theirproduction. 

Leaving the robust novelties, we may now take up a couple of forms, which are equally constants and differentiated from the parent speciesin exactly the same manner, though by other characters, but which are soobviously weak as to have no manifest chance of self maintenance in thewild state. These are the whitish and the oblong-leavedevening-primroses or the _Oenothera albida_ and _oblonga_. 

_Oenothera albida_ is a very weak species, with whitish, narrow leaves, which are evidently incapable [538] of producing sufficient quantitiesof organic food. The young seedling-plants are soon seen to lag behind, and if no care is taken of them they are overgrown by their neighbors. It is necessary to take them out, to transplant them into pots withrichly manured soil, and to give them all the care that should be givento weak and sickly plants. If this is done fully grown rosettes may beproduced, which are strong enough to keep through the winter. In thiscase the individual leaves become stronger and broader, with oblongblades and long stalks, but retain their characteristic whitish color. 

In the second year the stems become relatively stout. Not that theybecome equal to those of _lamarckiana_, but they become taller thanmight have been expected from the weakness of the plants in the previousstages. The flowers and racemes are nearly as large as those of theparent-form, the fruits only a little thinner and containing a smallerquantity of seed. From these seeds I have grown a second and a thirdgeneration, and observed that the plants remain true to their type. 

_O. Oblonga_ may be grown either as an annual, or as a biennial. In thefirst case it is very slender and weak, bearing only small fruits andfew seeds. In the alternative case however, it [539] becomes denselybranched, bearing flowers on quite a number of racemes and yielding afull harvest of seeds. But it always remains a small plant, reachingabout half the height of that of _lamarckiana_. 

When very young it has broader leaves, but in the adult rosettes theleaves become very narrow, but fleshy and of a bright green color. Theyare so crowded as to leave no space between them unoccupied. Theflowering spikes of the second year bear long leaf-like bracts under thefirst few flowers, but those arising later are much shorter. Numerouslittle capsules cover the axis of the spike after the fading away of thepetals, constituting a very striking differentiating mark. This speciesalso was found to be quite constant, if grown from pure seed. 

We have now given the descriptions of seven new forms, which diverge indifferent ways from the parent-type. All were absolutely constant fromseed. Hundreds or thousands of seedlings may have arisen, but theyalways come true and never revert to the original _O. Lamarckiana_ type. From this they have inherited the condition of mutability, eithercompletely or partly, and according to this they may be able to producenew forms themselves. But this occurs only rarely, and combinations ofmore than one [540] type in one single plant seem to be limited to theadmixture of the dwarf stature with the characters of the other newspecies. 

These seven novelties do not comprise the whole range of the newproductions of my _O. Lamarckiana_. But they are the most interestingones. Others, as the _O. Semilata_ and the _O. Leptocarpa_ are quite asconstant and quite as distinct, but have no special claims for a closerdescription. Others again were sterile, or too weak to reach the adultstage and to yield seeds, and no reliable description or appreciationcan be given on the ground of the appearance of a single individual. 

Contrasted with these groups of constant forms are three inconstanttypes which we now take up. They belong to two different groups, according to the cause of their inconstancy. In one species which I call_O. Lata_, the question of stability or instability must remain whollyunsolved, as only pistillate flowers are produced, and no seed can befertilized save by the use of the pollen of another form, and thereforeby hybridization. The other head comprises two fertile forms, _O. Scintillans_ and _O. Elliptica_, which may easily be fertilized withtheir own pollen, but which gave a progeny only partly similar to theparents. 

The _Oenothera lata_ is a very distinct form [541] which was found morethan once in the field, and recently (1902) in a luxuriant floweringspecimen. It has likewise been raised from seeds collected in differentyears at the original station. It is also wholly pistillate. Apparentlythe anthers are robust, but they are dry, wrinkled and nearly devoid ofcontents. The inner wall of cells around the groups of pollen grow outinstead of being resorbed, partly filling the cavity which is left freeby the miscarriage of the pollen-grains. This miscarriage does notaffect all the grains in the same degree, and under the microscope a fewof them with an apparently normal structure may be seen. But thecontents are not normally developed, and I have tried in vain to obtainfertilization with a large number of flowers. Only bycross-fertilization does _O. Lata_ produce seeds, and then as freely asthe other species when self-fertilized. Of course its chance of everfounding a wild type is precluded by this defect. 

_O. Lata_ is a low plant, with a limp stem, bent tips and branches, allvery brittle, but with dense foliage and luxuriant growth. It has brightyellow flowers and thick flower-buds. But for an unknown reason thepetals are apt to unfold only partially and to remain wrinkledthroughout the flowering time. The stigmas are slightly divergent fromthe normal type, [542] also being partly united with one another, andlaterally with the summit of the style, but without detriment to theirfunction. 

Young seedlings of _lata_ may be recognized by the very first leaves. They have a nearly orbicular shape and are very sharply set off againsttheir stalk. The surface is very uneven, with convexities andconcavities on both sides. This difference is lessened in the laterleaves, but remains visible throughout the whole life of the plant, evenduring the flowering season. Broad, sinuate leaves with rounded tips area sure mark of _O. Lata_. On the summits of the stems and branches theyare crowded so as to form rosettes. 

Concerning inheritance of these characteristics nothing can be directlyasserted because of the lack of pollen. The new type can only beperpetuated by crosses, either with the parent form or some othermutant. I have fertilized it, as a rule, with _lamarckiana_ pollen, buthave often also used that from _nanella_ and others. In doing so, the_lata_ repeats its character in part of its offspring. This part seemsto be independent of the nature of the pollen used, but is very variableaccording to external circumstances. On the average one-fourth of theoffspring become _lata_, the others assuming the type of thepollen-parent, if this was a _lamarckiana_ or [543] partly this type andpartly that of any other of the new species derived from _lamarckiana_, that might have been used as the pollen-parent. This average seems to bea general rule, recurring in all experiments, and remaining unchangedthrough a long series of successive generations. The fluctuations aroundthis mean go up to nearly 50% and down nearly to 1%, but, as in othercases, such extreme deviations from the average are met with onlyexceptionally. 

The second category includes the inconstant but perfectly fertilespecies. I have already given the names of the only two forms, whichdeserve to be mentioned here. 

One of them is called _scintillans_ or the shiny evening-primrose, because its leaves are of a deep green color with smooth surfaces, glistening in the sunshine. On the young rosettes these leaves aresomewhat broader, and afterwards somewhat narrower than those of _O. Lamarckiana_ at the corresponding ages. The plants themselves alwaysremain small, never reaching the stature of the ancestral type. They arelikewise much less branched. They can easily be cultivated in annualgenerations, but then do not become as fully developed and as fertile, as when flowering in the second year. The flowers have the samestructure as those of the _lamarckiana_, but are of a smaller size. 

[544] Fertilizing the flowers artificially with their own pollen, excluding the visiting insects by means of paper bags, and saving andsowing the seed of each individual separately, furnishes all therequisites for the estimation of the degree of stability of thisspecies. In the first few weeks the seed-pans do not show anyunequality, and often the young plants must be replanted at widerintervals, before anything can be made out with certainty. But as soonas the rosettes begin to fill it becomes manifest that some of them aremore backward than others in size. Soon the smaller ones show theirdeeper green and broader leaves, and thereby display the attributes ofthe _scintillans_. The other grow faster and stronger and exhibit allthe characteristics of ordinary _lamarckiana_s. 

The numerical proportion of these two groups has been found different ondifferent occasions. Some plants give about one-third _scintillans_ andtwo-thirds _lamarckiana_, while the progeny of individuals of anotherstrain show exactly the reverse proportion. 

Two points deserve to be noticed. First the progeny of the _scintillans_appears to be mutable in a large degree, exceeding even the_lamarckiana_. The same forms that are produced most often by theparent-family are also most ordinarily [545] met with among theoffspring of the shiny evening-primrose. They are _oblonga_, _lata_ and_nanella_. _Oblonga_ was observed at times to constitute as much as 1%or more of the sowings of _scintillans_, while _lata_ and _nanella_ werecommonly seen only in a few scattering individuals, although seldomlacking in experiments of a sufficient size. 

Secondly the instability seems to be a constant quality, although thewords themselves are at first sight, contradictory. I mean to convey theconception that the degree of instability remains unchanged duringsuccessive generations. This is a very curious fact, and stronglyreminds us of the hereditary conditions of striped-flower varieties. But, on the contrary, the atavists, which are here the individuals withthe stature and the characteristics of the _lamarckiana_, have become_lamarckiana_s in their hereditary qualities, too. If their seed issaved and sown, their progeny does not contain any _scintillans_, or atleast no more than might arise by ordinary mutations. 

One other inconstant new species is to be noted, but as it was very rareboth in the field and in my cultures, and as it was difficult ofcultivation, little can as yet be said about it. It is the _Oenotheraelliptica_, with narrow elliptical leaves and also with ellipticalpetals. It repeats [546] its type only in a very small proportion of itsseed. 

All in all we thus have a group of a dozen new types, springing from anoriginal form in one restricted locality, and seen to grow there, orarising in the garden from seeds collected from the original locality. Without any doubt the germs of the new types are fully developed withinthe seed, ready to be evolved at the time of germination. More favorableconditions in the field would no doubt allow all of the described newspecies to unfold their attributes there, and to come into competitionwith each other and with the common parents. But obviously this is onlyof secondary importance, and has no influence on the fact that a numberof new types, analogous to the older swarms of _Draba_, _Viola_ and ofmany other polymorphous species, have been seen to arise directly in thewild state. 

[547]

LECTURE XIX

EXPERIMENTAL PEDIGREE-CULTURES

The observation of the production of mutants in the field at Hilversum, and the subsequent cultivation of the new types in the garden atAmsterdam, gives ample proof of the mutability of plants. Furthermore itfurnishes an analogy with the hypothetical origin of the swarms ofspecies of _Draba_ and _Viola_. Last but not least important it affordsmaterial for a complete systematic and morphologic study of the newlyarisen group of forms. 

The physiologic laws, however, which govern this process are only veryimperfectly revealed by such a study. The instances are too few. Moreover the seeds from which the mutants spring, escape observation. Itis simply impossible to tell from which individual plants they have beenderived. The laevifolia and the brevistylis have been found almost everyyear, the first always recurring on the same spot, the second on variousparts of the original field. It is therefore allowable to assume acommon [548] origin for all the observed individuals of either strain. But whether, besides this, similar strains are produced anew by the old_lamarckiana_ group, it is impossible to decide on the sole ground ofthese field-observations. 

The same holds good with the other novelties. Even if one of them shouldgerminate repeatedly, without ever opening its flowers, the possibilitycould not be excluded that the seeds might have come originally from thesame capsule but lain dormant in the earth during periods of unequallength. 

Other objections might be cited that can only be met by direct and fullycontrolled experiments. Next to the native locality comes theexperimental garden. Here the rule prevails that every plant must befertilized with pollen of its own, or with pollen of other individualsof known and recorded origin. The visits of insects must be guardedagainst, and no seeds should be saved from flowers which have beenallowed to open without this precaution. Then the seeds of eachindividual must be saved and sown separately, so as to admit of anappreciation, and if necessary, a numerical determination of the natureof its progeny. And last but not least the experiments should beconducted in a similar manner during a series of successive years. 

[549] I have made four such experiments, each comprising the handling ofmany thousands of individual plants, and lasting through five to ninegenerations. At the beginning the plants were biennial, as in the nativelocality, but later I learned to cultivate them in annual generations. They have been started from different plants and seeds, introduced fromthe original field into my garden at Amsterdam. 

It seems sufficient to describe here one of these pedigree-cultures, asthe results of all four were similar. In the fall of 1886 I took ninelarge rosettes from the field, planted them together on an isolated spotin the garden, and harvested their seeds the next year. These nineoriginal plants are therefore to be considered as constituting the firstgeneration of my race. The second generation was sown in 1888 andflowered in 1889. It at once yielded the expected result. 15, 000seedlings were tested and examined, and among them 10 showed divergingcharacters. They were properly protected, and proved to belong to twonew types. 5 of them were _lata_ and 5 _nanella_. They flowered nextyear and displayed all the characters as described in our precedinglecture. Intermediates between them and the general type were not found, and no indication of their appearance was noted in their parents. [550]They came into existence at once, fully equipped, without preparation orintermediate steps. No series of generations, no selection, no strugglefor existence was needed. It was a sudden leap into another type, asport in the best acceptation of the word. It fulfilled my hopes, and atonce gave proof of the possibility of the direct observation of theorigin of species, and of the experimental control thereof. 

The third generation was in the main a repetition of the second. I triedsome 10, 000 seedlings and found three _lata_ and three _nanella_, ornearly the same proportion as in the first instance. But besides these a_rubrinervis_ made its appearance and flowered the following year. Thisfact at once revealed the possibility that the instability of_lamarckiana_ might not be restricted to the three new types now underobservation. Hence the question arose how it would be possible to obtainother types or to find them if they were present. It was necessary tohave better methods of cultivation and examination of the young plants. Accordingly I devoted the three succeeding years to working on thisproblem. 

I found that it was not at all necessary to sow any larger quantities ofseed, but that the young plants must have room enough to develop intofull and free rosettes. Moreover I observed [551] that the attributes of_lata_ and _nanella_, which I now studied in the offspring of my firstmutants, were clearly discernible in extreme youth, while those of_rubrinervis_ remained concealed some weeks longer. Hence I concludedthat the young plants should be examined from time to time until theyproved clearly to be only normal _lamarckiana_. Individuals exhibitingany deviation from the type, or even giving only a slight indication ofit, were forthwith taken out of the beds and planted separately, undercircumstances as favorable as possible. They were established in potswith well-manured soil and kept under glass, but fully exposed tosunshine. As a rule they grew very fast, and could be planted out earlyin June. Some of them, of course, proved to have been erroneously takenfor mutants, but many exhibited new characters. 

All in all I had 334 young plants which did not agree with the parentaltype. As I examined some 14, 000 seedlings altogether, the result wasestimated at about 2. 5%. This proportion is much larger than in theyields of the two first generations and illustrates the value ofimproved methods. No doubt many good mutations had been overlooked inthe earlier observations. 

As was to be expected, _lata_ and _nanella_ [552] were repeated in thisthird generation (1895). I was sure to get nearly all of them, withoutany important exceptions, as I now knew how to detect them at almost anyage. In fact, I found many of them; as many as 60 _nanella_ and 73_lata_, or nearly 5% of each. _Rubrinervis_ also recurred, and was seenin 8 specimens. It was much more rare than the two first-named types. 

But the most curious fact in that year was the appearance of _oblonga_. No doubt I had often seen it in former years, but had not attached anyvalue to the very slight differences from the type, as they then seemedto me. I knew now that any divergence was to be esteemed as important, and should be isolated for further observation. This showed that amongthe selected specimens not less than 176, or more than 1% belonged tothe _oblonga_ type. This type was at that time quite new to me, and ithad to be kept through the winter, to obtain stems and flowers. Itproved to be as uniform as its three predecessors, and especially assharply contrasted with _lamarckiana_. The opportunity for the discoveryof any intermediates was as favorable as could be, because thedistinguishing marks were hardly beyond doubt at the time of theselection and removal of the young plants. But no connecting links werefound. 

[553] The same holds good for _albida_, which appeared in 15 specimens, or in 0. 1%, of the whole culture. By careful cultivation these plantsproved not to be sickly, but to belong to a new, though weak type. Itwas evident that I had already seen them in former years, but havingfailed to recognize them had allowed them to be destroyed at an earlyage, not knowing how to protect them against adverse circumstances. Eventhis time I did not succeed in getting them strong enough to keepthrough the winter. 

Besides these, two new types were observed, completing the range of allthat have since been recorded to regularly occur in this family. Theywere _scintillans_ and _gigas_. The first was obtained in the way justdescribed. The other hardly escaped being destroyed, not having showeditself early enough, and being left in the bed after the end of theselection. But as it was necessary to keep some rosettes through thewinter in order to have biennial flowering plants to furnish seeds, Iselected in August about 30 of the most vigorous plants, planted them onanother bed and gave them sufficient room for their stems and branchesin the following summer. Most of them sent up robust shoots, but nodifference was noted till the first flowers opened. One plant had a muchlarger crown of bright blossoms than any of the others. [554] As soon asthese flowers faded away, and the young fruits grew out, it became clearthat a new type was showing itself. On that indication I removed all thealready fertilized flowers and young fruits, and protected the buds fromthe visits of insects. Thus the isolated flowers were fertilized withtheir own pollen only, and I could rely upon the purity of the seedsaved. This lot of seeds was sown in the spring of 1897 and yielded auniform crop of nearly 300 young _gigas_ plants. 

Having found how much depends upon the treatment, I could graduallydecrease the size of my cultures. Evidently the chance of discoveringnew types would be lessened thereby, but the question as to the repeatedproduction of the same new forms could more easily and more clearly beanswered in this way. In the following year (1896) I sowed half as manyseeds as formerly, and the result proved quite the same. With theexception of _gigas_ all the described forms sprang anew from the purelyfertilized ancestry of normal _lamarckiana_s. It was now the fifthgeneration of my pedigree, and thus I was absolutely sure that thedescendants of the mutants of this year had been pure and withoutdeviation for at least four successive generations. 

Owing partly to improved methods of selection, [555] partly no doubt tochance, even more mutants were found this year than in the former. Outof some 8, 000 seedlings I counted 377 deviating ones, or nearly 5%, which is a high proportion. Most of them were _oblonga_ and _lata_, thesame types that had constituted the majority in the former year. 

_Albida_, _nanella_ and _rubrinervis_ appeared in large numbers, andeven _scintillans_, of which I had but a single plant in the previousgeneration, was repeated sixfold. 

New forms did not arise, and the capacity of my strain seemed exhausted. This conclusion was strengthened by the results of the next threegenerations, which were made on a much smaller scale and yielded thesame, or at least the mutants most commonly seen in previous years. 

Instead of giving the figures for these last two years separately, Iwill now summarize my whole experiment in the form of a pedigree. Inthis the normal _lamarckiana_ was the main line, and seeds were onlysown from plants after sufficient isolation either of the plantsthemselves, or in the latter years by means of paper bags enclosing theinflorescences. I have given the number of seedlings of _lamarckiana_which were examined each year in the table below. Of course by far thelargest number of them were [556] thrown away as soon as they showedtheir differentiating characters in order to make room for the remainingones. At last only a few plants were left to blossom in order toperpetuate the race. I have indicated for each generation the number ofmutants of each of the observed forms, placing them in vertical columnsunderneath their respective heads. The three first generations werebiennial, but the five last annual. 

 PEDIGREE OF A MUTATING FAMILY OF _OENOTHERA LAMARCKIANA_ IN THE EXPERIMENTAL GARDEN AT AMSTERDAM

 Gener: O. Gig. Albida obl. Rubrin. Lam. Nanella lata. Scint. VIII. 5 1 0 1700 21 1 VII. 9 0 3000 11 VI. 11 29 3 1800 9 5 1 V. 25 135 20 8000 49 142 6 IV. 1 15 176 8 14000 60 73 1 III. 1 10000 3 3 II. 15000 5 5 I. 9

It is most striking that the various mutations of the evening-primrosedisplay a great degree of regularity. There is no chaos of forms, noindefinite varying in all degrees and in all directions. Quite on thecontrary, it is at once evident that very simple rules govern the wholephenomenon. 

I shall now attempt to deduce these laws from [557] my experiment. Obviously they apply not only to our evening-primroses, but may beexpected to be of general validity. This is at once manifest, if wecompare the group of new mutants with the swarms of elementary formswhich compose some of the youngest systematic species, and which, as wehave seen before, are to be considered as the results of previousmutations. The difference lies in the fact that the evening-primroseshave been seen to spring from their ancestors and that the _drabas_ havenot. Hence the conclusion that in comparing the two we must leave outthe pedigree of the evening-primroses and consider only the group offorms as they finally show themselves. If in doing so we find sufficientsimilarity, we are justified in the conclusion that the _drabas_ andothers have probably originated in the same way as theevening-primroses. Minor points of course will differ, but the mainlines cannot have complied with wholly different laws. All so-calledswarms of elementary species obviously pertain to a single type, andthis type includes our evening-primroses as the only controlled case. 

Formulating the laws of mutability for the evening-primroses wetherefore assume that they hold good for numerous other correspondingcases. 

[558] I. The first law is, that new elementary species appear suddenly, without intermediate steps. 

This is a striking point, and the one that is in the most immediatecontradiction to current scientific belief. The ordinary conceptionassumes very slow changes, in fact so slow that centuries are supposedto be required to make the differences appreciable. If this were true, all chance of ever seeing a new species arise would be hopelessly small. Fortunately the evening-primroses exhibit contrary tendencies. One ofthe great points of pedigree-culture is the fact that the ancestors ofevery mutant have been controlled and recorded. Those of the last yearhave seven generations of known _lamarckiana_ parents preceding them. Ifthere had been any visible preparation towards the coming mutation, itcould not have escaped observation. Moreover, if visible preparationwere the rule, it could hardly go on at the same time and in the sameindividuals in five or six diverging directions, producing from oneparent, _gigas_ and _nanella_, _lata_ and _rubrinervis_, _oblonga_ and_albida_ and even _scintillans_. 

On the other hand the mutants, that constitute the first representativesof their race, exhibit all the attributes of the new type in fulldisplay at once. No series of generations, no selection, [559] nostruggle for existence are needed to reach this end. In previouslectures I have mentioned that I have saved the seeds of the mutantswhenever possible, and have always obtained repetitions of the prototypeonly. Reversions are as absolutely lacking as is also a furtherdevelopment of the new type. Even in the case of the inconstant forms, where part of the progeny yearly return to the stature of _lamarckiana_, intermediates are not found. So it is also with _lata_, which ispistillate and can only be propagated by cross-fertilization. But thoughthe current belief would expect intermediates at least in this case, they do not occur. I made a pedigree-culture of lata during eightsuccessive generations, pollinating them in different ways, and alwaysobtained cultures which were partly constituted of _lata_ and partly of_lamarckiana_ specimens. But the _lata_s remained _lata_ in all thevarious and most noticeable characters, never showing any tendency togradually revert into the original form. 

Intermediate forms, if not occurring in the direct line from one speciesto another, might be expected to appear perhaps on lateral branches. Inthis case the mutants of one type, appearing in the same year, would notbe a pure type, but would exhibit different degrees of deviation fromthe parent. The best would then have to [560] be chosen in order to getthe new type in its pure condition. Nothing of the kind, however, wasobserved. All the _oblonga_-mutants were pure _oblongas_. The pedigreeshows hundreds of them in the succeeding years, but no difference wasseen and no material for selection was afforded. All were as nearlyequal as the individuals of old elementary species. 

II. New forms spring laterally from the main stem. 

The current conception concerning the origin of species assumes thatspecies are slowly converted into others. The conversion is assumed toaffect all the individuals in the same direction and in the same degree. The whole group changes its character, acquiring new attributes. Byinter-crossing they maintain a common line of progress, one individualnever being able to proceed much ahead of the others. 

The birth of the new species necessarily seemed to involve the death ofthe old one. This last conclusion, however, is hard to understand. Itmay be justifiable to assume that all the individuals of one localityare ordinarily intercrossed, and are moreover subjected to the sameexternal conditions. They might be supposed to vary in the samedirection if these conditions were changed slowly. But this could ofcourse have no possible influence on the plants of the [561] samespecies growing in distant localities, and it would be improbable theyshould be affected in the same way. Hence we should conclude that when aspecies is converted into a new type in one locality this is only to beconsidered as one of numerous possible ones, and its alteration wouldnot in the least change the aspect of the remainder of the species. 

But even with this restriction the general belief is not supported bythe evidence of the evening-primroses. There is neither a slow nor asudden change of all the individuals. On the contrary, the vast majorityremain unchanged; thousands are seen exactly repeating the originalprototype yearly, both in the native field and in my garden. There is nodanger that _lamarckiana_ might die out from the act of mutating, northat the mutating strain itself would be exposed to ultimate destructionfrom this cause. 

In older swarms, such as _Draba_ or _Helianthemum_, no such center, around which the various forms are grouped, is known. Are we to concludetherefore that the main strain has died out? Or is it perhaps concealedamong the throng, being distinguished by no peculiar character? If our_gigas_ and _rubrinervis_ were growing in equal numbers with the_lamarckiana_ in the native field, would it be possible to decide [562]which of them was the progenitor of the others? Of course this could bedone by long and tedious crossing experiments, showing atavism in theprogeny, and thereby indicating the common ancestor. But even thiscapacity seems to be doubtful and connected only with the state ofmutability and to be lost afterwards. Therefore if this period ofmutation were ended, probably there would be no way to decide concerningthe mutual relationship of the single species. 

Hence the lack of a recognizable main stem in swarms of elementaryspecies makes it impossible to answer the question concerning theircommon origin. 

Another phase of the opposition between the prevailing view and my ownresults seems far more important. According to the current belief theconversion of a group of plants growing in any locality and floweringsimultaneously would be restricted to one type. In my own experimentsseveral new species arose from the parental form at once, giving a widerange of new forms at the same time and under the same conditions. 

III. New elementary species attain their full constancy at once. 

Constancy is not the result of selection or of improvement. It is aquality of its own. It can neither be constrained by selection if it isabsent [563] from the beginning, nor does it need any natural orartificial aid if it is present. Most of my new species have provedconstant from the first. Whenever possible, the original mutants havebeen isolated during the flowering period and artificiallyself-fertilized. Such plants have always given a uniform progeny, allchildren exhibiting the type of the parent. No atavism was observed andtherefore no selection was needed or even practicable. 

Briefly considering the different forms, we may state that the fullexperimental proof has been given for the origin of _gigas_ and_rubrinervis_, for _albida_ and _oblonga_, and even for _nanella_, whichis to be considered as of a varietal nature; with _lata_ the decisiveexperiment is excluded by its unisexuality. _laevifolia_ and_brevistylis_ were found originally in the field, and never appeared inmy cultures. No observations were made as to their origin, and seedshave only been sown from later generations. But these have yieldeduniform crops, thereby showing that there is no ground for theassumption that these two older varieties might behave otherwise thanthe more recent derivatives. 

_Scintillans_ and _elliptica_ constitute exceptions to the rule given. They repeat their character, from pure seed, only in part of theoffspring. I have tried to deliver the _scintillans_ from this [564]incompleteness of heredity, but in vain. The succeeding generations, ifproduced from true representatives of the new type, and with purefertilization, have repeated the splitting in the same numericalproportions. The instability seems to be here as permanent a quality asthe stability in other instances. Even here no selection has beenadequate to change the original form. 

IV. Some of the new strains are evidently elementary species, whileothers are to be considered as retrograde varieties. 

It is often difficult to decide whether a given form belongs to one oranother of these two groups. I have tried to show that the best andstrictest conception of varieties limits them to those forms that haveprobably originated by retrograde or degressive steps. Elementaryspecies are assumed to have been produced in a progressive way, addingone new element to the store. Varieties differ from their speciesclearly in one point, and this is either a distinct loss, or theassumption of a character, which may be met with in other species andgenera. _laevifolia_ is distinguished by the loss of the crinkling ofthe leaves, _brevistylis_ by the partial loss of the epigynous qualitiesof the flowers, and _nanella_ is a dwarf. These three new forms aretherefore [565] considered to constitute only retrograde steps, and noadvance. This conclusion has been fully justified by some crossingexperiments with _brevistylis_, which wholly complies with Mendel's law, and in one instance with _nanella_, which behaves in the same manner, ifcrossed with _rubrinervis_. 

On the other hand, _gigas_ and _rubrinervis_, _oblonga_ and _albida_obviously bear the characters of progressive elementary species. Theyare not differentiated from _lamarckiana_ by one or two main features. They diverge from it in nearly all organs, and in all in a definitethough small degree. They may be recognized as soon as they havedeveloped their first leaves and remain discernible throughout life. Their characters refer chiefly to the foliage, but no less to thestature, and even the seeds have peculiarities. There can be littledoubt but that all the attributes of every new species are derived fromone principal change. But why this should affect the foliage in onemanner, the flowers in another and the fruits in a third direction, remains obscure. To gain ever so little an insight into the nature ofthese changes, we may best compare the differences of ourevening-primroses with those between the two hundred elementary speciesof _Draba_ and other similar instances. In doing so we find the samemain [566] feature, the minute differences in nearly all points. 

V. The same new species are produced in a large number of individuals. 

This is a very curious fact. It embraces two minor points, viz: themultitude of similar mutants in the same year, and the repetitionthereof in succeeding generations. Obviously there must be some commoncause. This cause must be assumed to lie dormant in the _Lamarckiana_sof my strain, and probably in all of them, as no single parent-plantproved ever to be wholly destitute of mutability. Furthermore thedifferent causes for the sundry mutations must lie latent together inthe same parent-plant. They obey the same general laws, become activeunder similar conditions, some of them being more easily awakened thanothers. The germs of the _oblonga_, _lata_ and _nanella_ are especiallyirritable, and are ready to spring into activity at the least summons, while those of _gigas_, _rubrinervis_ and _scintillans_ are far moredifficult to arouse. 

These germs must be assumed to lie dormant during many successivegenerations. This is especially evident in the case of _lata_ and_nanella__, which appeared in the first year of the pedigree culture andwhich since have been repeated yearly, and have been seen to arise bymutation [567] also during the last season (1903). Only _gigas_ appearedbut once, but then there is every reason to assume that in largersowings or by a prolongation of the experiments it might have made asecond appearance. 

Is the number of such germs to be supposed to be limited or unlimited?My experiment has produced about a dozen new forms. Without doubt Icould easily have succeeded in getting more, if I had had any definitereason to search for them. But such figures are far from favoring theassumption of indefinite mutability. The group of possible new forms isno doubt sharply circumscribed. Partly so by the morphologicpeculiarities of _lamarckiana_, which seem to exclude red flowers, composite leaves, etc. No doubt there are more direct reasons for theselimits, some changes having taken place initially and others later, while the present mutations are only repetitions of previous ones, anddo not contribute new lines of development to those already existing. This leads us to the supposition of some common original cause, whichproduced a number of changes, but which itself is no longer at work, buthas left the affected qualities, and only these, in the state ofmutability. 

In nature, repeated mutations must be of far greater significance thanisolated ones. How [568] great is the chance for a single individual tobe destroyed in the struggle for life? Hundreds of thousands of seedsare produced by _lamarckiana_ annually in the field, and only some slowincrease of the number of specimens can be observed. Many seeds do notfind the proper circumstances for germination, or the young seedlingsare destroyed by lack of water, of air, or of space. Thousands of themare so crowded when becoming rosettes that only a few succeed inproducing stems. Any weakness would have destroyed them. As a matter offact they are much oftener produced in the seed than seen in the fieldwith the usual unfavorable conditions; the careful sowing of collectedseeds has given proof of this fact many times. 

The experimental proof of this frequency in the origin of new types, seems to overcome many difficulties offered by the current theories onthe probable origin of species at large. 

VI. The relation between mutability and fluctuating variability hasalways been one of the chief difficulties of the followers of Darwin. The majority assumed that species arise by the slow accumulation ofslight fluctuating deviations, and the mutations were only to beconsidered as extreme fluctuations, obtained, in the main, by acontinuous selection of small differences in a constant direction. 

[569] My cultures show that quite the opposite is to be regarded asfact. All organs and all qualities of _lamarckiana_ fluctuate and varyin a more or less evident manner, and those which I had the opportunityof examining more closely were found to comply with the general laws offluctuation. But such oscillating changes have nothing in common withthe mutations. Their essential character is the heaping up of slightdeviations around a mean, and the occurrence of continuous lines ofincreasing deviations, linking the extremes with this group. Nothing ofthe kind is observed in the case of mutations. There is no mean for themto be grouped around and the extreme only is to be seen, and it iswholly unconnected with the original type. It might be supposed that oncloser inspection each mutation might be brought into connection withsome feature of the fluctuating variability. But this is not the case. The dwarfs are not at all the extreme variants of structure, as thefluctuation of the height of the _lamarckiana_ never decreases or evenapproaches that of the dwarfs. There is always a gap. The smallestspecimens of the tall type are commonly the weakest, according to thegeneral rule of the relationship between nourishment and variation, butthe tallest dwarfs are of course the most robust specimens of theirgroup. [570] Fluctuating variability, as a rule, is subject toreversion. The seeds of the extremes do not produce an offspring whichfluctuates around their parents as a center, but around some point onthe line which combines their attributes with the correspondingcharacteristic of their ancestors, as Vilmorin has put it. No reversionaccompanies mutation, and this fact is perhaps the completest contrastin which these two great types of variability are opposed to each other. 

The offspring of my mutants are, of course, subject to the general lawsof fluctuating variability. They vary, however, around their own mean, and this mean is simply the type of the new elementary species. 

VII. The mutations take place in nearly all directions. 

Many authors assume that the origin of species is directed by unknowncauses. These causes are assumed to work in each single case for theimprovement of the animals and plants, changing them in a mannercorresponding in a useful way to the changes that take place in theirenvironment. It is not easy to imagine the nature of these influencesnor how they would bring about the desired effect. 

This difficulty was strongly felt by Darwin, and one of the chiefpurposes of his selection theory may be said to have been the attempt[571] to surmount it. Darwin tried to replace the unknown cause bynatural agencies, which lie under our immediate observation. On thispoint Darwin was superior to his predecessors, and it is chiefly due tothe clear conception of this point that his theory has gained itsdeserved general acceptance. According to Darwin, changes occur in alldirections, quite independently of the prevailing circumstances. Somemay be favorable, others detrimental, many of them without significance, neither useful nor injurious. Some of them will sooner or later bedestroyed, while others will survive, but which of them will survive, isobviously dependent upon whether their particular changes agree with theexisting environic conditions or not. This is what Darwin has called thestruggle for life. It is a large sieve, and it only acts as such. Somefall through and are annihilated, others remain above and are selected, as the phrase goes. Many are selected, but more are destroyed; dailyobservation does not leave any doubt upon this point. 

How the differences originate is quite another question. It has nothingto do with the theory of natural selection nor with the struggle forlife. These have an active part only in the accumulation of usefulqualities, and only in so [572] far as they protect the bearers of suchcharacters against being crowded out by their more poorly constitutedcompetitors. 

However, the differentiating characteristics of elementary species areonly very small. How widely distant they are from the beautifuladaptative organizations of orchids, of insectivorous plants and of somany others! Here the difference lies in the accumulation of numerouselementary characters, which all contribute to the same end. Chance musthave produced them, and this would seem absolutely improbable, evenimpossible, were it not for Darwin's ingenious theory. Chance there is, but no more than anywhere else. It is not by mere chance that thevariations move in the required direction. They do go, according toDarwin's view, in all directions, or at least in many. If these includethe useful ones, and if this is repeated a number of times, cumulationis possible; if not, there is simply no progression, and the typeremains stable through the ages. Natural selection is continually actingas a sieve, throwing out the useless changes and retaining the realimprovements. Hence the accumulation in apparently predisposeddirections, and hence the increasing adaptations to the more specializedconditions of life. It must be obvious to any one who can free himselffrom the current ideas, [573] that this theory of natural selectionleaves the question as to how the changes themselves are brought about, quite undecided. There are two possibilities, and both have beenpropounded by Darwin. One is the accumulation of the slight deviationsof fluctuating variability, the other consists of successive sports orleaps taking place in the same direction. 

In further lectures a critical comparison of the two views will begiven. Today I have only to show that the mutations of theevening-primroses, though sudden, comply with the demands made by Darwinas to the form of variability which is to be accepted as the cause ofevolution and as the origin of species. 

Some of my new types are stouter and others weaker than their parents, as shown by _gigas_ and _albida_. Some have broader leaves and somenarrower, _lata_ and _oblonga_. Some have larger flowers (_gigas_) ordeeper yellow ones (_rubrinervis_), or smaller blossoms (_scintillans_), or of a paler hue (_albida_). In some the capsules are longer(_rubrinervis_), or thicker (_gigas_), or more rounded (_lata_), orsmall (_oblonga_), and nearly destitute of seeds (_brevistylis_). Theunevenness of the surface of the leaves may increase as in _lata_, ordecrease as in _laevifolia_. The tendency to become annual prevails in_rubrinervis_, but _gigas_ tends to become [574] biennial. Some are richin pollen, while _scintillans_ is poor. Some have large seeds, otherssmall. _Lata_ has become pistillate, while _brevistylis_ has nearly lostthe faculty to produce seeds. Some undescribed forms were quite sterile, and some I observed which produced no flowers at all. From thisstatement it may be seen that nearly all qualities vary in oppositedirections and that our group of mutants affords wide material for thesifting process of natural selection. On the original field the_laevifolia_ and _brevistylis_ have held their own during sixteen yearsand probably more, without, however, being able to increase theirnumbers to any noticeable extent. Others perish as soon as they maketheir appearance, or a few individuals are allowed to bloom, butprobably leave no progeny. 

But perhaps the circumstances may change, or the whole strain may bedispersed and spread to new localities with different conditions. Someof the latter might be found to be favorable to the robust _gigas_, orto _rubrinervis_, which requires a drier air, with rainfall in thespringtime and sunshine during the summer. It would be worth while tosee whether the climate of California, where neither _O. Lamarckiana_nor _O. Biennis_ are found wild, would not exactly [575] suit therequirements of the new species _rubrinervis_ and _gigas_. 

NOTE. _Oenothera_s are native to America and all of the species growingin Europe have escaped from gardens directly, or may have arisen bymutation, or by hybridization of introduced species. A fixed hybridbetween _O. Cruciata_ and _O. Biennis_ constituting a species has beenin cultivation for many years. The form known as _O. Biennis_ in Europe, and used by de Vries in all of the experiments described in theselectures, has not yet been found growing wild in America and is notidentical with the species bearing that name among American botanists. Concerning this matter Professor de Vries writes under date of Sept. 12, 1905: "The '_biennis_' which I collected in America has proved to be amotley collection of forms, which at that time I had no means ofdistinguishing. No one of them, so far as they are now growing in mygarden is identical with our _biennis_ of the sand dunes. " The sameappears to be the case with _O. Muricata_. Plants from the NortheasternAmerican seaboard, identifiable with the species do not entirely agreewith those raised from seed received from Holland. 

_O. Lamarckiana_ has not been found growing wild in America in recentyears although the evidence at hand seems to favor the conclusion thatit was seen and collected in the southern states in the last century. (See MacDougal, Vail, Shull, and Small: Mutants and Hybrids of the_Oenotheras_. Publication 24. Carnegie Institution. Washington, D. C. , 1905. ) EDITOR. 

[576]LECTURE XX

THE ORIGIN OF WILD SPECIES AND VARIETIES

New species and varieties occur from time to time in the wild state. Setting aside all theoretical conceptions as to the common origin ofspecies at large, the undoubted fact remains that new forms aresometimes met with. In the case of the peloric toad-flax the mutationsare so numerous that they seem to be quite regular. The production ofnew species of evening-primroses was observed on the field andafterwards duplicated in the garden. There is no reason to think thatthese cases are isolated instances. Quite on the contrary they seem tobe the prototypes of repeated occurrences in nature. 

If this conception is granted, the question at once arises, how are weto deal with analogous cases, when fortune offers them, and what can weexpect to learn from them?

A critical study of the existing evidence seems to be of greatimportance in order to ascertain the best way of dealing with new facts, and of estimating the value of the factors concerned. [577] It ismanifest that we must be very careful and conservative in dealing withnew facts that are brought to our attention, and every effort should bemade to bring additional evidence to light. Many vegetable anomalies areso rare that they are met with only by the purest chance, and are thenbelieved to be wholly new. When a white variety of some common plant ismet with for the first time we generally assume that it originated onthat very spot and only a short time previously. The discovery of asecond locality for the same variety at once raises the question as to acommon origin in the two instances. Could not the plants of the secondlocality have arisen from seeds transported from the first?

White varieties of many species of blue-bells and gentians are found notrarely, white-flowering plants of heather, both of _Erica Tetralix_ and_Calluna vulgaris_ occur on European heaths; white flowers of _Brunellavulgaris_, _Ononis repens_, _Thymus vulgaris_ and others may be seen inmany localities in the habitats of the colored species. Pelories oflabiates seem to occur often in Austria, but are rare in Holland; whitebilberries (_Vaccinium Myrtillus_) have many known localities throughoutEurope, and nearly all the berry-bearing species in the large heathfamily are recorded as having white varieties. 

[578] Are we to assume a single origin for all the representatives ofsuch a variety, as we have done customarily for all the representativesof a wild species? Or can the same mutation have been repeated atdifferent times and in distant localities? If a distinct mutation from agiven species is once possible, why should it not occur twice or thrice?

A variety which seems to be new to us may only appear so, because thespot where it grows had hitherto escaped observation. _Lychnis preslii_is a smooth variety of _Lychnis diurna_ and was observed for the firsttime in the year 1842 by Sekera. It grew abundantly in a grove nearMunchengratz in southern Hungary. It was accompanied by the ordinaryhairy type of the species. Since then it has been observed to be quiteconstant in the same locality, and some specimens have been collectedfor me there lately by Dr. Nemec, of Prague. No other native localitiesof this variety have been discovered, and there can be no doubt that itmust have arisen from the ordinary campion near the spot where it stillgrows. But this change may have taken place some years before the firstdiscovery, or perhaps one or more centuries ago. This could only beknown if it could be proved that the locality had been satisfactorilyinvestigated previously, and that the variety had not [579] been metwith. Even in this case only something would be discovered about thetime of the change, but nothing about its real nature. 

So it is in many cases. If a variety is observed in a number ofspecimens at the time of its first discovery, and at a locality notstudied previously, it takes the aspect of an old form of limiteddistribution, and little can be learned as to the circumstances underwhich it arose. If on the contrary it occurs in very small numbers orperhaps even in a single individual, and if the spot where it is foundis located so that it could hardly have escaped previous observation, then the presumption of a recent origin seems justified. 

What has to be ascertained on such occasions to give them scientificvalue? Three points strike me as being of the highest importance. First, the constancy of the new type; secondly, the occurrence or lack ofintermediates, and last, but not least, the direct observation of arepeated production. 

The first two points are easily ascertained. Whether the new type islinked with its more common supposed ancestor by intermediate steps is aquery which at once strikes the botanist. It is usually recorded in suchcases, and we may state at once that the general result is, that suchintermediates do not occur. This is [580] of the highest importance andadmits of only two explanations. One is that intermediates may beassumed to have preceded the existent developed form, and to have diedout afterwards. But why should they have done so, especially in cases ofrecent changes? On the other hand the intermediates may be lackingbecause they have never existed, the change having taken place by asudden leap, such as the mutations described in our former lectures. Itis manifest that the assumption of hypothetical intermediates could onlygain some probability if they had been found in some instance. Sincethey do not occur, the hypothesis seems wholly unsupported. 

The second point is the constancy of the new type. Seeds should be savedand sown. If the plant fertilizes itself without the aid of insects, asdo some evening-primroses, the seed saved from the native locality mayprove wholly pure, and if it does give rise to a uniform progeny theconstancy of the race may be assumed to be proved, provided thatrepeated trials do not bring to light any exceptions. If the offspringshows more than one type, cross-fertilization is always to be looked toas the most probable cause, and should be excluded, in order to sow pureseeds. Garden-experiments of this kind, and repeated trials, shouldalways be combined [581] with the discovery of a presumed mutation. Inmany instances the authors have realized the importance of this point, and new types have been found constant from the very beginning. Manycases are known which show no reversions and even no partial reversions. This fact throws a distinct light on our first point, as it makes thehypothesis of a slow and gradual development still more improbable. 

My third point is of quite another nature and has not as yet been dealtwith. But as it appeals to me as the very soul of the problem, it seemsnecessary to describe it in some detail. It does not refer to the newtype itself, nor to any of its morphologic or hereditary attributes, butdirectly concerns the presumed ancestors themselves. 

The peloric toad-flax in my experiment was seen to arise thrice from thesame strain. Three different individuals of my original race showed atendency to produce peloric mutations, and they did so in a number oftheir seeds, exactly as the mutations of the evening-primroses wererepeated nearly every year. Hence the inference, that whenever we find anovelty which is really of very recent date, the parent-strain which hasproduced it might still be in existence on the same spot. In the case ofshrubs or perennials the very parents might yet be found. [582] But itseems probable, and is especially proved in the case of theevening-primroses, that all or the majority of the representatives ofthe whole strain have the same tendency to mutate. If this were ageneral rule, it would suffice to take some pure seeds from specimens ofthe presumed parents and to sow and multiply the individuals to such anextent that the mutation might have a chance to be repeated. 

Unfortunately, this has not as yet been done, but in my opinion itshould be the first effort of any one who has the good luck to discovera new wild mutation. Specimens of the parents should be transplantedinto a garden and fertilized under isolated conditions. Seeds saved fromthe wild plant would have little worth, as they might have been partlyfertilized by the new type itself. 

After this somewhat lengthy discussion of the value of observationssurrounding the discovery of new wild mutations, we now come to thedescription of some of the more interesting cases. As a first example, Iwill take the globular fruited shepherd's purse, described by SolmsLaubach as _Capsella heegeri_. Professor Heeger discovered one plantwith deviating fruits, in a group of common shepherd's purses in themarket-place near Landau in Germany, in the fall of 1897. They werenearly spherical, [583] instead of flat and purse-shaped. Their valveswere thick and fleshy, while those of the ordinary form aremembranaceous and dry. The capsules hardly opened and therefore differedin this point from the shepherd's purse, which readily loosens both itsvalves as soon as it is ripe. 

Only one plant was observed; whence it came could not be determined, norwhether it had arisen from the neighboring stock of C_apsella_ or not. The discoverer took some seed to his garden and sent some to thebotanical garden at Strassburg, of which Solms-Laubach is the director. The majority of the seeds of course were sowed naturally on the originalspot. The following year some of the seeds germinated and repeated thenovelty. The leaves, stems and flowers were those of the commonshepherd's purse, but no decision could be reached concerning the typeof this generation before the first flowers had faded and the roundedcapsules had developed. Then it was seen that the _heegeri_ came truefrom seed. It did so both in the gardens and on the market-place, whereit was observed to have multiplied and spread in some small measure. Thesame was noted the following year, but then the place was covered withgravel and all the plants destroyed. It is not recorded to have beenseen wild since. 

[584] Intermediate forms have not been met with. Some slight reversionsmay occur in the autumn on the smallest and weakest lateral branches. Such reversions, however, seem to be very rare, as I have tried in vainto produce them on large and richly branched individuals, by applyingall possible inducements in the form of manure and of cutting, tostimulate the production of successive generations of weaker sidebranches. 

This constancy was proved by the experiments of Solms-Laubach, which Ihave repeated in my own garden during several years with seed receivedfrom him. No atavists or deviating specimens have been found among manyhundreds of flowering plants. 

It is important to note that within the family of the crucifers the formof the capsule and the attributes of the valves and seeds are usuallyconsidered to furnish the characteristics of genera, and this point hasbeen elucidated at some length by Solms-Laubach. There is, however, nosufficient reason to construe a new genus on the ground of Heeger'sglobular fruited shepherd's purse; but as a true elementary species, andeven as a good systematic species it has proved itself, and as such itis described by Solms-Laubach, who named it in honor of its discoverer. 

Exactly analogous discoveries have been [586] instead of displaying abright yellow cup. _O. Cruciata_ grows in the Adirondack Mountains, inthe states of New York and Vermont, and seems to be abundant there. Ithas been introduced into botanical gardens and yielded a number ofhybrids, especially with _O. Biennis and _O. Lamarckiana_, and thenarrow petals of the parent-species may be met with in combination withthe stature and vegetative characteristics of these last named species. _O. Cruciata_ has a purple foliage, while _biennis_ and _lamarckiana_are green, and many of the hybrids may instantly be recognized by theirpurple color. 

The curious attribute of the petals is not to be considered simply as areduction in size. On anatomical inquiry it has been found that thesenarrow petals bear some characteristics which, on the normal plants, arelimited to the calyx. Stomata and hairs, and the whole structure of thesurface and inner tissues on some parts of these petals are exactlysimilar to those of the calyx, while on others they have retained thecharacteristics of petals. Sometimes there may even be seen by the nakedeye green longitudinal stripes of calyx-like structure alternating withbright yellow petaloid parts. For these reasons the cruciata charactermay be considered as a case of sepalody of the petals, or of the petalsbeing partly converted into sepals. 

[587] It is worth while to note that as a monstrosity this occurrence isextremely rare throughout the whole vegetable kingdom, and only very fewinstances have been recorded. 

Two cases of sudden mutations have come to my knowledge, producing thissame anomaly in allied species. One has been already alluded to; itpertains to the common evening-primrose or _Oenothera biennis_, and oneis a species belonging to another genus of the same family, the greathairy willow-herb or _Epilobium _hirsutum_. I propose to designate bothnew forms by the varietal name of _cruciata_, or _cruciatum_. 

_Oenothera biennis cruciata_ was found in a native locality of the _O. Biennis itself. It consisted of only one plant, showing in all itsflowers the _cruciata_ marks. In all other respects it resembled whollythe _biennis_, especially in the pure green color of its foliage, whichat once excluded all suspicion of hybrid origin with the purple _O. Cruciata_. Moreover in our country this last occurs only in thecultivated state in botanical gardens. 

Intermediates were not seen, and as the plant bore some pods, it waspossible to test its constancy. I raised about 500 plants from itsseeds, out of which more than 100 flowered in the first year. The otherswere partly kept through the winter and flowered next year. Seeds savedin [588] both seasons were sown on a large scale. Both the first and thesucceeding generations of the offspring of the original plant came truewithout any exception. Intermediates are often found in hybrid cultures, and in them the character is a very variable one, but as yet they werenot met with in progeny of this mutant. All these plants were exactlylike _O. Biennis_, with the single exception of their petals. 

_Epilobium hirsutum cruciatum_ was discovered by John Rasor nearWoolpit, Bury St. Edmunds, in England. It flowered in one spot, producing about a dozen stems, among large quantities of theparent-species, which is very common there, as it is elsewhere inEurope. This species is a perennial, multiplying itself by undergroundrunners, and the stems of the new variety were observed to stand soclose to each other that they might be considered as the shoots of oneindividual. In this case this specimen might probably be the originalmutant, as the variety had not been seen on that spot in previous years, even as it has not been found elsewhere in the vicinity. 

Intermediates were not observed, though the difference is a verystriking one. In the cruciate flowers the broad and bright purple petalsseem at first sight to be wholly wanting. They are too weak to expandand to reflex the calyx [589] as in the normal flowers of the species. The sepals adhere to one another, and are only opened at their summit bythe protruding pistils. Even the stamens hardly come to light. At theperiod of full bloom the flowers convey only the idea of closed budscrowned by the conspicuous white cross of the stigma. Any intermediateform would have at once betrayed itself by larger colored petals, comingout of the calyx-sheath. The cruciate petals are small and linear andgreenish, recalling thereby the color of the sepals. 

Mr. Rasor having sent me some flowers and some ripe capsules of hisnovelty, I sowed the latter in my experimental garden, where the plantflowered in large numbers and with many thousands of flowers both in1902 and 1903. All of these plants and all of these flowers repeated thecruciate type exactly, and not the slightest impurity or tendency topartial reversion has been observed. 

Thus true and constant cruciate varieties have been produced fromaccidentally observed initial plants, and because of their very curiouscharacters they will no doubt be kept in botanical gardens, even if theyshould eventually become lost in their native localities. 

At this point I might note another observation made on the wild speciesof _Oenothera cruciata_ [590] from the Adirondacks. Through the kindnessof Dr. MacDougal, of the New York Botanical Garden, I received seedsfrom Sandy Hill near Lake George. When the plants, grown from theseseeds, flowered, they were not a uniform lot, but exhibited two distincttypes. Some had linear petals and thin flower-buds, and in others thepetals were a little broader and the buds more swollen. The differencewas small, but constant on all the flowers, each single plant clearlybelonging to one or the other of the two types. Probably two elementaryspecies were intermixed here, but whether one is the systematic type andthe other a mutation, remains to be seen. 

Nor seem these two types to exhaust the range of variability of_Oenothera cruciata_. Dr. B. L. Robinson of Cambridge, Mass. , had thekindness to send me seeds from another locality in the same region. Theseeds were collected in New Hampshire and in my garden produced a trueand constant _cruciata_, but with quite different secondary charactersfrom both the aforesaid varieties. The stems and flower-spikes and eventhe whole foliage were much more slender, and the calyx-tubes of theflowers were noticeably more elongated. It seems not improbable that_Oenothera cruciata_ includes a group of lesser unities, and may proveto comprise a [591] swarm of elementary species, while the originalstrain might even now be still in a condition of mutability. A closescrutiny in the native region is likely to reveal many unexpectedfeatures. 

A very interesting novelty has already been described in a formerlecture. It is the _Xanthium wootoni_, discovered in the region aboutLas Vegas, New Mexico, by T. D. A. Cockerell. It is similar in allrespects to _X. Commune_, but the burrs are more slender and theprickles much less numerous, and mostly stouter at their base. It growsin the same localities as the _X. Commune_, and is not recorded to occurelsewhere. Whether it is an old variety or a recent mutation it is ofcourse impossible to decide. In a culture made in my garden from theseed sent me by Mr. Cockerell, I observed (1903) that both forms had asubvariety with brownish foliage, and, besides this, one of a puregreen. Possibly this species, too, is still in a mutable condition. 

Perhaps the same may be asserted concerning the beautiful shrub, _Hibiscus Moscheutos_, observed in quite a number of divergent types byJohn W. Harshberger. They grew in a small meadow at Seaside Park, NewJersey, in a locality which had been undisturbed for years. Theydiffered from each other in nearly all the [592] organs, in size, in thediameter of the stems, which were woody in some and more fleshy inothers, in the shape of the foliage and in the flowers. More than twentytypes could be distinguished and seeds were saved from a number of them, in order to ascertain whether they are constant, or whether perhaps amain stem in a mutating condition might be found among them. If thisshould prove to be the case, the relations between the observed formswould probably be analogous to those between the _O. Lamarckiana_ andits derivatives. 

Many other varieties have sprung from the type-species under similarconditions from time to time. A fern-leaved mercury, _Mercurialis annualaciniata_, was discovered in the year 1719 by Marchant. The type wasquite new at the time and maintained itself during a series of years. The yellow deadly nightshade or _Atropa Belladonna lutea_ was foundabout 1850 in the Black Forest in Germany in a single spot, and hassince been multiplied by seeds. It is now dispersed in botanicalgardens, and seems to be quite constant. A dwarf variety of a bean, _Phaseolus lunatus_, was observed to spring from the ordinary type by asudden leap about 1895 by W. W. Tracy, and many similar cases could begiven. 

The annual habit is not very favorable for [593] the discovery of newforms in the wild state. New varieties may appear, but may be crowdedout the first year. The chances are much greater with perennials, andstill greater with shrubs or trees. A single aberrant specimen may livefor years and even for centuries, and under such conditions is prettysure to be discovered sooner or later. Hence it is no wonder that manysuch cases are on record. They have this in common that the originalplant of the variety has been found among a vast majority ofrepresentatives of the corresponding species. Nothing of course isdirectly known about its origin. Intermediate links have as a rule beenwanting, and the seeds, which have often been sown, have not yieldedreliable results, as no care was taken to preserve the blossoms fromintercrossing with their parent-forms. 

Stress should be laid upon one feature of these curious occurrences. Relatively often the same novelty has been found twice or thrice, oreven more frequently, and under conditions which make it very improbablethat any relation between such occurrences might exist. The samemutation must have taken place more than once from the same main stem. 

The most interesting of these facts are connected with the origin of thepurple beech, which [594] is now so universally cultivated. I take thefollowing statements from an interesting historical essay of Prof. Jaggi. He describes three original localities. One is near the Swissvillage, Buch am Irchel, and is located on the Stammberg. During the17th century five purple beeches are recorded to have grown on thisspot. Four of them have died, but one is still alive. Seedlings havegerminated around this little group, and have been mostly dug up andtransplanted into neighboring gardens. Nothing is known about the realorigin of these plants, but according to an old document, it seems thatabout the year 1190 the purple beeches of Buch were already enjoyingsome renown, and attracting large numbers of pilgrims, owing to some oldlegend. The church of Embrach is said to have been built in connectionwith this legend, and was a goal for pilgrimages during many centuries. 

A second native locality of the purple beech is found in a forest nearSondershausen in Thuringen, Germany, where a fine group of these treesis to be seen. They were mentioned for the first time in the latter halfof the eighteenth century, but must have been old specimens long beforethat time. The third locality seems to be of much later origin. It is aforest near Roveredo in South Tyrol, where a new [595] university isbeing erected. It is only a century ago that the first specimens of thepurple beech were discovered there. 

As it is very improbable that the two last named localities should havereceived their purple beeches from the first named forest, it seemsreasonable to assume that the variety must have been produced at leastthrice. 

The purple beech is now exceedingly common in cultivation. But Jaggisucceeded in showing that all the plants owe their origin to theoriginal trees mentioned above, and are, including nearly all cultivatedspecimens with the sole exception of the vicinity of Buch, probablyderived from the trees in Thuringen. They are easily multiplied bygrafting, and come true from seed, at least often, and in a highproportion. Whether the original trees would yield a pure progeny iffertilized by their own pollen has as yet not been tested. The youngseedlings have purple seed-leaves, and may easily be selected by thischaracter, but they seem to be always subjected in a large measure tovicinism. 

Many other instances of trees and shrubs, found in accidental specimensconstituting a new variety in the wild state, might be given. Theoak-leaved beech has been found in a forest of Lippe-Detmold in Germanyand near Versailles, [596] whence it was introduced into horticulture byCarriere. Similarly divided and cleft leaves seem to have occurred moreoften in the wild state, and cut-leaved hazels are recorded from Rouenin France, birches and alders from Sweden and Lapland, where both aresaid to have been met with in several forests. The purple barberry wasfound about 1830 by Bertin, near Versailles. Weeping varieties of asheswere found wild in England and in Germany, and broom-like oaks, _Quercuspedunculata fastigiata_, are recorded from Hessen-Darmstadt, Calabria, the Pyrenees and other localities. About the real origin of all thesevarieties nothing is definitely known. 

The "single-leaved" strawberry is a variety often seen in botanicalgardens, as it is easily propagated by its runners. It was discoveredwild in Lapland at the time of Linnaeus, and appeared afterwardsunexpectedly in a nursery near Versailles. This happened about the year1760 and Duchesne tested it from seeds and found it constant. Thisstrain, however, seems to have died out before the end of the 18thcentury. In a picture painted by Holbein (1495-1543), strawberry leavescan be seen agreeing exactly with the monophyllous type. The variety maythus be assumed to have arisen independently [597] at least thrice, atdifferent periods and in distant localities. 

From all these statements and a good many others which can be found inhorticultural and botanical literature, it may be inferred thatmutations are not so very rare in nature as is often supposed. Moreoverwe may conclude that it is a general rule that they are neither precedednor accompanied by intermediate steps, and that they are ordinarilyconstant from seed from the first. 

Why then are they not met with more often? In my opinion it is thestruggle for life which is the cause of this apparent rarity; which isnothing else than the premature death of all the individuals that sovary from the common type of their species as to be incapable ofdevelopment under prevailing circumstances. It is obviously withoutconsequence whether these deviations are of a fluctuating or of amutating nature. Hence we may conclude that useless mutations will soondie out and will disappear without leaving any progeny. Even if they areproduced again and again by the same strain, but under the sameunfavorable conditions, there will be no appreciable result. 

Thousands of mutations may perhaps take place yearly among the plants ofour immediate vicinity without any chance of being discovered. [598] Weare trained to the appreciation of the differentiating marks ofsystematic species. When we have succeeded in discerning these as givenby our local flora lists, we rest content. Meeting them again we are inthe habit of greeting them with their proper names. Such is thesatisfaction ensuing from this knowledge that we do not feel anyinclination for further inquiry. Striking deviations, such as manyvarietal characters, may be remarked, but then they are considered asbeing of only secondary interest. Our minds are turned from thedelicately shaded features which differentiate elementary species. 

Even in the native field of the evening-primroses, no botanist wouldhave discovered the rosettes with smaller or paler leaves, constitutingthe first signs of the new species. Only by the guidance of a distincttheoretical idea were they discovered, and having once been pointed outa closer inspection soon disclosed their number. 

Variability seems to us to be very general, but very limited. The limitshowever, are distinctly drawn by the struggle for existence. Of coursethe chance for useful mutations is a very small one. We have seen thatthe same mutations are as a rule repeated from time to time by the samespecies. Now, if a useful mutation, [599] or even a wholly indifferentone, might easily be produced, it would have been so, long ago, andwould at the present time simply exist as a systematic variety. Ifproduced anew somewhere the botanist, would take it for the old varietyand would omit to make any inquiry as to its local origin. 

Thousands of seeds with perhaps wide circles of variability are ripenedeach year, but only those that belong to the existing old narrow circlessurvive. How different would Nature appear to us if she were free toevolve all her potentialities!

Darwin himself was struck with this lack of harmony between commonobservations and the probable real state of things. He discussed it inconnection with the cranesbill of the Pyrenees (_Geranium pyrenaicum_). He described how this fine little plant, which has never beenextensively cultivated, had escaped from a garden in Staffordshire andhad succeeded in multiplying itself so as to occupy a large area. 

In doing so it had evidently found place for an uncommonly large numberof plantlets from its seeds and correspondingly it had commenced to varyin almost all organs and qualities and nearly in all imaginabledirections. It displayed under these exceptional circumstances acapacity which never had been exceeded and [600] which of course wouldhave remained concealed if its multiplication had been checked in theordinary way. 

Many species have had occasion to invade new regions and cover them withhundreds of thousands of individuals. First are to be cited thosespecies which have been introduced from America into Europe since thetime of Columbus, or from Europe into this country. Some of them havebecome very common. In my own country the evening-primroses and Canadafleabane or are examples, and many others could be given. They should beexpected to vary under these circumstances in a larger degree. Have theydone so? Manifestly they have not struck out useful new characters thatwould enable their bearers to found new elementary species. At leastnone have been observed. But poor types might have been produced, andperiods of mutability might have been gone through similar to that whichis now under observation for Lamarck's evening primrose in Holland. 

From this discussion we may infer that the chances of discovering newmutating species are great enough to justify the utmost efforts tosecure them. It is only necessary to observe large numbers of plants, grown under circumstances which allow the best opportunities for [601]all the seeds. And as nature affords such opportunities only at rareintervals, we should make use of artificial methods. Large quantities ofseed should be gathered from wild plants and sowed under very favorableconditions, giving all the nourishment and space required to the youngseedlings. It is recommended that they be sown under glass, either in aglass-house or protected against cold and rain by glass-frames. The samelot of seed will be seen to yield twice or thrice as many seedlings ifthus protected, compared with what it would have produced when sown inthe field or in the garden. I have nearly wholly given up sowing seedsin my garden, as circumstances can be controlled and determined withgreater exactitude when the sowing is done in a glasshouse. 

The best proof perhaps, of the unfavorable influence of externalconditions for slightly deteriorated deviations is afforded byvariegated leaves. Many beautiful varieties are seen in our gardens andparks, and even corn has a variety with striped leaves. They are easilyreproduced, both by buds and by seeds, and they are the most ordinary ofall varietal deviations. They may be expected to occur wild also. But noreal variegated species, nor even good varieties with this attributeoccurs in nature. [602] On the other hand occasional specimens with asingle variegated leaf, or with some few of them, are actually met with, and if attention is once drawn to this question, perhaps a dozen or soinstances might be brought together in a summer. But they never seem tobe capable of further evolution, or of reproducing themselvessufficiently and of repeating their peculiarity in their progeny. Theymake their appearance, are seen during a season, and then disappear. Even this slight incompleteness of some spots on one or two leaves maybe enough to be their doom. 

It is a common belief that new varieties owe their origin to the directaction of external conditions and moreover it is often assumed thatsimilar deviations must have similar causes, and that these causes mayact repeatedly in the same species, or in allied, or even systematicallydistant genera. No doubt in the end all things must have their causes, and the same causes will lead under the same circumstances to the sameresults. But we are not justified in deducing a direct relation betweenthe external conditions and the internal changes of plants. Theserelations may be of so remote a nature that they cannot as yet beguessed at. Therefore only direct experience may be our guide. Summingup the result of our facts and discussions [603] we may state that wildnew elementary species and varieties are recorded to have appeared fromtime to time. Invariably this happened by sudden leaps and withoutintermediates. The mutants are constant when propagated by seed, and atonce constitute a new race. In rare instances this may be of sufficientsuperiority to win a place for itself in nature, but more often it hasqualities which have led to its introduction into gardens as anornamental plant or into botanical gardens by reason of the interestafforded by their novelty, or by their anomaly. 

Many more mutations may be supposed to be taking place all around us, but artificial sowings on a large scale, combined with a closeexamination of the seedlings and a keen appreciation of the slightestindications of deviation seem required to bring them to light. 

[604]LECTURE XXI

MUTATIONS IN HORTICULTURE

It is well known that Darwin based his theory of natural selection to alarge extent upon the experience of breeders. Natural and artificialselection exhibit the same general features, yet it was impossible inDarwin's time to make a critical and comparative analysis of the twoprocesses. 

In accordance with our present conception there is selection of speciesand selection within the species. The struggle for life determines whichof a group of elementary species shall survive and which shalldisappear. In agricultural practice the corresponding process is usuallydesignated by the name of variety-testing. Within the species, or withinthe variety, the sieve of natural selection is constantly eliminatingpoor specimens and preserving those that are best adapted to live underthe given conditions. Some amelioration and some local races are theresult, but this does not appear to be of much importance. On thecontrary, the selection [605] within the race holds a prominent place inagriculture, where it is known by the imposing term, race-breeding. 

Experience and methods in horticulture differ from those in agriculturein many points. Garden varieties have been tested and separated for along time, but neither vegetables nor flowers are known to exhibit suchmotley groups of types as may be seen in large forage crops. 

New varieties which appear from time to time may be ornamental orotherwise in flowers, and more or less profitable than their parents invegetables and fruits. In either case the difference is usuallystriking, or if not, its culture would be unprofitable. 

The recognition of useful new varieties being thus made easy, the wholeattention of the breeder is reduced to isolating the seeds of themutants that are to be saved and sown separately, and this process mustbe repeated during a few years, in order to produce the quantity of seedthat is needed for a profitable introduction of the variety intocommerce. In proportion to the abundance of the harvest of each yearthis period is shorter for some and longer for other species. 

Isolation in practice is not so simple nor so easy an affair as it is inthe experimental garden. Hence we have constant and nearly unavoidable[606] cross-fertilizations with the parent form or with neighboringvarieties, and consequent impurity of the new strain. This impurity wehave called vicinism, and in a previous lecture have shown its effectsupon the horticultural races on one hand, and on the other, on thescientific value that can be ascribed to the experience of the breeder. We have established the general rule that stability is seldom met with, but that the observed instability is always open to the objection ofbeing the result of vicinism. Often this last agency is its sole cause;or it may be complicated with other factors without our being able todiscern them. 

Though our assertion that the practice of the horticulturist inproducing new varieties is limited to isolation, whenever chance affordsthem, is theoretically valid, it is not always so. We may discernbetween the two chief groups of varieties. The retrograde varieties areconstant, the individuals not differing more from one another than thoseof any ordinary species. The highly variable varieties play an importantpart in horticulture. Double flowers, striped flowers, variegated leavesand some others yield the most striking instances. Such forms have beenincluded in previous lectures among the ever-sporting varieties, becausetheir peculiar characters oscillate between two extremes, viz: [607] thenew one of the variety and the corresponding character of the originalspecies. 

In such cases isolation is usually accompanied by selection: rarely hasthe first of a double, striped or variegated race well filled or richlystriped flowers or highly spotted leaves. Usually minor degrees of theanomaly are seen first, and the breeder expects the novelty to developits features more completely and more beautifully in subsequentgenerations. Some varieties need selection only in the beginning, inothers the most perfect specimens must be chosen every year asseed-bearers. For striped flowers, it has been prescribed by Vilmorin, that seeds should be taken only from those with the smallest stripes, because there is always reversion. Mixed seed or seed from medium typeswould soon yield plants with too broad stripes, and therefore lessdiversified flowers. 

In horticulture, new varieties, both retrograde and ever-sporting, areknown to occur almost yearly. Nevertheless, not every novelty of thegardener is to be considered as a mutation in the scientific sense ofthe word. First of all, the novelties of perennial and woody species areto be excluded. Any extreme case of fluctuating variability may bepreserved and multiplied in the vegetative way. Such types aredesignated [608] in horticulture as varieties, though obviously they areof quite another nature than the varieties reproduced by seed. Secondly, a large number, no doubt the greater number of novelties, are of hybridorigin. Here we may discern two cases. Hybrids may be produced by thecrossing of old types, either of two old cultivated forms or newlyintroduced species, or ordinarily between an old and an introducedvariety. Such novelties are excluded from our present discussion. Secondly, hybrids may be produced between a true, new mutation and someof the already existing varieties of the same species. Examples of thisobvious and usual practice will be given further on, but it must bepointed out now that by such crosses a single mutation may produce asmany novelties as there are available varieties of the same species. 

Summarizing these introductory remarks we must lay stress on the factthat only a small part of the horticultural novelties are realmutations, although they do occur from time to time. If useful, they areas a rule isolated and multiplied, and if necessary, improved byselection. They are in many instances, as constant from seed as theunavoidable influence of vicinism allows them to be. Exact observationson the origin, or on the degree of constancy, are usually lacking, [609]the notes being ordinarily made for commercial purposes, and often onlyat the date of introduction into trade, when the preceding stages of thenovelty may have been partly forgotten. 

With this necessary prelude I will now give a condensed survey of thehistorical facts relating to the origin of new horticultural varieties. An ample description has been given recently by Korshinsky, a Russianwriter, who has brought together considerable historical material asevidence of the sudden appearance of novelties throughout the wholerealm of garden plants. 

The oldest known, and at the same time one of the most accuratelydescribed mutations is the origin of the cut-leaved variety of thegreater celandine or _Chelidonium majus_. This variety has beendescribed either as such, or as a distinct species, called _Chelidoniumlaciniatum_ Miller. 

It is distinguished from the ordinary species, by the leaves being cutinto narrow lobes, with almost linear tips, a character which is, as wehave seen on a previous occasion, repeated in the petals. It is atpresent nearly as commonly cultivated in botanical gardens as the _C. Majus_, and has escaped in many localities and is observed to thrive asreadily as the native wild [610] plants. It was not known until a fewyears before the close of the 16th century. Its history has beendescribed by the French botanist, Rose. It was seen for the first timein the garden of Sprenger, an apothecary of Heidelberg, where the _C. Majus_ had been cultivated for many years. Sprenger discovered it in theyear 1590, and was struck by its peculiar and sharply deviatingcharacters. He was anxious to know whether it was a new plant and sentspecimens to Clusius and to Plater, the last of whom transmitted them toCaspar Bauhin. These botanists recognized the type as quite new andBauhin described it some years afterwards in his Phytopinax under thename of _Chelidonium majus foliis quernis_, or oak-leaved celandine. Thenew variety soon provoked general interest and was introduced into mostof the botanical gardens of Europe. It was recognized as quite new, andrepeated search has been made for it in a wild state, but in vain. Noother origin has been discovered than that of Sprenger's garden. Afterwards it became naturalized in England and elsewhere, but there isnot the least doubt as to its derivation in all the observed cases. 

Hence its origin at Heidelberg is to be considered as historicallyproven, and it is of course only legitimate to assume that it originatedin [611] the year 1590 from the seeds of the _C. Majus_. Nevertheless, this was not ascertained by Sprenger, and some doubt as to a possibleintroduction from elsewhere might arise. If not, then the mutation musthave been sudden, occurring without visible preparation and without theappearance of intermediates. 

From the very first, the cut-leaved celandine has been constant fromseed. Or at least it has been propagated by seed largely and withoutdifficulty. Nothing, however, is known about it in the first few yearsof its existence. Later careful tests were made by Miller, Rose andothers and later by myself, which have shown its stability to beabsolute and without reversion, and it has probably been so from thebeginning. The fact of its constancy has led to its specific distinctionby Miller, as varieties were in his time universally, and up to thepresent time not rarely, though erroneously, believed to be less stablethan true species. 

Before leaving the laciniate celandine it is to be noted that in crosseswith _C. Majus_ it follows the law of Mendel, and for this reason shouldbe considered as a retrograde variety, the more so, as it is alsotreated as such from a morphological point of view by Stahl and others. 

We now come to an enumeration of those cases in which the date of thefirst appearance [612] of a new horticultural variety has been recorded, and I must apologize for the necessity of again quoting many variations, which have previously been dealt with from another point of view. Insuch cases I shall limit myself as closely as possible to historicalfacts. They have been recorded chiefly by Verlot and Carriere, who wrotein Paris shortly after the middle of the past century, and afterwards byDarwin, Korshinsky, and others. It is from their writings and fromhorticultural literature at large that the following evidence is broughttogether. 

A very well-known instance is that of the dwarf variety of _Tagetessignata_, which arose in the nursery of Vilmorin in the year 1860. Itwas observed for the first time in a single individual among a lot ofthe ordinary _Tagetes signata_. It was found impossible to isolate it, but the seeds were saved separately. The majority of the offspringreturned to the parental type, but two plants were true dwarfs. Fromthese the requisite degree of purity for commercial purposes wasreached, the vicinists not being more numerous than 10% of the entirenumber. The same mutation had been observed a year earlier in the samenursery in a lot of _Saponaria calabrica_. The seeds of this dwarfrepeated the variety in the next generation, but in the third none wereobserved. Then the variety was [613] thought to be lost, and the culturewas given up, as the Mendelian law of the splitting of varietal hybridswas not known. According to our present knowledge we might expect theatavistic descendants of the first dwarf to be hybrids, and to be liableto split in their progeny into one-fourth dwarfs and three-fourthsnormal specimens. From this it is obvious that the dwarfs would haveappeared a second time if the strain had been continued by means of theseeds of the vicinistic progeny. 

In order to avoid a return to this phase of the question, another use ofthe vicinists should at once be pointed out. It is the possibility ofincreasing the yield of the new variety. If space admits of sowing theseeds of the vicinists, a quarter of the progeny may be expected to cometrue to the new type, and if they were partly pollinated by the dwarfs, even a larger number would do so. Hence it should be made a rule to sowthese seeds also, at least when those of the true representatives of thenovelty do not give seed enough for a rapid multiplication. 

Other dwarfs are recorded to have sprung from species in the same suddenand unexpected manner, as for instance _Ageratum coeruleum_ of the samenursery, further _Clematis Viticella nana_ and _Acer campestre nanum_. _Prunus Mahaleb nana_ was discovered in 1828 in one [614] specimen nearOrleans by Mme. LeBrun in a large culture of Mahaleb. _Lonicera tataricanana_ appeared in 1825 at Fontenay-aux Roses. A tall variety of thestrawberry is called "Giant of Zuidwijk" and originated at Boskoop inHolland in the nursery of Mr. Van de Water, in a lot of seedlings of theordinary strawberry. It was very large, but produced few runners, andwas propagated with much difficulty, for after six years only 15 plantswere available. It proved to be a late variety with abundant largefruit, and was sold at a high price. For a long time it was prominent incultures in Holland only. 

Varieties without prickles are known to have originated all of a suddenin sundry cases. _Gleditschia sinensis_, introduced in 1774 from China, gave two seedlings without spines in the year 1823, in the nursery ofCaumzet. It is curious in being one of the rare instances where asimultaneous mutation in two specimens is acknowledged, because as arule, such records comply with the prevailing, though inexact, beliefthat horticultural mutations always appear in single individuals. 

From Korshinsky's survey of varieties with cut leaves or laciniate formsthe following cases may be quoted. In the year 1830 a nurseryman namedJacques had sown a large lot of elms, [615] _Ulmus pedunculata_. One ofthe seedlings had cut leaves. He multiplied it by grafting and gave itto the trade under the name of _U. Pedunculata urticaefolia_. It hassince been lost. 

Laciniate alders seem to have been produced by mutation at sundry times. Mirbel says that the _Alnus glutinosa laciniata_ is found wild inNormandy and in the forests of Montmorency near Paris. A similar varietyhas been met with in a nursery near Orleans in the year 1855. Inconnection with this discovery some discussion has arisen concerning thequestion whether it was probable that the Orleans strain was a newmutation, or derived in some way from the trees cited by Mirbel. Ofcourse, as always in such cases, any doubt, once pronounced, affects theimportance of the observation for all time, since it is impossible togather sufficient historical evidence to fully decide the point. Thesame variety had appeared under similar circumstances in a nursery atLyons previously (1812). 

Laciniated maples are said to be of relatively frequent occurrence innurseries, among seedlings of the typical species. Loudon says that once100 laciniated seedlings were seen to originate from seed of some normaltrees. But in this case it is rather probable that the presumed [616]normal parents were in reality hybrids between the type and thelaciniated form, and simply split according to Mendel's law. Thishypothesis is partly founded on general considerations and partly onexperiments made by myself with the cut-leaved celandine, previouslyalluded to, which I crossed with the type. The hybrids repeated thefeatures of the species and showed no signs of their internal hybridconstitution. But the following year one-fourth of their progenyreturned to the cut-leaved form. If the same thing has taken place inthe case of Loudon's maples, but without their hybrid origin beingknown, the result would have been precisely what he observed. 

_Broussonetia papyriffera dissecta_ originated about 1830 at Lyons, anda second time in 1866 at Fontenay-aux-Roses. The cut-leaved hazelnuts, birches, beeches and others have mostly been found in the wild state, asI have already pointed out in a previous lecture. A similar variety ofthe elder, _Sambucus nigra laciniata_, and its near ally, _Sambucusracemosa laciniata_, are often to be seen in our gardens. They have beenon record since 1886 and come true from seed, but their exact originseems to have been forgotten. Cut-leaved walnuts have been known since1812; they come true from seed, but are extremely liable to vicinism, anuisance which is [617] ascribed by some authors to the fact that oftenon the same tree the male catkins flower and fall off several weeksbefore the ripening of the pistils of the other form of flowers. 

Weeping varieties afford similar instances. _Sophora japonica pendula_originated about 1850, and _Gleditschia triacanthos pendula_ some timelater in a nursery at Chateau-Thierry (Aisne, France). In the year 1821the bird's cherry, or _Prunus Padus_, produced a weeping variety, and in1847 the same mutation was observed for the allied _Prunus Mahaleb_. Numerous other instances of the sudden origin of weeping trees, both ofconifers and of others, have been brought together in Korshinsky'spaper. This striking type of variation includes perhaps the bestexamples of the whole historical evidence. As a rule they appear inlarge sowings, only one, or only a few at a time. Many of them have notbeen observed during their youth, but only after having been planted outin parks and forests, since the weeping characters show only afterseveral years. 

The monophyllous bastard-acacia originated in the same way. Itspeculiarities will be dealt with on another occasion, but thecircumstances of its birth may as well be given here. In 1855 in thenursery of Deniau, at Brain-sur-l'Authion (Maine et Loire), it appearedin a lot of [618] seedlings of the typical species in a singleindividual. This was transplanted into the Jardin des Plantes at Paris, where it flowered and bore seeds in 1865. It must have been partlypollinated by the surrounding normal representatives of the species, since the seeds yielded only one-fourth of true offspring. Thisproportion, however, has varied in succeeding years. Briot remarks thatthe monophyllous bastard acacia is liable to petaloid alterations of itsstamens, which deficiency may encroach upon its fertility andaccordingly upon the purity of its offspring. 

Broom-like varieties often occur among trees, and some are known fortheir very striking reversions by buds, as we have seen on a previousoccasion. They are ordinarily called pyramidal or fastigiate forms, andas far as their history goes, they arise suddenly in large sowings ofthe normal species. The fastigiate birch was produced in this way byBaumann, the _Abies concolor fastigiata_ by Thibault and Keteleer atParis, the pyramidal cedar by Paillat, the analogous form of_Wellingtonia_ by Otin. Other instances could easily be added, though ofcourse some of the most highly prized broom-like trees are so old thatnothing is known about their origin. This, for instance, is the casewith the pyramidal yew-tree, _Taxus baccata fastigiata_. [619] Othershave been found wild, as already mentioned in a former lecture. 

An analogous case is afforded by the purpleleaved plums, of which themost known form is Prunus Pissardi. It is said to be a purple variety of_Prunus cerasifera_, and was introduced at the close of the seventiesfrom Persia, where it is said to have been found in Tabris. A similarvariety arose independently and unexpectedly in the nursery of Spath, near Berlin, about 1880, but it seems to differ in some minor pointsfrom the Persian prototype. 

A white variety of _Cyclamen vernum_ made its appearance in the year1836 in Holland. A single individual was observed for the first timeamong a large lot of seedlings, in a nursery near Haarlem. It yielded asatisfactory amount of seed, and the progeny was true to the new type. Such plants propagate slowly, and it was only twenty-seven years later(1863) that the bulbs were offered for sale by the Haarlem firm ofKrelage & Son. The price of each bulb was $5. 00 in that year, but soonafterwards was reduced to $1. 00 each, which was about thrice theordinary price of the red variety. 

The firm of Messrs. Krelage & Son has brought into commerce a wide rangeof new bulb-varieties, all due to occasional mutations, some by seed andothers by buds, or to the accidental [620] transference of new qualitiesinto the already existing varieties by cross-pollination through theagency of insects. Instead of giving long lists of these novelties, Imay cite the black tulips, which cost during the first few years oftheir introduction about $25. 00 apiece. 

Horticultural mutations are as a rule very rare, especially in genera orspecies which have not yet been brought to a high degree of variability. In these the wide range of varieties and the large scale in which theyare multiplied of course give a greater chance for new varieties. Butthen the possibilities of crossing are likewise much larger, andapparent changes due to this cause may easily be taken for originalmutations. 

The rarity of the mutations is often proved by the lapse of time betweenthe introduction of a species and its first sport. Some instances may begiven. They afford a proof of the length of the period during which thespecies remained unaltered, although some of these alterations may bedue to a cross with an allied form. _Erythrina Crista-galli_ wasintroduced about 1770, and produced its first sport in 1884, after morethan a century of cultivation. _Begonia semperflorens_ has beencultivated since 1829, and for half a century before it commencedsporting. The same length of time has elapsed [621] between the firstculture and the first variation of _Crambe maritima_. Other cases are onrecord in which the variability exhibited itself much sooner, perhapswithin a few years after the original discovery of the species. But suchinstances seem, as a rule, to be subject to doubt as to the concurrenceof hybridization. So for instance the _Iris lortetii_, introduced in theyear 1895 from the Lebanon, which produced a white variety from its veryfirst seeds. If by chance the introduced plants were natural hybridsbetween the species and the white variety, this apparent and ratherimprobable mutation would find a very simple explanation. The length ofthe period preceding the first signs of variability is largely, ofcourse, due to divergent methods of culture. Such species as_Erythrina_, which are perennial and only sown on a small scale, shouldnot be expected to show varieties very soon. Annual species, which arecultivated yearly in thousands or even hundreds of thousands ofindividuals, have a much better chance. Perhaps the observed differencesare largely due to this cause. 

Monstrosities have, from time to time, given rise to cultivated races. The cockscomb or _Celosia_ is one of the most notorious instances. Cauliflowers, turnips and varieties of cabbages are recorded by DeCandolle to have arisen in [622] culture, more than a century ago, asisolated monstrous individuals. They come true from seed, but showdeviations from time to time which seem to be intimately linked withtheir abnormal characters. Apetalous flowers may be considered asanother form of monstrosity, and in _Salpiglossis sinuata_ such avariety without a corolla made its appearance in the year 1892 in thenursery of Vilmorin. It appeared suddenly, yielded a good crop of seedand was constant from the outset, without any sign of vicinism orimpurity. 

In several cases the origin of a variety is obscure, while thesubsequent historical evidence is such as to make an original suddenappearance quite probable. Although these instances offer but indirectevidence, and will sooner or later lose their importance, it seemsdesirable to lay some stress on them here, because most of these casesare very obvious and more striking than purely historical facts. Sterilevarieties belong to this heading. Sometimes they bear fruit withoutkernels, sometimes flowers without sexual organs, or even no flowers atall. Instances have been given in the lecture on retrograde varieties;they are ordinarily assumed to have originated by a leap, because it isnot quite clear how a loss of the capacity for the formation of seedscould have been slowly accumulated [623] in preceding generations. Aninteresting case is afforded by a sterile variety of corn, whichoriginated some time ago in my own pedigree-cultures made for anotherpurpose, and which had begun with an ear of 1886. The first generationfrom the original seeds showed nothing particular, but the second atonce produced quite a number of sterile plants. The sterility was causedby the total lack of branches, including those bearing the pistillateflowers. The terminal spikes themselves were reduced to naked spindles, without branches, without flowers and even almost without bracts. 

In some individuals, however, this negative character was seen to giveway at the tip, showing a few small naked branches. Of course it wasimpossible to propagate this curious form, but my observations showedthat it sprang into existence from known ancestors by a single step orsudden leap. This leap, however, was not confined to a single specimen;on the contrary it affected 40 plants out of a culture of 340individuals. The same phenomenon was repeated from the seeds of thenormal plants in the following year, but afterwards the monstrositydisappeared. 

The Italian poplar affords another instance. It is considered by someauthors as a distinct species, _Populus italica_, and by others as a[624] broom-like variety of the _Populus nigra_, from which it isdistinguished by its erect branches and other characters of minorimportance. It is often called the pyramidal or fastigiate poplar. Itsorigin is absolutely unknown and it occurs only in the cultivated state. In Italy it seems to have been cultivated from the earliest historicaltimes, but it was not introduced into other countries till theeighteenth century. In 1749 it was brought into France, and in 1758 intoEngland, and to day it may be seen along roads throughout central Europeand in a large part of Asia. But the most curious fact is that it isonly observed in staminate specimens; pistillate trees have not beenfound, although often sought for. This circumstance makes it veryprobable that the origin of the broom-like poplar was a sudden mutation, producing only one individual. This being staminate, it has beenpropagated exclusively by cuttings. It is to be admitted, however, thatno material evidence is at hand to prove that it is not an original wildspecies, the pistillate form of which has been lost by vegetativemultiplication. One form only of many dioecious plants is to be found incultivation, as, for instance some South American species of _Ribes_. 

Total lack of historical evidence concerning [625] the origin of avariety has sometimes been considered as sufficient proof of a suddenorigin. The best known instance is that of the renowned cactus-dahliawith its recurved instead of incurved ray-florets. It was introducedfrom Mexico into the Netherlands by Van den Berg of Jutphaas, under thefollowing remarkable circumstances. In the autumn of 1872 one of hisfriends had sent him a small case, containing seeds, bulbs and rootsfrom Mexico. From one of these roots a _Dahlia_ shoot developed. It wascultivated with great care and bloomed next year. It surprised all whosaw it by the unexpected peculiarity of its large rich crimson flowers, the rays of which were reversed tubular. The margins of the narrow rayswere curved backwards, showing the bright color of the upper surface. Itwas a very showy novelty, rapidly multiplied by cuttings, and was soonintroduced into commerce. It has since been crossed with nearly allother available varieties of the _Dahlia_, giving a large and rich groupof forms, bound together by the curious curling of the petals. It hasnever been observed to grow in Mexico, either wild or in gardens, andthus the introduced individual has come to be considered as the first ofits race. 

I have already mentioned that the rapid production of large numbers ofnew varieties, by [626] means of the crossing of the offspring of asingle mutant with previously existing sorts, is a very common featurein horticultural practice. It warns us that only a small part of thenovelties introduced yearly are due to real mutations. Further instancesof novelties with such a common origin are the purple-leaved dahlias, the gooseberries without prickles, the double petunias, erect gloxiniasand many others. Accumulation of characters, acquired in different racesof a species, may easily be effected in this way; in fact it is one ofthe important factors in the breeding of horticultural novelties. 

I have alluded more than once in this lecture to the question, whetherit is probable that mutations occur in one individual or in more. Thecommon belief among horticulturists is that, as a rule, they appear in asingle plant. This belief is so widespread that whenever a novelty isseen for the first time in two or more specimens it is at once suggestedthat it might have originated and been overlooked in a previousgeneration. Not caring to confess a lack of close observation, thenumber of mutants in such cases is usually kept secret. At least thisstatement has been made to me by some of the horticulturists at Erfurt, whom I visited some years ago in order to learn as much as [627]possible about the methods of production of their novelties. Hence it issimply impossible to decide the question on the basis of the experienceof the breeders. Even in the case of the same novelty arising in sundryvarieties of the same species, the question as to common origin, bymeans of crossing, is often hard to decide, as for instance inmoss-roses and nectarines. On the other hand, instances are on recordwhere the same novelty has appeared at different times, often at longintervals. Such is the case with the butterfly-cyclamen, a form withwide-spreading petals which originated in Martin's nursery in England. The first time it was seen it was thought to be of no value, and wasthrown away, but when appearing for a second time it was multiplied andeventually placed on the market. Other varieties of _Cyclamen_, as forinstance the crested forms, are also known to have originatedrepeatedly. 

In concluding this series of examples of horticultural mutations, Imight mention two cases, which have occurred in my own experimentalgarden. The first refers to a tubular _Dahlia_. It has ray-florets, theligules of which have their margins grown together so as to form tubes, with the outer surface corresponding to the pale under-surface of thecorolla. 

This novelty originated in a single plant in a [628] culture from theseed of the dwarf variety "Jules Chretien. " The seeds were taken fromintroduced plants in my garden, and as the sport has no ornamental valueit is uncertain whether this was the first instance or whether it hadpreviously occurred in the nursery at Lyons, from whence the bulbs weresecured. Afterwards it proved true from seed, but was very variable, exhibiting rather the features of an ever-sporting variety. 

Another novelty was seen the first time in several individuals. It was apink sport of the European cranesbill, _Geranium pratense_. It arosequite unexpectedly in the summer of 1902 from a striped variety of theblue species. It was seen in seven specimens out of a lot of about ahundred plants. This strain was introduced into my garden in 1897, whenI bought two plants under the name of _Geranium pratense album_, whichhowever proved to belong to the striped variety. From their seeds Isowed in 1898 a first generation, of which a hundred plants flowered thenext year, and from their seeds I sowed in 1900 the lot which producedthe sport. Neither the introduced plants nor their offspring hadexhibited the least sign of a color-variation, besides the blue andwhite stripes. Hence it is very probable that my novelty was a truefirst mutation, the more probably [629] so since a pink variety wouldwithout doubt have a certain horticultural value and would have beenpreserved if it had occurred. But as far as I have been able toascertain, it is as yet unknown, nor has it been described until today. 

Summing up the results of this long, though very incomplete, list ofhorticultural novelties with a more or less well-known origin, we seethat sudden appearances are the rule. Having once sprung into existencethe new varieties are ordinarily constant, except as affected byvicinism. Details concerning the process are mostly unavailable or atleast are of very doubtful value. And to this it should be added thatreally progressive mutations have hardly been observed in horticulture. Hence the theoretical value of the facts is far less than might havebeen expected. 

[630]LECTURE XXII

SYSTEMATIC ATAVISM

The steady cooperation of progression and retrogression is one of theimportant principles of organic evolution. I have dwelt upon this pointmore than once in previous lectures. I have tried to show that both inthe more important lines of the general pedigree of the vegetablekingdom, and in the numerous lateral branches ending in the genera andspecies within the families, progression and retrogression are nearlyalways at work together. Your attention has been directed to themonocotyledons as an example, where retrogression is everywhere soactive that it can almost be said to be the prevailing movement. Reduction in the vegetative and generative organs, in the anatomicalstructure and growth of the stems, and in sundry other ways is themethod by which the monocotyledons have originated as a group from theirsupposed ancestors among the lower dicotyledonous families. Retrogression is the leading idea in the larger families of the group, [631] as for instance in the aroids and the grasses. Retrogradeevolution is also typical in the highest and most highly differentiatedfamily of the monocotyledons, the orchids, which have but one or twostamens. In the second place I have had occasion more than once toassert that retrogression, though seemingly consisting in thedisappearance of some quality, need not, as a rule, be considered as acomplete loss. Quite on the contrary, it is very probable that reallosses are extremely rare, if not wholly lacking. Ordinarily the loss isonly apparent, the capacity becomes inactive only, but is not destroyed. The character has become latent, as it is commonly stated, and thereforemay return to activity and to the full display of its peculiarity, whenever occasion offers. 

Such a return to activity was formerly called atavism. But as we haveseen, when dealing with the phenomena of latency at large, sundry casesof latency are to be distinguished, in order to get a clear insight intothese difficult processes. 

So it is with atavism, too. If any plant reverts to a known ancestor, wehave a positive and simple case. But ancestors with alternate specificmarks are as a rule neither historically nor experimentally manifest. They are only reputed to be such, and the presumption rests [632] uponthe systematic affinity between the derivative species and its nearestprobable allies. Such reversions are now to be examined at some lengthand may be adequately treated under the head of systematic atavism. Tothis form of atavism pertain, on the basis of our definition, thosephenomena by which species assume one or more characters of allies, fromwhich they are understood to have descended by the loss of the characterunder discussion. The phenomena themselves consist in the production ofanomalies and varieties, and as the genetic relation of the latter isoften hardly beyond doubt, the anomalies seem to afford the bestinstances for the study of systematic atavism. This study has for itschief aim the demonstration of the presence of the latent characters, and to show that they return to activity suddenly and not by a slow andgradual recovery of the former features. It supports the assertion thatthe visible elementary characters are essentially an external display ofqualities carried by the bearers of heredity, and that these bearers areseparate entities, which may be mingled together, but are not fused intoa chaotic primitive life-substance. Systematic atavism by this meansleads us to a closer examination of the internal and concealed causes, which rule the affinities and divergencies of [633] allied species. Itbrings before us and emphasizes the importance of the conception of theso-called unit-characters. 

The primrose will serve as an example. In the second lecture we haveseen that the old species of Linnaeus, the _Primula veris_, was split upby Jacquin into three smaller ones, which are called _P. Officinalis_, _P. Elatior_ and _P. Acaulis_. From this systematic treatment we caninfer that these three forms are assumed to be derived from a commonancestor. Now two of them bear their flowers in bracted whorls, condensed into umbels at the summits of a scape. The scapes themselvesare inserted in the axils of the basal leaves, and produce the flowersabove them. In the third species, _Primula acaulis_, this scape islacking and the flowers are inserted singly in the axils on long slenderstalks. For this reason the species is called acaulescent, indicatingthat it has no other stem than the subterranean rootstock. But on closerinspection we observe that the flower stalks are combined into littlegroups, each group occupying the aril of one of the basal leaves. Thisfact at once points to an analogy with the umbellate allies, and inducesus to examine the insertion of the flowers more critically. In doing sowe find that they are united at their base so as to constitute a sessileumbel. [634] The scapes are not absolutely lacking, but only reduced toalmost invisible rudiments. 

Relying upon this conclusion we infer that all of the three elementaryspecies have umbels, some pedunculate and the others not. On this pointthey agree with the majority of the allied species in the genus and inother genera, as for instance in _Androsace_. Hence the conclusion thatthe common ancestors were perennial plants with a rootstock bearingtheir flowers in umbels or whorls on scapes. Lacking in the _Primulaveris_, these scapes must obviously have been lost at the time of theevolution of this form. 

Proceeding on this line of speculation we at once see that a veryadequate opportunity for systematic atavism is offered here. Accordingto our general conception the apparent loss of a scape is no proof of acorresponding internal loss, but might as well be caused simply by thereduction of the scape-growing capacity to a latent or inactive state. It might be awakened afterwards by some unknown agency, and return toactivity. 

Now this is exactly what happens from time to time. In Holland theacaulescent primrose is quite a common plant, filling the woods in thespring with thousands of clusters of bright yellow flowers. It is a veryuniform type, but in [635] some years it is seen to return to atavisticconditions in some rare individuals. More than once I have observed suchcases myself, and found that the variation is only a partial one, producing one or rarely two umbels on the same plant, and liable to failof repetition when the varying specimens are transplanted into thegarden for further observation. But the fact remains that scapes occur. The scapes themselves are of varying length, often very short, andseldom long, and their umbels display the involucre of bracts in amanner quite analogous to that of the _Primula officinalis_ and _P. Elatior_. To my mind this curious anomaly strongly supports the view ofthe latent condition of the scape in the acaulescent species, and thatsuch a dormant character must be due to a descent from ancestors withactive scapes, seems to be in no need of further reiteration. Returningto activity the scapes at once show a full development, in no wayinferior to that of the allied forms, and only unstable in respect totheir length. 

A second example is afforded by the bracts of the crucifers. This groupis easily distinguished by its cruciform petals and the grouping of theflowers into long racemes. In other families each flower of such aninflorescence would be subtended by a bract, according to the [636]general rule that in the higher plants side branches are situated in thearils of leaves. Bracts are reduced leaves, but the spikes of thecruciferous plants are generally devoid of them. The flower-stalks, withnaked bases, seem to arise from the common axis at indefinite points. 

Hence the inference that crucifers are an exception to a general rule, and that they must have originated from other types which did complywith this rule, and accordingly were in the possession of floral bracts. Or, in other words, that the bracts must have been lost during theoriginal evolution of the whole family. This conclusion being accepted, the accidental re-apparition of bracts within the family must beconsidered as a case of systematic atavism, quite analogous to there-appearance of the scapes in the acaulescent primrose. The systematicimportance of this phenomenon, however, is far greater than in the firstcase, in which we had only to deal with a specific character, while theabolition of the bracts has become a feature of a whole family. 

This reversion is observed to take place according to two widelydifferent principles. On one hand, bracts may be met with in a few strayspecies, assuming the rank of a specific character. On the other handthey may be seen [637] to occur as an anomaly, incompletely developed, often very rare and with all the appearance of an accidental variation, but sometimes so common as to seem nearly normal. 

Coming now to particular instances, we may turn our attention in thefirst place to the genus _Sisymbrium_. This is a group of about 50species, of wide geographic distribution, among which the hedge mustard(_S. Officinalis_) is perhaps the most common of weeds. Two species arereputed to have bracts, _Sisymbrium hirsutum_ and _S. Supinum_. Eachflower-stalk of their long racemes is situated in the aril of such abract, and the peculiarity is quite a natural one, corresponding exactlyto what is seen in the inflorescence of other families. Besides the_Sisymbrium some six other genera afford similar structures. 

_Erucastrum pollichii_ has been already alluded to in a former lecturewhen dealing with the same problem from another point of view. Aspreviously stated, it is one of the most manifest and most easilyaccessible examples of a latent character becoming active throughsystematic atavism. In fact, its bracts are found so often as to beconsidered by some authors as of quite normal occurrence. Contrastedwith those of the above mentioned species of _Sisymbrium_, they are notseen at the base of all the flower [638] stalks, but are limited to thelowermost part of the raceme, adorning a few, often ten or twelve, andrarely more flower-stalks. Moreover they exhibit a feature which isindicative of the presence of an abnormality. They are not all of thesame size, but decrease in length from the base of the raceme upward, and finally slowly disappear. 

Besides these rare cases there are quite a number of cruciferous specieson record, which have been observed to bear bracts. Penzig in hisvaluable work on teratology gives a list of 33 such genera, many of themrepeating the anomaly in more than one species. Ordinary cabbages areperhaps the best known instance, and any unusual abundance ofnourishment, or anomalous cause of growth seems to be liable to incitethe development of bracts. The hedge garlic or garlic mustard(_Alliaria_), the shepherd's purse, the wormseed or _Erysimumcheiranthoides_ and many others afford instances. In my cultures ofHeeger's shepherd's purse, the new species derived at Landau in Germanyfrom the common shepherd's purse, the anomaly was observed to occur morethan once, showing that the mutation, which changed the fruits, had notin the least affected this subordinate anomalous peculiarity. In allthese cases the bracts behave as with the Erucastrum, [639] beinglimited to the base of the spike, and decreasing in size from the lowerflowers upward. Connected with these atavistic bracts is a feature ofminor importance, which however, by its almost universal accompanimentof the bracts, deserves our attention, as it is indicative of anotherlatent character. As a rule, the bracts are grown together with theiraxillary flower-stalk. This cohesion is not complete, nor is it alwaysdeveloped in the same degree. Sometimes it extends over a large part ofthe two organs, leaving only their tips free, but on other occasions itis limited to a small part of the base. But it is very interesting thatthis same cohesion is to be seen in the shepherd's purse, in thewormseed and in the cabbage, as well as in the case of the _Erucastrum_and most of the other observed cases of atavistic bracts. This factsuggests the idea of a common origin for these anomalies, and would leadto the hypothesis that the original ancestors of the whole family, before losing the bracts, exhibited this peculiar mode of cohesion. 

Bracts and analogous organs afford similar cases of systematic atavismin quite a number of other families. Aroids sometimes produce longbracts from various places on their spadix, as may be seen in thecultivated greenhouse species, _Anthurium scherzerianum_. [640] Poppieshave been recorded to bear bracts analogous to the little scales on theflower-stalks of the pansies, on the middle of their flower stalks. Asimilar case is shown by the yellow foxglove or _Digitalis parviflora_. The foxgloves as a rule have naked flower-stalks, without the two littleopposite leafy organs seen in so many other instances. The yellowspecies, however, has been seen to produce such scales from time totime. The honeysuckle genus is, as a rule, devoid of the stipules at thebase of the petiole, but _Lonicera etrusca_ has been observed to developsuch organs, which were seen to be free in some, but in other specimenswere adnate to the base of the leaf, and even connate with those of theopposite leaf. 

Other instances could be given proving that bracts and stipules, whensystematically lacking, are liable to reappear as anomalies. In doingso, they generally assume the peculiar characters that would be expectedof them by comparison with allied genera in which they are of normaloccurrence. There can be no doubt that their absence is due to anapparent loss, resulting from the reduction of a formerly active qualityto inactivity. Resuming this effective state, the case attains the valueand significance accorded to systematic atavism. 

A very curious instance of reduced bracts, developing [641] to unusualsize, is afforded by a variety of corn, which is called _Zea Mayscryptosperma_, or _Zea Mays tunicata_. In ordinary corn the kernels aresurrounded by small and thin, inconspicuous and membranaceous scales. Invisible on the integrate spikes, when ripe, they are easily detectedby pulling the kernels out. In _cryptosperma_ they are so stronglydeveloped as to completely hide the kernels. Obviously they constitute acase of reversion to the characters of some unknown ancestor, since thecorn is the only member of the grass-family with naked kernels. The var. _tunicata_, for this same reason, has been considered to be the originalwild form, from which the other varieties of corn have originated. Butas no historical evidence on this point is at hand, we must leave it asit is, notwithstanding the high degree of attractiveness attached to thesuggestion. 

The horsetail-family may be taken as a further support of our assertion. Some species have stems of two kinds, the fertile being brownish andappearing in early spring before the green or sterile ones. In othersthe stems are all alike, green and crowned with a conelike spike ofsporangia-bearing scales. Manifestly the dimorphous cases are to beconsidered as the younger ones, partly because they are obviousexceptions to the common rule, and [642] partly because the division oflabor is indicative of a higher degree of evolution. But sometimes thesedimorphic species are seen to revert to the primary condition, developing a fertile cone at the summit of the green summer-stem. I havehad the opportunity of collecting an instance of this anomaly on thetall _Equisetum telmateja_ in Switzerland, and other cases are on recordin teratological literature. It is an obvious example of systematicatavism, occurring suddenly and with the full development of all thequalities needed for the normal production of sporangia and spores. Allof these must be concealed in a latent condition within the youngtissues of the green stems. 

More than once I have had occasion to deal with the phenomenon oftorsions, as exhibited by the teasels and some other plants. Thisanomaly has been shown to be analogous to the cases described as doubleadaptations. The capacity of evolving antagonistic characters isprominent in both. The antagonists are assumed to lie quietly togetherwhile inactive. But as soon as evolution calls them into activity theybecome mutually exclusive, because only one of them can come to fulldisplay in the same organ. External influences decide which of the twobecomes dominant and which remains dormant. This decision must takeplace separately [643] for each stem and each branch, but as a rule, thestronger ages are more liable to furnish anomalies than the weaker. 

Exactly the same thing is true of double adaptations. Every bud of thewater-persicaria may develop either into an erect or into a floatingstem, according as it is surrounded by water or by relatively dry soil. In other cases utility is often less manifest, but some use may eitherbe proved, or shown to be very probable. At all events the termadaptation includes the idea of utility, and obviously uselesscontrivances could hardly be brought under the same head. 

We have also dealt with the question of heredity. It is obvious thatfrom the flowers of the floating and erect stems of the water-persicariaseeds will result, each capable of yielding both forms. Quite the samething was the case with the teasels. Some 40% of the progeny producebeautifully twisted stems, but whether the seed was saved from the mostcompletely twisted specimens or from the straight plants of the race wasof no importance. 

This phenomenon of twisting may now be considered from quite anotherpoint of view. It is a case of systematic atavism, or of thereacquirement of some ancient and long-lost quality. This quality is thealternate position of [644] the leaves, which has been replaced in theteasel family by a grouping in pairs. In order to prove the validity ofthis assertion, it will be necessary to discuss two points separately, viz. : relative positions of the leaves, and the manner in which thealternate position causes the stems to become twisted. 

Leaves are affixed to their stems and branches in various ways. Amongthem one is of wide occurrence throughout the whole realm of the higherplants, while all the others are more rare. Moreover these subordinatearrangements are, as a rule, confined to definite systematic groups. Such groups may be large, as for instance, the monocotyledons, that havetheir leaves arranged in two opposite rows in many families, or small, as genera or subdivisions of genera. Apart from these special cases themain stem and the greater part of the branches of the pedigree of thehigher plants exhibit a spiral condition or a screw arrangement, allleaves being inserted at different points and on different sides of thestem. This condition is assumed to be the original one, from which themore specialized types have been derived. As is usual with characters ingeneral, it is seen to vary around an average, the spiral becomingnarrower and looser. A narrow spiral condenses the leaves, while a [645]loose one disperses them. According to such fluctuating deviations thenumber of leaves, inserted upon a given number of spiral circuits, isdifferent in different species. In a vast majority of cases 13 leavesare found on 5 circuits, and as we have only to deal with thisproportion in the teasels we will not consider others. 

In the teasels this screw-arrangement has disappeared, and has beenreplaced by a decussate grouping. The leaves are combined into pairs, each pair occupying the opposite sides of one node. The succeeding pairsalternate with one another, so as to place their leaves at right angles. The leaves are thus arranged on the whole stem in four equidistant rows. 

On the normal stem of a teasel the two members of a pair are tied to oneanother in a comparatively complicated way. The leaves are broadlysessile and their bases are united so as to constitute a sort of cup. The margins of these cups are bent upward, thereby enabling them to holdwater, and after a rainfall they may be seen filled to the brim. It isbelieved that these little reservoirs are useful to the plant during theflowering period, because they keep the ants away from the honey. Considering the internal structure of the stem at the base of these cupswe find that the vascular bundles of the two opposite leaves arestrongly connected [646] with one another, constituting a ring whichnarrowly surrounds the stem, and which would impede an increase inthickness, if such were in the nature of the plant. But since the stemsend their existence during the summer of their development, thisstructure is of no real harm. 

The grouping of the leaves in alternate pairs may be seen within the budas well as on the adult stems. In order to do this, it is necessary tomake transverse sections through the heart of the rosette of the leavesof the first year. If cut through the base, the pair exhibit connatewings, corresponding to the water-cups; if cut above these, the leavesseem to be free from one another. 

In order to compare the position of leaves of the twisted plants withthis normal arrangement, the best way is to make a corresponding sectionthrough the heart of the rosette of the first year. It is not necessaryto make a microscopic preparation. In the fall the changed dispositionmay at once be seen to affect the central leaves of the group. All therosettes of the whole race commence with opposite leaves; those that areto produce straight stems remain in this condition, but the preparationfor twisting begins at the end of the first year as shown by a specialarrangement of the leaves. This [647] disposition may then be seen toextend to the very center of the rosette, by use of microscopicalsections. Examining sections made in the spring, the originalarrangement of the leaves of the stem is observed to continue until thebeginning of the growth of the shoot. It is easy to estimate the numberof leaves corresponding to a given number of spiral circuits in thesesections and the proportion is found to indicate 13 leaves on 5 turns. These figures are the same as those given above for the ordinaryarrangement of alternate leaves in the main lines of the pedigree of thevegetable kingdom. 

Leaving aside for the moment the subsequent changes of this spiralarrangement, it becomes at once clear that here we have a case ofsystematic atavism. The twisted teasels lose their decussation, but indoing so the leaves are not left in a disorderly dispersion, but adistinct new arrangement takes its place, which is to be assumed as thenormal one for the ancestors of the teasel family. The case is to beconsidered as one of atavism. Obviously no other explanation ispossible, than the supposition that the 5-13 spiral is still latent, though not displayed by the teasels. But in the very moment when thefaculty of decussation disappears, it resumes its place, and becomes[648] as prominent as it must once have been in the ancestors, and isstill in that part of their offspring, which has not become changed inthis respect. Thus the proof of our assertion of systematic atavism is, in this case, not obtained by the inspection of the adult, but by theinvestigation of the conditions in an early stage. It remains to beexplained how the twisting may finally be caused by this incipientgrouping of the leaves. Before doing so, it may be as well to state thatthe case of the teasel is not an isolated one, and that the sameconclusions are supported by the valerian, and a large number of otherexamples. In early spring some rosettes show a special condition of theleaves, indicating thereby at once their atavism and their tendency tobecome twisted as soon as they begin to expand. The Sweet William or_Dianthus barbatus_ affords another instance; it is very interestingbecause a twisted race is available, which may produce thousands ofinstances developed in all imaginable degrees, in a single lot ofplants. _Viscaria oculata_ is another instance belonging to the samefamily. 

The bedstraw (_Galium_) also includes many species which from time totime produce twisted stems. I have found them myself in Holland on_Galium verum_ and _G. Aparine_. Both seem [649] to be of rareoccurrence, as I have not succeeded in getting any repetition byprolonged culture. 

Species, which generally bear their leaves in whorls, are also subjectedto casual atavisms of this kind, as for instance the tall Europeanhorsetail, _Equisetum Telmateja_, which occasionally bears cones on itsgreen summer stems. Its whorls are changed on the twisted parts intoclearly visible spirals. The ironwood or _Casuarina quadrivalvis_ issometimes observed to produce the same anomaly on its smaller lateralbranches. 

Coming now to the discussion of the way in which the twisting is theresult of the spiral disposition of the leaves, we may consider thisarrangement on stems in the adult state. These at once show the spiralline and it is easy to follow this line from the base up to the apex. Inthe most marked cases it continues without interruption, not rarelyhowever, ending in a whorl of three leaves and a subsequent straightinternode, of which there may even be two or three. The spiral exhibitsthe basal parts of the leaves, with the axillary lateral branches. Thedirection of the screw is opposed to that of the twisting, and thespiral ribs are seen to cross the line of insertion of the leaves atnearly right angles. On this line the leaves are nearer [650] to oneanother than would correspond to the original proportion of 5 turns for13 leaves. In fact, 10 or even 13 leaves may not rarely be counted on asingle turn. Or the twist may become so strong locally as to change thespiral into a longitudinal line. On this line all inserted leaves extendthemselves in the same direction, resembling an extended flag. 

The spiral on the stem is simply the continuation of the spiral linefrom within the rosettes of the first year. Accordingly it is seen tobecome gradually less steep at the base. For this reason it must be oneand the same with this line, and in extreme youth it must have producedits leaves at the same mutual distances as this line. Transversesections of the growing summits of the stems support this conclusion. 

From these several facts we may infer that the steepness of the spiralline increases on the stem, as it is gradually changed into a screw. Originally 5 turns were needed for 13 leaves, but this number diminishesand 4 or 3 or even 2 turns may take the same number of foliar organs, until the screw itself is changed into a straight line. 

This change consists in an unwinding of the whole spiral, and in orderto effect this the stem must become wound up in the opposite direction. The winding of the foliar screw must [651] curve the longitudinal ribs. The straighter and steeper the screw becomes, the more the ribs willbecome twisted. That this happens in the opposite direction is obvious, without further discussion. The twisting is the inevitable consequenceof the reversal of the screw. 

Two points remain to be dealt with. One is the direct proof of thereversal of the screw, the other the discussion of its cause. The firstmay be observed by a simple experiment. Of course it proceeds onlyslowly, but all that is necessary is to mark the position of one of theyounger leaves of a growing stem of a twisting individual and to observethe change in its position in a few hours. It will be seen to haveturned some way around the stem, and finally may be seen to make acomplete revolution in the direction opposite to the screw, and therebydemonstrating the fact of its uncurling. 

The cause of this phenomenon is to be sought in the intimate connectionof the basal parts of the leaves, which we have detailed above. Thefibrovascular strands constitute a strong rope, which is twisted aroundthe stem along the line on which the leaves are inserted. Thestrengthening of the internodes may stretch this rope to some extent, but it is too strong to be rent asunder. Hence it opposes the normalgrowth, and the only manner in which the internodes [652] may adjustthemselves to the forces which tend to cause their expansion is bystraightening the rope. In doing so they may find the required space, bygrowing out in an unusual direction, bending their axes and twisting theribs. 

To prove the validity of this explanation, a simple experiment may begiven. If the fibrovascular rope is the mechanical impediment whichhinders the normal growth, we may try the effect of cutting through thisrope. By this means the hindrance may at least locally be removed. Now, of course, the operation must be made in an early stage before, or atthe beginning of the period of growth, in every case before theuncurling of the rope begins. Wounds made at this time are apt to giverise to malformations, but notwithstanding this difficulty I havesucceeded in giving the necessary proof. Stems operated upon becomestraight where the rope is cut through, though above and under thewounded part they go on twisting in the usual way. 

Sometimes the plants themselves succeed in tearing the rope asunder, andlong straight internodes divide the twisted stems in two or more partsin a very striking manner. A line of torn leaf-bases connects the twoparts of the screw and gives testimony of what has passed within [653]the tissues. At other times the straightening may have taken placedirectly internal to a leaf, and it is torn and may be seen to beattached to the stem by two distinct bases. 

Summing up this description of the hereditary qualities of our twistedteasels and of their mechanical consequences, we may say that the lossof the normal decussation is the cause of all the observed changes. Thisspecial adaptation, which places the leaves in alternating pairs, replaced and concealed the old and universal arrangement on a screwline. In disappearing, it leaves the latter free, and according to therule of systematic atavism, this now becomes active and takes its place. If the fibrovascular connection of the leaf-bases were lost at the sametime the stems would grow and become straight and tall. This changehowever, does not occur, and the bases of the leaves now constitute acontinuous rope instead of separate rings, and thereby impede thestretching of the internodes. These in their turn avoid the difficultyby twisting themselves in a direction opposite to that of the spiral ofthe leaves. 

As a last example of systematic atavism I will refer to the reversionarychanges, afforded by the tomatoes. Though the culture of this plant is arecent one, it seems to be at present in a state of mutability, producing new strains, or [654] assuming the features of theirpresumable ancestors. In his work "The Survival of the Unlike, " Baileyhas given a detailed description of these various types. Moreover, hehas closely studied the causes of the changes, and shown the greattendency of the tomatoes to vicinism. By far the larger part of theobserved cases of running out of varieties are caused by accidentalcrosses through the agency of insects. Even improvements are not rarelydue to this cause. Besides these common and often unavoidable changes, others of greater importance occur from time to time. Two of themdeserve to be mentioned. They are called the "Upright" and the "Mikado"types, and differ as much or even more from their parents than thelatter do from any one of their wild congeners. Their characters cometrue from seed. The "Mikado" race or the _Lycopersicum grandifolium_(_L. Latifolium_) has larger and fewer leaflets than the slender andsomewhat flimsy foliage of the common form. Flat or plane blades withdecurrent margins constitute another character. This variety, however, does not concern our present discussion. The upright type has stiff andself-sustaining stems and branches, resembling rather a potato-plantthan a tomato. Hence the name _Lycopersicum solanopsis_ or _L. Validum_, under which it is usually described. [655] The foliage of the plant isso distinct as to yield botanical characters of sufficient importance tojustify this specific designation. The leaflets are reduced in numbersand greatly modified, and the flowers in the inflorescence are reducedto two or three. This curious race came in suddenly, without anypremonition, and the locality and date of its mutation are still onrecord. Until some years ago it had not made its appearance for a secondtime. Obviously it is to be considered as a reversionary form. The limpstems of the common tomatoes are in all respects indicative of thecultivated condition. They cannot hold themselves erect, but must betied up to supports. The color of the leaves is a paler green thanshould be expected from a wild plant. Considering other species of thegenus _Solanum_, of which the _Lycopersicum_ is a subdivision, the stemsare as a rule erect and self-supporting, with some few exceptions. These, however, are special adaptations as shown by the winding stems ofthe bitter-sweet. 

From this discussion we seem justified in concluding that the originalappearance of the upright type was of the nature of systematic atavism. It differs however, from the already detailed cases in that it is not amonstrosity, nor an ever-sporting race, but is as constant a form [656]as the best variety or species. Even on this ground it must beconsidered as a representative of a separate group of instances of theuniversal rule of systematic reversions. 

Of late the same mutation has occurred in the garden of C. A. White atWashington. The parent form in this case was the "Acme, " of the ordinaryweak and spreading habit of growth. It is known as one of the best andmost stable of the varieties and was grown by Mr. White for many years, and had not given any sign of a tendency towards change. Seeds from someof the best plants in 1899 were sown the following spring, and the youngseedlings unexpectedly exhibited a marked difference from their parents. From the very outset they were more strong and erect, more compact andof a darker green than the "Acme. " When they reached the fruiting stagethey had developed into typical representatives of the _Lycopersicumsolanopsis_ or upright division. The whole lot of plants comprised onlysome 30 specimens, and this number, of course, is too small to basefar-reaching conclusions upon. But all of the lot showed this type, notrue "Acme" being seen among them. The fruit differed in flavor, consistency and color from that of the parent, and it also ripenedearlier than the latter. No seed was saved from [657] these plants, butthe following year the "Acme" was sown again and found true to its type. Seeds saved from this generation in 1900 have, however, repeated themutation, giving rise to exactly the same new upright form in 1901. Thiswas called by its originator "The Washington. " Seeds from this secondmutation were kindly sent to me by Mr. White, and proved true to theirtype when sown in my garden. 

Obviously it is to be assumed in the case of the tomatoes as well as ininstances from other genera cited, that characters of ancestors, whichare not displayed in their progeny, have not been entirely lost, but arestill present, though in a latent condition. They may resume theiractivity unexpectedly, and at once develop all the features which theyformerly had borne. 

Latency, from this point of view, must be one of the most common thingsin nature. All organisms are to be considered as internally formed of ahost of units, partly active and partly inactive. Extremely minute andalmost inconceivably numerous, these units must have their materialrepresentatives within the most intimate parts of the cells. 

[658]LECTURE XXIII

TAXONOMIC ANOMALIES

The theory of descent is founded mainly on comparative studies, whichhave the advantage of affording a broad base and the convincing effectof concurrent evidence brought together from widely different sources. The theory of mutation on the other hand rests directly uponexperimental investigations, and facts concerning the actual descent ofone form from another are as yet exceedingly rare. It is alwaysdifficult to estimate the validity of conclusions drawn from isolatedinstances selected from the whole range of contingent phenomena, andthis is especially true of the present case. Systematic and physiologicfacts seem to indicate the existence of universal laws, and it is notprobable that the process of production of new species would bedifferent in the various parts of the animal and vegetable kingdoms. Moreover the principle of unit-characters, the preeminent significanceof which has come to be more fully recognized of late, is in fullharmony [659] with the theory of sudden mutations. Together these twoconceptions go to strengthen the probability of the sudden origin of allspecific characters. 

Experimental researches are limited in their extent, and the number ofcases of direct observation of the process of mutation will probablynever become large enough to cover the whole field of the theory ofdescent. Therefore it will always be necessary to show that thesimilarity between observed and other cases is such as to lift above alldoubt the assertion of their resulting from the same causes. 

Besides the direct comparison of the mutations described in our formerlectures, with the analogous cases of the horticultural and naturalproduction of species and varieties at large, another way is open toobtain the required proof. It is the study of the phenomena, designatedby Casimir de Candolle by the name of taxonomic anomalies. It is theassertion that characters, which are specific in one case, may beobserved to arise as anomalies or as varieties in other instances. Ifthey can be shown to be identical or nearly so in both, it is obviouslyallowable to assume the same origin for the specific character and forthe anomaly. In other terms, the specific marks may be considered ashaving originated according to the laws [660] that govern the productionof anomalies, and we may assume them to lie within reach of ourexperiments. The experimental treatment of the origin of species mayalso be looked upon as a method within our grasp. 

The validity and the significance of these considerations will at oncebecome clear, if we choose a definite example. The broadest and mostconvincing one appears to me to be afforded by the cohesion of thepetals in gamopetalous flowers. According to the current views thefamilies with the petals of their flowers united are regarded as one ortwo main branches of the whole pedigree of the vegetable kingdom. Eichler and others assume them to constitute one branch, and thereforeone large subdivision of the system. Bessey, on the other hand, hasshown the probability of a separate origin for those groups which haveinferior ovaries. Apart from such divergencies the connation of thepetals is universally recognized as one of the most important systematiccharacters. 

How may this character have originated? The heath-family or theEricaceae and their nearest allies are usually considered to be thelowest of the gamopetalous plants. In them the cohesion of the petals isstill subject to reversionary exceptions. Such cases of atavism may[661] be observed either as specific marks, or in the way of anomalies. _Ledum_, _Monotropa_ and _Pyrola_, or the Labrador tea, the Indian pipeand wintergreen are instances of reversionary gamopetalism with freepetals. In heaths (_Erica Tetralix_) and in rhododendrons the samedeviation is observed to occur from time to time as an anomaly, and eventhe common _Rhododendron ponticum_ of our gardens has a variety in whichthe corolla is more or less split. Sometimes it exhibits five freepetals, while at other times only one or two are entirely free, theremaining four being incompletely loosened. 

Such cases of atavism make it probable that the coherence of the petalshas originally arisen by the same method, but by action in the oppositedirection. The direct proof of this conclusion is afforded by a curiousobservation, made by Vilmorin upon the bright and large-floweredgarden-poppy, _Papaver bracteatum_. Like all poppies it has four petals, which are free from one another. In the fields of Messrs. Vilmorin, where it is largely cultivated for its seeds, individuals occur fromtime to time which are anomalous in this respect. They exhibit atendency to produce connate petals. Their flowers become monopetalous, and the whole strain is designated by the name of _Papaver_ [662]_bracteatum monopetalum_. Henry de Vilmorin had the kindness to send mesome of these plants, and they have flowered in my garden during severalyears. The anomaly is highly variable. Some flowers are quite normal, exhibiting no sign of connation; others are wholly gamopetalous, thefour petals being united from their base to the very margin of the cupformed. In consequence of the broadness of the petals however, this cupis so wide as to be very shallow. 

Intermediate states occur, and not infrequently. Sometimes only two orthree petals are united, or the connation does not extend the entirelength of the petals. These cases are quite analogous to the imperfectsplitting of the corolla of the rhododendron. Giving free rein to ourimagination, we may for a moment assume the possibility of a newsubdivision of the vegetable kingdom, arising from Vilmorin's poppy andhaving gamopetalous flowers for its chief character. If the characterbecame fixed, so as to lose its present state of variability, such agroup of supposititious gamopetalous plants might be quite analogous tothe corresponding real gamopetalous families. Hence there can be noobjection to the view, that the heaths have arisen in an analogousmanner from their polypetalous ancestors. Other species of [663] thesame genus have also been recorded to produce gamopetalous flowers, asfor instance, _Papaver hybridum_, by Hoffmann. Poppies are not the soleexample of accidental gamopetaly. Linnaeus observed the same deviationlong ago for _Saponaria officinalis_, and since, it has been seen in_Clematis Vitalba_ by Jaeger, in _Peltaria alliacea_ by Schimper, in_Silene annulata_ by Boreau and in other instances. No doubt it is notat all of rare occurrence, and the origin of the present gamopetalousfamilies is to be considered as nothing extraordinary. It is, as amatter of fact, remarkable that it has not taken place in more numerousinstances, and the mallows show that such opportunities have beenavailable at least more than once. 

Other instances of taxonomic anomalies are afforded by leaves. Manygenera, the species of which mainly bear pinnate or palmate leaves, havestray types with undivided leaves. Among the brambles, _Rubus odoratus_and _R. Flexuosus_ may be cited, among the aralias, _Aralia crassifolia_and _A. Papyrifera_, and among the jasmines, the deliciously scentedsambac (_Jasminum Sambac_). But the most curious instance is that of thetelegraph-plant, or _Desmodium gyrans_, each complete leaf of whichconsists of a large terminal leaflet and two little lateral ones. Theselatter keep up, [664] night and day, an irregular jerking movement, which has been compared to the movements of a semaphore. _Desmodium_ isa papilionaceous plant and closely allied to the genus _Hedysarum_, which has pinnate leaves with numerous pairs of leaflets. Its place inthe system leaves no doubt concerning its origin from pinnate-leavedancestors. At the time of its origination its leaves must have becomereduced as to the number of the blades, while the size of the terminalleaflet was correspondingly increased. 

It might seem difficult to imagine this great change taking placesuddenly. However, we are compelled to familiarize ourselves with suchhypothetical assumptions. Strange as they may seem to those who areaccustomed to the conception of continuous slow improvements, they arenevertheless in complete agreement with what really occurs. Fortunatelythe direct proof of this assertion can be given, and in a case which isnarrowly related, and quite parallel to that of the _Desmodium_, sinceit affects a plant of the same family. It is the case of themonophyllous variety of the bastard-acacia or _Robinia Pseud-Acacia_. Ina previous lecture we have seen that it originated suddenly in a Frenchnursery in the year 1855. It can be propagated by seed, and exhibits acurious degree [665] of variability of its leaves. In some instancesthese are one-bladed, the blade reaching a length of 15 cm. , and hardlyresembling those of the common bastard-acacia. Other leaves produce oneor two small leaflets at the base of the large terminal one, and by thiscontrivance are seen to be very similar to those of the _Desmodium_, repeating its chief characters nearly exactly, and only differingsomewhat in the relative size of the various parts. Lastly realintermediates are seen between the monophyllous and the pinnate types. As far as I have been able to ascertain, these are produced on weaktwigs under unfavorable conditions; the size of the terminal leafletdecreases and the number of the lateral blades increases, showingthereby the presence of the original pinnate type in a latent condition. 

The sudden origin of this "one-leaved" acacia in a nursery may be takenas a prototype of the ancient origin of _Desmodium_. Of course thecomparison only relates to a single character, and the movements of theleaflets are not affected by it. But the monophylly, or rather the sizeof the terminal blade and the reduction of the lateral ones, may be heldto be sufficiently illustrated by the bastard-acacia. It is worth whileto state, that analogous varieties have also arisen in other genera. The"one-leaved" [666] strawberry has already been referred to. Itoriginated from the ordinary type in Norway and at Paris. The walnutlikewise, has its monophyllous variety. It was mentioned for the firsttime as a cultivated tree about 1864, but its origin is unknown. Asimilar variety of the walnut, with "one-bladed" leaves but of varyingshapes, was found wild in a forest near Dieppe in France some years ago, and appeared to be due to a sudden mutation. 

Something more is known concerning the "one-bladed" ashes, varieties ofwhich are often seen in our parks and gardens. The common form has broadand deeply serrate leaves, which are far more rounded than the leafletsof the ordinary ash. The majority of the leaves are simple, but someproduce one or two smaller leaflets at their base, closely correspondingin this respect to the variations of the "one-bladed" bastard-acacia, and evidently indicating the same latent and atavistic character. Insome instances this analogy goes still further, and incompletely pinnateleaves are produced with two or more pairs of leaflets. Besides thisvariable type another has been described by Willdenow. It has singleleaves exclusively, never producing smaller lateral leaflets, and it issaid to be absolutely constant from seed, while the more variable types[667] seem to be also more inconstant when propagated sexually. Thedifference is so striking and affords such a reliable feature that Kochproposed to make two distinct varieties of them, calling the pure type_Fraxinus excelsior monophylla_, and the varying trees _F. Excelsiorexheterophylla_. Some writers, and among them Willdenow, have preferredto separate the "one-leaved" forms from the species, and to call them_Fraxinus simplicifolia_. 

According to Smith and to Loudon, the "one-leaved" ashes are found wildin different districts in-England. Intermediate forms have not beenrecorded from these localities. This mode of origin is that alreadydetailed for the laciniate varieties of alders and so many other trees. Hence it may be assumed that the "one-leaved" ashes have sprung suddenlybut frequently from the original pinnate species. The pure type ofWilldenow should, in this case, be considered as due to a slightlydifferent mutation, perhaps as a pure retrograde variety, while thevarying strains may only be eversporting forms. This would likewiseexplain part of their observed inconstancy. 

In this respect the historic dates, as collected by Korshinsky, are notvery convincing. Vicinism has of course, almost never been excluded, andpart of the multiformity of the offspring [668] must obviously be due tothis most universal agency. Indirect vicinism also plays some part, andprobably affords the explanation of some reputed mutative productions ofthe variety. So, for instance, in the case of Sinning, who after sowingthe seeds of the common ash, got as large a proportion as 2% ofmonophyllous trees in a culture of some thousand plants. It is probablethat his seeds were taken partly from normal plants, and partly fromhybrids between the normal and the "one-bladed" type, assuming thatthese hybrids have pinnate leaves like their specific parent, and bearthe characters of the other parent only in a latent condition. 

Our third example relates to peltate leaves. They have the stalkinserted in the middle of the blade, a contrivance produced by theconnation of the two basal lobes. The water-lilies are a well knowninstance, exhibiting sagittate leaves in the juvenile stage and changingin many species, into nearly circular peltate forms, of which _Victoriaregia_ is a very good example, although its younger stages do not alwaysexcite all the interest they deserve. The Indian cress (_Tropaeolum_), the marsh pennywort or _Hydrocotyle_, and many other instances could bequoted. Sometimes the peltate leaves are not at all orbicular, but areelongated, oblong or elliptic, and with only the lobes [669] at the baseunited. The lemon-scented _Eucalyptus citriodora_ is one of the mostwidely known cases. In other instances the peltate leaves become more orless hollow, constituting broad ascidia as in the case of thecrassulaceous genus _Umbilicus_. 

This connation of the basal lobes is universally considered as a goodand normal specific character. Nevertheless it has its manifest analogyin the realm of the anomalies. This is the pitcher or ascidium. On sometrees it is of quite common occurrence, as on the lime-tree (_Tiliaparvifolia_) and the magnolia (_Magnolia obovata_ and its hybrids). Itis probable that both these forms have varieties with, and otherswithout, ascidia. Of the lime-tree, instances are known of single treeswhich produce hundreds of such anomalous leaves yearly, and one such atree is growing in the neighborhood of Amsterdam at Lage Vuursche. Ihave alluded to these cases more than once, but on this occasion acloser inspection of the structure of the ascidium is required. For thispurpose we may take the lime-tree as an example. Take the shape of thenormal leaves in the first place. These are cordate at their base andmainly inequilateral, but the general shape varies to a considerableextent. This variation is closely related to the position of the leaveson the twigs, and shows [670] distinct indications of complying with thegeneral law of periodicity. The first leaves are smaller, with morerounded lobes, the subsequent leaves attain a larger size, and theirlobes slightly change their forms. In the first leaves the lobes are sobroad as to touch one another along a large part of their margins, butin organs formed later this contact gradually diminishes and the typicalleaves have the lobes widely separated. Now it is easily understood thatthe contact or the separation of the lobes must play a part in theconstruction of the ascidia, as soon as the margins grow together. Leaves which touch one-another, may be affected by the connation withoutany further malformation. They remain flat, become peltate and exhibit ashape which in some way holds a middle position between the pennywortsand the lemon-scented eucalyptus. Here we have the repetition of thespecific characters of these plants by the anomaly of another. Wheneverthe margins are not in contact, and become connate, notwithstandingtheir separation, the blade must be folded together in some slightdegree, in order to produce the required contact. This is the origin ofthe ascidium. It is quite superfluous to insist upon the fact that theirwidth or narrowness must depend upon the corresponding normal form. Themore distant the [671] lobes, the deeper the ascidium will become. Itshould be added that this explanation of the different shapes of ascidiais of general validity. 

Ascidia of the snake-plantain or _Plantago lanceolata_ are narrow tubes, because the leaves are oblong or lanceolate, while those of the broadleaved species of arrowhead, as for instance, the _Sagittaria japonica_, are of a conical shape. 

From the evidence of the lime-tree we may conclude that normal peltateleaves may have originated in the same way. And from the fact thatpitchers are one of the most frequent anomalies, we may conclude thatthe chance of producing peltate leaves must have been a very great one, and wholly sufficient to account for all observed cases. In everyinstance the previously existing shape of the leaf must have decidedwhether peltate or pitcher-like leaves would be formed. As far as we canjudge peltate anomalies are quite uninjurious, while ascidia are formswhich must impede the effect of the light on the leaf, as they concealquite an important part of the upper surface. In this way it is easilyconceivable that peltate leaves are a frequent specific character, whileascidia are not, as they only appear in the special cases of limitedadaptation, as in the instances of the so called pitcher-plants. Thegenera _Nepenthes_, [672] _Sarracenia_ and some others are very wellknown and perhaps even the bladderworts or _Utricularia_ might beincluded here. 

The reproduction of specific characters by anomalous ascidia is not atall limited to the general case as described above. More minute detailsmay be seen to be duplicated in the same way. Proofs are afforded on oneside by incomplete ascidia, and on the other by the double cups. 

Incomplete ascidia are those of the _Nepenthes_. The leaf is dividedinto three parts, a blade, a tendril and the pitcher. Or in other words, the limb produces a tendril at its summit, by means of which the plantis enabled to fasten itself to surrounding shrubs and to climb betweentheir branches. But the end of this tendril bears a well-formed urn, which however, is produced only after the revolving and graspingmovements of the tendril have been made. Some species have more roundedand some more elongated ascidia and often the shape is seen to changewith the development of the stem. The mouth of the urn is strengthenedby a thick rim and covered with a lid. Numerous curious contrivances inthese structures to catch ants and other insects have been described, but as they have no relation to our present discussion, we shall abstainfrom dealing with them. [673] Likewise we must refrain from aconsideration of the physiologic qualities of the tendril, and confineour attention to the combination of a limb, a naked midvein and anascidium. This combination is to be the basis of our discussion. It isliable to be produced all of a sudden. This assertion is proved by itsoccurrence as a varietal mark in one of our most ordinary cultivatedplants. It is the group known as _Croton_, belonging to the genus_Codiaeum_. A variety is called _interruptum_ and another_appendiculatum_, and these names both relate to the interruption of theleaves by a naked midvein. The leaves are seen to be built up of threeparts. The lower half retains the aspect of a limb; it is crowned by avein without lateral nerves or blade-like expansions, and this stalk inits turn bears a short limb on its summit. The base of this apical limbexhibits two connate lobes, forming together a wide cup or ascidium. Itshould be stated that these _interruptum_ varieties are highly variable, especially in the relative size of the three principal parts of theleaf. Though it is of course conceded that the ascidium of _Nepenthes_has many secondary devices which are lacking in _Croton_, it seemshardly allowable to deny the possibility of an analogous origin forboth. Those of the _Croton_, according to our knowledge regardingsimilar cases, must [674] have arisen at once, and hence the conclusionthat the ascidia of _Nepenthes_ are also originally due to a suddenmutation. Interrupted leaves, with an ascidium on a naked prolongationof the midvein, are by no means limited to the _Croton_ varieties. Asstray anomalies they have often been observed, and I myself had theopportunity of collecting them on magnolia, on clover and on some otherspecies. They are additional evidence in support of the explanationgiven above. 

In the same way double ascidia may be made use of to explain the foliarcups of the teasels and some other plants, as for instance, someEuropean snakeroots (_Eryngium maritimum_ and _E. Campestre_), or thefloral leaves of the honeysuckle. The leaves on the stems of the teaselsare disposed in pairs, and the bases of the two leaves of each pair areconnate so as to constitute large cups. We have already mentioned thesecups, and recall them in the present connection to use them as aprototype of the double ascidia. These are constituted of two oppositeleaves, accidentally connated at their base or along some part of theirmargins. If the leaves are sessile, the analogy with the teasels iscomplete, as shown, for instance, in a case of _Cotyledon_, acrassulaceous plant which is [675] known to produce such cups from timeto time. They are narrower than those of the teasel, but this depends, as we have seen for the "one-leaved" ascidium, on the shape of theoriginal leaf. In other respects they exactly imitate the teasel cupsshowing thereby how these cups may probably have originated. 

In numerous cases of anomalies some accidental structures are parallelto specific characters, while others are not, being obviously injuriousto their bearers. So it is also with the double ascidia. In the case ofstalked leaves the two opposite stalks must, of course, constitute along and very narrow tube, when growing together. This tube must bear atits summit the conical ascidium produced by the two connate limbs. Atits base however, it includes the terminal bud of the stem, andfrequently the tube is so narrow as to impede its further development. By this contrivance the double ascidium assumes a terminal position. Instances have been observed on magnolia, in _Boehmeria_ and in othercases. 

Flowers on leaves are of rare occurrence. Notwithstanding this, theyconstitute specific characters in some instances, accidental anomaliesin others. _Helwingia rusciflora flora_ is the most curious and bestknown instance. It is a little shrub, belonging to the Cornaceae, and[676] has broad elliptical undivided leaves. On the middle of themidvein these leaves are seen to bear small clusters of flowers; indeedthis is the only place where flowers are produced. Each cluster has from13-15 flowers, of which some are staminate and borne on stalks, whileothers are pistillate and nearly sessile. These flowers are small and ofa pale greenish color and yield small stone-fruits, with a thin coatingof pulpy tissue. As the name indicates, this mode of flowering isclosely similar to that of _Ruscus_, which however, does not bear itsflowers and berries on real leaves, but on leaflike expansions of thetwigs. _Phyllonoma ruscifolia_, a saxifragaceous plant, bears the samespecific name, indicating a similar origin of the flowers. Otherinstances have been collected by Casimir de Candolle, but their numberis very small. 

As a varietal mark, flowers on leaves likewise rarely occur. Oneinstance however, is very remarkable, and we have already dealt with it, when treating of constant varieties, and of the lack of vicinism in thecase of species with exclusive self-fertilization. 

It is the "Nepaul-barley" or _Hordeum trifurcatum_. The leaves, which inthis case bear the adventitious flowers, are the inner scales of thespikelets, and not on green leaves as in the [677] cases already alludedto. But this of course makes no real difference. The character isvariable to a high degree, and this fact indicates its varietal nature, though it should be recalled that at least with the _Helwingia_, themajority of the leaves are destitute of flowers, and that in this waysome degree of variability is present in this normal case too. 

All in all there are three sorts of "Nepaul-barley. " They have the samevarietal mark, but belong to different species of barley. These aredifferentiated according to the number of the rows in which the grainsare seen on the spikes. These numbers may be two, four or six, givingrise to the specific names of _Hordeum distichum_, _tetrastichum_ and_hexastichum_. Whether these three varieties are of independent, butparallel origin, or are to be considered as due to a single mutation andsubsequent crosses is not known, all of them being of ancient origin. Historic evidence concerning their birth is wholly wanting. From analogyit would seem probable that the character had arisen by a mutation inone of the three named species, and had been transferred to others bymeans of accidental crosses, even as it has been artificiallytransmitted of late to quite a number of other sorts. But howeveradmissible this conception may seem, there is of course no realobjection [678] to the assumption of independent and parallel mutations. 

For the purpose of a comparison with the _Helwingia_ type we arehowever, not at all concerned with the species to which the_trifurcatum_ variety belongs, but only with the varietal mark itself. The spikelets may be one-, two- or three-flowered, according to thespecies. If we choose for further consideration the _hexastichum_ type, each spikelet produces three normal flowers and afterwards three normalgrains. Morphologically however, the spikelet is not homologous to thoseparts of other grasses which have the same name. It is constituted ofthree real spikelets, and thus deserves the name of a tripleconstruction. Each of these three little organs has its normal pair ofouter scales or glumae. These are linear and short, ending in a long andnarrow spine. Those of the middle-most spikelets stand on its outerside, while those of the lateral part are placed transversely. In thisway they form a kind of involucre around the central parts. The latterconsist of the inner and outer palets or scales, each two of whichinclude one of the flowers. The outer palet is to be considered as themetamorphosed leaf, in the aril of which the flower is produced. In thecommon sorts of barley it bears a long awn, giving thereby its typicalaspect to the [679] whole spike. The axillary flower is protected on theopposite side by a two-keeled inner palet. Each flower exhibits threestamens and an ovary. In the six-rowed barley all the three flowers of atriple spikelet are fertile, and each of them has a long awn on the topof the outer palet. But in the two-rowed species only the middle-mostflower is normal and has an awn, the two remaining being sterile andmore or less rudimentary and with only very short awns. From thisdescription it is easily seen that the species of barley may bedistinguished from one another, even at a casual glance, by the numberof the rows of the awns, and therefore by the shape of the entirespikes. This striking feature, however, does not exist in the"Nepaul-barley. " The awns are replaced by curiously shaped appendices, which are three-lobed. The central lobe is oblong and hollow, and formsa kind of hood, which covers a small supernumerary floret. The twolateral lobes are narrower, often linear and extended into a smaller orlonger awn. These awns are mostly turned away from the center of thespike. The central lobe may sometimes bear two small florets, butordinarily only one is to be found, and this is often incomplete, havingonly one or two stamens, or is different in some other way. [680] Thesenarrow lateral lobes heighten the abnormal aspect of the whole spike. 

They are only produced at a somewhat advanced stage of the developmentof the palet, are united to one another and to the central part bystrong veins, which form transversal anastomoses at their insertion. Thelength of these awns is very variable, and this quality is perhaps themost striking of the whole variety. Often they reach only 1-2 mm. , orthe majority may become longer and attain even 1 cm. , while here andthere, between them, longer ones are inserted, extending in someinstances even as far as 3 cm. From the spike. Their transverse positionin such cases is strikingly contrasted with the ordinary erect type ofthe awns. 

These lateral lobes are to be regarded, from the morphologic point ofview, as differentiated parts of the blade of the leaf. Before they areformed, or coincidently with the beginning of their development, thesummit of the central lobe becomes hollow, and the development of thesupernumerary flower commences. In different varieties, and especiallyin the most recent crosses of them, this development is excessivelyvariable. 

The accidental flower arises at some distance beneath the summit of thescale, on its middle [681] vein. The development begins with theprotrusion of a little scale, and the flower itself is situated beneaththis scale, and is to be protected by it and by the primary scale, butis turned upside down at the same time. Opposite to this organ, whichrepresents the outer palet of the adventitious flower, two littleswollen bodies are evolved. In the normal flowers of barley and othergrains and grasses their function is to open the flowers by swelling, and afterwards collapse and allow them to close. 

In the adventitious flowers of the "Nepaul-barley, " however, thisfunction is quite superfluous. The stamens occur in varying numbers;typically there are three, but not rarely less, or more, are seen. Insome instances the complete double whorl of six, corresponding to theancestral monocotyledonous type, has been found. This is a very curiouscase of systematic atavism, quite analogous to the _Iris pallidaabavia_, previously alluded to, which likewise has six stamens, and tothe cases given in a previous lecture. But for our present discussion itis of no further interest. The ovary is situated in the middle of theflower, and in some instances two have been observed. This is also to beconsidered as a case of atavism. 

All these parts of the adventitious flower are more or less subject toarrest of development, [682] in a later stage. They may even sometimesbecome abnormal. Stamens may unite into pairs, or carpels bear fourstigmas. The pollen-sacs are as a rule barren, the mother-cellsundergoing atrophy, while normal grains are seen but rarely. Likewisethe ovaries are rudimentary, but Wittmack has observed the occasionalproduction of ripe grains from these abnormal florets. 

The scale is seldom seen to extend any farther upwards than thesupernumerary flower. But in the rare instances where it does prolongits growth, it may repeat the abnormality and bear a second floret abovethe first. This of course is generally much weaker, and morerudimentary. 

Raciborsky, who has lately given a full and very accurate description ofthis anomaly, lays great stress upon the fact that it is quite useless. It is perhaps the most obviously useless structure in the wholevegetable kingdom. Notwithstanding this, it has come to be as completelyhereditary as any of the most beautiful adaptations in nature. Thereforeit is one of the most serious objections to the hypothesis of slow andgradual improvements on the sole ground of their usefulness. Thestruggle for life and natural selection are manifestly inadequate togive even the slightest indication of [683] an explanation of this case. It is simply impossible to imagine the causes that might have producedsuch a character. The only way out of this difficulty is to assume thatit has arisen at once, in its present apparently differentiated and veryvariable condition, and that, being quite uninjurious and since it doesnot decrease the fertility of the race, it has never been subjected tonatural selection, and so has saved itself from destruction. 

But if we once grant the probability of the origin of the"Nepaul-barley" by a sudden mutation, we obviously must assume the samein the case of the _Helwingia_ and other normal instances. In this waywe gain a further support for our assertion, that even the strangestspecific characters may have arisen suddenly. 

After having detailed at some length those proofs which seem to be themost striking, and which had not been previously described withsufficient detail, we may now take a hasty survey of other contingentcases. In the first place the cruciate flowers of some onagraceousplants should be remembered. Small linear petals occur as a specificcharacter in _Oenothera cruciata_ of the Adirondacks, but have been seento arise as sudden mutations in the common evening-primrose (_O. Biennis_) in Holland, and in the willow-herb (_Epilobium hirsutum_) inEngland. [684] Leaves placed in whorls of three are very rare. Theoleander, juniper and some few other plants have ternate whorls as aspecific character. As an anomaly, ternate whorls are far more common, and perhaps any plant with opposite leaves may from time to time producethem. Races rich in this abnormality are found in the wild state in theyellow loosestrife or _Lysimachia vulgaris_, in which it is a veryvariable specific character, the whorls varying from two to four leaves. In the cultivated state it is met with in the myrtle or _Myrtuscommunis_, where it has come to be of some importance in Israeliticritual. Crisped leaves are known in a mallow, _Malva crispa_, and as avariety in cabbages, parsley, lettuce and others. The orbicular fruitsof Heeger's shepherd's purse (_Capsella heegeri_) recall similar fruitsof other cruciferous genera, as for instance, _Camelina_. Screw-likestems with wide spirals are specific in the flower-stalks of _Cyclamen_and _Vallisneria_, varietal in _Juncus effusus spiralis_ and accidentalin _Scirpus lacustris_. Dormant buds or small bulbs in inflorescencesare normal for wild onions, _Polygonum viviparum_ and others, varietalin _Poa alpina vivipara_ and perhaps in _Agave vivipara_, and accidentalin plantains (_Plantago lanceolata_), _Saxifraga umbrosa_ and others. [685] Cleft leaves, one of the most general anomalies, are typical in_Boehmeria biloba_. The adnation of the peduncles of the inflorescencesto the stem is typical in _Solanum_ and accidental in many other cases. 

It seems quite superfluous to add further proof. It is a very generalphenomenon that specific characters occur in other genera as anomalies, and under such circumstances that the idea of a slow evolution on theground of utility is absolutely excluded. No other explanation remainsthan that of a sudden mutation, and once granted for the abnormal cases, this explanation must obviously likewise be granted for the analogousspecific characters. 

Our whole discussion shows that mutations, once observed in definiteinstances, afford the most probable basis for the explanation ofspecific characters at large. 

[686]LECTURE XXIV

THE HYPOTHESIS OF PERIODIC MUTATIONS

The prevailing belief that slow and gradual, nearly invisible changesconstitute the process of evolution in the animal and vegetable kingdom, did not offer a strong stimulus for experimental research. Noappreciable response to any external agency was of course to beexpected. Responses were supposed to be produced, but the correspondingoutward changes would be too small to betray themselves to theinvestigator. 

The direct observation of the mutations of the evening-primrose haschanged the whole aspect of the problem at once. It is no longer amatter dealing with purely hypothetical conditions. Instead of the vaguenotions, uncertain hopes, and a priori conceptions, that have hithertoconfused the investigator, methods of observation have been formulated, suitable for the attainment of definite results, the general nature ofwhich is already known. 

To my mind the real value of the discovery [687] of the mutability ofthe evening-primrose lies in its usefulness as a guide for further work. The view that it might be an isolated case, lying outside of the usualprocedure of nature, can hardly be sustained. On such a supposition itwould be far too rare to be disclosed by the investigation of a smallnumber of plants from a limited area. Its appearance within the limitedfield of inquiry of a single man would have been almost a miracle. 

The assumption seems justified that analogous cases will be met with, perhaps even in larger numbers, when similar methods of observation areused in the investigation of plants of other regions. The mutablecondition may not be predicated of the evening-primroses alone. It mustbe a universal phenomenon, although affecting a small proportion of theinhabitants of any region at one time: perhaps not more than one in ahundred species, or perhaps not more than one in a thousand, or evenfewer may be expected to exhibit it. The exact proportion is immaterial, because the number of mutable instances among the many thousands ofspecies in existence must be far too large for all of them to besubmitted to close scrutiny. 

It is evident from the above discussion that next in importance to thediscovery of the prototype of mutation is the formulation of methods[688] for bringing additional instances to light. These methods maydirect effort toward two different modes of investigation. We may searchfor mutable plants in nature, or we may hope to induce species to becomemutable by artificial methods. The first promises to yield results mostquickly, but the scope of the second is much greater and it may yieldresults of far more importance. Indeed, if it should once becomepossible to bring plants to mutate at our will and perhaps even inarbitrarily chosen directions, there is no limit to the power we mayfinally hope to gain over nature. 

What is to guide us in this new line of work? Is it the minuteinspection of the features of the process in the case of theevening-primroses? Or are we to base our hopes and our methods onbroader conceptions of nature's laws? Is it the systematic study ofspecies and varieties, and the biologic inquiry into their realhereditary units? Or is the theory of descent to be our starting-point?Are we to rest our conceptions on the experience of the breeder, or isperhaps the geologic pedigree of all organic life to open to us betterprospects of success?

The answer to all such questions is a very simple one. All possibilitiesmust be considered, and no line of investigation ignored. For myself Ihave based my field-researches and my [689] testing of native plants onthe hypothesis of unit-characters as deduced from Darwin's _Pangenesis_. This conception led to the expectation of two different kinds ofvariability, one slow and one sudden. The sudden ones known at the timewere considered as sports, and seemed limited to retrograde changes, orto cases of minor importance. The idea that sudden steps might be takenas the principal method of evolution could be derived from thehypothesis of unit characters, but the evidence might be too remote fora starting point for experimental investigation. 

The success of my test has given proof to the contrary. Hence theassertion that no evidence is to be considered as inadequate for thepurpose under discussion. Sometime a method of discovering, or ofproducing, mutable plants may be found, but until this is done, allfacts of whatever nature or direction must be made use of. A very slightindication may change forever the whole aspect of the problem. 

The probabilities are now greatly in favor of our finding out the causesof evolution by a close scrutiny of what really happens in nature. Apersistent study of the physiologic factors of this evolution is thechief condition of success. To this study field-observations maycontribute as well as direct experiments, [690] microscopicalinvestigations as well as extended pedigree-cultures. The cooperation ofmany workers is required to cover the field. Somewhere no doubt thedesired principle lies hidden, but until it is discovered, all methodsmust be tried. 

With this conception as the best starting point for furtherinvestigation, we may now make a brief survey of the other phase of theproblem. We shall try to connect our observations on theevening-primroses with the theory of descent at large. 

We start with two main facts. One is the mutability of Lamarck'sprimrose, and the second is the immutable condition of quite a number ofother species. Among them are some of its near allies, the common andthe small flowered evening-primrose, or _Oenothera biennis_ and _O. Muricata_. 

From these facts, a very important question arises in connection withthe theory of descent. Is the mutability of our evening-primrosestemporary, or is it a permanent condition? A discussion of this problemwill give us the means of reaching a definite idea as to the scope ofour inquiries. 

Let us consider the present first. If mutability is a permanentcondition, it has of course no beginning, and moreover is not due to the[691] agency of external circumstances. Should this be granted for theevening-primrose, it would have to be predicated for other species foundin a mutable state. Then, of course, it would be useless to investigatethe causes of mutability at large, and we should have to limit ourselvesto the testing of large numbers of plants in order to ascertain whichare mutable and which not. 

If, on the other hand, mutability is not a permanent feature, it mustonce have had a beginning, and this beginning itself must have had anexternal cause. The amount of mutability and its possible directions maybe assumed to be due to internal causes. The determination of the momentat which they will become active can never be the result of internalcauses. It must be assigned to some external factor, and as soon as thisis discovered the way for experimental investigation is open. 

In the second place we must consider the past. On the supposition ofpermanency all the ancestors of the evening-primrose must have beenmutable. By the alternative view mutability must have been a periodicphenomenon, producing at times new qualities, and at other times leavingthe plants unchanged during long successions of generations. The presentmutable state must then have been preceded by an immutable [692]condition, but of course thousands of mutations must have been requiredto produce the evening-primroses from their most remote ancestors. 

If we take the species into consideration that are not mutable atpresent, we may ask how we are to harmonize them with each of the twotheories proposed. If mutability is permanent, it is manifest that thewhole pedigree of the animal and vegetable kingdom is to be consideredas built up of main mutable lines, and that the thousands of constantspecies can only be taken to represent lateral branches of thegenealogic tree. 

These lateral branches would have lost the capacity of mutating, possessed by all their ancestors. And as the principle of the hypothesisunder discussion does not allow a resumption of this habit, they wouldbe doomed to eternal constancy until they finally die out. Loss ofmutability, under this conception, means loss of the capacity for allfurther development. Only those lines of the main pedigree which haveretained this capacity would have a future; all others would die outwithout any chance of progression. 

If, on the other hand, mutability is not permanent, but a periodiccondition, all lines of the genealogic tree must be assumed to showalternatively [693] mutating and constant species. Some lines may bemutating at the present moment; others may momentarily be constant. Themutating lines will probably sooner or later revert to the inactivestate, while the powers of development now dormant may then becomeawakened on other branches. 

The view of permanency represents life as being surrounded withunavoidable death, the principle of periodicity, on the contrary, follows the idea of resurrection, granting the possibility of futureprogression for all living beings. At the same time it yields a morehopeful prospect for experimental inquiry. 

Experience must decide between the two main theories. It demonstratesthe existence of polymorphous genera, such as _Draba_ and _Viola_ andhundreds of others. They clearly indicate a previous state ofmutability. Their systematic relation is exactly what would be expected, if they were the result of such a period. Perhaps mutability has notwholly ceased in them, but, might be found to survive in some of theirmembers. Such very rich genera however, are not the rule, but areexceptional cases, indicating the rarity of powerful mutative changes. 

On the other hand, species may remain in a state of constancy duringlong, apparently during indefinite, ages. 

[694] Many facts plead in favor of the constancy of species. Thisprinciple has always been recognized by systematists. Temporarily thecurrent form of the theory of natural selection has assumed species tobe inconstant, ever changing and continuously being improved and adaptedto the requirements of the life-conditions. The followers of the theoryof descent believed that this conclusion was unavoidable, and wereinduced to deny the manifest fact that species are constant entities. The mutation theory gives a clew to the final combination of the twocontending ideas. Reducing the changeability of the species to distinctand probably short periods, it at once explains how the stability ofspecies perfectly agrees with the principle of descent throughmodification. 

On the other hand, the hypothesis of mutative periods is by no meansirreconcilable with the observed facts of constancy. Such casual changescan be proved by observations such as those upon the evening-primrose, but it is obvious that a disproof can never be given. The principlegrants the present constancy of the vast majority of living forms, andonly claims the exceptional occurrence of definite changes. 

Proofs of the constancy of species have been given in different ways. The high degree of similarity of the individuals of most of our [695]species has never been denied. It is observed throughout extendedlocalities, and during long series of years. Other proofs are affordedby those plants which have been transported to distant localities sometime since, but do not exhibit any change as a result of this migration. Widely dispersed plants remain the same throughout their range, providedthat they belong to a single elementary species. Many species have beenintroduced from America into Europe and have spread rapidly and widely. The Canadian horsetail (_Erigeron canadensis_), the evening-primrose andmany other instances could be given. They have not developed any specialEuropean features after their introduction. Though exposed to otherenvironmental conditions and to competition with other species, theyhave not succeeded in developing a new character. Such species as provedadequate to the new environment have succeeded, while those which didnot have succumbed. 

Much farther back is the separation of the species which now live bothin arctic regions and on the summits of our highest mountaintops. If wecompare the alpine flora with the arctic plants, a high degree ofsimilarity at once strikes us. Some forms are quite identical; othersare slightly different, manifestly representing elementary species ofthe same systematic [696] type. Still others are more distant or evenbelong to different genera. The latter, and even the diverging, thoughnearly allied, elementary species, do not yield adequate evidence in anydirection. 

They may as well have lived together in the long ages before theseparation of the now widely distant floras, or have sprung from acommon ancestor living at that time, and subsequently have changed theirhabits. After excluding these unreliable instances, a good number ofspecies remain, which are quite the same in the arctic and alpineregions and on the summits of distant mountain ranges. As notransportation over such large distances can have brought them from onelocality to the other, no other explanation is left than that they havebeen wholly constant and unchanged ever since the glacial period whichseparated them. Obviously they must have been subjected to widelychanging conditions. The fact of their stability through all theseoutward changes is the best proof that the ordinary external conditionsdo not necessarily have an influence on specific evolution. They mayhave such a result in some instances, in others they obviously have not. Many arctic forms bearing the specific name of _alpinus_ justify thisconclusion. _Astragalus alpinus_, _Phleum alpinum_, _Hieracium alpinum_and [697] others from the northern parts of Norway may be cited asexamples. 

Thus Primula imperialis has been found in the Himalayas, and many otherplants of the high mountains of Java, Ceylon and northern India areidentical forms. Some species from the Cameroons and from Abyssinia havebeen found on the mountains of Madagascar. Some peculiar Australiantypes are represented on the summit of Kini Balu in Borneo. None ofthese species, of course, are found in the intervening lowlands, and theonly possible explanation of their identity is the conception of acommon post-glacial origin, coupled with complete stability. Thisstability is all the more remarkable as nearly allied but slightlydivergent forms have also been reported from almost all of theselocalities. Other evidence is obtained by the comparison of ancientplants with their living representatives. The remains in tombs ofancient Egypt have always afforded strong support of the views of theadherents of the theory of stability, and to my mind they still do so. The cereals and fruits and even the flowers and leaves in the funeralwreaths of Rameses and Amen-Hotep are the same that are still nowcultivated in Egypt. Nearly a hundred or more species have beenidentified. Flowers of _Acacia_, leaves of _Mimusops_, [698] petals of_Nymphaea_ may be cited as instances, and they are as perfectlypreserved as the best herbarium-specimens of the present time. Thepetals and stamens retain their original colors, displaying them asbrightly as is consistent with their dry state. 

Paleontologic evidence points to the same conclusion. Of course theremains are incomplete, and rarely adequate for a close comparison. Therange of fluctuating variability should be examined first, but the testof elementary species given by their constancy from seed cannot, ofcourse, be applied. Apart from these difficulties, paleontologists agreein recognizing the very great age of large numbers of species. It wouldrequire a too close survey of geologic facts to go into details on thispoint. Suffice it to say that in more recent Tertiary deposits manyspecies have been identified with living forms. In the Miocene periodespecially, the similarity of the types of phanerogamic plants withtheir present offspring, becomes so striking that in a large number ofcases specific distinctions rest in greater part on theoreticalconceptions rather than on real facts. For a long time the ideaprevailed that the same species could not have existed through more thanone geologic period. Many distinctions founded on this belief have sincehad to be abandoned. [699] Species of algae belonging to thewell-preserved group of the diatoms, are said to have remained unchangedfrom the Carboniferous period up to the present time. 

Summing up the results of this very hasty survey, we may assert thatspecies remain unchanged for indefinite periods, while at times they arein the alternative condition. Then at once they produce new forms oftenin large numbers, giving rise to swarms of subspecies. All facts pointto the conclusion that these periods of stability and mutabilityalternate more or less regularly with one another. Of course a directproof of this view cannot, as yet, be given, but this conclusion isforced upon us by a consideration of known facts bearing on theprinciple of constancy and evolution. 

If we are right in this general conception, we may ask further, what isto be the exact place of our group of new evening-primroses in thistheory? In order to give an adequate answer, we must consider the wholerange of the observations from a broader point of view. First of all itis evident that the real mutating period must be assumed to be muchlonger than the time covered by my observations. Neither the beginningnor the end have been seen. It is quite obvious that _Oenotheralamarckiana_ was in a mutating condition when I first [700] saw it, seventeen years ago. How long had it been so? Had it commenced to mutateafter its introduction into Europe, some time ago, or was it alreadypreviously in this state? It is as yet impossible to decide this point. Perhaps the mutable state is very old, and dates from the time of thefirst importation of the species into Europe. 

Apart from all such considerations the period of the directobservations, and the possible duration of the mutability through evenmore than a century, would constitute only a moment, if compared withthe whole geologic time. Starting from this conception the pedigree ofour mutations must be considered as only one small group. Instead offiguring a fan of mutants for each year, we must condense all thesucceeding swarms into one single fan, as might be done also for _Drabaverna_ and other polymorphous species. In _Oenothera_ the main stem isprolonged upwards beyond the fan; in the others the main stem is lackingor at least undiscernable, but this feature manifestly is only ofsecondary importance. We might even prefer the image of a fan, adjustedlaterally to a stem, which itself is not interrupted by this branch. 

On this principle two further considerations are to be discussed. Firstthe structure of the [701] fan itself, and secondly the combination ofsucceeding fans into a common genealogic tree. 

The composition of the fan as a whole includes more than is directlyindicated by the facts concerning the birth of new species. They arisein considerable quantities, and each of them in large numbers ofindividuals, either in the same or in succeeding years. This multipleorigin must obviously have the effect of strengthening the new types, and of heightening their chances in the struggle for life. Arising in asingle specimen they would have little chance of success, since in thefield among thousands of seeds perhaps one only survives and attainscomplete development. Thousands or at least hundreds of mutated seedsare thus required to produce one mutated individual, and then, how smallare its chances of surviving! The mutations proceed in all directions, as I have pointed out in a former lecture. Some are useful, others mightbecome so if the circumstances were accidentally changed in definitedirections, or if a migration from the original locality might takeplace. Many others are without any real worth, or even injurious. Harmless or even slightly useless ones have been seen to maintainthemselves in the field during the seventeen years of my research, asproved by _Oenothera laevifolia_ and _Oenothera_ [702] _brevistylis_. Most of the others quickly disappear. 

This failure of a large part of the productions of nature deserves to beconsidered at some length. It may be elevated to a principle, and may bemade use of to explain many difficult points of the theory of descent. If, in order to secure one good novelty, nature must produce ten ortwenty or perhaps more bad ones at the same time, the possibility ofimprovements coming by pure chance must be granted at once. Allhypotheses concerning the direct causes of adaptation at once becomesuperfluous, and the great principle enunciated by Darwin once morereigns supreme. 

In this way too, the mutation-period of the evening-primrose is to beconsidered as a prototype. Assuming it as such provisionally, it may aidus in arranging the facts of descent so as to allow of a deeper insightand a closer scrutiny. All swarms of elementary species are the remainsof far larger initial groups. All species containing only a fewsubspecies may be supposed to have thrown off at the outset far morenumerous lateral branches, out of which however, the greater part havebeen lost, being unfit for the surrounding conditions. It is theprinciple of the struggle for life between elementary species, followedby the survival of the [703] fittest, the law of the selection ofspecies, which we have already laid stress upon more than once. 

Our second consideration is also based upon the frequent repetition ofthe several mutations. Obviously a common cause must prevail. Thefaculty of producing _nanella_ or _lata_ remains the same through allthe years. This faculty must be one and the same for all the hundreds ofmutative productions of the same form. When and how did it originate? Atthe outset it must have been produced in a latent condition, and evenyet it must be assumed to be continuously present in this state, andonly to become active at distant intervals. But it is manifest that theoriginal production of the characters of _Oenothera gigas_ was aphenomenon of far greater importance than the subsequent accidentaltransition of this quality into the active state. Hence the conclusionthat at the beginning of each series of analogous mutations there musthave been one greater and more intrinsic mutation, which opened thepossibility to all its successors. This was the origination of the newcharacter itself, and it is easily seen that this incipient change is tobe considered as the real one. All others are only its visibleexpressions. 

Considering the mutative period of our evening-primrose [704] as oneunit-stride section in the great genealogic tree, this period includestwo nearly related, but not identical changes. One is the production ofnew specific characters in the latent condition, and the other is thebringing of them to light and putting them into active existence. Thesetwo main factors are consequently to be assumed in all hypotheticconceptions of previous mutative periods. 

Are all mutations to be considered as limited to such periods? Of coursenot. Stray mutations may occur as well. Our knowledge concerning thispoint is inadequate for any definite statement. Swarms of variablespecies are easily recognized, if the remnants are not too few. But ifonly one or two new species have survived, how can we tell whether theyhave originated-alone or together with others. This difficulty is stillmore pronounced in regard to paleontologic facts, as the remains ofgeologic swarms are often found, but the absence of numerous mutationscan hardly be proved in any case. 

I have more than once found occasion to lay stress on the importance ofa distinction between progressive and retrograde mutations in previouslectures. All improvement is, of course, by the first of these modes ofevolution, but apparent losses of organs or qualities are [705] perhapsof still more universal occurrence. Progression and regression are seento go hand in hand everywhere. No large group and probably even no genusor large species has been evolved without the joint agency of these twogreat principles. In the mutation-period of the evening-primroses theobserved facts give direct support to this conclusion, since some of thenew species proved, on closer inspection, to be retrograde varieties, while others manifestly owe their origin to progressive steps. Suchsteps may be small and in a wrong direction; notwithstanding this theymay be due to the acquisition of a wholly new character and thereforebelong to the process of progression at large. 

Between them however, there is a definite contrast, which possibly is inintimate connection with the question of periodic and stray mutations. Obviously each progressive change is dependent upon the production of anew character, for whenever this is lacking, no such mutation ispossible. Retrograde changes, on the other hand, do not require suchelaborate preliminary work. Each character may be converted into thelatent condition, and for all we know, a special preparation for thispurpose is not at all necessary. It is readily granted that such specialpreparation may occur, because the [706] great numbers in which ourdwarf variety of the _Oenothera_ are yearly produced are suggestive ofsuch a condition. On the other hand, the _laevifolia_ and _brevistylis_mutations have not been repeated, at least not in a visible way. 

From this discussion we may infer that it is quite possible that a largepart of the progressive changes, and a smaller part of the retrogrademutations, are combined into groups, owing their origin to commonexternal agencies. The periods in which such groups occur wouldconstitute the mutative periods. Besides them the majority of theretrograde changes and some progressive steps might occur separately, each being due to some special cause. Degressive mutations, or thosewhich arise by the return of latent qualities to activity, would ofcourse belong with the latter group. 

This assumption of a stray and isolated production of varieties is to alarge degree supported by experience in horticulture. Here there are noreal swarms of mutations. Sudden leaps in variability are not rare, butthen they are due to hybridization. Apart from this mixture ofcharacters, varieties as a rule appear separately, often with intervalsof dozens of years, and without the least suggestion of a common cause. It is quite superfluous to go into details, as we have dealt with thehorticultural [707] mutations at sufficient length on a previousoccasion. Only the instance of the peloric toadflax might be recalledhere, because the historic and geographic evidence, combined with theresults of our pedigree-experiment, plainly show that peloric mutationsare quite independent of any periodic condition. They may occur anywherein the wide range of the toad-flax, and the capacity of repeatedlyproducing them has lasted some centuries at least, and is perhaps evenas old as the species itself. 

Leaving aside such stray mutations, we may now consider the probableconstitution of the great lines of the genealogic tree of the eveningprimroses, and of the whole vegetable and animal kingdom at large. Theidea of drawing up a pedigree for the chief groups of living organismsis originally due to Haeckel, who used this graphic method to supportthe Darwinian theory of descent. Of course, Haeckel's genealogic treesare of a purely hypothetic nature, and have no other purpose than toconvey a clear conception of the notion of descent, and of the greatlines of evolution at large. Obviously all details are subject to doubt, and many have accordingly been changed by his successors. These changesmay be considered as partial improvements, and the somewhat picturesqueform of Haeckel's pedigree might well be replaced by [708] more simpleplans. But the changes have by no means removed the doubts, nor havethey been able to supplant the general impression of distinct groups, united by broad lines. This feature is very essential, and it is easilyseen to correspond with the conception of swarms, as we have deduced itfrom the study of the lesser groups. 

Genealogic trees are the result of comparative studies; they are farremoved from the results of experimental inquiry concerning the originof species. What are the links which bind them together? Obviously theymust be sought in the mutative periods, which have immediately precededthe present one. In the case of the evening-primrose the systematicarrangement of the allied species readily guides us in the delimitationsof such periods. For manifestly the species of the large genus of_Oenothera_ are grouped in swarms, the youngest or most recent of whichwe have under observation. Its immediate predecessor must have been thesubgenus _Onagra_, which is considered by some authors as consisting ofa single systematic species, _Oenothera biennis_. Its multifarious formspoint to a common origin, not only morphologically but alsohistorically. Following this line backward or downward we reach anotherapparent mutation-period, which includes the origin of [709] the groupcalled _Oenothera_, with a large number of species of the same generaltype as the _Onagra-forms, Still farther downward comes the old genus_Oenothera_ itself, with numerous subgenera diverging in sundrycharacters and directions. 

Proceeding still farther we might easily construct a main stem withnumerous succeeding fans of lateral branches, and thus reach, from ournew empirical point of view, the theoretical conclusion alreadyformulated. 

Paleontologic facts readily agree with this conception. The swarms ofspecies and varieties are found to succeed one another like so manystories. The same images are repeated, and the single stories seem to beconnected by the main stems, which in each tier produce the whole numberof allied forms. Only a few prevailing lines are prolonged throughnumerous geologic periods; the vast majority of the lateral branches arelimited each to its own storey. It is simply the extension of thepedigree of the evening-primroses backward through ages, with the sameconstruction and the same leading features. There can be no doubt thatwe are quite justified in assuming that evolution has followed the samegeneral laws through the whole duration of life on earth. Only a momentof their lifetime is disclosed to us, but it [710] is quite sufficientto enable us to discern the laws and to conjecture the outlines of thewhole scheme of evolution. 

A grave objection which has, often, and from the very outset, been urgedagainst Darwin's conception of very slow and nearly imperceptiblechanges, is the enormously long time required. If evolution does notproceed any faster than what we can see at present, and if the processmust be assumed to have gone on in the same slow manner always, thousands of millions of years would have been needed to develop thehigher types of animals and plants from their earliest ancestors. 

Now it is not at all probable that the duration of life on earthincludes such an incredibly long time. Quite on the contrary thelifetime of the earth seems to be limited to a few millions of years. The researches of Lord Kelvin and other eminent physicists seem to leaveno doubt on this point. Of course all estimates of this kind are onlyvague and approximate, but for our present purposes they may beconsidered as sufficiently exact. 

In a paper published in 1862 Sir William Thomson (now Lord Kelvin) firstendeavored to show that great limitation had to be put upon the enormousdemand for time made by Lyell, Darwin and other biologists. From aconsideration [711] of the secular cooling of the earth, as deduced fromthe increasing temperature in deep mines, he concluded that the entireage of the earth must have been more than twenty and less than fortymillions of years, and probably much nearer twenty than forty. His viewshave been much criticised by other physicists, but in the main they havegained an ever-increasing support in the way of evidence. New mines ofgreater depth have been bored, and their temperatures have proved thatthe figures of Lord Kelvin are strikingly near the truth. George Darwinhas calculated that the separation of the moon from the earth must havetaken place some fifty-six millions of years ago. Geikie has estimatedthe existence of the solid crust of the earth at the most as a hundredmillion years. The first appearance of the crust must soon have beensucceeded by the formation of the seas, and a long time does not seem tohave been required to cool the seas to such a degree that life becamepossible. It is very probable that life originally commenced in thegreat seas, and that the forms which are now usually included in theplankton or floating-life included the very first living beings. According to Brooks, life must have existed in this floating conditionduring long primeval epochs, and evolved nearly all the main branches ofthe animal and vegetable kingdom [712] before sinking to the bottom ofthe sea, and later producing the vast number of diverse forms which nowadorn the sea and land. 

All these evolutions, however, must have been very rapid, especially atthe beginning, and together cannot have taken more time than the figuresgiven above. 

The agency of the larger streams, and the deposits which they bring intothe seas, afford further evidence. The amount of dissolved salts, especially of sodium chloride, has been made the subject of acalculation by Joly, and the amount of lime has been estimated by EugeneDubois. Joly found fifty-five and Dubois thirty-six millions of years asthe probable duration of the age of the rivers, and both figurescorrespond to the above dates as closely as might be expected from thediscussion of evidence so very incomplete and limited. 

All in all it seems evident that the duration of life does not complywith the demands of the conception of very slow and continuousevolution. Now it is easily seen, that the idea of successive mutationsis quite independent of this difficulty. Even assuming that somethousands of characters must have been acquired in order to produce thehigher animals and plants of the present time, no valid objection israised. The demands of the biologists and the results of [713] thephysicists are harmonized on the ground of the theory of mutation. 

The steps may be surmised to have never been essentially larger than inthe mutations now going on under our eyes, and some thousands of themmay be estimated as sufficient to account for the entire organization ofthe higher forms. Granting between twenty and forty millions of yearssince the beginning of life, the intervals between two successivemutations may have been centuries and even thousands of years. As yetthere has been no objection cited against this assumption, and hence wesee that the lack of harmony between the demands of biologists and theresults of the physicists disappears in the light of the theory ofmutation. 

Summing up the results of this discussion, we may justifiably assertthat the conclusions derived from the observations and experiments madewith evening-primroses and other plants in the main agree satisfactorilywith the inferences drawn from paleontologic, geologic and systematicevidence. Obviously these experiments are wonderfully supported by thewhole of our knowledge concerning evolution. For this reason the lawsdiscovered in the experimental garden may be considered of greatimportance, and they may guide us in our further inquiries. Withoutdoubt many minor [714] points are in need of correction and elaboration, but such improvements of our knowledge will gradually increase our meansof discovering new instances and, new proofs. 

The conception of mutation periods producing swarms of species from timeto time, among which only a few have a chance of survival, promises tobecome the basis for speculative pedigree-diagrams, as well as forexperimental investigations. 

[715]

LECTURE XXV

GENERAL LAWS OF FLUCTUATION

The principle of unit-characters and of elementary species leads at onceto the recognition of two kinds of variability. The changes of wideramplitude consist of the acquisition of new units, or the loss ofalready existing ones. The lesser variations are due to the degree ofactivity of the units themselves. 

Facts illustrative of these distinctions were almost wholly lacking atthe time of the first publication of Darwin's theories. It was a boldconception to point out the necessity for such distinction on purelytheoretical grounds. Of course some sports were well known andfluctuations were evident, but no exact analysis of the details waspossible, a fact that was of great importance in the demonstration ofthe theory of descent. The lack of more definite knowledge upon thismatter was keenly felt by Darwin, [716] and exercised much influenceupon his views at various times. 

Quetelet's famous discovery of the law of fluctuating variabilitychanged the entire situation and cleared up many difficulties. While aclear conception of fluctuations was thus gained, mutations wereexcluded from consideration, being considered as very rare, ornon-existent. They seemed wholly superfluous for the theory of descent, and very little importance was attached to their study. Currentscientific belief in the matter has changed only in recent years. Mendel's law of varietal hybrids is based upon the principle ofunit-characters, and the validity of this conception has thus beenbrought home to many investigators. 

A study of fluctuating or individual variability, as it was formerlycalled, is now carried on chiefly by mathematical methods. It is not mypurpose to go into details, as it would require a separate course oflectures. I shall consider the limits between fluctuation and mutationonly, and attempt to set forth an adequate idea of the principles of thefirst as far as they touch these limits. The mathematical treatment ofthe facts is no doubt of very great value, but the violent discussionsnow going on between mathematicians such as Pearson, Kapteyn and othersshould warn biologists to abstain [717] from the use of methods whichare not necessary for the furtherance of experimental work. 

Fortunately, Quetelet's law is a very clear and simple one, and quitesufficient for our considerations. It claims that for biologic phenomenathe deviations from the average comply with the same laws as thedeviations from the average in any other case, if ruled by chance only. The meaning of this assertion will become clear by a further discussionof the facts. First of all, fluctuating variability is an almostuniversal phenomenon. Every organ and every quality may exhibit it. Someare very variable, while others seem quite constant. Shape and size varyalmost indefinitely, and the chemical composition is subject to the samelaw, as is well known for the amount of sugar in sugar-beets. Numbersare of course less liable to changes, but the numbers of the rays ofumbels, or ray-florets in the composites, of pairs of blades in pinnateleaves, and even of stamens and carpels are known to be oftenexceedingly variable. The smaller numbers however, are more constant, and deviations from the quinate structure of flowers are rare. Complicated structures are generally capable of only slight deviations. 

From a broad point of view, fluctuating variability [718] falls undertwo heads. They obey quite the same laws and are therefore easilyconfused, but with respect to questions of heredity they should becarefully separated. They are designated by the terms individual andpartial fluctuation. Individual variability indicates the differencesbetween individuals, while partial variability is limited to thedeviations shown by the parts of one organism from the averagestructure. The same qualities in some cases vary individually and inothers partially. Even stature, which is as markedly individual forannual and biennial plants as it is for man, becomes partially variantin the case of perennial herbs with numbers of stems. Often a characteris only developed once in the whole course of evolution, as forinstance, the degree of connation of the seed-leaves in tricotyls and innumerous cases it is impossible to tell whether a character isindividual or partial. Consequently such minute details are generallyconsidered to have no real importance for the hereditary transmission ofthe character under discussion. 

Fluctuations are observed to take place only in two directions. Thequality may increase or decrease, but is not seen to vary in any otherway. This rule is now widely established by numerous investigations, andis fundamental to [719] the whole method of statistical investigation. It is equally important for the discussion of the contrast betweenfluctuations and mutations, and for the appreciation of their part inthe general progress of organization. Mutations are going on in alldirections, producing, if they are progressive, something quite newevery time. Fluctuations are limited to increase and decrease of what isalready available. They may produce plants with higher stems, morepetals in the flowers, larger and more palatable fruits, but obviouslythe first petal and the first berry, cannot have originated by thesimple increase of some older quality. Intermediates may be found, andthey may mark the limit, but the demonstration of the absence of a limitis quite another question. It would require the two extremes to be shownto belong to one unit, complying with the simple law of Quetelet. 

Nourishment is the potent factor of fluctuating variability. Of coursein thousands of cases our knowledge is not sufficient to allow us toanalyze this relation, and a number of phases of the phenomenon havebeen discovered only quite recently. But the fact itself is thoroughlymanifest, and its appreciation is as old as horticultural science. Knight, who lived at the beginning of the last century, has laid greatstress upon it, and it has since influenced practice in a [720] largemeasure. Moreover, Knight pointed out more than once that it is theamount of nourishment, not the quality of the various factors, thatexercises the determinative influence. Nourishment is to be taken in thewidest sense of the word, including all favorable and injuriouselements. Light and temperature, soil and space, water and salts areequally active, and it is the harmonious cooperation of them all thatrules growth. 

We treated this important question at some length, when dealing with theanomalies of the opium-poppies, consisting of the conversion of stamensinto supernumerary pistils. The dependency upon external influenceswhich this change exhibited is quite the same as that shown byfluctuating variability at large. We inquired into the influence of goodand bad soil, of sunlight and moisture and of other concurrent factors. Especial emphasis was laid upon the great differences to which thevarious individuals of the same lot may be exposed, if moisture andmanure differ on different portions of the same bed in a way unavoidableeven by the most careful preparation. Some seeds germinate on moist andrich spots, while their neighbors are impeded by local dryness, or bydistance from manure. Some come to light on a sunny day, and increasetheir first leaves rapidly, while on [721] the following day the weathermay be unfavorable and greatly retard growth. The individual differencesseem to be due, at least in a very great measure, to such apparenttrifles. 

On the other hand partial differences are often manifestly due tosimilar causes. Considering the various stems of plants, which multiplythemselves by runners or by buds on the roots, the assertion is in noneed of further proof. The same holds good for all cases of artificialmultiplication by cuttings, or by other vegetative methods. But even ifwe limit ourselves to the leaves of a single tree, or the branches of ashrub, or the flowers on a plant, the same rule prevails. Thedevelopment of the leaves is dependent on their position, whetherinserted on strong or weak branches, exposed to more or less light, ornourished by strong or weak roots. The vigor of the axillary buds and ofthe branches which they may produce is dependent upon the growth andactivity of the leaves to which the buds are axillary. 

This dependency on local nutrition leads to the general law ofperiodicity, which, broadly speaking, governs the occurrence of thefluctuating deviations of the organs. This law of periodicity involvesthe general principle that every axis, as a rule, increases in strengthwhen [722] growing, but sooner or later reaches a maximum and mayafterwards decrease. 

This periodic augmentation and declination is often boldly manifest, though in other cases it may be hidden by the effect of alternateinfluences. Pinnate leaves generally have their lower blades smallerthan the upper ones, the longest being seen sometimes near the apex andsometimes at a distance from it. Branches bearing their leaves in tworows often afford quite as obvious examples, and shoots in generalcomply with the same rule. Germinating plants are very easy ofobservation on this point. When they are very weak they produce onlysmall leaves. But their strength gradually increases and the subsequentorgans reach fuller dimensions until the maximum is attained. Thephenomenon is so common that its importance is usually overlooked. Itshould be considered as only one instance of a rule, which holds goodfor all stems and all branches, and which is everywhere dependent on therelation of growth to nutrition. 

The rule of periodicity not only affects the size of the organs, butalso their number, whenever these are largely variable. Umbellate plantshave numerous rays on the umbels of strong stems, but the number is seento decrease and to become very small on the weakest lateral [723]branches. The same holds good for the number of ray-florets in theflower-heads of the composites, even for the number of stigmas on theovaries of the poppies, which on weak branches may be reduced to as fewas three or four. Many other instances could be given. 

One of the best authenticated cases is the dependency of partialfluctuation on the season and on the weather. Flowers decline when theseason comes to an end, become smaller and less brightly colored. Thenumber of ray-florets in the flower-heads is seen to decrease towardsthe fall. Extremes become rarer, and often the deviations from theaverage seem nearly to disappear. Double flowers comply with this rulevery closely, and many other cases will easily occur to any student ofnature. 

Of course, the relation to nourishment is different for individual andpartial fluctuations. Concerning the first, the period of development ofthe germ within the seed is decisive. Even the sexual cells may be inwidely different conditions at the moment of fusion, and perhaps thisstate of the sexual cells includes the whole matter of the decision forthe average characters of the new individual. Partial fluctuationcommences as soon as the leaves and buds begin to form, and all laterchanges in nutrition can only cause partial differences. All leaves, [724] buds, branches, and flowers must come under the influence ofexternal conditions during the juvenile period, and so are liable toattain a development determined in part by the action of these factors. 

Before leaving these general considerations, we must direct ourattention to the question of utility. Obviously, fluctuating variabilityis a very useful contrivance, in many cases at least. It appears all themore so, as its relation to nutrition becomes manifest. Here two aspectsare intimately combined. More nutrient matter produces larger leaves andthese are in their turn more fit to profit by the abundance ofnourishment. So it is with the number of flowers and flower-groups, andeven with the numbers of their constituent organs. Better nourishmentproduces more of them, and thereby makes the plant adequate to make afuller use of the available nutrient substances. Without fluctuationsuch an adjustment would hardly be possible, and from all our notions ofusefulness in nature, we therefore must recognize the efficiency of thisform of variability. 

In other respects the fluctuations often strike us as quite useless oreven as injurious. The numbers of stamens, or of carpels are dependenton nutrition, but their fluctuation is not known to have any attractionfor the visiting insects. 

[725] If the deviations become greater, they might even becomedetrimental. The flowers of the St. Johnswort, or _Hypericumperforatum_, usually have five petals, but the number varies from threeto eight or more. Bees could hardly be misled by such deviations. Thecarpels of buttercups and columbines, the cells in the capsules ofcotton and many other plants are variable in number. The number of seedsis thereby regulated in accordance with the available nourishment, butwhether any other useful purpose is served, remains an open question. Variations in the honey-guides or in the pattern of color-designs mighteasily become injurious by deceiving insects, and such instances as thegreat variability of the spots on the corolla of some cultivated speciesof monkey-flowers, for instance, the _Mimulus quinquevulnerus_, couldhardly be expected to occur in wild plants. For here the dark brownspots vary between nearly complete deficiency up to such predominancy asalmost to hide the pale yellow ground-color. 

After this hasty survey of the causes of fluctuating variability, we nowcome to a discussion of Quetelet's law. It asserts that the deviationsfrom the average obey the law of probability. They behave as if theywere dependent on chance only. 

Everyone knows that the law of Quetelet can [726] be demonstrated themost readily by placing a sufficient number of adult men in a row, arranging them according to their size. The line passing over theirheads proves to be identical with that given by the law of probability. Quite in the same way, stems and branches, leaves and petals and evenfruits can be arranged, and they will in the main exhibit the same lineof variability. Such groups are very striking, and at the first glanceshow that the large majority of the specimens deviate from the mean onlyto a very small extent. Wider deviations are far more rare, and theirnumber lessens, the greater the deviation, as is shown by the curvatureof the line. It is almost straight and horizontal in the middle portion, while at the ends it rapidly declines, going sharply downward at oneextreme and upward at the other. 

It is obvious however, that in these groups the leaves and other organscould conveniently be replaced by simple lines, indicating their size. The result would be quite the same, and the lines could be placed atarbitrary, but equal distances. Or the sizes could be expressed byfigures, the compliance of which with the general law could bedemonstrated by simple methods of calculation. In this manner thevariability of different organs can easily be compared. Another methodof demonstration consists in [727] grouping the deviations intopreviously fixed divisions. For this purpose the variations are measuredby standard units, and all the instances that fall between two limitsare considered to constitute one group. Seeds and small fruits, berriesand many other organs may conveniently be dealt with in this way. As anexample we take ordinary beans and select them according to their size. This can be done in different ways. On a small piece of board a longwedge-shaped slit is made, into which seeds are pushed as far aspossible. The margin of the wedge is calibrated in such a manner thatthe figures indicate the width of the wedge at the corresponding place. By this device the figure up to which a bean is pushed at once shows itslength. Fractions of millimeters are neglected, and the beans, afterhaving been measured, are thrown into cylindrical glasses of the samewidth, each glass receiving only beans of equal length. It is clear thatby this method the height to which beans fill the glasses isapproximately a measure of their number. If now the glasses are put in arow in the proper sequence, they at once exhibit the shape of a linewhich corresponds to the law of chance. In this case however, the lineis drawn in a different manner from the first. It is to be pointed outthat the glasses may be replaced by lines indicating [728] the height oftheir contents, and that, in order to reach a more easy and correctstatement, the length of the lines may simply be made proportionate tothe number of the beans in each glass. If such lines are erected on acommon base and at equal distances, the line which unites their upperends will be the expression of the fluctuating variability of thecharacter under discussion. 

The same inquiry may be made with other seeds, with fruits, or otherorgans. It is quite superfluous to arrange the objects themselves, andit is sufficient to arrange the figures indicating their value. In orderto do this a basal line is divided into equal parts, the demarcationscorresponding to the standard-units chosen for the test. The observedvalues are then written above this line, each finding its place betweenthe two demarcations, which include its value. It is very interestingand stimulating to construct such a group. The first figures may fallhere and there, but very soon the vertical rows on the middle part ofthe basal line begin to increase. Sometimes ten or twenty measurementswill suffice to make the line of chance appear, but often indentationswill remain. With the increasing number of the observations theirregularities gradually [729] disappear, and the line becomes smootherand more uniformly curved. 

This method of arranging the figures directly on a basal line is veryconvenient, whenever observations are made in the field or garden. Veryfew instances need be recorded to obtain an appreciation of the meanvalue, and to show what may be expected from a continuance of the test. The method is so simple and so striking, and so wholly independent ofany mathematical development that it should be applied in all cases inwhich it is desired to ascertain the average value of any organ, and themeasure of the attendant deviations. 

I cite an instance, secured by counting the ray-florets on theflower-heads of the corn-marigold or _Chrysanthemum segetum_. It wasthat, by which I was enabled to select the plant, which afterwardsshowed the first signs of a double head. I noted them in this way;

 47 47 52 41 54 68 44 50 62 75 36 45 58 65 72 __ 99

Of course the figures might be replaced in this work by equidistant dotsor by lines, but experience teaches that the chance of making mistakesis noticeably lessened by writing down [730] the figures themselves. Whenever decimals are made use of it is obviously the best plan to keepthe figures themselves. For afterwards it often becomes necessary toarrange them according to a somewhat different standard. 

Uniting the heads of the vertical rows of figures by a line, the formcorresponding to Quetelet's law is easily seen. In the main it is alwaysthe same as the line shown by the measurements of beans and seeds. Itproves a dense crowding of the single instances around the average, andon both sides of the mass of the observations, a few wide deviations. These become more rare in proportion to the amount of their divergency. On both sides of the average the line begins by falling very rapidly, but then bends slowly so as to assume a nearly horizontal direction. Itreaches the basal line only beyond the extreme instances. 

It is quite evident that all qualities, which can be expressed byfigures, may be treated in this way. First, of all the organs occurringin varying numbers, as for instance the ray-florets of composites, therays of umbels, the blades of pinnate and palmate leaves, the numbers ofveins, etc. , are easily shown to comply with the same general rule. Likewise the amount of chemical substances can be expressed inpercentage numbers, as is done on a large [731] scale with sugar inbeets and sugar-cane, with starch in potatoes and in other instances. These figures are also found to follow the same law. 

All qualities which are seen to increase and to decrease may be dealtwith in the same manner, if a standard unit for their measurement can befixed. Even the colors of flowers may not escape our inquiry. 

If we now compare the lines, compiled from the most divergent cases, they will be found to exhibit the same features in the main. Ordinarilythe curve is symmetrical, the line sloping down on both sides after thesame manner. But it is not at all rare that the inclination is steep onone side and gradual on the other. This is noticeably the case if theobservations relate to numbers, the average of which is near zero. Hereof course the allowance for variation is only small on one side, whileit may increase with out distinct limits on the alternate slope. So itis for instance with the numbers of ray-florets in the example given onp. 729. Such divergent cases, however, are to be considered asexceptions to the rule, due to some unknown cause. 

Heretofore we have discussed the empirical side of the problem only. Forthe purpose of experimental study of questions of heredity this isordinarily quite sufficient. The inquiry [732] into the phenomenon ofregression, or of the relation of the degree of deviation of the progenyto that of their parents, and the selection of extreme instances formultiplication are obviously independent of mathematical considerations. On the other hand an important inquiry lies in the statistical treatmentof these phenomena, and such treatment requires the use of mathematicalmethods. 

Statistics however, are not included in the object of these lectures, and therefore I shall refrain from an explanation of the method of theirpreparation and limit myself to a general comparison of the observedlines with the law of chance. Before going into the details, it shouldbe repeated once more that the empirical result is quite the same forindividual and for partial fluctuations. As a rule, the latter occur infar greater number, and are thus more easily investigated, butindividual or personal averages have also been studied. 

Newton discovered that the law of chance can be expressed by very simplemathematical calculations. Without going into details, we may at oncestate that these calculations are based upon his binomium. If the form(a + b) is calculated for some value of the exponent, and if the valuesof the coefficients after development are alone considered, they yieldthe basis [733] for the construction of what is called the line or curveof probability. For this construction the coefficients are used asordinates, the length of which is to be made proportionate to theirvalue. If this is done, and the ordinates are arranged at equaldistances, the line which unites their summits is the desired curve. Atfirst glance it exhibits a form quite analogous to the curves offluctuating variability, obtained by the measurements of beans and inother instances. Both lines are symmetrical and slope rapidly down inthe region of the average, while with increasing distance they graduallylose their steep inclination, becoming nearly parallel to the base attheir termination. 

This similarity between such empirical and theoretical lines is initself an empirical fact. The causes of chance are assumed to beinnumerable, and the whole calculation is based on this assumption. Thecauses of the fluctuations of biological phenomena have not as yet beencritically examined to such an extent as to allow of definiteconceptions. The term nourishment manifestly includes quite a number ofseparate factors, as light, space, temperature, moisture, the physicaland chemical conditions of the soil and the changes of the weather. Without doubt the single factors are very numerous, but whether they arenumerous enough to be treated [734] as innumerable, and thereby toexplain the laws of fluctuations, remains uncertain. Of course theeasiest way is to assume that they combine in the same manner as thecauses of chance, and that this is the ground of the similarity of thecurves. On the other hand, it is manifestly of the highest importance toinquire into the part the several factors play in the determination ofthe curves. It is not at all improbable that some of them have a largerinfluence on individual, and others on partial, fluctuations. If thiswere the case, their importance with respect to questions of hereditymight be widely different. In the present state of our knowledge thefluctuation-curves do not contribute in any large measure to anelucidation of the causes. Where these are obvious, they are so withoutstatistics, exactly as they were, previous to Quetelet's discovery. 

In behalf of a large number of questions concerning heredity andselection, it is very desirable to have a somewhat closer knowledge ofthese curves. Therefore I shall try to point out their more essentialfeatures, as far as this can be done without mathematical calculations. 

At a first glance three points strike us, the average or the summit ofthe curve, and the extremes. If the general shape is once denoted by theresults of observations or by the coefficients [735] of the binomium, all further details seem to depend upon them. In respect to the averagethis is no doubt the case; it is an empirical value without need of anyfurther discussion. The more the number of the observations increases, the more assured and the more correct is this mean value, but generallyit is the same for smaller and for larger groups of observations. 

This however, is not the case with the extremes. It is quite evidentthat small groups have a chance of containing neither of them. The morethe number of the observations increases, the larger is the chance ofextremes. As a rule, and excluding exceptional cases, the extremedeviations will increase in proportion to the number of cases examined. In a hundred thousand beans the smallest one and the largest one may beexpected to differ more widely from one another than in a few hundredbeans of the same sample. Hence the conclusion that extremes are not asafe criterion for the discussion of the curves, and not at all adequatefor calculations, which must be based upon more definite values. 

A real standard is afforded by the steepness of the slope. This may beunequal on the two sides of one curve, and likewise it may differ fordifferent cases. This steepness is usually measured by means of a pointon the half curve and [736 ] for this purpose a point is chosen whichlies exactly half way between the average and the extreme. Not howeverhalf way with respect to the amplitude of the extreme deviation, for onthis ground it would partake of the uncertainty of the extreme itself. It is the point on the curve which is surpassed by half the number, andnot reached by the other half of the number of the observations includedin the half of the curve. This point corresponds to the important valuecalled the probable error, and was designated by Galton as the quartile. For it is evident that the average and the two quartiles divide thewhole of the observations into four equal parts. 

Choosing the quartiles as the basis for calculations we are independentof all the secondary causes of error, which necessarily are inherent inthe extremes. At a casual examination, or for demonstrative purposes, the extremes may be prominent, but for all further considerations thequartiles are the real values upon which to rest calculations. 

Moreover if the agreement with the law of probability is once conceded, the whole curve is defined by the average and the quartiles, and theresult of hundreds of measurements or countings may be summed up inthree, or, in [737] the case of symmetrical curves, perhaps in twofigures. 

Also in comparing different curves with one another, the quartiles areof great importance. Whenever an empirical fluctuation-curve is to becompared with the theoretical form, or when two or more cases ofvariability are to be considered under one head, the lines are to bedrawn on the same base. It is manifest that the averages must be broughtupon the same ordinate, but as to the steepness of the line, muchdepends on the manner of plotting. Here we must remember that the mutualdistance of the ordinates has been a wholly arbitrary one in all ourprevious considerations. And so it is, as long as only one curve isconsidered at a time. But as soon as two are to be compared, it isobvious that free choice is no longer allowed. The comparison must bemade on a common basis, and to this effect the quartiles must be broughttogether. They are to lie on the same ordinates. If this is done, eachdivision of the base corresponds to the same proportionate number ofindividuals, and a complete comparison is made possible. 

On the ground of such a comparison we may thus assert that, fluctuations, however different the organs or qualities observed, arethe same whenever their curves are seen to overlap one [738] another. Furthermore, whenever an empirical curve agrees in this manner with thetheoretical one, the fluctuation complies with Quetelet's law, and maybe ascribed to quite ordinary and universal causes. But if it seems todiverge from this line, the cause of this divergence should be inquiredinto. 

Such abnormal curves occur from time to time, but are rare. Unsymmetrical instances have already been alluded to, and seem to bequite frequent. Another deviation from the rule is the presence of morethan one summit. This case falls under two headings. If the ray floretsof a composite are counted, and the figures brought into a curve, aprominent summit usually corresponds to the average. But next to this, and on both sides, smaller summits are to be seen. On a close inspectionthese summits are observed to fall on the same ordinates, on which, inthe case of allied species, the main apex lies. The specific characterof one form is thus repeated as a secondary character on an alliedspecies. Ludwig discovered that these secondary summits comply with therule discovered by Braun and Schimper, stating the relation of thesubsequent figures of the series. This series gives the terms of thedisposition of leaves in general, and of the bracts and flowers on thecomposite flower [739] heads in our particular case. It is the series towhich we have already alluded when dealing with the arrangement of theleaves on the twisted teasels. It commences with 1 and 2 and eachfollowing figure is equal to the sum of its two precedents. The mostcommon figures are 3, 5, 8, 13, 18, 21, higher cases seldom coming underobservation. Now the secondary summits of the ray-curves of thecomposites are seen to agree, as a rule, with these figures. Otherinstances could readily be given. 

Our second heading includes those cases which exhibit two summits ofequal or nearly equal height. Such cases occur when different races aremixed, each retaining its own average and its own curve-summit. We havealready demonstrated such a case when dealing with the origin of ourdouble corn-chrysanthemum. The wild species culminates with 13 rays, andthe grandiflorum variety with 21. Often the latter is found to beimpure, being mixed with the typical species to a varying extent. Thisis not easily ascertained by a casual inspection of the cultures, butthe true condition will promptly betray itself, if curves areconstructed. In this way curves may in many instances be made use of todiscover mixed races. Double curves may also result from theinvestigation [740] of true double races, or ever-sporting varieties. The striped snapdragon shows a curve of its stripes with two summits, one corresponding to the average striped flowers, and the other to thepure red ones. Such cases may be discovered by means of curves, but theconstituents cannot be separated by culture-experiments. 

A curious peculiarity is afforded by half curves. The number of petalsis often seen to vary only in one direction from what should be expectedto be the mean condition. With buttercups and brambles and many othersthere is only an increase above the typical five; quaternate flowers arewanting or at least are very rare. With weigelias and many others thenumber of the tips of the corolla varies downwards, going from five tofour and three. Hundreds of flowers show the typical five, and determinethe summit of the curve. This drops down on one side only, indicatingunilateral variability, which in many cases is due to a very intimateconnection of a concealed secondary summit and the main one. In the caseof the bulbous buttercup, _Ranunculus bulbosus_, I have succeeded inisolating this secondary summit, although not in a separate variety, butonly in a form corresponding to the type of ever-sporting varieties. 

[741] Recapitulating the results of this too condensed discussion, wemay state that fluctuations are linear, being limited to an increase andto a decrease of the characters. These changes are mainly due todifferences in nourishment, either of the whole organism or of itsparts. In the first case, the deviations from the mean are calledindividual; they are of great importance for the hereditary charactersof the offspring. In the second case the deviations are far moreuniversal and far more striking, but of lesser importance. They arecalled partial fluctuations. 

All these fluctuations comply, in the main, with the law of probability, and behave as if their causes were influenced only by chance. 

[742]

LECTURE XXVI

ASEXUAL MULTIPLICATION OF EXTREMES

Fluctuating variability may be regarded from two different points ofview. The multiformity of a bed of flowers is often a desirable feature, and all means which widen the range of fluctuation are therefore used toenhance this feature, and variability affords specimens, which surpassthe average, by yielding a better or larger product. 

In the case of fruits and other cultivated forms, it is of courseprofitable to propagate from the better specimens only, and if possibleonly from the very best. Obviously the best are the extremes of thewhole range of diverging forms, and moreover the extremes on one side ofthe group. Almost always the best for practical purposes is that inwhich some quality is strengthened. Cases occur however, in which it isdesirable to diminish an injurious peculiarity as far as possible, andin these instances the opposite extreme is the most profitable one. 

These considerations lead us to a discussion [743] of the results of thechoice of extremes, which it may be easily seen is a matter of thegreatest practical importance. This choice is generally designated asselection, but as with most of the terms in the domain of variability, the word selection has come to have more than one meaning. Facts haveaccumulated enormously since the time of Darwin, a more thoroughknowledge has brought about distinctions, and divisions at a rapidlyincreasing rate, with which terminology has not kept pace. Selectionincludes all kinds of choice. Darwin distinguished between natural andartificial selection, but proper subdivisions of these conceptions areneeded. 

In the fourth lecture we dealt with this same question, and saw thatselection must, in the first place, make a choice between the elementaryspecies of the same systematic form. This selection of species orspecies-selection was the work of Le Couteur and Patrick Shirreff, andis now in general use in practice where it has received the name ofvariety-testing. This clear and unequivocal term however, can hardly beincluded under the head of natural selection. The poetic terminology ofselection by nature has already brought about many difficulties thatshould be avoided in the future. On the other hand, the designation ofthe process as a natural [744] selection of species complies as closelyas possible with existing terminology, and does not seem liable to anymisunderstanding. 

It is a selection between species. Opposed to it is the selection withinthe species. Manifestly the first should precede the second, and if thissequence is not conscientiously followed it will result in confusion. This is evident when it is considered that fluctuations can only appearwith their pure and normal type in pure strains, and that each admixtureof other units is liable to be shown by the form of the curves. Moreover, selection chooses single individuals, and a single plant, if it isnot a hybrid, can scarcely pertain to two different species. The firstchoice therefore is apt to make the strain pure. 

In contrasting selection between species with that within the species, of course elementary species are meant, including varieties. The termswould be of no consequence if only rightly understood. For the sake ofclearness we might designate the last named process with the term ofintra-specific selection, and it is obvious that this term is applicableboth to natural and to artificial selection. 

Having previously dealt with species-selection at sufficient length, wemay now confine ourselves to the consideration of the intra-specific[745] selection process. In practice it is of secondary importance, andin nature it takes a very subordinate position. For this reason it willbe best to confine further discussions to the experience of thebreeders. 

Two different ways are open to make fluctuating variability profitable. Both consist in the multiplication of the chosen extremes, and thisincrease may be attained in a vegetative manner, or by the use of seeds. Asexual and sexual propagation are different in many respects, and sothey are also in the domain of variability. 

In order to obtain a clear comprehension of this difference, it isnecessary to start from the distinction between individual and partialfluctuations, as given in the last lecture. This distinction may bediscussed more understandingly if the causes of the variability aretaken into consideration. We have dealt with them at some length, andare now aware that inner conditions only, determine averages, while somefluctuation around them is allowable, as influenced by externalconditions. These outward influences act throughout life. At the veryfirst they impress their stamp on the whole organism, and incite alasting change in distinct directions. This is the period of thedevelopment of the germ within the seed; it begins with the fusion ofthe sexual cells, and each of them may be influenced [746] to anoticeable degree before this union. This is the period of thedetermination of individual variability. As soon as ramifications begin, the external conditions act separately on every part, influencing someto a greater and others to a lesser degree. Here we have the beginningof partial variability. At the outset all parts may be affected in thesame way and in the same measure, but the chances of such an agreement, of course, rapidly diminish. This is partly due to differences inexposure, but mainly to alterations of the sensibility of the organsthemselves. 

It is difficult to gain a clear conception of the contrast betweenindividual and partial variability, and neither is it easy to appreciatetheir cooperation rightly. Perhaps the best way is to consider theiractivity as a gradual narrowing of possibilities. At the outset theplant may develop its qualities in any measure, nothing being as yetfixed. Gradually however, the development takes a definite direction, for better or for worse. Is a direction once taken, then it becomes theaverage, around which the remaining possibilities are grouped. The plantor the organ goes on in this way, until finally it reaches maturity withone of the thousands of degrees of development, between which at thebeginning it had a free choice. 

[747] Putting this discussion in other terms, we find every individualand every organ in the adult state corresponding with a single ordinateof the curve. The curve indicates the range of possibilities, theordinate shows the choice that has been made. Now it is clear at oncethat this choice has not been made suddenly but gradually. Halfway ofthe development, the choice is halfway determined, but the other half isstill undefined. The first half is the same for all the organs of theplant, and is therefore termed individual; the second differs in theseparate members, and consequently is known as partial. Which of the twohalves is the greater and which the lesser, of course depends on thecases considered. 

Finally we may describe a single example, the length of the capsules ofthe evening-primrose. This is highly variable, the longest reaching morethan twice the length of the smallest. Many capsules are borne on thesame spike, and they are easily seen to be of unequal size. They varyaccording to their position, the size diminishing in the main from thebase upwards, especially on the higher parts. Likewise the fruits ofweaker lateral branches are smaller. Curves are easily made by measuringa few hundred capsules from corresponding parts of different plants, oreven by limiting the [748] inquiry to a single individual. These curvesgive the partial variability, and are found to comply with Quetelet'slaw. 

Besides this limited study, we may compare the numerous individuals ofone locality or of a large plot of cultivated plants with one another. In doing so, we are struck with the fact that some plants have large andothers small fruits. We now limit ourselves to the main spike of eachplant, and perhaps to its lower parts, so as to avoid as far as possiblethe impression made by the partial fluctuations. The differences remain, and are sufficient to furnish an easy comparison with the general law. In order to do this, we take from each plant a definite number ofcapsules and measure their average length. In some experiments I tookthe twenty lowermost capsules of the main spikes. In this way oneaverage was obtained for each plant, and combining these into a curve, it was found that these fluctuations also came under Quetelet's law. Thus the individual averages, and the fluctuations around each of them, follow the same rule. The first are a measure for the whole plant, thesecond only for its parts. As a general resume we can assert that, as arule, a quality is determined in some degree during the earlier stagesof the organism, and that this determination is valid throughout its[749] life. Afterwards only the minor points remain to be regulated. This makes it at once clear that the range of individual and partialvariability together must be wider than that of either of them, takenalone. Partial fluctuations cannot, of course, be excluded. Thus ourcomparison is limited to individual and partial variability on one side, and partial fluctuations alone on the other side. 

Intra-specific selection is thus seen to fall under two heads: aselection between the individuals, and a choice within each of them. Thefirst affords a wider and the latter a narrower field. 

Individual variability, considered as the result of outward influencesoperative during extreme youth, can be excluded in a very simple manner. Obviously it suffices to exclude extreme youth, in other words, toexclude the use of seeds. Multiplication in a vegetative way, bygrafting and budding, by runners or roots, or by simple division ofrootstocks and bulbs is the way in which to limit variability to thepartial half. This is all we may hope to attain, but experience showsthat it is a very efficient means of limitation. Partial fluctuationsare generally far smaller than individual and partial fluctuationstogether. 

Individual variability in the vegetable kingdom [750] might be calledseed-variation, as opposed to partial or bud-fluctuation. And perhapsthese terms are more apt to convey a clear conception of the distinctionthan any other. The germ within the unripe seed is easily understood tobe far more sensitive to external conditions than a bud. 

Multiplication of extremes by seed is thus always counteracted byindividual variability, which at once reopens all, or nearly all, theinitial possibilities. Multiplication by buds is exempt from this dangerand thus leads to a high degree of uniformity. And this uniformity is inmany cases exactly what the breeder endeavors to obtain. 

We will treat of this reopening of previous possibilities under the headof regression in the next lecture. It is not at all absolute, at leastnot in one generation. Part of the improvement remains, and favors thenext generation. This part may be estimated approximately as being aboutone-third or one-half of the improvement attained. Hence the conclusionthat vegetative multiplication gives rise to varieties which are as arule twice or thrice as good as selected varieties of plants propagatedby seeds. Hence, likewise the inference that breeders generally prefervegetative multiplication of improved forms, and apply it in allpossible cases. [751] Of course the application is limited, and foragecrops and the greater number of vegetables will always necessarily bepropagated by seed. 

Nature ordinarily prefers the sexual way. Asexual multiplications, although very common with perennial plants, appear not to offerimportant material for selection. Hence it follows that in comparing thework of nature with that of man, the results of selection followed byvegetative propagation should always be carefully excluded. Our largebulb-flowers and delicious fruits have nothing in common with naturalproducts, and do not yield a standard by which to judge nature's work. 

It is very difficult for a botanist to give a survey of what practicehas attained by the asexual multiplication of extremes. Nearly all ofthe large and more palatable fruits are due to such efforts. Someflowers and garden-plants afford further instances. By far the greatestmajority of improved asexual varieties, however, are not the result ofpure intra-specific selection. They are due largely to the choice of thebest existing elementary species, and to some extent to crosses betweenthem, or between distinct systematic species. In practice selection andhybridization go hand in hand and it is often difficult to ascertainwhat part of [752] the result is due to the one, and what to the otherfactor. 

The scientist, on the contrary, has nothing to do with the industrialproduct. His task is the analysis of the methods, in order to reach aclear appreciation of the influence of all the competing factors. Thisstudy of the working causes leads to a better understanding of thepractical processes, and may become the basis of improvement in methods. 

Starting from these considerations, we will now give some illustrativeexamples, and for the first, choose one in which hybridization is almostcompletely excluded. 

Sugar-canes have long been considered to be plants without seed. Theirnumerous varieties are propagated only in a vegetative way. The stemsare cut into pieces, each bearing one or two or more nodes with theirbuds. An entire variety, though it may be cultivated in large districtsand even in various countries, behaves with respect to variability as asingle individual. Its individual fluctuability has been limited to theearliest period of its life, when it arose from an unknown seed. Thepersonal characters that have been stamped on this one seed, partly byits descent, and partly in the development of its germ during the periodof ripening, have become the indelible characters [753] of the variety, and only the partial fluctuability, due to the effect of laterinfluences, can now be studied statistically. 

This study has for its main object the production of sugar in the stems, and the curves, which indicate the percentage of this importantsubstance in different stems of the same variety, comply with Quetelet'slaw. Each variety has its own average, and around this the data of themajority of the stems are densely crowded, while deviations on bothsides are rare and become the rarer the wider they are. The "Cheribon"cane is the richest variety cultivated in Java, and has an average of19% sugar, while it fluctuates between 11% and 28%. "Chunnic" averages14%, "Black Manilla" 13% and "White Manilla" 10%; their highest andlowest extremes diverge in the same manner, being for the last namedvariety 1% and 15%. 

This partial variability is of high practical interest, because on it aselection may be founded. According to the conceptions described in aprevious lecture, fluctuating variability is the result of those outwardfactors that determine the strength of development of the plant or theorgan. The inconstancy of the degree of sensibility, combined with theever-varying weather conditions preclude any close proportionality, butapart from this difficulty there is, in the [754] main, a distinctrelation between organic strength and the development of singlequalities. This correlation has not escaped observation in the case ofthe sugar-cane, and it is known that the best grown stocks are generallythe richest in sugar. Now it is evident that the best grown and richeststems will have the greater chance of transmitting these qualities tothe lateral-buds. This at once gives, a basis for vegetative selection, upon which it is not necessary to choose a small number of veryexcellent stems, but simply to avoid the planting of all those that arebelow the average. By this means the yield of the cultures has oftennoticeably been enhanced. 

As far as experience goes, this sort of selection, however profitable, does not conduce to the production of improved races. Only temporaryameliorations are obtained, and the selection must be made in the samemanner every year. Moreover the improvement is very limited and does notgive any promise of further increase. In order to reach this, one has torecur to the individual fluctuability, and therefore to seed. 

Nearly half a century ago, Parris discovered, on the island of Barbados, that seeds might occasionally be gathered from the canes. These, however, yielded only grass-like plants of no real value. The sameobservation was made [755] shortly afterwards in Java and in other sugarproducing countries. In the year 1885, Soltwedel, the director of one ofthe experiment stations for the culture of sugar-cane in Java, conceivedthe idea of making use of seedlings for the production of improvedraces. This idea is a very practical one, precisely because of thepossibility of vegetative propagation. If individuals would show thesame range as that of partial fluctuability, then the choice of theextremes would at once bring the average up to the richness of the beststocks. Once attained, this average would be fixed, without furtherefforts. 

Unfortunately there is one great drawback. This is the infertility ofthe best variety, that of the "Cheribon" cane. It flowers abundantly insome years, but it has never been known to produce ripe seeds. For thisreason Soltwedel had to start from the second best sort, and chose the"Hawaii" cane. This variety usually yields about 14% sugar, andSoltwedel found among his seedlings one that showed 15%. This fact wasquite unexpected at that time, and excited widespread interest in thenew method, and since then it has been applied to numerous varieties, and many thousands of seedlings have been raised and tested as to theirsugar-production. 

[756] From a scientific point of view the results are quite striking. From the practical standpoint, however, the question is, whether the"Hawaii" and other fertile varieties are adequate to yield seedlings, which will surpass the infertile "Cheribon" cane. Now "Hawaii" averages14% and "Cheribon" 19%, and it is easily understood that a "Hawaii"seedling with more than 19% can be expected only from very largesowings. Hundreds of thousands of seedlings must be cultivated, andtheir juice tested, before this improvement can be reached. Even then, it may have no significance for practical purposes. Next to the amountof sugar comes the resistance to the disease called "Sereh, " and the newrace requires to be ameliorated in this important direction, too. Otherqualities must also be considered, and any casual deterioration in othercharacters would make all progress illusory. For these reasons much timeis required to attain distinct improvements. 

These great difficulties in the way of selecting extremes for vegetativepropagation are of course met with everywhere. They impede the work ofthe breeder to such a degree, that but few men are able to surmountthem. Breeding new varieties necessitates the bending of every effort tothis purpose, and a clear conception of [757] the manifold aspects ofthis intricate problem. These fall under two heads, the exigencies ofpractice, and the physiologic laws of variability. Of course, only thelatter heading comes within the limits of our discussion which includestwo main points. First comes the general law of fluctuation that, thoughslight deviations from the average may be found by thousands, or ratherin nearly every individual, larger and therefore important deviationsare very rare. Thousands of seedlings must be examined carefully inorder to find one or two from which it might be profitable to start anew race. This point is the same for practical and for scientificinvestigation. In the second place however, a digression is met with. The practical man must take into consideration all the varying qualitiesof his improved strains. Some of them must be increased and others bedecreased, and their common dependency on external conditions oftenmakes it very difficult to discover the desired combinations. It isobvious, however, that the neglect of one quality may make allimprovement of other characters wholly useless. No augmentation ofsugar-percentage, of size and flavor of fruits can counterbalance anincrease in sensitiveness to disease, and so it is with other qualitiesalso. 

[758] Improved races for scientific investigation can be kept free frominfection, and protected against numerous other injuries. In theexperimental garden they may find conditions which cannot be realizedelsewhere. They may show a luxuriant growth, and prove to be excellentmaterial for research, but have features which, having been overlookedat the period of selection, would at once condemn them if left toordinary conditions, or to the competition of other species. 

Considering all these obstacles, it is only natural that breeders shoulduse every means to reach their goal. Only in very rare instances do theyfollow methods analogous to scientific processes, which tend to simplifythe questions as much as possible. As a rule, the practical way is thecombination of as many causes of variability as possible. Now the threegreat sources of variability are, as has been pointed out on severaloccasions, the original multiformity of the species, fluctuatingvariability, and hybridization. Hence, in practical experiments, allthree are combined. Together they yield results of the highest value, and Burbank's improved fruits and flowers give testimony to thepractical significance of this combination. 

From a scientific point of view however, it is [759] ordinarilydifficult, if not impossible, to discern the part which each of thethree great branches of variability has taken in the origination of theproduct. A full analysis is rarely possible, and the treatment of one ofthe three factors must necessarily remain incomplete. 

Notwithstanding these considerations, I will now give some examples inorder to show that fluctuating variability plays a prominent part inthese improvements. Of course it is the third in importance in theseries. First comes the choice of the material from the assemblage ofspecies, elementary species and varieties. Hybridization comes next inimportance. But even the hybrids of the best parents may be improved, because they are no less subject to Quetelet's law than any otherstrain. Any large number of hybrids of the same ancestry will provethis, and often the excellency of a hybrid variety depends chiefly, orat least definitely, on the selection of the best individuals. Beingpropagated only in a vegetative way, they retain their original goodqualities through all further culture and multiplication. 

As an illustrative example I will take the genus _Canna_. Originallycultivated for its large and bright foliage only, it has since become aflowering plant of value. Our garden strains have originated by thecrossing of [760] a number of introduced wild species, among which the_Canna indica_ is the oldest, now giving its name to the whole group. Ithas tall stems and spikes with rather inconspicuous flowers with narrowpetals. It has been crossed with _C. Nepalensis_ and _C. Warczewiczii_, and the available historic evidence points to the year 1846 as that ofthe first cross. This was made by Annee between the _indica_ and the_nepalensis_; it took ten years to multiply them to the required degreefor introduction into commerce. These first hybrids had bright foliageand were tall plants, but their flowers were by no means remarkable. 

Once begun, hybridization was widely practiced. About the year 1889Crozy exhibited at Paris the first beautifully flowering form, which henamed for his wife, "Madame Crozy. " Since that time he and many others, have improved the flowers in the shape and size, as well as in color andits patterns. In the main, these ameliorations have been due to thediscovery and introduction of new wild species possessing the requiredcharacters. This is illustrated by the following incident. In the year1892 I visited Mr. Crozy at Lyons. He showed me his nursery and numerousacquisitions, those of former years as well as those that were quitenew, and which were in the process of rapid [761] multiplication, previous to being given to the trade. I wondered, and asked, why no purewhite variety was present. His answer was "Because no white species hadbeen found up to the present time, and there is no other means ofproducing white varieties than by crossing the existing forms with a newwhite type. "

Comparing the varieties produced in successive periods, it is very easyto appreciate their gradual improvement. On most points this is notreadily put into words, but the size of the petals can be measured, andthe figures may convey at least some idea of the real state of things. Leaving aside the types with small flowers and cultivated exclusivelyfor their foliage, the oldest flowers of _Canna_ had petals of 45 mm. Length and 13 mm. Breadth. The ordinary types at the time of my visithad reached 61 by 21 mm. , and the "Madame Crozy" showed 66 by 30 mm. Ithad however, already been surpassed by a few commercial varieties, whichhad the same length but a breadth of 35 mm. And the latest production, which required some years of propagation before being put on the market, measured 83 by 43 mm. Thus in the lapse of some thirty years the lengthhad been doubled and the breadth tripled, giving flowers with broadcorollas and with petals [762] joined all around, resembling the besttypes of lilies and _Amaryllis_. 

Striking as this result unquestionably is, it remains doubtful as towhat part of it is due to the discovery and introduction of new largeflowered species, and what to the selection of the extremes offluctuating variability. As far as I have been able to ascertainhowever, and according to the evidence given to me by Mr. Crozy, selection has had the largest part in regard to the size, while thecolor-patterns are introduced qualities. 

The scientific analysis of other intricate examples is still moredifficult. To the practical breeder they often seem very simple, but thestudent of heredity, who wishes to discern the different factors, isoften quite puzzled by this apparent simplicity. So it is in the case ofthe double lilacs, a large number of varieties of which have recentlybeen originated and introduced into commerce by Lemoine of Nancy. In themain they owe their origin to the crossing and recrossing of a singleplant of the old double variety with the numerous existingsingle-flowered sorts. 

This double variety seems to be as old as the culture of the lilacs. Itwas already known to Munting, who described it in the year 1671. Twocenturies afterwards, in 1870, a new description [763] was given byMorren, and though more than one varietal name is recorded in his paper, it appears from the facts given that even at that time only one varietyexisted. It was commonly called _Syringa vulgaris azurea plena_, andseems to have been very rare and without real ornamental value. 

Lemoine, however, conceived the desirability of a combination of thedoubling with the bright colors and large flower-racemes of otherlilacs, and performed a series of crosses. The "_azurea plena_" has nostamens, and therefore must be used in all crosses as the pistil-parent;its ovary is narrowly inclosed in the tube of the flower, and difficultto fertilize. On the other hand, new crosses could be made every year, and the total number of hybrids with different pollen-parents wasrapidly increased. After five years the hybrids began to flower andcould be used for new crosses, yielding a series of compound hybrids, which however, were not kept separate from the products of the firstcrosses. 

Gradually the number of the flowering specimens increased, and thecharacter of doubling was observed to be variable to a high degree. Sometimes only one supernumerary petal was produced, sometimes a wholenew typical corolla was extruded from within the first. In the same[764] way the color and the number of the flowers on each raceme wereseen to vary. Thousands of hybrids were produced, and only those whichexhibited real advantages were selected for trade. These were multipliedby grafting, and each variety at present consists only of the buds ofone original individual and their products. No constancy from seed isassumed, many varieties are even quite sterile. 

Of course, no description was given of the rejected forms. It is onlystated that many of them bore either single or poorly filled flowers, orwere objectionable in some other way. The range of variability, fromwhich the choices were made, is obscure and only the fact of theselection is prominent. What part is due to the combination of theparental features and what to the individual fluctuation of the hybriditself cannot be ascertained. 

So it is in numerous other instances. The dahlias have been derived fromthree or more original species, and been subjected to cultivation andhybridization in an ever-increasing scale for a century. The bestvarieties are only propagated in the vegetative way, by the roots andbuds, or by grafting and cutting. Each of them is, with regard to itshereditary qualities, only one individual, and the individual characterswere selected at the same time with the [765] varietal and hybridcharacters. Most of them are very inconstant from seed and as a rule, only mixtures are offered for sale in seed-lists. Which of theirornamental features are due to fluctuating deviation from an average isof course unknown. _Amaryllis_ and _Gladiolus_ are surrounded with thesame scientific uncertainties. Eight or ten, or even more, species havebeen combined into one large and multiform strain, each bringing itspeculiar qualities into the mixed mass. Every hybrid variety is oneindividual, being propagated by bulbs only. Colors and color-patterns, shape of petals and other marks, have been derived from the wildancestors, but the large size of many of the best varieties is probablydue to the selection of the extremes of fluctuating variability. So itis with the begonias of our gardens, which are also composite hybrids, but are usually sown on a very large scale. Flowers of 15 cm. Diameterare very showy, but there can be no doubt about the manner in which theyare produced, as the wild species fall far short of this size. 

Among vegetables the potatoes afford another instance. Originally quitea number of good species were in culture, most of them having smalltubers. Our present varieties are due to hybridization and selection, each of them being propagated only in the vegetative way. 

[766] Selection is founded upon different qualities, according to theuse to be made of the new sort. Potatoes for the factory have even beenselected for their amount of starch, and in this case at least, fluctuating variability has played a very important part in theimprovement of the race. 

Vegetative propagation has the great advantage of exempting thevarieties from regression to mediocrity, which always followsmultiplication by seeds. It affords the possibility of keeping theextremes constant, and this is not its only advantage. Another, likewisehighly interesting, side of the question is the uniformity of the wholestrain. This is especially important in the case of fruits, thoughordinarily it is regarded as a matter of course, but there are someexceptions which give proof of the real importance of the usualcondition. For example, the walnut-tree. Thousands of acres ofwalnut-orchards consist of seedling trees grown from nuts of unknownparentage. The result is a great diversity in the types of trees and inthe size and shape of the nuts, and this diversity is an obviousdisadvantage to the industry. The cause lies in the enormousdifficulties attached to grafting or budding of these trees, which makethis method very expensive and to a high degree uncertain andunsatisfactory. 

[767] After this hasty survey of the more reliable facts of the practiceof an asexual multiplication of the extremes of fluctuating variability, we may now return to the previously mentioned theoreticalconsiderations. These are concerned with an estimation of the chances ofthe occurrence of deviations, large enough to exhibit commercial value. This chance may be calculated on the basis of Quetelet's law, wheneverthe agreement of the fluctuation of the quality under consideration hasbeen empirically determined. In the discussion of the methods ofcomparing two curves, we have pointed to the quartiles as the decisivepoints, and to the necessity of drawing the curves so that these pointsare made to overlie one another, on each side of the average. If now wecalculate the binomium of Newton for different values of the exponent, the sum of the coefficients is doubled for each higher unit of theexponent, and at the same time the extreme limit of the curve isextended one step farther. Hence it is possible to calculate a relationbetween the value of the extreme and the number of cases required. Itwould take us too long to give this calculation in detail, but it iseasily seen that for each succeeding step the number of individuals mustbe doubled, though the length of the steps, or the amount of increase ofthe quality [768] remains the same. The result is that many thousands ofseedlings are required to go beyond the ordinary range of variations, and that every further improvement requires the doubling of the wholeculture. If ten thousand do not give a profitable deviation, the nextstep requires twenty thousand, the following forty thousand, and so on. And all this work would be necessary for the improvement of a singlequality, while practice requires the examination and amelioration ofnearly all the variable characters of the strain. 

Hence the rule that great results can only be obtained by the use oflarge numbers, but it is of no avail to state this conclusion from ascientific point of view. Scientific experimenters will rarely be ableto sacrifice fifty thousand plants to a single selection. The problem isto introduce the principle into practice and to prove its directusefulness and reliability. It is to Luther Burbank that we owe thisgreat achievement. His principles are in full harmony with the teachingsof science. His methods are hybridization and selection in the broadestsense and on the largest scale. One very illustrative example of hismethods must suffice to convey an idea of the work necessary to producea new race of superlative excellency. Forty thousand blackberry andraspberry [769] hybrids were produced and grown until the fruit matured. Then from the whole lot a single variety was chosen as the best. It isnow known under the name of "Paradox. " All others were uprooted withtheir crop of ripening berries, heaped up into a pile twelve feet wide, fourteen feet high and twenty-two feet long, and burned. Nothingremained of that expensive and lengthy experiment, except the oneparent-plant of the new variety. Similar selections and similar amountof work have produced the famous plums, the brambles and theblackberries, the Shasta daisy, the peach almond, the improvedblueberries, the hybrid lilies, and the many other valuable fruits andgarden-flowers that have made the fame of Burbank and the glory ofhorticultural California. 

[770]

LECTURE XXVII

INCONSTANCY OF IMPROVED RACES

The greater advantages of the asexual multiplication of extremes are ofcourse restricted to perennial and woody plants. Annual and biennialspecies cannot as a rule, be propagated in this way, and even with someperennials horticulturists prefer the sale of seeds to that of roots andbulbs. In all these cases it is clear that the exclusion of theindividual variability, which was shown to be an important point in thelast lecture, must be sacrificed. 

Seed-propagation is subject to individual as well as to fluctuatingvariability. The first could perhaps be designated by another term, embryonic variability, since it indicates the fluctuations occurringduring the period of development of the germ. This period begins withthe fusion of the male and female elements and is largely dependent uponthe vigor of these cells at the moment, and on the varying qualitiesthey may have acquired. It comprises in the main the time of theripening of the seed, and [771] might perhaps best be considered to endwith the beginning of the resting stage of the ripe seed. Hence it isclear that the variability of seed-propagated annual races has a widerrange than that of perennials, shrubs and trees. At present it isdifficult to discern exactly the part each of these two main factorsplays in the process. Many indications are found however, that make itprobable that embryonic variability is wider, and perhaps of far greaterimportance than the subsequent partial fluctuations. The high degree ofsimilarity between the single specimens of a vegetative variety, and thelarge amount of variability in seed-races strongly supports this view. The propagation and multiplication of the extremes of fluctuatingvariability by means of seeds requires a close consideration of therelation between seedling and parent. The easiest way to get a clearconception of this relation is to make use of the ideas concerning thedependency of variability upon nourishment. Assuming these to be correctin the main, and leaving aside all minor questions, we may conclude thatthe chosen extreme individual is one of the best nourished andintrinsically most vigorous of the whole culture. On account of thesevery qualities it is capable of nourishing all of its organs better andalso its seeds. In other words, the seeds [772] of the extremeindividuals have exceptional chances of becoming better nourished thanthe average of the seeds of the race. Applying the same rule to them, itis easily understood that they will vary, by reason of this betternourishment, in a direction corresponding to that of their parent. 

This discussion gives a very simple explanation of the acknowledged factthat the seeds of the extremes are in the main the best for thepropagation of the race. It does not include however, all the causes forthis preferment. Some are of older date and due to previous influences. 

A second point in our discussion is the appreciation of the fact that asingle individual may be chosen to gather the seed from, and that theseseeds, and the young plants they yield, are as a rule, numerous. Henceit follows that we are to compare their average and their extremes withthe qualities of the parents. Both are of practical as well as oftheoretical interest. The average of the progeny is to be considered asthe chief result of the selection in the previous generation, while theextremes, at least those which depart in the same direction, areobviously the means of further improvement of the race. 

Thus our discussion should be divided into [773] two heads. One of thesecomprises the relation of the average of the progeny to the exceptionalqualities of the chosen parent, and the other the relation ofexceptional offspring to the exceptional parents. 

Let us consider the averages first. Are they to be expected to be equalto the unique quality of the parent, or perhaps to be the same as theaverage of the whole unselected race? Neither of these cases occur. Experience is clear and definite on this important point. Vilmorin, whenmaking the first selections to improve the amount of sugar in beets, wasstruck with the fact that the average of the progeny lies between thatof the original strain and the quality of the chosen parent. Heexpressed his observation by stating that the progeny are grouped aroundand diverge in all directions from some point, placed on the line whichunites their parent with the type from which it sprang. All breedersagree on this point, and in scientific experiments it has often beenconfirmed. We shall take up some illustrative examples presently, but inorder to make them clear, it is necessary to give a closer considerationto the results of Vilmorin. 

From his experience it follows that the average of the progeny is higherthan that of the race at large, but lower than the chosen parent. [774]In other words, there is a progression and a regression. A progressionin relation to the whole race, and a regression in comparison with theparent. The significance of this becomes clear at once, if we recall theconstancy of the variety which could be obtained from the selectedextreme in the case of vegetative multiplication. The progression iswhat the breeder wants, the regression what he detests. Regression isthe permanency of part of the mediocrity which the selection was invokedto overcome. Manifestly it is of the highest interest that theprogression should be as large, and the regression as small as possible. In order to attain this goal the first question is to know the exactmeasure of progression and regression as they are exhibiting themselvesin the given cases, and the second is to inquire into the influences, onwhich this proportion may be incumbent. 

At present our notions concerning the first point are still very limitedand those concerning the second extremely vague. Statistical inquirieshave led to some definite ideas about the importance of regression, andthese furnish a basis for experimental researches concerning the causesof the phenomenon. Very advantageous material for the study ofprogression and regression in the realm of fluctuating variability isafforded by the [775] ears of corn or maize. The kernels are arranged inlongitudinal rows, and these rows are observed to occur in varying, butalways even, numbers. This latter circumstance is due to the fact thateach two neighboring rows contain the lateral branches of a single rowof spikelets, the ages of which however, are included in the fleshy bodyof the ear. The variation of the number of the rows is easily seen tocomply with Quetelet's law, and often 30 or 40 ears suffice to give atrustworthy curve. Fritz Muller made some experiments upon theinheritance of the number of the rows, in Brazil. He chose a race whichaveraged 12 rows, selected ears with 14, 16 and 18 rows, etc. , and sowedtheir kernels separately. In each of-these cultures he counted the rowsof the seeds on the ears of all the plants when ripe, and calculatedtheir average. This average, of course, does not necessarily correspondto a whole number, and fractions should not be neglected. 

According to Vilmorin's rule he always found some progression of theaverage and some regression. Both were the larger, the more theparent-ear differed from the general average, but the proportion betweenboth remained the same, and seems independent of the amount of thedeviation. Putting the deviation at 5, the progression calculated fromhis figures is [776] 2 and the regression 3. In other words the averageof the progeny has gained over the average of the original varietyslightly more than one-third, and slightly less than one-half of theparental deviation. I have repeated this experiment of Fritz Miller'sand obtained nearly the same regression of three-fifths, though workingwith another variety, and under widely different climatic conditions. 

The figures of Fritz Muller were, as given below, in one experiment. Inthe last column I put the improvement calculated for a proportion oftwo-fifths above the initial average of 12. 

 Rows on Average of rows 12 + 2/5 of parent ears of progeny Difference 14 12. 6 12. 8 16 14. 1 13. 6 18 15. 2 14. 4 20 15. 8 15. 2 22 16. 1 16. 0

Galton, in his work on natural inheritance, describes an experiment withthe seeds of the sweet pea or _Lathyrus odoratus_. He determined theaverage size in a lot of purchased seeds, and selected groups of seedsof different, but for each group constant, sizes. These were sown, andthe average of the seeds was determined anew in the subsequent harvestthey yielded. These figures agreed with the rule of Vilmorin and werecalculated in the manner [777] given for the test of the corn. Theprogression and regression were found to be proportionate to the amountof the deviation. The progression of the average was one-third, and theregression in consequence two-thirds of the total deviation. Theamelioration is thus seen to be nearly, though not exactly, the same asin the previous case. 

From the evidence of the other corresponding experiments, and fromvarious statistical inquiries it seems that the value of the progressionis nearly the same in most cases, irrespective of the species used andthe quality considered. It may be said to be from one-third to one-halfof the parental deviation, and in this form the statement is obviouslyof wide and easy applicability. 

Our figures also demonstrate the great preeminence of vegetativevarieties above the improved strains multiplied by seeds. They have adefinite relation. Asexually multiplied strains may be said to begenerally two times or even three times superior to the commonoffspring. This is a difference of great practical importance, andshould never be lost sight of in theoretical considerations of theproductive capacity of selection. Multiplication by seed however, hasone great advantage over the asexual method; it may be repeated. The[778] selection is not limited to a single choice, but may be applied intwo or more succeeding generations. Obviously such a repetition affordsa better chance of increasing the progression of the average and ofameliorating the race to a greater degree than would be possible by asingle choice. This principle of repeated selection is at present theprominent feature of race improvement. Next to variety-testing andhybridizing it is the great source of the steady progression ofagricultural crops. From a practical standpoint the method is clear andas perfect as might be expected, but this is not the side of the problemwith which we are concerned here. The theoretical analysis andexplanation of the results obtained, however, is subject to much doubt, and to a great divergence of conceptions. So it is also with theapplication of the practical processes to those occurring in nature. Some assume that here repeated selection is only of subordinateimportance, while others declare that the whole process of evolution isdue to this agency. This very important point however, will be reservedfor the next lecture, and only the facts available at present will beconsidered here. 

As a first example we may take the ray-florets of the composites. On aformer occasion we have dealt with their fluctuation in number and [779]found that it is highly variable and complies in the main withQuetelet's law. _Madia elegans_, a garden species, has on the average 21rays on each head, fluctuating between 16 and 25 or more. I saved theseeds of a plant with only 17 rays on the terminal head, and got fromthem a culture which averaged 19 rays, which is the mean between 21 and17. In this second generation I observed the extremes to be 22 and 12, and selected a plant with 13 rays as the parent for a continuation ofthe experiment. The plants, which I got from its seeds, averaged 18 andshowed 22 and 13 as extremes. The total progression of the average wasthus, in two generations, from 21 to 18, and the total regression from13 to 18, and the proportion is thus seen to diminish by the repetitionrather than to increase. 

This experiment, however, is of course too imperfect upon which to foundgeneral conclusions. It only proves the important fact that the improvedaverage of the second generation is not the starting-point for thefurther improvement. But the second generation allows a choice of anextreme, which diverges noticeably more from the mean than anyindividual of the first culture, and thereby gives a larger amount ofabsolute progression, even if the proportion between progression andregression remains [780] the same. The repetition is only an easy methodof getting more widely deviating extremes; whether it has, besides this, another effect, remains doubtful. In order to be able to decide thisquestion, it is necessary to repeat the selection during a series ofgenerations. In this way the individual faults may be removed as far aspossible. I chose an experiment of Fritz Muller, relating to the numberof rows of grains on the ears exactly as in the case above referred to, and which I have repeated in my experimental garden at Amsterdam. 

I started from a variety known to fructify fairly regularly in ourclimate, and exhibiting in the mean 12-14 rows, but varying between 8and 20 as exceptional cases. I chose an ear with 16 rows and sowed itsseeds in 1887. A number of plants were obtained, from each of which, oneear was chosen in order to count its rows. An average of 15 rows wasfound with variations complying with Quetelet's law. One ear reached 22rows, but had not been fertilized, some others had 20 rows, and the bestof these was chosen for the continuation of the experiment. I repeatedthe sowing during 6 subsequent generations in the same way, choosingeach time the most beautiful ear from among those with the greatestnumber of rows. Unfortunately with the increase of the number the [781]size of the grains decreases, the total amount of nourishment availablefor all of them remaining about the same. Thus the kernels andconsequently the new plants became smaller and weaker, and the chance offertilization was diminished in the ears with the highest number ofrows. Consequently the choice was limited, and after having twice chosena spike with 20 and once one with 24 rows, I finally preferred thosewith the intermediate number of 22. 

This repeated choice has brought the average of my race up from 13 to20, and thus to the extreme limit of the original variety. Seven yearswere required to attain this result, or on an average the progressionwas one row in a year. This augmentation was accompanied by anaccompanying movement of the whole group in the same direction. Theextreme on the side of the small numbers came up from 8 to 12 rows, andcobs with 8 or 10 rows did not appear in my race later than the thirdgeneration. On the other side the extreme reached 28, a figure neverreached by the original variety as cultivated with us, and ears with 24and 26 rows have been seen during the four last generations inincreasing numbers. 

This slow and gradual amelioration was partly due to the mode ofpollination of the corn. [782] The pollen falls from the male spikes onthe ears of the same plant, but also is easily blown on surroundingspikes. In order to get the required amount of seed it is necessary inour climate to encroach as little as possible upon free pollination, aiding the self-pollination, but taking no precautions againstintercrossing. It is assumed that the choice of the best ears indicatesthe plants which have had the best pollen-parents as well as the bestpistil parents, and that selection here, as in other cases, corrects thefaults of free intercrossing. But it is granted that this correction isonly a slow one, and accounts in a great degree for the slowness of theprogression. Under better climatic conditions and with a more entireisolation of the individuals, it seems very probable that the sameresult could have been reached in fewer generations. 

However this may be, the fact is that by repeated selection the straincan be ameliorated to a greater extent than by a single choice. Thisresult completely agrees with the general experience of breeders and theexample given is only an instance of a universal rule. It has theadvantage of being capable of being recorded in a numerical way, and ofallowing a detailed and definite description of all the succeedinggenerations. The entire harvest of all [783] of them has been countedand the figures combined into curves, which at once show the wholecourse of the pedigree-experiment. These curves have in the main takenthe same shape, and have only gradually been moved in the chosendirection. 

Three points are now to be considered in connection with thisexperiment. The first is the size of the cultures required for theresulting amelioration. In other words, would it have been possible toattain an average of 20 rows in a single experiment? This is a matter ofcalculation, and the calculation must be based upon the experiencerelated above, that the progression in the case of maize is equal totwo-fifths of the parental deviation. A cob with 20 rows means adeviation of 7 from the average of 13, the incipient value of my race. To reach such an average at once, an ear would be required with 7 x 5/2= 17-1/2 rows above the average, or an ear with 30-32 rows. These neveroccur, but the rule given in a previous lecture gives a method ofcalculating the probability of their occurrence, or in other words, thenumber of ears required to give a chance of finding such an ear. Itwould take too long to give this calculation here, but I find thatapproximately 12, 000 ears would be required to give one with 28 rows, which was the highest number attained in [784] my experiment, while100, 000 ears would afford a chance of one with 32 rows*. Had I been ableto secure and inspect this number of ears, perhaps I would have neededonly a year to get an average of 20 rows. This however, not being thecase, I have worked for seven years, but on the other hand havecultivated all in all only about one thousand individuals for the entireexperiment. 

Obviously this reduction of the size of the experiment is of importance. One hundred thousand ears of corn could of course, be secured directlyfrom trade or from some industrial culture, but corn is cultivated onlyto a small extent in Holland, and in most cases the requisite number ofindividuals would be larger than that afforded by any single plantation. 

Repeated selection is thereby seen to be the means of reducing the sizeof the required cultures to possible measures, not only in theexperimental-garden, but also for industrial purposes. A selection fromamong 60, 000-100, 000 individuals may be within reach of Burbank, but offew others. As a rule they prefer a longer time with a smaller lot ofplants. This

 * On about 200 ears the variability ranges from 8-22 rows, and this leads approximately to one row more by each doubling of the numbers of instances. One ear with 22 rows in 200 would thus lead to the expectation of one ear with 32 rows in 100, 000 ears. 

[785] is exactly what is gained by repeated selections. To my mind thisreduction of the size of the cultures is probably the sole effect of therepetition. But experience is lacking on this point, and exactcomparisons should be made whenever possible, between the descendants ofa unique but extreme choice, and a repeated but smaller selection. Theeffect of the repetition on the nourishment of the chosenrepresentatives should be studied, for it is clear that a plant with 22rows, the parents and grandparents of which had the same number, indicates a better condition of internal qualities than one with thesame number of rows, produced accidentally from the common race. In thisway it may perhaps be possible to explain, why in my experiment an earwith 22 rows gave an average offspring with 20, while the calculation, founded on the regression alone would require a parental ear with 32rows. 

However, as already stated, this discussion is only intended to conveysome general idea as to the reduction of the cultures by means ofrepeated selections, as the material at hand is wholly inadequate forany closer calculation. This important point of the reduction may beillustrated in still another manner. 

The sowing of very large numbers is only required because it isimpossible to tell from the [786] inspection of the seeds which of themwill yield the desired individual. But what is impossible in theinspection of the seeds may be feasible, at least in important measure, in the inspection of the plants which bear the seeds. Whenever such aninspection demonstrates differences, in manifest connection with thequality under consideration, any one will readily grant that it would beuseless to sow the seeds of the worst plants, and that even the wholeaverage might be thrown over, if it were only possible to point out anumber of the best. But it is clear that by this inspection of theparent plants the principle of repeated selection is introduced for twosucceeding generations, and that its application to a larger series ofgenerations is only a question of secondary importance. 

Summing up our discussion of this first point we may assert thatrepeated selection is only selection on a small and practical scale, while a single choice would require numbers of individuals higher thanare ordinarily available. 

A second discussion in connection with our pedigree-culture of corn isthe question whether the amelioration obtained was of a durable nature, or only temporary. In other words, whether the progeny of the race wouldremain constant, if cultivated after cessation of the selection. Inorder to ascertain this, [787] I continued the culture during severalgenerations, choosing ears with less than the average number of rows. The excellence of the race at once disappeared, and the ordinary averageof the variety from which I had started seven years before, returnedwithin two or three seasons. This shows that the attained improvement isneither fixed nor assured and is dependent on continued selection. Thisresult only confirms the universal experience of breeders, which teachesthe general dependency of improved races on continued selection. Here astriking contrast with elementary species or true varieties is obvious. The strains which nature affords are true to their type; their averagecondition remains the same during all the succeeding generations, andeven if it should be slightly altered by changes in the externalconditions, it returns to the type, as soon as these changes come to anend. It is a real average, being the sum of the contribution of all themembers of the strain. Improved races have only an apparent average, which is in fact biased by the exclusion of whole groups of individuals. If left to themselves, their appearance changes, and the real averagesoon returns. This is the common experience of breeders. 

A third point is to be discussed in connection [788] with the detailedpedigree-cultures. It is the question as to what might be expected froma continuation of improvement selection. Would it be possible to obtainany imaginable deviation from the original type, and to reachindependency from further selection? This point has not until nowattracted any practical interest, and from a practical point of view andwithin the limits of ordinary cultures, it seems impossible to obtain apositive answer. But in the theoretical discussion of the problems ofdescent it has become of the highest importance, and therefore requiresa separate treatment, which will be reserved for the next lecture. 

Here we come upon another equally difficult problem. It relates to theproportion of embryonic or individual fluctuation, to partial variationas involved in the process of selection. Probably all qualities whichmay be subjected to selection vary according to both principles, theembryonic decision giving only a more definite average, around which theparts of the individual are still allowed to oscillate. It is so withthe corn, and whenever two or more ears are ripening or even onlyflowering on the same plant, differences of a partial nature may be seenin the number of their rows. These fluctuations are only small however, ordinarily not exceeding two and rarely four [789] rows. Choosing alwaysthe principal ear, the figures may be taken to indicate the degree ofpersonal deviation from the average of the race. But whenever we make amistake, and perchance sow from an ear, the deviation of which waslargely due to partial variation, the regression should be expected tobecome considerably larger. Hence it must be conceded that exactcalculations of the phenomena of inheritance are subject to muchuncertainty, resulting from our very imperfect knowledge concerning thereal proportion of the contributing factors, and the difficulty ofascertaining their influence in any given case. Here also we encountermore doubts than real facts, and much remains to be done before exactcalculations may become of real scientific value. 

Returning to the question of the effects of selection in the long run, two essentially different cases are to be considered. Extremes may beselected from among the variants of ordinary fluctuating variability, orfrom ever-sporting varieties. These last we have shown to be doubleraces. Their peculiar and wide range of variability is due to thesubstitution of two characters, which exclude one another, or ifcombined, are diminished in various degrees. Striped flowers and stocks, "five-leaved" clover, pistilloid opium-poppies and numerous other [790]monstrosities have been dealt with as instances of such ever-sportingvarieties. 

Now the question may be put, what would be the effect of selection if inlong series of years one of the two characters of such a double racewere preferred continuously, to the complete exclusion of the other. Would the race become changed thereby? Could it be affected to such adegree as to gradually lose the inactive quality, and cease to be adouble race?

Here manifestly we have a means by which to determine what selection isable to accomplish. Physiologic experiments may be said to be too shortto give any definite evidence. But cases may be cited where nature hasselected during long centuries and with absolute constancy in herchoice. Moreover unconscious selections by man have often worked in ananalogous manner, and many cultivated plants may be put to the testconcerning the evidence they might give on this point. Statingbeforehand the result of this inquiry, we may assert that long-continuedselection has absolutely no appreciable effect. Of course I do not denythe splendid results of selection during the first few years, nor thenecessity of continued selection to keep the improved races to theheight of their ameliorated qualities. I only wish to state that thework [791] of selection here finds its limit and that centuries andperhaps geologic periods of continued effort in the same direction arenot capable of adding anything more to the initial effect. Someillustrative examples may suffice to prove the validity of thisassertion. Every botanist who has studied the agricultural practice ofplant-breeding, or the causes of the geographic distribution of plants, will easily recall to his mind numerous similar cases. Perhaps the moststriking instance is afforded by cultivated biennial plants. The mostimportant of them are forage-beets and sugar-beets. They are, of course, cultivated only as biennials, but some annual specimens may be seen eachyear and in nearly every field. They arise from the same seed as thenormal individuals, and their number is obviously dependent on externalconditions, and especially on the time of sowing. Ordinary culturesoften show as much as 1% of these useless plants, but the exigencies oftime and available labor often compel the cultivator to have a largepart of his fields sown before spring. In central Europe, where theclimate is unfavorable at this season, the beets respond by theproduction of far larger proportions of annual specimens, their numbercoming often up to 20% or more, thus constituting noticeable losses inthe product [792] of the whole field. Rimpau, who has made a thoroughstudy of this evil and has shown its dependency on various externalconditions, has also tried to find methods of selection with the aim ofovercoming it, or at least of reducing it to uninjurious proportions. But in these efforts he has reached no practical result. The annuals aresimply inexterminable. 

Coming to the alternative side of the problem it is clear that annualshave always been excluded in the selection. Their seeds cannot be mixedwith the good harvest, not even accidentally, since they have ripened ina previous year. In order to bear seeds in the second year beets must betaken from the field, and kept free from frost through the winter. Thefollowing spring they are planted out, and it is obvious that even themost careless farmer is not liable to mix them with annual specimens. Hence we may conclude that a strict and unexcelled process of selectionhas been applied to the destruction of this tendency, not only forsugar-beets, since Vilmorin's time, when selection had become a wellunderstood process, but also for forage-beets since the beginning ofbeet culture. Although unconscious, the selection of biennials must havebeen uninterrupted and strict throughout many centuries. 

It has had no effect at all. Annuals are seen [793] to return everyyear. They are ineradicable. Every individual is in the possession ofthis latent quality and liable to convert it into activity as soon asthe circumstances provoke its appearance, as proved by the increase ofannuals in the early sowings. Hence the conclusion that selection in thelong run is not adequate to deliver plants from injurious qualities. Other proofs could be given by other biennials, and among them the strayannual plants of common carrots are perhaps the most notorious. In myown cultures of evening-primroses I have preferred the annuals andexcluded the biennials, but without being able to produce a pure annualrace. As soon as circumstances are favorable, the biennials return inlarge numbers. Cereals give analogous proofs. Summer and wintervarieties have been cultivated separately for centuries, but in trialsit is often easy to convert the one into the other. No real and definiteisolation has resulted from the effect of the long continued unconsciousselection. 

Striped flowers, striped fruits, and especially striped radishes affordfurther examples. It would be quite superfluous to dwell upon them. Selection always tends to exclude the monochromatic specimens, but doesnot prevent their return in every generation. Numerous [794] raremonstrosities are in the same category, especially when they are of sorare occurrence as not to give any noticeable contribution to theseed-production, or even if they render their bearers incapable ofreproduction. In such cases the selection of normal plants is verysevere or even absolute, but the anomalies are by no means exterminated. Any favorable circumstances, or experimental selection in their behalfshows them to be still capable of full development. Numerous cases ofsuch subordinate hereditary characters constitute the greater part ofthe science of vegetable teratology. 

If it should be objected that all these cases cover too short a time tobe decisive, or at least fail in giving evidence relative to formertimes, alpine plants afford a proof which one can hardly expect to besurpassed. During the whole present geologic epoch they have beensubjected to the never failing selection of their climate and otherexternal conditions. They exhibit a full and striking adaptation tothese conditions, but also possess the latent capacity for assuminglowland characters as soon as they are transported into suchenvironment. Obviously this capacity never becomes active on themountains, and is always counteracted by selection. This agency isevidently without any effect, for as we have seen when dealing [795]with the experiments of Nageli, Bonnier and others, each singleindividual may change its habits and its aspect in response totransplantation. The climate has an exceedingly great influence on eachindividual, but the continuance of this influence is without permanentresult. 

So much concerning ever-sporting varieties and double adaptations. Wenow come to the effects of a continuous selection of simple characters. 

Here the sugar-beets stand preeminent. Since Vilmorin's time they havebeen selected according to the amount of sugar in their roots, and theresult has been the most striking that has ever been attained, ifconsidered from the standpoint of practice. But if critically examined, with no other aim than a scientific appreciation of the improvement incomparison with other processes of selection, the support of theevidence for the theory of accumulative influence proves to be verysmall. 

The amount of sugar is expressed by percentage-figures. These however, are dependent on various causes, besides the real quantity of sugarproduced. One of these causes is the quantity of watery fluid in thetissues, and this in its turn is dependent on the culture in dryer ormoister soil, and on the amount of moisture in the air, and the samevariety of sugar-beets [796] yields higher percentage-figures in a dryregion than in a wet one. This is seen when comparing, for instance, theresults of the analyses from the sandy provinces of Holland with thosefrom the clay-meadows, and it is very well known that Californian beetsaverage as high as 26% or more, while the best European beets remain atabout 20%. As far as I have been able to ascertain, these figureshowever, are not indicative of any difference of race, but simply directresponses to the conditions of climate and of soil. 

Apart from these considerations the improvement reached in half acentury or in about twenty to thirty generations is not suggestive ofanything absolute. Everything is fluctuating now, even as it was at theoutset, and equally dependent on continual care. Vilmorin has given somefigures for the beets of the first generations from which he started hisrace. He quotes 14% as a recommendable amount, and 7 and 21 as theextreme instances of his analyses. However incorrect these figures maybe, they coincide to a striking degree with the present condition of thebest European races. Of course minor values are excluded each year bythe selection, and in consequence the average value has increased. Forthe year 1874 we find a standard of 10-14% considered as normal, [797]bad years giving 10%, good years from 12% to 14% in the average. Extremeinstances exceeded 17%. From that time the practice of the polarizationof the juice for the estimate of the sugar has rapidly spread throughoutEurope, and a definite increase of the average value soon resulted. Thishowever, often does not exceed 14%, and beets selected in the field forthe purpose of polarization come up to an average of 15 to 16%, varyingdownward to less than 10% and upward to 20 and 21%. In the main thefigures are the same as those of Vilmorin, the range of variability hasnot been reduced, and higher extremes are not reached. An averageincrease of 1% is of great practical importance, and nothing can excelthe industry and care displayed in the improvement of the beet-races. Notwithstanding this a lasting influence has not been exercised; themethods of selection have been improved, and the number of polarizedbeets has been brought up to some hundreds of thousands in singlefactories, but the improvement is still as dependent upon continuousselection as it was half a century ago. 

The process is practically very successful, but the support afforded byit to the selection theory vanishes on critical examination. 

[798]

LECTURE XXVIII

ARTIFICIAL AND NATURAL SELECTION

The comparison of artificial and natural selection has furnishedmaterial support for the theory of descent, and in turn been the objectof constant criticism since the time of Darwin. The criticisms, ingreater part, have arisen chiefly from an imperfect knowledge of bothprocesses. By the aid of distinctions recently made possible, thecontrast between elementary species and improved races has become muchmore vivid, and promises to yield better results on which to basecomparisons of artificial and natural selection. 

Elementary species, as we have seen in earlier lectures, occur in wildand in cultivated plants. In older genera and systematic species theyare often present in small numbers only, but many of the more recentwild types and also many of the cultivated forms are very rich in thisrespect. In agriculture the choice of the most adequate elementary formsfor any special purpose is acknowledged [799] as the first step in theway of selection, and is designated by the name of variety-testing, applying the term variety to all the subdivisions of systematic speciesindiscriminately. In natural processes it bears the title of survival ofspecies. The fact that recent types show large numbers, and in someinstances even hundreds of minor constant forms, while the older generaare considerably reduced in this respect, is commonly explained by theassumption of extinction of species on a correspondingly large scale. This extinction is considered to affect the unfit in a higher measurethan the fit. Consequently the former vanish, often without leaving anytrace of their existence, and only those that prove to be sufficientlyadapted to the surrounding external conditions, resist and survive. 

This selection exhibits far-reaching analogies between the artificialand the natural processes, and is in both cases of the very highestimportance. In nature the dying out of unfit mutations is the result ofthe great struggle for life. In a previous lecture we have compared itsagency with that of a sieve. All elements which are too small or tooweak fall through, and only those are preserved which resist the siftingprocess. Reduced in number they thrive and multiply and are thus enabledto [800] strike out new mutative changes. These are again submitted tothe sifting tests, and the frequent repetition of this process isconsidered to give a good explanation of the manifold, highlycomplicated, and admirable structures which strike the beginner as theonly real adaptations in nature. 

Exactly in the same way artificial selection isolates and preserves someelementary species, while it destroys others. Of course the time is notsufficient to secure new mutations, or at least these are only rare atpresent, and their occurrence is doubtful in historic periods. Apartfrom this unavoidable difference the analogy between natural andartificial selection appears to me to be very striking. 

This form of selection may be termed selection between species. Opposedto it stands the selection within the elementary species or variety. Ithas of late, alone come to be known as selection, though in reality itdoes not deserve this distinction. I have already detailed thehistorical evidence which gives preference to selection between species. The process can best be designated by the name of intraspecificselection, if it is understood that the term intraspecific is meant toapply to the conception of small or elementary species. 

I do not wish to propose new terms, but [801] I think that the principaldifferences might better become understood by the introduction of theword election into the discussion of questions of heredity. Electionmeant formerly the preferential choice of single individuals, while thederivation of the word selection points to a segregation of assembliesinto their larger parts. Or to state it in a shorter way, individualselection is exactly what is usually termed election. Choosing one manfrom among thousands is to elect him, but a select party is a group ofchosen persons. There would be no great difficulty in the introductionof the word election, as breeders are already in the habit of callingtheir choice individuals "elite, " at least in the case of beets and ofcereals. 

This intraspecific selection affords a second point for the comparisonbetween natural and artificial processes. This case is readily grantedto be more difficult than the first, but there can be no doubt that thesimilarity is due to strictly comparable causes. In practice thisprocess is scarcely second in importance to the selection betweenspecies, and in numerous cases it rests upon it, and crowns it, bringingthe isolated forms up to their highest possible degree of usefulness. Innature it does quite the same, adapting strains of individuals to thelocal conditions of their environment. Improved [802] races do notgenerally last very long in practice; sooner or later they are surpassedby new selections. Exactly so we may imagine the agency of naturalintraspecific selection. It produces the local races, the marks of whichdisappear as soon as the special external conditions cease to act. It isresponsible only for the smallest lateral branches of the pedigree, buthas nothing in common with the evolution on the main stems. It is ofvery subordinate importance. 

These assertions of course, are directly opposed to the current run ofscientific belief, but they are supported by facts. A considerable partof the evidence has already been dealt with and for our closingdiscussion only an exact comparison remains to be made between the twodetailed types of intraspecific selection. In coming to this I willfirst dwell upon some intermediate types and conclude with a criticaldiscussion of the features of artificial selection, which to my mindprove the invalidity of the conclusions drawn from it in behalf of anexplanation of the processes of nature. 

Natural selection occurs not only in the wild state, but is also activein cultivated fields. Here it regulates the struggle of the selectedvarieties and improved races with the older types, and even with thewild species. In a previous [803] lecture I have detailed the rapidincrease of the wild oats in certain years, and described theexperiments of Risler and Rimpau in the running out of select varieties. The agency is always the same. The preferred forms, which give a largerharvest, are generally more sensitive to injurious influences, moredependent on rich manure and on adequate treatment. The native varietieshave therefore the advantage, when climatic or cultural conditions areunfavorable for the fields at large. They suffer in a minor degree, andare thereby enabled to propagate themselves afterwards more rapidly andto defeat the finer types. This struggle for life is a constant one, andcan easily be followed, whenever the composition of a strain is noted insuccessive years. It is well appreciated by breeders and farmers, because it is always liable to counteract their endeavors and to claimtheir utmost efforts to keep their races pure. There can be no doubtthat exactly the same struggle exempt from man's intrusion is fought outin the wild state. 

Local races of wild plants have not been the object for fieldobservations recently. Some facts however, are known concerning them. Onthe East Friesian Islands in the North Sea the flowers are strikinglylarger and brighter colored than those of the same species on the [804]neighboring continent. This local difference is ascribed by Behrens to amore severe selection by the pollinating insects in consequence of theirlesser frequency on these very windy isles. Seeds of the pines from theHimalayas yield cold-resisting young plants if gathered from trees in ahigh altitude, while the seeds of the same species from lower regionsyield more sensitive seedlings. Similar instances are afforded by_Rhododendron_ and other mountain species. According to Cieslarcorresponding differences are shown by seeds of firs and larches fromalpine and lowland provinces. 

Such changes are directly dependent on external influences. This isespecially manifest in experiments extending the cultures in higher orin more northern regions. The shorter summer is a natural agent ofselection; it excludes all individuals which cannot ripen their seedsduring so short a period. Only the short lived ones survive. Schubelermade very striking experiments with corn and other different cereals, and has succeeded in making their culture possible in regions of Norwaywhere it formerly failed. In the district of Christiania, corn hadwithin some few years reduced its lifetime from 123 to 90 days, yieldingsmaller stems and fewer kernels, but still sufficient to make itsculture profitable under the existing conditions. [805] This change wasnot permanent, but was observed to diminish rapidly and to disappearentirely, whenever the Norwegian strain was cultivated in the southernpart of Germany. It was a typical improved race, dependent on continualselection by the short summers which had produced it. Similar resultshave been reached by Von Wettstein in the comparison of kinds of flaxfrom different countries. The analogy between such cultivated localraces and the local races of nature is quite striking. The practice ofseed exchange rests for a large part on the experience that thecharacters, acquired under the definite climatic and cultural conditionsof some select regions, hold good for one or two, and sometimes evenmore generations, before they decrease to practical uselessness. TheProbstei, the Hanna and other districts owe their wealth to thistemporary superiority of their wheat and other cereals. 

Leaving these intermediate forms of selection, we now come to ourprincipal point. It has already been discussed at some length in theprevious lecture, but needs further consideration. It is the questionwhether intraspecific selection may be regarded as a cause of lastingand ever-increasing improvement. This is assumed by biologists whoconsider fluctuating variability as the main source of progression [806]in the organic world. But the experience of the breeders does notsupport this view, since the results of practice prove that selectionaccording to a constant standard soon reaches a limit which it is notcapable of transgressing. In order to attain further improvements themethod of selection itself must be improved. A better and sharper methodassures the choice of more valuable representatives of the race, even ifthese must be sought for in far larger numbers of individuals, as isindicated by the law of Quetelet. 

Continuous or even prolonged improvement of a cultivated race is not theresult of frequently repeated selection, but of the improvement of thestandard of appreciation. Nature, as far as we know, changes herstandard from time to time only in consequence of the migrations of thespecies, or of local changes of climate. Afterwards the new standardremains unchanged for centuries. 

Selection, according to a constant standard, reaches its results in fewgenerations. The experience of Van Mons and other breeders of applesshows that the limit of size and lusciousness may be soon attained. Vilmorin's experiments with wild carrots and those of Carriere withradishes lead to the same conclusion as regards roots. Improvements offlowers in [807] size and color are usually easy and rapid in thebeginning, but an impassable limit is soon reached. Numerous otherinstances could be given. 

Contrasted with these simple cases is the method of selecting sugarbeets. More than once I have alluded to this splendid example of theinfluence of man upon domestic races, and tried to point out how littlesupport it affords to the current scientific opinion concerning thepower of natural selection. For this reason it is interesting to see howa gradual development of the methods of selection has been, from thevery outset, one of the chief aims of the breeders. None of them doubtsthat an improvement of the method alone is adequate to obtain results. This result, in the main, is the securing of a few percent more ofsugar, a change hardly comparable with that progress in evolution, whichour theories are destined to explain. 

Vilmorin's original method was a very simple one. Polarization was stillundiscovered in his time. He determined the specific weight of hisbeets, either by weighing them as a whole, or by using a piece cut fromthe base of the roots and deprived of its bark, in order to test onlythe sugar tissues. The pieces were floated in solutions of salt, whichwere diluted until the pieces [808] began to sink. Their specific weightat that moment was determined and considered to be a measure of thecorresponding value of the beet. This principle was afterwards improvedin two ways. The first was a selection after the salt solution method, but performed on a large scale. After some few determinations, asolution was made of such strength as to allow the greater number of thebeets to float, and only the best to sink down. In large vesselsthousands of beets could be tested in this way, to select a few of thevery heaviest. The other improvement was the determination of thespecific weight of the sap, pressed out from the tissue. It was moretedious and more expensive, but more direct, as the influence of the aircavities of the tissue was excluded. It prepared the way forpolarization. 

This was introduced about the year 1874 in Germany, and soon becamegenerally accepted. It allowed the amount of sugar to be measureddirectly, and with but slight trouble. Thousands of beets could betested yearly by this method, and the best selected for the productionof seed. In some factories a standard percentage is determined byprevious inquiries, and the mass of the beets is tested only by it. Inothers the methods of taking samples and clearing the sap have beenimproved so far as to allow the [809] exact determination of threehundred thousand polarization values of beets within a few weeks. Suchfigures give the richest material for statistical studies, and at onceindicate the best roots, while they enable the breeder to change hisstandard in accordance with the results at any time. Furthermore theyallow the mass of the beets to be divided into groups of differentquality, and to produce, besides the seeds for the continuation of therace, a first class and second-class product and so on. In the factoryof Messrs. Kuhn & Co. , at Naarden, Holland, the grinding machine hasbeen markedly improved, so as to tear all cell walls asunder, open allcells, and secure the whole of the sap within less than a minute, andwithout heating. 

It would take too long to go into further details, or to describe thesimultaneous changes that have been applied to the culture of the elitestrains. The detailed features suffice to show that the chief care ofthe breeder in this case is a continuous amelioration of the method ofselecting. It is manifest that the progression of the race is in themain due to great technical improvements, and not solely to therepetition of the selection. 

Similar facts may be seen on all the great lines of industrialselection. An increasing appreciation [810] of all the qualities of theselected plants is the common feature. Morphological characters, and thecapacity of yielding the desired products, are the first points thatstrike the breeder. The relation to climate and the dependence on manuresoon follow; but the physiological and chemical sides of the problem areusually slow of recognition in the methods of selection. When visitingMr. De Vilmorin at Paris some years ago, I inspected his laboratory forthe selection of potatoes. In the method in use, the tubers were rubbedto pulp and the starch was extracted and measured. A starch percentagefigure was determined for each plant, and the selection of the tubersfor planting was founded upon this result. In the same way wheat hasbeen selected by Dippe at Quedlinburg, first by a determination of itsnitrogenous contents in general, and secondly by the amount of thesubstances which determine its value for baking purposes. 

The celebrated rye of Schlanstedt was produced by the late Mr. Rimpau ina similar manner and was put on the market between 1880 and 1890 and wasreceived with great favor throughout central Europe, especially inGermany and in France. It is a tall variety, with vigorous stems andvery long heads, the kernels of which are nearly double the size ofthose of the [811] ordinary rye, and are seen protruding, when ripe, from between the scales of the spikelets. It is unfit for poor soils, but is one of the very best varieties for soils of medium fertility in atemperate climate. It is equal in the production of grain to the bestFrench sorts, but far surpassing them in its amount of straw. It wasperfected at the farm of Schlanstedt very slowly, according to thecurrent conceptions of the period. The experiment was started in theyear 1866, at which time Rimpau collected the most beautiful heads fromamong his fields, and sowed their kernels in his experiment garden. Fromthis first culture the whole race was derived. Every year the best earsof the strain were chosen for repeated culture, under experimental care, while the remainder was multiplied in a field to furnish the seeds forlarge and continually increasing areas of his farms. 

Two or three years were required to produce the quantity of seed of eachkind required for all the fields of Schlanstedt. The experiment garden, which through the kindness of Mr. Rimpau I had the good fortune ofvisiting more than once between 1875 and 1878, was situated in themiddle of his farm, at some distance from the dwellings. Of course itwas treated with more care, and especially kept [812] in betterconditions of fertility than was possible for the fields at large. Acontinued study of the qualities and exigencies of the elite plantsaccompanied this selection, and gave the means of gradually increasingthe standard. Resistance against disease was observed and otherqualities were ameliorated in the same manner. Mr. Rimpau repeatedlytold me that he was most anxious not to overlook any single character, because he feared that if any of them might become selected in the wrongway, perchance unconsciously, the whole strain might suffer to such adegree as to make all the other ameliorations quite useless. With thispurpose the number of plants per acre was kept nearly the same as thosein the fields, and the size of the culture was large enough every yearto include the best kernels of quite a number of heads. These were neverseparated, and exact individual pedigrees were not included in the plan. This mixture seemed to have the advantage of keeping up an average valueof the larger number of the characters, which either from their natureor from their apparent unimportance had necessarily to be neglected. 

After ten years of continuous labor, the rye of Rimpau caught theattention of his neighbors, being manifestly better than that ofordinary [813] sowings. Originally he had made his cultures for theimprovement of his own fields only. Gradually however, he began to sellhis product as seed to others, though he found the difference still veryslight. After ten years more, about 1886, he was able to sell all hisrye as seed, thereby making of course large profits. It is nowacknowledged as one of the best sorts, though in his last letter Mr. Rimpau announced to me that the profits began to decline as otherselected varieties of rye became known. The limit of productiveness wasreached, and to surmount this, selection had to be begun again from somenew and better starting point. 

This new starting point invokes quite another principle of selection, aprinciple which threatens to make the contrast between artificial andnatural selection still greater. In fact it is nothing new, being in useformerly in the selection of domestic animals, and having been appliedby Vilmorin to his sugar beets more than half a century ago. Why itshould ever have been overlooked and neglected in the selection of sugarbeets now is not clear. 

 The principle in itself is very simple. It agrees that the visible characters of an animal or a plant are only an imperfect measure for its hereditary qualities, instead of being the real criterion to be relied upon, as is the current belief. [814] It further reasons that a direct appreciation of the capacity of inheritance can only be derived from the observation of the inheritance itself. Hence it concludes that the average value of the offspring is the only real standard by which to judge the representatives of a race and to found selection upon. 

These statements are so directly opposed to views prevalent among plantbreeders, that it seems necessary to deal with them from the theoreticaland experimental, as well as from the practical side. 

The theoretical arguments rest on the division of the fluctuatingvariability into the two large classes of individual or embryonic, andof partial deviations. We have dealt with this division at some lengthin the previous lecture. It will be apparent at once, if we choose adefinite example. Let us ask what is the real significance of thepercentage figure of a single plant in sugar beets. This value dependsin the first place, on the strain or family from which the beet has beenderived, but this primary point may be neglected here, because it is thesame for all the beets of any lot, and determines the average, aroundwhich all are fluctuating. 

The deviation of the percentage figure of a single beet depends on twomain groups of external [815] causes. First come those that haveinfluenced the young germs of the plant during its most sensitiveperiod, when still an embryo within the ripening seed. They give a newlimitation to the average condition, which once and forever becomesfixed for this special individual. In the second place the youngseedling is affected during the development of its crown of leaves, andof its roots, by numerous factors, which cannot change this average, butmay induce deviations from it, increasing or decreasing the amount ofsugar, which will eventually be laid down in the root. The best youngbeet may be injured in many ways during periods of its lifetime, andproduce less sugar than could reasonably be expected from it. It may besurpassed by beets of inferior constitution, but growing under morefavorable circumstances. 

Considered from this point of view the result of the polarization testis not a single value, but consists of at least two different factors. It may be equal to the algebraic sum of these, or to their difference, according to whether the external conditions on the field were locallyand individually favorable or unfavorable. A large amount of sugar maybe due to high individual value, with slight subsequent deviation fromit, [816] or to a less prominent character combined with an extremesubordinate deviation. 

Hence it is manifest that even the results of such a highly improvedtechnical method do not deserve the confidence usually put in them. Theyare open to doubt, and the highest figures do not really indicate thebest representatives of the race. In order to convey this conception toyou in a still stronger manner, let us consider the partial variabilityas it usually shows itself. The various leaves of a plant may noticeablyvary in size, the flowers in color, the fruits in flavor. They fluctuatearound an average, which is assumed to represent the approximate valueof the whole plant. But if we were allowed to measure only one leaf, orto estimate only one flower or fruit, and be compelled to conclude fromit the worth of the whole plant, what mistakes we could make! We mightindeed hit upon an average case, but we might as easily get an extreme, either in the way of increase or of decrease. In both cases our judgmentwould be badly founded. Now who can assure us that the single root of agiven beet is an average representative of the partial variability? Thefact that there is only one main root does not prove anything. An annualplant has only one stem, but a perennial species has many. The averageheight of the last is a [817] reliable character, but the casual heightof the former is very uncertain. 

So it is with the beets. A beet may be divided by its buds and givequite a number of roots, belonging to the same individual. Thesesecondary roots have been tested for the amount of sugar, and found toexhibit a manifest degree of variability. If the first root correspondedto their average, it might be considered as reliable, but if not anyonewill grant that an average is more reliable than a single determination. Deviations have as a fact been observed, proving the validity of ourassertion. These considerations at once explain the disappointment sooften experienced by breeders. Some facts may be quoted from the Belgianprofessor of agriculture at Gembloux, the late Mr. Laurent. He selectedtwo beets, from a strain, with the exceptional amount of 23% sugar, butkept their offspring separate and analyzed some 60 of each. In bothgroups the average was only 11-12%, the extremes not surpassing 14-15%. Evidently the choice was a bad one, notwithstanding the highpolarization value of the parent. Analogous cases are often observed, and my countrymen, Messrs. Kuhn & Co. , go so far as to doubt allexcessive variants, and to prefer beets with high, but lessextraordinary percentages. Such are to be had in larger numbers [818]and their average has a good chance of exemption from a considerableportion of the doubts adhering to single excessive cases. 

It is curious to note here what Louis de Vilmorin taught concerning thispoint in the year 1850. I quote his own words: "I have observed that inexperiments on heredity it is necessary to individualize as much aspossible. So I have taken to the habit of saving and sowing separatelythe seeds of every individual beet, and I have always found that amongthe chosen parent plants some had an offspring with a better averageyield than others. At the end I have come to consider this characteronly, as a standard for amelioration. "

The words are clear and their author is the originator of the wholemethod of plant breeding selection. Yet the principle has beenabandoned, and nearly forgotten under the impression that polarizationalone was the supreme guide to be relied upon. However, if I understandthe signs rightly, the time is soon coming when Vilmorin's experiencewill become once more the foundation for progress in breeding. 

Leaving the theoretical and historical aspects of the problem, we willnow recall the experimental evidence, given in a former lecture, dealingwith the inheritance of monstrosities. I have shown that in manyinstances monstrosities [819] constitute double races, consisting ofmonstrous and of normal individuals. At first sight one might be inducedto surmise that the monstrous ones are the true representatives of therace, and that their seeds should be exclusively sown, in order to keepthe strain up to its normal standard. One might even suppose that thenormal individuals, or the so-called atavists, had really reverted tothe original type of the species and that their progeny would remaintrue to this. 

My experiments, however, have shown that quite the contrary is the case. No doubt, the seeds of the monstrous specimens are trustworthy, but theseeds of the atavists are not less so. Fasciated hawkweeds and twistedteasels gave the same average constitution of the offspring from highlymonstrous, and from apparently wholly normal individuals. In other wordsthe fullest development of the visible characteristic was not in theslightest degree an indication of better hereditary tendencies. Inunfavorable years a whole generation of a fasciated race may exhibitexclusively normal plants, without transmitting a trace of thisdeficiency to the following generation. As soon as the suitableconditions return, the monstrosity reassumes its full development. Theaccordance of these facts with the experience [820] of breeders ofdomestic animals, and of Louis de Vilmorin, and with the result of thetheoretical considerations concerning the factors of fluctuation has ledme to suggest the method of selecting, which I have made use of in myexperiments with tricotyls and syncotyls. 

Seedling variations afford a means of counting many hundreds ofindividuals in a single germinating pan. If seed from one parent plantis sown only in each pan, a percentage figure for the amount ofdeviating seedlings may be obtained. These figures we have called thehereditary percentages. I have been able to select the parent plantsafter their death on the sole ground of these values. And the result hasbeen that from varieties which, on an average, exhibited 50-55%deviating seedlings, after one or two years of selection this proportionin the offspring was brought up to about 90% in most of the cases. _Phacelia_ and mercury with tricotylous seedlings, and the Russiansunflower with connate seed leaves, may be cited as instances. 

Besides these tests, others were performed, based only on the visiblecharacters of the seedlings. The result was that this characteristic wasalmost useless as a criterion. The atavists gave, in the main, nearlythe same hereditary percentages as the tricotyls and syncotyls, and[821] their extremes were in each case far better constituted than theaverage of the chosen type. Hence, for selection purposes, the atavistsmust be considered to be in no way inferior to the typical specimens. 

If it had been possible to apply this principle to twisted and fasciatedplants, and perhaps even to other monstrosities, I think that it willreadily be granted that the chance of bringing even these races up to apercentage of 90% would have been large enough. But the large size ofthe cultures required for the counting of numerous groups of offspringin the adult state has deterred me from making such trials. Recentlyhowever, I have discovered a species, _Viscaria oculata_ which allows ofcounting twisted specimens in the pans, and I may soon be able to obtainproofs of this assertion. The validity of the hereditary percentage as astandard of selection has, within the last few years, been recognizedand defended by two eminent breeders, W. A. Hays in this country and VonLochow in Germany. Both of them have started from the experience ofbreeders of domestic animals. Von Lochow applied the principle to rye. He first showed how fallacious the visible characters often are. Forinstance the size of the kernels is often dependent on their number inthe head, and if this number is [822] reduced by the injurious varietalmark of lacunae (Luckigkeit), the whole harvest will rapidly deteriorateby the selection of the largest kernels from varieties which are notquite free from this hereditary deficiency. 

In order to estimate the value of his rye plants, he gathers the seed ofeach one separately and sows them in rows. Each row corresponds to aparent plant and receives 200 or 150 seeds, according to the availablequantity. In this way from 700 to 800 parent plants are tested yearly. Each row is harvested separately. The number of plants gives the averagemeasure of resistance to frost, this being the only important cause ofloss. Then the yield in grain and straw is determined and calculated, and other qualities are taken into consideration. Finally one or moregroups stand prominent above all others and are chosen for thecontinuation of the race. All other groups are wholly excluded from the"elite, " but among them the best groups and the very best individualsfrom lesser groups are considered adequate for further cultivation, inorder to produce the commercial product of the race. 

As a matter of fact the rye of Von Lochow is now one of the bestvarieties, and even surpasses the celebrated variety of Schlanstedt. Itwas only after obtaining proof of the validity [823] of his method thatVon Lochow decided to give it to the public. 

W. M. Hays has made experiments with wheat at the Minnesota AgriculturalExperiment Station. He chose a hundred grains as a proper number for theappreciation of each parent plant, and hence has adopted the name of"centgener power" for the hereditary percentage. 

The average of the hundred offspring is the standard to judge the parentby. Experience shows at once that this average is not at allproportional to the visible qualities of the parent. Hence theconclusion that the yield of the parent plant is a very uncertainindication of its value as a parent for the succeeding generation. Onlythe parents with the largest power in the centgener of offspring arechosen, while all others are wholly discarded. Afterwards the seeds ofthe chosen groups are propagated in the field until the requiredquantities of seed are obtained. 

This centgener power, or breeding ability, is tested and compared forthe various parent plants as to yield, grade, and percentage ofnitrogenous content in the grain, and as to the ability of the plant tostand erect, resist rust, and other important qualities. It is evidentthat by this test of a hundred specimens a far better [824] and muchmore reliable determination can be made than on the ground of theminutest examination of one single plant. From this point of view themethod of Hays commands attention. But the chief advantage lies in thefact that it is a direct proof of that which it is desired to prove, while the visible marks give only very indirect information. 

Thus the results of the men of practice are in full accordance withthose of theory and scientific experiment, and there can be little doubtthat they open the way for a rapid and important improvement. Onceattained, progress however, will be dependent on the selectionprinciple, and the hereditary percentage, or centgener power or breedingability, must be determined in each generation anew. Without this therace would soon regress to its former condition. 

To return to our starting point, the comparison of artificial andnatural selection. Here we are at once struck by the fact that it ishardly imaginable, how nature can make use of this principle. In somemeasure the members of the best centgener will manifestly be at anadvantage, because they contain more fit specimens than the othergroups. But the struggle for existence goes on between individuals, andnot between groups of brethren against groups of [825] cousins. In everygroup the best adapted individuals will survive, and soon the breedingdifferences between the parents must vanish altogether. Manifestly theycan, as a rule, have no lasting result on the issue of the struggle farexistence. 

If now we remember that in Darwin's time this principle, breedingability, enjoyed a far more general appreciation than at present, andthat Darwin must have given it full consideration, it becomes at onceclear that this old, but recently revived principle, is not adequate tosupport the current comparison between artificial and natural selection. 

In conclusion, summing up all our arguments, we may state that there isa broad analogy between breeding selection in the widest sense of theword, including variety testing, race improvement and the trial of thebreeding ability on one side, and natural selection on the other. Thisanalogy however, points to the importance of the selection betweenelementary species, and the very subordinate role of intraspecificselection in nature. It strongly supports our view of the origin ofspecies by mutation instead of continuous selection. Or, to put it inthe terms chosen lately by Mr. Arthur Harris in a friendly criticism ofmy views: "Natural selection may explain the survival [826] of thefittest, but it cannot explain the arrival of the fittest. "

A

_Abies concolor fastigiata_, 618_Acacia_, 176, 196, 217, 458, 697 bastard, 343, 617, 618, 664, 665, 666_Acer compestre nanum_, 612_Achillea millefolium_, 131, 132, 441Adaptation, 702 double, 430, 451, 452, 454, 455, 457, 458, 642_Aegilops ovata_, 265 _speltaeformis_, 265_Agave vivipara_, 684_Ageratum coeruleum_, 612_Agrostemma Coronaries bicolor_, 125 _Githago_, 282 _nicaeensis_, 162_Agrotis_, 204Alder, cut-leaved, 147, 596Alfalfa, 264Algae, 699Allen, Grant, 237_Alliaria_, 638_Alnus glutinosa laciniata_, 615Alpine plants, 437, 695, 794_Althaea_, 490Amaranth, 282, 452_Amaranthus caudatus_, 282_Amaryllis_, 272, 275, 762 brasiliensis_, 275 leopoldi_, 275 pardina_, 275 psittacina_, 275 vittata_, 275Amen-Hotep, 697_Ampelopsis_, 239_Amygdalus persica laevis_, 126_Anagallis arvensis_, 162_Androsace_, 634_Anemone_, 266, 331 _coronaria_, 241, 491 var. "Bride, " 510 _magellanica_, 266 _sylvestris_, 266_Anemone_, garden, 241Annee, 760Anomalies, taxonomic, 658, 685_Anthemis_, 236 _nobilis_, 130_Anthurium scherzerianum_, 639_Antirrhinum majus_, 315 _luteum rubro-striatum_, 315Apetalous flowers, 622Apples, 134, 240, 328, 454, 806 elementary species, 75 method of cultivating, 76 origin of cultivated varieties, 73 use by the Romans, 74 "Wealthy, " 78, 79 wild, 73, 74, 75, 76_Aquilegia chrysantha_, 161_Arabis ciliata glabrata_ _hirsuta glaberrima_, 126_Aralia crassifolia_, 662Arbres fruitiers ou Pomonomie belge, 76_Aralia papyrifera_, 662Arctic flora, 695_Arnica_, 494 _montana_, 236Aroids, 222, 631, 639Artemisias, 131Artificial selection, 18, 71, 77, 93, 95, 743, 744, 798, 826 first employed, 72, 92 nature of, 19_Arum maculatum immaculatum_, 125Ascidia, 310, 366, 367, 427, 428, 669, 670, 671, 672, 673, 674, 675Ash, 135, 341 one-bladed, 666, 667 weeping, 196, 596Ashe, 343Aster, 132, 152, 242 seashore, 200, 282_Aster Tripolium_, 132, 200, 236, 282, 410_Astragalus alpinus_, 696Atavism, 154, 170, 172, 175, 176, 178, 182, 185, 187, 188, 198, 220, 222, 226, 235, 344, 354, 399, 405, 411, 660, 661 bud, 183, 226 definition of, 170, 631 false, 185, 187 negative, 344 positive, 344 seed, 176 systematic, 174, 222, 630-657Atavists, 156, 201 heredity of, 412_Atropa Belladonna lutea_, 592_Aubretia_, 241_Avena fatua_, 100, 207_Azalea_, 178, 322_Azolla caroliniana_, 239

B

Babington, _Manual of British Botany_, 36, Bailey, 78, 306, 684Balsams, 334Bananas, 90, 134Banyan, 244Barberry, 133, 180 European, 270 purple, 596_Barbarea vulgaris_, 427Barley, 98, 105, 133, 203, 678, 679 "Nepaul, " 203, 676, 677, 679, 681, 682Bastard-acacia, 133, 136, 140Bateson, 250Bauhin, Caspar, 72, 610Baumann, 618Beans, 90, 152, 327, 727, 735Bedstraw, 648Beech, 133, 135, 242 cut-leaved, 179, 196, 616 laciniated, 196 oak-leaved, 595 purple, 196, 593, 595Beeches, 427 fern-leaved, 147Beets, 68, 72, 92, 93, 792, 796, 801, 815, 817, 818 Californian, 796 European, 796 forage, 71, 72, 791 salad, 71Beet-sugar, 67, 68, 69, 70, 71, 109, 165, 717, 791, 807, 813, 814_Begonia_, 218, 366, 509, 765 ever-flowering, 148 tuberous, 272 _clarkii_, 272 _davisii_, 272 _rosiflora_, 272 _sedeni_, 273 _semperflorens_, 133, 148, 620_Begonia_ bulbous, 372 _veitchi_, 272Behrens, 804Belladonna, 145_Bellis perennis_, 236 _perennis plena_, 195Bentham, 237Bentham & Hooker, _Handbook of British Flora_, 36_Berberis_, 133, 180, 455 _ilicifolia_, 270 _vulgaris_, 270Bertin, 596_Berula angustifolia_, 457Bessey, 660_Beta maritima_, 69 _patula_, 69, 70 _vulgaris_, 69, 70_Betula_, 132Between-race, 358Bewirkung, Theorie der directen (Nageli), 448_Biastrepsis_, 402_Bidens_, 131 _atropurpurea_, 131 _cernua_, 131, 158 _leucantha_, 131 _tripartite_, 131Bilberries, 577Bindweed, 41924Binomium, of Newton, 767Birch, 133, 243 cut-leaved, 596, 616 fastigiate, 618 fern-leaved, 179_Bisoutella_, 282 _laevigata glabra_, 125Bitter-sweet, 125Blackberry, 268, 768 "Paradox, " 769Blue-bells, variation in, 54, 491, 577Blueberries, 769Blue-bottle, 499, 507, 509, 510Blueflag, atavism of, 172_Boehmeria_, 675 _bilboa_, 685Bonnier, 439, 441, 442, 444, 451, 795Boreau, 663Brambles, 126, 127, 147, 239, 244, 245, 268, 740, 769, 663_Brassica_, 244Braun, 738Braun and Schimper, 494Bread-fruits, 90Briot, 618Britton and Brown's Flora, 162Brooks, 711Broom, 140 prickly, 217Broom-rape, 220_Broussonetia papyifera dissecta_, 616_Brunella_, 146, 268 _vulgaris_, 577 _vulgaris alba_, 201_Bryophyllum calycinum_, 218Buckwheat, 452Bud-variation, 750Buds, adventitious, 218Burbank, Luther, 57, 79, 116, 134, 268, 758, 768, 769, 784Buttercup, 331, 357, 410, 725, 740 Asiatic, 241

C

Cabbages, 428, 684 atavism in, 638 origin of varieties, 621Cactuses, 444Cactus-dahlia, 625_Calamintha Acinos_, 437, 452Calamus root, 222_Calendula officinalis_, 502_Calliopsis tinctoria_, 195_Calluna_, 146 _vulgaris_, 437, 577_Caltha_, 490 _palustris_, 331_Camelina_, 684_Camellia_, 178, 323 _japonica_, 368Camellias, 331Camomile, 130, 132, 156, 366, 494, 503, 509, 512_Campanula persicifolia_, 151, 234 _rotundifolia_, 437Campion, 283, 302, 304 evening, 281 red, 238_Canna_, 751, 759, 761 _indica_, 760 "Madame Crozy, " 760, 761 _nepalensis_, 760 _warczewiczii_, 760_Capsella Bursa-pastoris apetala_, 585 _heegeri_, 22, 582, 583, 684_Carex_, 53Carnation, 178, 241, 491 wheat-ear, 227_Carpinus Betulus heterophylla_, 180Carriere, 491, 596, 612, 806Carrots, 806Catch-fly, 419Carboniferous period, 699_Casuarina quadrivalvis_, 649Cauliflowers, origin of, 621Caumzet, 614Causation, theory of direct, (Nageli), 448Cedar, pyramidal, 618Celandine, 147, 245, 280, 365 oak-leaved, 603, 610, 611_Celosia_, 621_Celosia cristata_, 327, 411_Centaurea_, 242Centgener power, 20, 822_Centranthus macrosiphon_, 424_Cephalotaxus_, 170, 226 _pedunculata fastigiata_, 169Cereals, 105, 106, 107, 119, 801, 804 origin of cultivation, 104Character-units, 632Charlock, 424_Cheiranthus_, 490_Cheiri_, 370_Cheiri gynantherus_, 371_Chelidonium laciniatum_, 22, 609 _majus_, 147, 365, 600, 610, 611 _majus foliis quernis_, 610Cherries, 79Cherry, bird's, 617Chestnuts, 427Chromosomes, 306_Chrysanthemum_, 178, 274 corn, 739_Chrysanthemum carinatum_, 494 _coronarium_, 161, 202, 510 _grandiflorum_, 739 _imbricatum_, 494 _indicum_, 490 _inodorum_, 503 _inodorum plenissimum_, 336 new double, 501 _segetum_, 202, 493, 504, 729 _segetum_, var. _grandiflorum_, 43, 495, 498, 504, 504_Chrysopogon montanus_, 450Cieslar, 804_Cineraria cruenta_, 514Cinquefoil, 52_Clarkia_, 420 _elegans_, 198 _pulchella_, 282 _pulchella carnea_, 162_Clematis Vitalba_, 662 _Viticella nana_, 612Clover, 80, 102, 674 crimson (Italian), 353, 358, 359, 360 five-leaved, 340, 362, 374, 431, 509, 789 four-leaved, 340, 346, 352 red, 235, 281 white, 133, 366Clusius, 610_Cochlearia anglica_, 52 _danica_, 52 _officinalis_, 52Coconut, 67, 82, 83, 87, 88, 89 dispersal of, 85, 89 geographic origin of, 88, 89Coconut-palm, 84, 88Cockerell, T. D. A. , 139, 140, 591Cocklebur, 139Cockscomb, 165, 327, 356, 411, 621_Cocos nucifera stupposa_, 83, 84 _cupuliformis_, 82 _rutila_, 82_Codiaeum appendicularum_, 673_Colchicum_, 490_Coleus_, 132Columbine, 725 yellow, 161Columbus, 89, 118Columella, 106Composites, 130, 131, 336, 723, 778Conifers, 168, 226, 239, 455 weeping, 617Connation, of petals, 660, 661"Conquests, " 242Contra-selection, 425Cook, 84, 86, 88, 89Corn, 81, 90, 118, 119, 135, 283, 287, 288, 775, 786, 788, 804 American, 205Corn-cockle, 162Corn-chrysanthemum, 739Corn-flowers, 491, 92Corn, "Forty-day, " 118 "Harlequin, " 327 sterile variety of, 622 sugar, 135, 158 "Tuscarora, " 205Corn-marigold, 493, 494Cornel berry, yellow, 196Cornaceae, 675_Cornu_, 338_Cornus Mas_, 196Correlation, 142_Corylus_, 133 _Avellana_, 181 _tubulosa_, 181Cotton, 725Cotyledon, 674 variation in, 416_Crambe maritima_, 621Cranesbill, 599 European, 628 meadow, 322_Crataegus_, 196 _oxyacantha_, 132Crowfoot, 331 corn, 283_Crepis biennis_, 410, 411Cress, Indian, 192Crosses bisexual, 255, 276, 294, 298 reciprocal, 279 unisexual, 255, 261 varietal (see Hybrids)_Croton_, 673, 674Crozy, 760, 762Crucifers, 222, 635_Cryptomeria_, 169, 226 _japonica_, 239Cucumbers, 118_Cucumis_, 52_Cucurbita_, 52Cultivated plants, 65, 66 elementary species of, 62 improvement of, 92 mixed nature of, 96, 118 origin of, 91Currants, 79 Californian, 270 flowering, 166 "Gordon's, " 270 Missouri, 270 white, 158 white-flowered, 167Cuttings, 721_Cyclamen_, 323, 355, 627, 684 Butterfly, 627 _vernum_, 619_Cypripedium caudatum_, 487_Cytisus adami_, 271 _candicans Attleyanus_, 367 _Laburnum_, 271 _prostratus_, 139 _prostratus ciliata_, 125 _purpureus_, 271 _spinescens_, 139

D

_Dahlia_, 131, 241, 272, 625 cactus, 625 "Jules Chretien, " 628 purple-leaved, 626 "surprise, " 230 tubular, 627 [sic] 274, 490, 764 first double ones, 490 green, 227, 229, 230Daisies, 131, 132, 494 double, 195 hen-and-chicken, 514 ox-eye, 202Shasta, 769 yellow, 202Dandelion, 411 parthenogenesis, 61 variations in, 60Daphne Mezereum, 146Darwin, 1, 2, 3, 4, 5, 6, 7, 18, 76, 85, 93, 109, 110, 180, 196, 205, 206, 242, 306, 324, 338, 448, 571, 604, 612, 689, 702, 710, 715, 743, 798, 825Darwin, George, 711Darwinian theory, 461 basis of, 5Date, 134_Datura Stramonium_, 139, 142 _Stramonium inermis_, 300 _Tatula_, 139, 142, 300Dead-nettle, 237De Bary, 38, 47, 49De Candolle, 76, 84, 85, 89, 228, 370, 403, 621 Alphonse, 74, 129, 226 A. P. , 129 Casimir, 659, 676De Graaff, 275_Delphinium Ajacis_, 192Deniau, 617Descent, theory of, 690, 694, 702, 707, 716, 798De Serres, Olivier, 72_Desmodium gyrans_, 655, 656, 663, 664, 65Dewberry, California, 269_Dianthus barbatus_, 322, 648 twisted variety, 408Diatoms, 699Dictoyledons ancestors of monocotyledons, 15_Digitalis parviflora_, 161, 640 _purpurea_, 483 pelorism of, 482Dimorphism, 445, 447, 454, 457, 458Dippe, 810_Dipsacus fullonum_, 402 sylvestris_, 402, 402Dominant character, 280Double flowers poppies 490 production of, 489 types of, 330Double races (see also ever-sporting varieties), 419, 427, 428Dubois, Eugene, 712Duchesne, 185, 188, 596Duckweed, 222_Draba_, 692, 693 verna, 47, 50, 51, 53, 125, 126, 518, 533, 546, 547, 561_Dracocephalum moldavicum_, 419Dragon-head, 419_Drosera anglica_, 268 _filiformis_, 268 _intermedia_, 268 _obovata_, 267 _rotundifolia_, 268

E

Earth, age of, 710Edelweiss, 438Eichler, 660Election, 801Electric light, growth in, 442Elementary species, 11, 13, 32, 67, 74, 76, 77, 78, 79, 91, 95, 116, 119, 124, 126, 128, 129, 207, 238, 252, 256, 307, 430, 435, 695, 696, 698, 702, 715, 787, 798, 800, 825 apples, 75 coconut, 82 corn, 81 cultivated plants, 62 definition of, 12, 35, 127 flax, 80 how produced, 16, 248 hybrids of, 253, 255 mutation of, 141 origin of, 459, 603 origin of, how studied, 463 selection of, 92 varieties vs. , 14, 15, 141, 152, 224, 243, 247, 251, 495Elm, 136, 219, 239, 427_Epilobium_, 268 _hirsutum_, 683 _hirsutum cruciatum_, 588 _montanum_, 269 _tetragonum_, 269_Equisetum Telmateja_, 642, 649_Erica Tetralix_, 577, 661Ericaceae, 146, 660_Erigeron _Asteroides_, 450 _canadensis_, 132, 236, 453, 600, 695_Erodium_, 146 _cicutarium album_, 161_Erucastrum_, 630, 638, 639 _pollichii_, 222, 637_Eryngium campestre_, 674 _maritimum_, 674_Erysimum cheiranthoides_, 638_Erythraea pulchella_, 452_Erythrina_, 621 _Crista-galli_, 620Eschcholtzias, 59Esimpler, 337_Eucalyptus citriodora_, 669 _Globulus_, 217_Euphorbia Ipecacuanha_, 55Evening-primrose, 62, 204, 256, 424, 686, 687, 688, 690, 691, 694, 695, 699, 702, 703, 705, 707, 708, 713, 747, 793Evolution, 93, 685, 686, 689, 704, 707, 709, 710, 713, 718 degressive, 222, 223, 249 progression in, 630 progressive, 221, 222, 223, 248 regression in, 630 regressive, 221, 222; 223, 24 retrograde, 221, 631Extremes, asexual multiplication of, 742, 769

F

Fabre, 265_Fagus_, 133_Fagus sylvatica pectinata_, 179Fan, genealogical, 700Fasciated stems, 409, 412Ferns, 63 cristate, 427 plumose, 427_Ficaria_, 53_Ficus radicans_, 436 _religiosus_, 244 _repens_, 436 _stipulata_, 436 _ulmifolia_, 436Figs, 436_Filago_, 52Fir, 134, 804Fittest, survival of, 826Flax, 80, 805 springing, 80 threshing, 80 white-flowered, 158, 160Fleabane, Canada, 132, 236Flowers, gamopetalous, 660Fluctuability embryonic, see Fluctuation, individualFluctuation, 708, 715, 716, 718, 719, 724, 737, 741 curves of, 729, 794 defined, 191 individual, 718, 723, 732, 741, 745, 749, 788 mutation vs. 7, 16, 719 partial, 718, 723, 732, 741, 745, 748, 749, 771 inadequate for evolution, in elementary species, 19 nature of, 18 specific and varietal characters vs. 17Forget-me-not, 368Fothergill, John, 521Foxglove, 163 peloric, 164, 356, 367 yellow, 161, 640_Fraxinus excelsior monophylla_, 667 _exheterophylla_, 667 _simplici folio_, 667French flora (Grenier and Godron), 433Fries on _Hieracium_, 60Frostweed, 440 species of, 52_Fuchsia_, 272, 355Fuchsias, 491

G

Gaertner, 279_Galeopsis Ladanum canescens_, 139_Galium_, 648 _Aparine_, 409, 648 _elatum_, 52 _erectum_, 52 _Mollugo_, 62 _verum_, 648Gallesio, 138Galton, 736, 776Gamopetaly, 662Garden-pansy, origin of, 38Garlic, 638Gauchery, 452Geikie, 711Genera artificial character of, 36 polymorphous, 692_Gentiana punctata concolor_, 125Gentians, 577Georgics (Vergil), 106_Geranium pratense_, 323, 628 _album_, 628 _pyreniacum_, 599German flora (Koth), 432Geum, 282Gherkins, 118Gideon, Peter M. , 78Glacial period, 696_Gladiolus_, 241, 272, 274, 368, 765 _cardinalis_, 275 _gandavensis_, 275 _psittacinus_, 275 _purpureo-auratus_, 275_Glaucium_, 241_Gleditschia sinensis_, 614 _triacanthos pendula_, 617_Gloxinia_, 282, 485 erect, 626_Gloxinia erecta_, 485 peloric variety, 485_Gnaphalium Leontopodium_, 438_Godetia amoena_, 161Godetias, 59, 232Godron, 265, 432Goeppert, 370Gooseberry, 79, 140, 626 red, 133, 165, 241Grapes, 90, 158, 328Grape-hyacinth, _plumosa_, 134Grasses, 102, 631, 681Grenier, 433Groundsel, 132Growth, nutrition and, 714, 720, 722Guelder-rose, 134, 239Gum-tree, Australian, 217_Gypsophila paniculata_ twisted variety, 409

H

Haeckel, 707Half-races, 358, 372, 409, 419, 424, 427, 428Hall, 444Hallet, F. F. , 109Harebell, 232 peach-leaved, 234Harris, Arthur, 825Harshberger, John W. , 591 on _Euphorbia_ in New Jersey, 55Hawksbeard, 410, 411, 412Hawkweed, 411, 439, 443, 819Hawkweeds seeding without fertilization, 61Hawthorn, white, 132Hays, W. M. On individual selection, 20, 94, 95, 117, 821, 823, 824Hazelnut, 133, 181, 242Hazels, cut-leaved, 596, -616Heath family, 146, 222, 660Heaths, origin of, 662Heather, 577_Hedera Helix arborea_, 437Hedgehog burweed, 140_Hedys_Arum_, 664Heeger, 582Heer, Oswald, 74, 105Heinricher, 172, 173, 174_Helianthemum_, 53, 125, 126, 561 _apenninum_, 52 _pilosum_, 52 _polifolium_, 52 _pulverulentum_, 52 _vulgare_, 440_Helichrysum_, 420_Helwingia_, 678, 678, 682 _rusciflora_, 675Hemp, 419Henbane, 282_Hepatica_, 322, 490Heredity, 731, 734, 818 bearers of, 632 in teasels, 642_Hesperis_, 241, 322 _matronalis_, 323, 411_Heylandia latebrosa_, 450_Hibiscus Moscheutos_, 591_Hieracium_, 59, 439 _alpinum_, 696Hildebrand, 160, 240, 241Hoffman, 160, 662Hofmeister, 160, 370, 480Holbein, 164, 596Holly, 140, 196Holtermann, 449, 451Hollyhock, 427Honeysuckle, 674 ground, 443_Hordeum distichum_, 677 _hexastichum_, 677, 678 _tetrastichum_, 677 _trifurcatum_, 676, 678 _vulgare trifurcatum_, 203Hornbeam, European, 180Horse-chestnut, 219 thornless, 234Horsetail, Canadian, 695 European, 649Horsetail, family, 641Horse-weed, 132 Canadian, 452_Hortensia_, 134, 181Horticulture, mutations in, 604Houseleek, 370, 371Hunneman, John, 521Hyacinths, 178, 322 white, 160Hybrids, 58, 201, 202, 206, 250, 575 between elementary species, 253 constant, 263, 264, 265, 266, 267, 268, 269 law of varietal, 716 Mendelian, 324 nature of, 20 species, 256, 260 splitting of, 210 varietal, 208, 209, 247, 277, 278, 279, 281, 285, 293, 294Hybridization, 706, 751, 752, 758, 759, 764_Hydrocotyle_, 668_Hyoscyamus niger_, 282 _pallidus_, 283_Hypericum perforatum_, 725_Hyssopus officinalis_, 161

I

_Iberis umbellata rosea_, 195Improved races, inconstancy of 770-797Indian cress, 668 pelorism of, 485Indian pipe, 661Ipecac spurge, 55_Iris_, 456 _falcifolis_, 172 _kaempferi_, 174 _lortetii_, 521 _pallida_, 172 _pallida abavia_, 681Isolation, 108Ivy, 436

J

Jacob's ladder, 200, 202Jacques, 614Jacquin, 52, 632Jaggi, 594, 595Jaeger, 228, 662Jalappa, 165Janczewski, 266Japanese plum, 58_Jasminum Sambac_, 662Joly, 712Jordan, Alexis, 45, 47, 49, 50, 129 experiments with species, 37, 40_Juncus effusus spiralis_, 684Juniper, 684

K

Kapteyn, 716Kelvin, Lord, 720, 711Kerner von Marilaun, 266, 267Keteleer, 618Knight, 390, 719, 720Koch, 433, 667Koelreuter, 279Korshinsky, 609, 612, 614, 617, 667Krelage, 510, 619Kuhn & Co. , Messrs. , 801, 809, 817

L

_Labiates_, 237 pelories of, 577_Labiatiflorae_, pelorism of, 468Labrador tea, 661_Laburnum_, 270, 284, 342 oak-leaved 147, 179 pelorism of, 485_Lactuca_, 52 _Scariola_, 456Lagasca, Mariano, 96, 97, 114Lamarck, 1, 447, 461, 522, 522Lamarckism objections to, 449_Lamium album_, 237 _maculatum_, 237 pelorism of, 486 _purpureum_, 237Larch, 804Larkspur, 124, 192, 311, 452 hybrid, 213 white, 160Latency, 657 individual, 219 specific, 246 systematic, 219, 220, 235 varietal, 246Latent characters, 216_Lathyrus odoratus_, 776_Laurea pinnatifida_, 450Laurel, lady's, 146Laurent, 802Leaves, cleft, 685 variegated, 426, 431LeBrun, Mme. , 614Le Couteur, 96, 97, 107, 108, 114, 115, 116, 742_Ledum_, 222, 661_Lemna_, 222Lemoine, 762, 762Lettuce, 684 crisped, 158 prickly, 456Life, struggle for, 103, 119, 120Lilacs, 59, 769 double, 762_Lilium candidum flore pleno_, 331 _pardalium_, 116Lime-tree, 355, 366, 428, 669 fern-leaved, 147_Linaria_, 467, 471, 480 _dalmatica_, 482 _genistifolia_, 267 _italica_, 267 _vulgaris_, 267, 471 _vulgaris peloria_, 464Lindley, 63, 129, 506Linnaeus, 32, 33, 129, 132, 256, 663 on the idea of species, 11, 13 on origin of species, 2, 34 on primroses, 52_Linum angustifolium_, 80 _crepitans_, 81 _usitatissimum_, 80, 161Link, 466Liver-leaf, 322_Lobelia syphilitica_, 161_Lonicera etrusca_, 640 _tartarica nana_, 614Lorenz, Chr. , 482Lothelier, 454_Lotus corniculatus_, 442 _corniculatus hirsutus_, 139London, 615, 616, 667Lucerne, 264Ludwig, 738Lupines, 90_Lychnis_, 282 _chalcedonica_, 161 _diurna_, 238, 578 _preslii_, 578 _vespertina_, 238, 281, 585_Lycium_, 455_Lycopersicum_, 655 _grandifolium_, 654 _latifolium_ (see _L. Grandifolium_). _solanopsis_, 854, 656 _validum_ (see _L. Solanopsis_). Lyell, 1, 710_Lysimachia vulgaris_, 684

M

MacDougal, D. T. , 62, 575, 590Macfarlane, 56, 255, 268_Madia elegans_, 779_Magnolia_, 355, 366, 428, 674, 675 _obovata_, 355, 669_Magnus_, 228_Mahonia aquifolia_, 270Maize, 134, 775 "Cuzco, " 152 European, 206 "Gracillima, " 152 "Horse-dent, " 152 "Quarantino, " 118Mallow, 663, 684_Malva crispa_, 684Maples, laciniate, 615Marchant, 592Marigold, 131, 158 corn, 729 field, 503, 505, 508 garden, 503 Japanese, 490, 494, 495Marsh-marigold, 331Martinet, 80Measart, 434Masters, 228, 370, 372_Matricaria Chamomilla_, 130 _Chamomilla discoidea_, 156Matricaria discoidea, D. C. , 157May-thorn, red, 196_Medicago media_, 264 _falcata_, 264_Melanium_, 39Melons, 118Mendel, 6, 210, 294, 296, 306, 308Mendel's law, 276, 293, 294, 298, 299, 300, 301, 307, 612, 613, 616, 716Mendelism, 307Mentha, 52_Mercurialis annua_, 420 _annua laciniata_, 592Mercury, 420, 422, 425, 820Methods of investigation, 21Metzger, 205, 206Milde, 38Milfoil, 441Millardet, 266Miller, 611Millet, 105_Mimulus_, 151 _quinquevulnerus_, 725_Mimusops_, 697Miocene period, 698Miquel, 83_Mirabilis_, 241 _Jalappa_, 322Mirbel, 615_Monardella macrantha_, 444Monstrosities, 400, 401, 445, 446, 447Monkey-flower, 725Monocotyledons ancestry of, 1, 5 regression in, 630_Monotropa_, 222, 661Morphologic units, 145, 152Monstrosities, 818Morgan on mutation-theory, 9Morren, 244, 762Mountain-ash, 342Muller, Fritz, 775, 776, 780Multiplication, vegetative (see Asexual propagation)Munting, Abraham, 164, 165, 490, 762Munting's drawings, 512Murr, 158, 236_Muscari comosam_, 134Museum d'Histoire Naturelle, Paris, 522Mutability vs. Fluctuating variability, 568Mutation, 659, 674, 677, 685, 686, 694, 713, 716, 825 absence of intermediate steps in, 474, 480 conditions for observing, 601 decided within the seed, 28 definition of, 7 easily observed, 30 experimental, 688 few observations of, 8 fluctuation vs. , 7, 16, 719 influence of on variability, 335 iterative nature of, 476, , 703 laws of, 556, 558, 560, 562, 564, 566, 568, 570 limited in time, 29 observation of, 16 in _Oenothera_, 521, 525, 690 oldest known, 609 oldest recorded, 22 periodic, 690, 692, 694 perodicity of, 519 progressive, 307 repetition of, 476 in _Saponaria calabrica_, 612 simultaneous, 614 in tomato, 655Mutations, 141, 275, 280, 445, 449, 573, 608, 620, 626, 678, 685, 686, 701, 704, 712, 713, 716, 800 artificial, 402 chance for useful, 598 defined, 191 frequency of, 597 in garden-flowers, 488 in horticulture, 604, 706 latent, 703 mode of appearance, 517 numerical proportion of, 475 original production of, 702 peloric, 707 periodic, 686, 705 progressive, 704 retrograde, 704 stray, 704, 705, 706 synonyms of, 191Mutation-period, 714_Myosotis azorica_, 368_Myrtus communis_, 684

N

Nageli, 60, 439, 443, 448, 795Nagelian principle, 448, 450, 451Natural selection, 18, 119, 120, 445, 456, 682, 694, 703, 743, 744, 798-826 basis, 604 nature of, 6, 19Naudin, 118Nectarines, 137, 138, 226, 627Nemec, 578Neo-Lamarckians principle of, 8Neo-Lamarckism 447_Nepenthes_, 671, 672, 673, 674Newton, 1, 732, 767_Nicandra_, 152_Nigella_, 134Nightshade, 298 black, 282Nourishment meaning of, 732 variability and 771_Nuphar_, 268Nutrition and growth, 720, 722_Nymphaea_, 698

O

Oats, 98, 100, 101, 105, 112, 113, 115, 119, 133, 452 "Early Angus, " 115 "Early Fellow, " 115 "Fine Fellow, " 115 "Hopetown, " 112 "Longfellow, " 115 "Make-him-rich, " 112 wild, 207, 803Oak, 136, 239_Oenothera_, 260, 262, 279, 700, 706, 708, 709 European species, source of, 575 mutation in, 521, 525, 585, 690, 708 new species of, 516-546 _albida_, 537, 553, 555, 563, 565, 573 _biennia_, 82, 205, 256, 257, 258; 259, 262, 263, 264, 521, 524, 527, 574, 575, 586, 587, 683, 690, 708 _biennis cruciata_, 22, 587 _brevistylis_, 263, 280, 526, 529, 530, 547, 563, 564, 565, 573, 574, 702, 706 _cruciata_, 575, 585, 586, 589, 590, 683 _elliptica_, 540, 545, 555, 562 _gigas_, 533, 534, 535, 536; 537, 553, 554, 563, 565, 566, 567, 573, 574, 702 _glauca_, 424 _hirtella_, 262 _laevifolia_, 526, 528, 529, 547, 563, 564, 573, 574, 701, 706 _lamarckiana_, 17, 262, 262, 522, 523, 527, 528, 529, 533, 574, 575, 586, 690, 699 pollination of, 524 _lata_, 540, 541, 542, 549, 550, 551, 552, 555, 559, 563, 566, 573, 574, 702 _leptocarpa_, 540 _muricata_, 256, 257, 258, 259, 262, 263, 264, 513, 575, 690 pollination of, 524 _nanella_, 526, 531, 549, 50, 551, 552, 555, 563, 564, 565, 566, 703 _oblonga_, 537, 538, 552, 555, 563, 565, 566, 572 _rubrinervis_, 533, 534, 536, 537, 550, 551, 552, 555, 563, 565, 568, 573, 574 _scintillans_, 540, 543, 553, 555, 563, 566, 573, 574 mutability of, 544 _semilata_, 540 _suaveolens_, 521_Oleander_, 684_Onagra_, 262, 708, 709Onions, wild, 684_Ononis repens_, 577Orange, 90, 133, 134Orchids, 631Origin of species (Darwin), 109_Orobanche_, 220_Othonna crassifolia_, 442Otin, 618Oviedo, 89

P

_Paeonia corallina leiocarpa_, 126Paillat, 618Pangenes, 306Pangenesis, 306, 689_Panicum_, 105Pansies, 640Pansy, 118, 121_Papaver alpinum_, 139 _bracteatum_, 661 _bracteatum monopetalum_, 661 _commutatum_, 357 _dubium glabrum_, 126 hybridism, 662 _somniferum Danebrog, 162 _somniferum monstruosum_, 371 _somniferum polycephalum_, Parris, 754Parsley crisped, 158, 181Parsnip, water, 457Pea-family, 344Peach, 138, 226, 240Peach-almond, 769Pears, 79, 90, 134, 147, 152, 203, 283Pearson, Karl, 716Peas, sugar, 135, 158_Pedicularis_, 410 _palustris_, 410Pedigree-culture, 109 experimental, 547_Pelargonium_, 272, 355Peloria, definition of, 164Peloric toad-flax first record of, 466 origin of, 459, 464, 472 sterility of, 467Pelorism _Antirrhinum majus_ (see snapdragon) _Digitalis purpurea_, 482 _Gloxinia_, 484, 485 labiates, 486 _Laburnum_, 485 _Lamium_, 486. _Linaria_, see Toad-flax _Linaria dalmatica_, 482 _Linaria vulgaris_, 464 orchids, 479, 486, 487 _Salvia_, 486 _Scrophularia nodosa_, 486 snapdragon, 481 toad-flax, 459-487 _Tropaeolum majus_, 485 _Uropedium Lindenii_, 487 wild sage, 486_Peltaria alliacea_, 663Pennywort, marsh, 668Penzig, 638Periodicity, law of, 365, 368, 721, 722Periods, mutative, 706, 708Periwinkles, 322Persicaria, water, 433, 434, 435, 643Petalomany, 330Petunia, 491, 626_Phacelia_, 420, 422, 820_Phaseolus lunatus_, 592 _multiflorus_, 202 _nanus_, 202_Phleum alpinum_, 696_Phlox_, 232 _drummondi_, 161_Phyllonoma ruscifolia_, 676Physiologic units, 144, 153, 249_Picris hieraoioides_, 411Pimpernel, scarlet, 162Pinacothec, Munich, 164Pine, 368, 804Pine-apples, 90, 134Pinks, 178_Pinus sylvestris_, 368Pistillody in poppies, 369, 370, 372Pitcher-plants, 671Plankton, 711_Plantago_, 53 lanceolata_, 520, 671, 684Plantain, 684Plater, 610Plum, 79, 134, 789 beach, 58 Japanese, 58 purple-leaved, 619_Plusia_, 204_Poa alpina vivipara_, 684_Podocarpus koraiana_, 169_Polemonium coeruleum_, 282 _coeruleum album_, 200 _dissectum_, 161, 202_Polygala_, 242_Polygonum amphibium_, 432 var. _natans_ Moench, 433, 434 var. _terrestris_ Wench, 433, 434 _Convolvulus_, 419, 424 _viviparum_, 684Polymorphy, 188Pomegranate, 90Pond-lily, yellow, 268Poplar, fastigiate, 623, 624 Italian, 623_Populus italica_, 622 _nigra_, 624Poppy, 146, 151, 152, 163, 165, 241, 356, 640, 723 "Danebrog, " 283, 291 garden, 661 "Mephisto, " 283, 291 opium, 89, 189, 195, 198, 282, 291, 369, 371, 373, 379, 383, 391, 405, 406, 420, 452, 720, 789 pistillody in, 369 pistilloid, 508 polycephalous, 405Potatoes, 765, 810_Potentilla Tormentilla_, 52Pre-Linnean attitude, 2Primrose, 268, 372, 410 evening (see evening-primrose). _Primula acaulis_, 52, 632 _elatior_, 52, 633, 635 _grandiflora_, 268 _imperialis_, 697 _japonica_, 410 _officinalis_, 52, 268, 633, 635 _variabilis_, 268 _veris_, 52, 633, 634Prodromus (De Candolle) 370Progression, 430, 705, 774, 775, 777, 779, 805 in evolution, 630Propagation asexual, 745, 751, 766, 767, 770, 774, 777 sexual, 745, 777 vegetative (see asexual)Proskowetz, Em. Von, 70Prototype definition of, 170_Prunus_, 52 _cerasifera_, 619 _Mahaleb_, 617 _nana_, 612 _maritima_, 59 _Padus_, 617 _Pissardi_, 619 variation in, 56_Pyrethrum roseum_, 511_Pyrola_, 222, 661

Q

Quartile, 736, 737, 767_Quercus pedunculata fastigata_, 596Quetelet's law, 463, 716, 717, 725, 730, 734, 738, 748, 753, 759, 767, 775, 779, 780, 806

R

Races, inconstancy of improved, 770-797Raciborsky, 682Radishes, 325, 806Ragwort, tansy, 157Raisins, 134Rameses, 697_Ranunculus_, 331 _acris_, 331 _arvensis_, 282 _arvensis inermis_, 125 _asiaticus_, ,241 _bulbosus_, 357, 410, 740Ra-n-Woser, King, 104_Raphanus Raphanistrum_, 202, 424, 520 _caudatus_, 202Rasor, John, 588, 589Raspberry, 268, 768 "Phenomenal, " 268 "Primus, " 269 Siberian, 269Ratzeburg, 467Raunkiaer on variation in _Taraxacum_, 60Recessive character, 280 Sports, 191, 715, 689 bud, 427

S

Sprenger, 610, 611Stability, 155Stahl, 611_Stellaria Holostea apetala_, 585Stocks, 146, 322, 328, 329, 332, 334, 336, 338, 432Stock "Brompton, " 329 chamois-colored, 198 "Queen, " 324 white, 160Stork's-bill, white hemlock, 161Strasburger, 196, 448Strawberry, 158, 266, 342 "Gaillon, " 135 "Giant of Zuidwijk, " 614 one-leaved, 164, 596, 666 white, 158, 165Striped flowers, 309, 374, 431, 606, 607 races, types of, 328Struggle for life, 674, 571, 682, 702, 799, 803, 824, 825St. Johnswort, 725St. Sebastian, 164Sub-species (see also Elementary species), 224, 225Sugar-beets (see Beets, sugar)Sugar-cane, 731, 752 "Black Manilla, " 753 "Cheribon, " 753, 755, 756 "Chunnic, " 753 "Hawaii, " 755, 756 seeds of, 754 "White Manilla, " 752Sundew, 268Sunflower, 410, 425, 820Sweet-flag, 222Sweet-pea, 160, 776Sweet William, 163, 282, 322, 648 twisted variety, 408, 648Syncotyls, 417, 424_Syringa vulgaris axurea plena_, 763Systematic species, 12, 64, 101, 128 nature of, 54, 62Systematic units, 61, 91

T

_Tagetes africana_, 510 _signata_, 612"Talavera de Bellevue, " 97_Tanacetum vulgare_, 131, 132, 236Tansy, 131, 132, 236_Taraxacum_, 125, 126 officinale, 59, 411_Tares_, 105_Taxus_, 136 _baccata_, 169 _baccata fastigiata_, 170, 618 _minor_, 169Teasels, 402, 642, 645, 674, 675 twisted, 405, 412, 446, 447, 643, 646, 647, 648, 819_Tetragonia expansa_, 162Theatre d'Agriculture, 72Thibault, 618Thomson, Sir William (see Kelvin, Lord)Thorn-apples, 139, 142, 143, 145, 238, 283, 300, 452 thornless, 234Thorn-broom, 457_Thrincia hirta_, 411Thuret, 38, 47, 49Thyme, white creeping, 201_Thymus Serphyllum album_, 201 _vulgaris_, 577_Tilia parvifolia_, 355, 669Toad-flax, 267, 282, 707 cross pollination of, 471 experiment with, described, 468 invisible dimorphous state of, 470, 471, 478 latent tendency to mutation in, 479 peloric, see Peloric toad flax sterility of mutants, 477 unusual pelorism, 486Tomato, 653 "Acme, " 656, 657 "Mikado, " 654 mutation of, 655 upright, 654 "Washington, " 657Tournefort author of genera, 32Tracy, W. W. 592Trees, genealogic, 707, 708Tricotyls, 416, 419, 420_Trifolium incarnatum_, 352_Triticum dicoccum_, 105_Tropaeolum_, 193, 668 _majus_, pelorism of, 485"True Exercises with Plants" (hunting), 490Tulips, 149, 178, 274, 322 black, 620Turnip, 244, 621Twisted stems, 402, 403, 405, 413Twisted varieties atavists of, 406

U

_Ulex europaeus_, 140, 217_Ulmus pedunculata_, 615 _pedunculata urticaefolia_, 615Umbellifers, 457_Umbilicus_, 669Unger, 105Unit-characters, 249, 261, 306, 307, 313, 658, 689, 715, 716Urban, 265_Uropedium lindenii_, 487Utility, 685, 724Utricularia, 672

V

_Vaccinium Myrtillus_, 577Valerian, 402, 409, 648 twisted, 403_Valeriana officinalis_, 402_Vallisneria_, 684. Van den Berg, 625Van de Water, 614Van Mons, 76, 77, 78, 806Variability (see also Fluctuation ), 188, 190, 191 analogous, 244 apple, 75 asexual, 320 correlative, 142, 143, 148, 167 cultivated plants, 66 embryonic, 770, 771, 814 ever-recurring, 190 fluctuating (see also individual), 62, 142, 190, 233, 375, 416, 454, 698, 759, 762, 765, 766, 767, 770, 771, 789, 805, 814 fluctuating vs. Mutability 569 homologous, 244 individual (see also fluctuating), 190, 716, 718, 746, 749, 770, 814 influence of mutation on, 335 kinds of, 715 nutrition and, 390, 391, 719, 771 parallel, 243 partial, 440, 444, 718, 746, 748, 753, 814, 816 repeated, 242 restricted, 598 sectional, 317 sexual, 320 sources of, 758Variation bud, 176, 178, 180, 284, 317, 318, 321, 338, 427, 750 definition of, 188 partial, 788, 789 seed, 750 spontaneous, 191 use of term, 189Variegation, 426, 427Varietal marks, origin of, 275Varieties, 84, 95, 126, 127, 128, 129, 132, 142 broom-like, 618, 624 constancy of, 532 constant, 135 crosses of species with, 247, 277, 278, 281 elementary species vs. 459 ever-sporting, 178, 309, 310, 311, 312, 313, 321, 324, 328, 329, 332, 333, 334, 350, 358, 365, 368, 372, 399, 413, 420, 430, 431, 432, 434, 445, 606, 607, 628, 740, 789, 790, 795 fasciated (see Fasciated stems). Groups of, 606 horticultural, 607, 609 hybrid, 122, 190, 608 hybrids of, 210, 254, 255 inconstant, 135, 154; 155, 161 mutation of, 141 negative (retrogressive), 131, 132, 134, 224, 226, 238, 245, 277 positive, 131, 132, 134, 224, 238, 245 pure, 122, 190 retrograde, 14, 15, 16, 95, 121, 208, 430, 435, 606, 607 retrogressive (see negative). Seed, 122 single, 191 spontaneous crosses, 209 sporting (see inconstant) stability of, 207 sterile, 622 types of, 142 variable, 606 vegetative, 122 weeping, 617Variety, 130 definition of, 11, 12 elementary species vs. 141, 152, 154, 224, 243, 247, 251 origin of, 141, 152, 224 use of term, 189, 435Variety-testing, 95, 97, 116, 119, 743, 799, 825Varro, 106Veitch & Sons, 272Venus' looking-glass, 367Verlot, 186, 612Vernon, 132_Vernonia cinerea_, 450_Veronica longifolia_, 282, 284 _scutellata_, 139 _spicata nitens_, 126_Viburnum Opulus_, 134, 239Vicinism, 185, 188, 203, 205, 206, 213, 214, 776 definition of, 188, 192, 606Vicinist, 199, 201_Vicoa aurioulata_, 450_Victoria regia_, 668Villars on _Draba verna_, 49Vilmorin, 570, 607, 612, 622, 661, 662, 773, 775, 776; 792, 795, 796, 797, 806, 807, 810, 813, 818, 820Vilmorin, Louis de, 72, 92, 93, 97, 108, 109, 110, 114, 185, 818Vilmorin, Messrs. , 322_Vinca_, 242, 490 _minor_, 322Vine, parsley-leaved, 179_Viola_, 126, 546, 547, 692 _agrestis_, 45 _alpestris_, 40 _altaica_, 39 _anopetala_, 44 _arvensis_, 39, 40, 41, 44 _curtisepala_, 45 _striolata_, 45 _aurobadia_, 44 _caloarata_, 39 _cornuta_, 39, 281 _lutea_, 38 _lutescens_, 44 _nemausensis_, 45 _ornatissima_, 44 _palescens_, 45 _patens_, 45 _roseola_, 44 _segetatis_, 45 _stenochila_, 41 _tricolor_, 38, 40, 41, 44, 46 _ammotropha_, 41 _coniophila_, 41 _genuina_, 42 _versicolor_, 42Violets, 63, 232, 233, 490Violet, dame's, 322, 323, 411 long-spurred, 281Virgil, 105, 106, 108_Viscaria oculata_, 4, 648, 821 twisted variety, 408_Vitis_, 52Volckamer, 228Von Lochow, 821, 822, 822Von Rumker, 94Von Wettstein, 448, 805Vrolik, 164, 483

W

"Waare Oeffeninge der Planten" (Munting), 490Wallace, 5, 7, 8, 30, 205Wall-flower, 370, 371Walnut, 243, 766 cut-leaved, 616 one-bladed, 666Water-lilies, 668Weber, 228Weeping-willow, 180 crisped, 181Weigelias, 740_Wellingtonia_, 618Wheat, 96, 98, 105, 113, 119, 283, 810, 823 bearded, 98 "Blue-stem, " 117 "Galland, " 100, 207 "Hopetown, " 112, 112 "Hunter's, " 111, 112 "Minnesota No. 169, " 117 "Mungoswell's, " 110, 111 "Pedigree, " 109 "Pringle's, " 114 "Rivett's bearded, " 207 "Sheriff's bearded red, " 114 "Sheriff's bearded white, " 114 "White Hunter's, " 112Wheat-ear carnation, 227White, C. A. , 656, 657White varieties, 577Whitlow-grasses, 63, 118, 119Whorls, ternate, 684Wild sage (see Salvia)Willdenow, 468, 666, 667Williamson, 491Willows, 135, 267Willow weeping (see Weeping-Willow)Willow-herb, 268, 269, 682Wintercress, 427Wintergreen, 661Wittmack, 682Wittrock, 38, 40, 41, 42, 43, 44, 45, 46Wooton, E. O. , 140Wormseed, 638

X

_Xanthium canadense_, 140 _commune_, 140, 152, 591 _commune Wootoni_, 22Wootoni, 140, 152, 591

Y

Yarrow, 131, 132Yew, 136, 169 pyramidal, 618

Z

_Zea Mays cryptosperma_, 641 _tunicata_, 641_Zinnia_, 490Zioberg, 466Zocher & Co. , 230