HORMONES AND HEREDITY

 A Discussion Of The Evolution Of Adaptations And The Evolution Of Species

By J. T. CUNNINGHAM, M. A. (OXON), F. Z. S. 

Sometime Fellow of University College, OxfordLecturer in zoology at East London College, University of London

LONDONCONSTABLE AND CO. LTD. 1921

PREFACE

My chief object in writing this volume was to discuss the relations ofmodern discoveries concerning hormones or internal secretions to thequestion of the evolution of adaptations, and on the other hand to theresults of recent investigations of Mendelian heredity and mutations. Ihave frequently found, from verbal or written references to my opinions, that the evidence on these questions and my own conclusions from thatevidence were either imperfectly known or misunderstood. This is notsurprising in view of the fact that hitherto my only publications on thehormone theory have been a paper in a German periodical and a chapter inan elementary text-book. The present publication is by no means a thoroughor complete exposition of the subject, it is merely an attempt to statethe fundamental facts and conclusions, the importance of which it seems tome are not generally appreciated by biologists. 

I have reviewed some of the chief of the recent discoveries concerningmutations, Mendelism, chromosomes, etc. , but have not thought it necessaryto repeat the illustrations which are contained in many of the volumes towhich I have referred. I have made some Mendelian experiments myself, notalways with results in agreement with the strict Mendelian doctrine, sothat I am not venturing to criticise without experience. I have nothesitated to reprint the figure, published many years ago, of a Floundershowing the production of pigment under the influence of light, because Ithought it was desirable that the reader should have before him thisfigure and those of an example of mutation in the Turbot for comparisonwhen following the argument concerning mutation and recapitulation. 

I take this opportunity of expressing my thanks to the Councils of theRoyal Society and the Zoological Society for permission to reproduce thefigures in the Plates. I also desire to thank Professor Dendy, F. R. S. , ofKing's College for his sympathetic interest in the publication of thebook, and Messrs. Constable and Co. For the care they have taken in itsproduction. 

J. T. CUNNINGHAM. London, _June_ 1921. 

 CONTENTS

INTRODUCTION - Historical Survey Of Theories Or Suggestions Of Chemical Influence In Heredity

CHAPTER I - Classification And Adaptation

CHAPTER II - Mendelism And The Heredity Of Sex

CHAPTER III - Influence Of Hormones On Development Of Somatic Sex-Characters

CHAPTER IV - Origin Of Somatic Sex-Characters In Evolution

CHAPTER V - Mammalian Sexual Characters, Evidence Opposed To The Hormone Theory

CHAPTER VI - Origin Of Non-Sexual Characters: The Phenomena Of Mutation

CHAPTER VII - Metamorphosis and Recapitulation

INDEX

 LIST OF PLATES

PLATE I. Recessive Pile Fowls

PLATE II. Abnormal Specimen Of Turbot

PLATE III. Flounder, Showing Pigmentation Of Lower Side After Exposure To Light

INTRODUCTION

 Historical Survey Of Theories Or Suggestions Of Chemical Influence In Heredity

Weismann, strongly as he denied the possibility of the transmission ofsomatic modifications, admitted the possibility or even the fact of thesimultaneous modification of soma and germ by external conditionssuch as temperature. Yves Delage [Footnote: Yves Delage, _L'Hérédité_(Paris, 1895), pp. 806-812. ] in 1895, in discussing this question, pointedout how changes affecting the soma would produce an effect on the ovum(and presumably in a similar way on the sperm). He writes:--

'Ce qui empêche l'oeuf de recevoir la modification reversible c'estqu'étant constitué autrement que les cellules différenciées de l'organismeil est influencé autrement qu'elles par les mêmes causes perturbatrices. Mais est-il impossible que malgré la différence de constitutionphysico-chimiques il soit influencé de la même façon?'

The author's meaning would probably have been better expressed if he hadwritten 'ce qui paraît empêcher. ' By 'modification reversible' he means achange in the ovum which will produce in the next generation a somaticmodification similar to that by which it was produced. It seems natural tothink of the influence of the ovum on the body and of the body on the ovumas of similar kind but in opposite directions, but it must be rememberedalways that the development of the body from the ovum Is not an influenceat all but a direct conversion by cell-division and differentiation of theovum into the body. 

Delage argues that if the egg contains the substances characteristic ofcertain categories of cells of the organism it ought to be affected at thesame time as those cells and by the same agents. He thinks that the eggonly contains the substances or the arrangements characteristic of certaingeneral functions (nervous, muscular, perhaps glandular of divers kinds)but without attribution to localised organs. In his view there is norepresentation of parts or of functions in the ovum, but a simplequalitative conformity of constitution between the egg and the categoriesof cells which in the body are charged with the accomplishment of theprincipal functions. Thus mutilations of organs formed of tissuesoccurring also elsewhere in the body cannot be hereditary, but if theorgan affected contains the whole of a certain kind of tissue such asliver, spleen, kidney, then the blood undergoes a qualitative modificationwhich reacts on the constitution of the egg. 

Suppose the internal secretion of a gland (_e. G. _ glucose for the liver, glycolytic for the ferment for the pancreas) is the physiological excitantfor the gland. If the gland is removed in whole or in part the proportionof its internal secretion in the blood will be diminished. Then the gland, if the suppression is partial, will undergo a new diminution of activityBut in, the egg the specific substance of the gland will also be lessstimulated, and in the next generation a diminution of the gland mayresult. Thus Delage states Massin found that partial removal of the liverin rabbits had an inherited effect. In the case of excretory glands thecontrary will be the case, for their removal causes increase in the bloodof the exciting urea and uric acid. 

The effects of disuse are similar to those of mutilations and of use viceversa. Delage, as seen above, does not consider that increase or decreaseof particular muscles can be inherited, but only the muscular system ingeneral. If, however, in consequence of the disuse of a group of musclesthere was a general diminution of the inherited muscular system, thespecial group would remain diminished while the rest were developed by usein the individual: there would thus be a heredity produced indirectly. With regard to general conditions of life, Delage states that there areonly two of which we know anything--namely, climate and alimentation--andhe merely suggests that temperature and food act at the same time on thecells of the body and on the similar substances in the egg. 

H. M. Vernon (_Variation in Animals and Plants_, 1903, pp. 351 _seq. _)cites instances of the cumulative effects of changed conditions of life, and points out that they are not really instances of the inheritance ofacquired characters, but merely of the germ-plasm and the body tissuesbeing simultaneously affected. He then asks, Through what agency is theenvironment enabled to act on the germ-plasm? And answers that the onlyconceivable one is a chemical influence through products of metabolismand specific internal secretions. He cites several cases of specificinternal secretions, making one statement in particular which seemsunintelligible, viz. That extirpation of the total kidney substance of adog leads not to a diminished secretion of urine but to a largelyincreased secretion accompanied by a rapid wasting away which soon endsfatally. 

Whenever a changed environment acts upon the organism, therefore, it tosome extent affects the normal excretions and secretions of some or all ofthe various tissues, and these react not only on the tissues themselves, but also to a less degree upon the determinants representing them in thegerm-plasm. Thus the relative size of the brain has decreased in the tamerabbit. This may be due to disuse; the excretions and secretions of thenervous tissues would be diminished, and the corresponding determinantsless stimulated. Another instance is afforded by pigmentation of the skinin man; which varies with the amount of light and heat from the sun towhich the skin is exposed. Specific excretory products of pigment in theskin may stimulate the pigment determinants in the germ-plasm to vigour. But only those characters of which the corresponding tissues possess aspecific secretion or excretion could become hereditary in this way. Forinstance, the brawny arm of the blacksmith could not be transmitted, as itis scarcely possible that the arm muscles can have a secretion differentfrom that of the other muscles. 

In 1904, P. Schiefferdecker[Footnote: P. Schiefferdecker, _Ueber Symbiose_. S. B. D. Niederrhein. Gesellsch. Zu Bonn. Sitzung der Medicinischen Sektion, 13 Juni 1904. ]made the definite suggestion that the presence of specific internalsecretions could be very well used for the explanation of the inheritanceof acquired characters. When particular parts of the body were changed, these modifications must change the mixture of materials in the blood bythe substances secreted by the changed parts. Thereby would be found aconnexion between the modified parts of the body and the germ-cells, theonly connexion in existence. It is to be assumed, according to thisauthor, that only a qualitative change in the nutritive fluid of thegerm-cells could produce an effect: a quantitative change would only causeincreased or decreased nourishment of the entire germ cells. 

In my own volume on _Sexual Dimorphism in the Animal Kingdom_, publishedin 1900, I attempted to explain the limitation of secondary sexualcharacters not only to one sex, but usually to one period of theindividual life, namely, that of sexual maturity; and in some cases, as inmale Cervidae, to one season of the year in which alone the sexual organsare active. It had been known for centuries that the normal development ofmale sexual characters did not take place in castrated animals, but theexact nature of the influence of the male generative organs on thatdevelopment was not known till a year or two later than 1900, when it wasshown to be due to an internal secretion. My argument was that allselection theories failed to account for the limitation of secondarysexual characters in heredity, whereas the Lamarckian theory would explainthem if the assumption were made that the effects of stimulation havingbeen originally produced when the body and tissues were under theinfluence of the sexual organs in functional activity, these effects wereonly developed in heredity when the body was in the same condition. 

About the year 1906, when preparing two special lectures in LondonUniversity on the same subject, I became acquainted with the work ofStarling and others on internal secretions or hormones, and saw at oncethat the hormone from the testes was the actual agent which constitutedthe 'influence' assumed by me in 1900. In these lectures I elaborated adefinite Lamarckian theory of the origin of Secondary Sexual Characters inrelation to Hormones, extending the theory also to ordinary adaptivestructures and characters which are not related to sex. Having met withmany obstacles in endeavouring to get a paper founded on the originallectures published in England, I finally sent it to Professor WilhelmRoux, the editor of the _Archiv für Entwicklungsmechanik der Organismen_, in which it was published in 1908. 

In his volume on the Embryology of the Invertebrata, 1914 (_Text-Book ofEmbryology_, edited by Walter Heape, vol. I. ), Professor E. W. MacBride inhis general summary (chapter xviii. ) puts forward suggestions concerninghormones without any reference to those who have discussed the subjectpreviously. He considers the matter from the point of view of development, and after indicating the probability that hormones are given off by allthe tissues of the body, gives instances of organs being formed inregeneration (eye of shrimp) or larvae (common sea-urchin) as the resultof the presence of neighbouring organs, an influence which he thinks canonly be due to a hormone given off by the organ already present. He thenstates that Professor Langley had pointed out to him in correspondencethat if an animal changes its structure in response to a changedenvironment, the hormones produced by the altered organs will be changed. The altered hormones will circulate in the blood and bathe the growing andmaturing genital cells. Sooner or later, he assumes, some of thesehormones may become incorporated in the nuclear matter of the genitalcells, and when these cells develop into embryos the hormones will be setfree at the corresponding period of development at which they wereoriginally formed, and reinforce the action of the environment. In thisway MacBride attempts to explain recapitulation in development and thetendency to precocity in the development of ancestral structures. His ideathat the hormones act by 'incorporation' in the genital cells is differentfrom that of stimulation of determinants put forward by myself and others, but it is surprising that he should refer to unpublished suggestions ofProfessor Langley, and not to the publications of authors who hadpreviously discussed the possible action of hormones in connexion with theheredity of somatic modifications. 

Dr. J. G. Adami in 1918 published the Croonian Lectures, delivered by himin 1917 under the title 'Adaptation and Disease, ' together with reprintsof previous papers, in a volume entitled _Medical Contributions to theStudy of Evolution_. In this work (footnote, p. 71) the author claimsthat he preceded Professor Yves Delage by some two years in offering aphysico-chemical hypothesis in place of determinants, and also assertsthat 'the conclusions reached by him in 1901 regarding metabolites and, aswe subsequently became accustomed to term them, hormones, and theirinfluence on the germ-cells, have since been enunciated by Heape, Bourne, Cunningham, MacBride, and Dendy, although in each case without note of his(Adami's) earlier contribution. ' These somewhat extensive claims deservecareful and impartial examination. The paper to which Dr. Adami refers wasan Annual Address to the Brooklyn Medical Club, published in the _New YorkMedical Journal_ and the _British Medical Journal_ in 1901, and entitled'On Theories of Inheritance, with special reference to Inheritance ofAcquired Conditions in Man. ' The belief that this paper had two years'priority over the volume of Delage entitled _L'Hérédité_ appears to havearisen from the fact that Adami consulted the bibliographical list inThomson's compilation, _Heredity_ 1908, where the date of Delage's work isas 1903. But this was the second edition, the first having been published, as quoted above, in 1895, six years before the paper by Adami. 

Next, with regard to the claim that Adami's views as stated in the paperto which he refers were essentially the same as those brought forwardby myself and others many years later, we find on reading the paper thatits author discussed merely the effect of toxins in disease upon thebody-cells and the germ-cells, causing in the offspring either variousforms of arrested and imperfect development or some degree of immunity. Inthe latter case he argues that the action of the toxin of the disease hasbeen to set up certain molecular changes, certain alterations in thecomposition of the cell-substance so that the latter responds in adifferent manner when again brought into contact with the toxin. Once thismodification in the cell-substance is produced the descendants of thiscell retain the same properties, although not permanently. Inheritance ofthe acquired condition has to be granted, he says, in the case of thebody-cells in such cases. But this is not the question: inheritance in theproper sense of the word means the transmission to individuals of the nextgeneration. 

On this point Adami says we must logically admit the action of the toxinson the germ-cells, and the individuals developed from these must, subjectto the law of loss already noted, have the same properties. He admits thatinherited immunity is rare, but says that it has occasionally been noted. Here we have again merely the same influence, chemical in this case, acting simultaneously on somatic cells and germ-cells, which is notthe inheritance of acquired characters at all. Adami remarks that Weismannwould make the somewhat subtle distinction that the toxins produce theseresults not by acting on the body-cells but by direct action on thegerm-cells, that the inheritance is blastogenic not somatogenic, and callsthis 'a sorry and almost Jesuitic play upon words. ' On the contrary, it isthe essential point, which Adami fails to appreciate. However, he goesfurther and refers to endogenous intoxication, to disturbed states of theconstitution, due to disturbances in glandular activity or to excess ofcertain internal secretions. Such disturbances he says, acting on thegerm-cells, would be truly somatogenic. In the case of gout he considersthat defect in body metabolism has led to intoxication of the germ-cells, and the offspring show a peculiar liability to be the subjects ofintoxications of the same order. Now, however important these viewsand conclusions may be from the medical point of view, in relation tothe heredity of general physiological or pathological conditions, they throw no light on the problems considered by myself and otherbiologists--namely, the origin of species and of structural adaptations. 

There is no mention anywhere in Adami's short paper of the evolution orheredity of structural characters or adaptations such as wing of Bird orBat, lung of Frog, asymmetry of Flat-fish or of specific characters, stillless of secondary sexual characters, which formed the basis of the hormonetheory in my 1908 paper. He does not even consider the evolution of thestructural adaptations which enable man to maintain the erect position onthe two hind-limbs. He does not consider the action of externalstimulation, whether the direct action on epidermal or other externalstructures or the indirect action through stimulation of functionalactivity. All his examples of external agents are toxins produced bybacteria invading the body, except in the case of gout, for which hesuggests no external cause at all. 

Only once in the last of the part of the paper considered does Adamimention internal secretions. His actual words are: 'We recognise yearlymore and more the existence of auto-intoxications, of disturbed states ofthe constitution due to disturbances in glandular activity or to excess ofcertain internal secretions or of the substances ordinarily neutralised bythe same. ' The only example he gives is that of gout. How remote this isfrom the discoveries concerning the specific action of hormones on thegrowth of the body or of special parts of the body, or on the function ofglands, and from a definite hormone theory of heredity as proposed bymyself, is sufficiently obvious. 

CHAPTER I

 Classification And Adaptation

The study of the animals and plants now living on the earth naturallydivides itself into two branches, the one being concerned with theirstructure and classification, the other with their living activities, their habits, life histories, and reproduction. Both branches are usuallyincluded under the terms Natural History, or Zoology, or Botany, and awork on any group of animals usually attempts to describe their structure, their classification, and their habits. But these two branches ofbiological science are obviously distinct in their methods and aims, andeach has its own specialists. The pursuit, whose ultimate object is todistinguish the various kinds of organisms and show their true and notmerely apparent relations to one another in structure and descent, requires large collections of specimens for comparison and reference: itcan be carried on more successfully in the museum than among the animalsor plants in their natural surroundings. This study, which may be calledTaxonomics, deals, in fact, with organisms as dead specimens, and itemphasises especially the distinguishing characters of the ultimatesubdivisions of the various tribes of animals and plants--namely, speciesand varieties. The investigation, on the other hand, of the differentmodes of life of animals or plants is based on a different mentalconception of them: it regards them primarily as living active organisms, not as dead and preserved specimens, and it can only be carried onsuccessfully by observing them in their natural conditions, in the widespaces of nature, under the open sky. 

The object of this kind of inquiry is to ascertain what are the uses oforgans or structures, what they are for, as we say in colloquial language, to discover what are their functions and how these functions are useful ornecessary to the life of the animals or plants to which they belong. Forexample, some Cuttle-fishes or Cephalopoda have eight arms or tentaclesand others ten. The taxonomist notices the fact and distinguishes the twogroups of Octopoda and Decapoda. 

But it is also of interest to ascertain what is the use of the twoadditional arms in the Decapoda. They differ from the other arms in beingmuch longer, and provided with sockets into which they can be retracted, and suckers on them are limited to the terminal region. In the majority ofzoological books in which Cephalopoda are described, nothing is said ofthe use or function of these two special arms. Observation of the livinganimal in aquaria has shown that their functions is to capture active preysuch as prawns. They act as a kind of double lasso. Sepia, for instance, approaches gently and cautiously till it is within striking distance of aprawn, then the two long tentacles are suddenly and swiftly shot out fromtheir sockets and the prawn is caught between the suckers at the ends ofthem. Another example is afforded by the masked crab (_Corystescassivelaunus_). This species has unusually long and hairy antennae. Theseare usually tactile organs, but it has been found that the habit of_Corystes_ is to bury itself deep in the sand with only the tips of theantennae at the surface, and the two are placed close together so as toform a tube, down which a current of water, produced by movements ofcertain appendages, passes to the gill chamber and provides for therespiration of the crab while it is buried, to a depth of two or threeinches. The results of the investigation of habits and functions may becalled Bionomics. It may be aided by scientific institutions speciallydesigned to supplement mere observation in the field, such as menageries, aquaria, vivaria, marine laboratories, the objects of which are to bringthe living organism under closer and more accurate observation. Thedifferences between the methods and results of these two branches ofBiology may be illustrated by comparing a British Museum Catalogue withone of Darwin's studies, such as the 'Fertilisation of Orchids' or'Earthworms. '

Other speculations in Biology are related to Taxonomics or Bionomicsaccording as they deal with the structure of the dead organism or theaction of the living. Anatomy and its more theoretical interpretation, morphology, are related to Taxonomics, physiology and its branches toBionomics. In fact, the fundamental principles of physiology must beunderstood before the study of Bionomics can begin. We must know theessential nature of the process of respiration before we can appreciatethe different modes of respiration in a whale and a fish, an aquaticinsect and a crustacean. The more we know of the physiology ofreproduction, the better we can understand the sexual and parental habitsof different kinds of animals. 

The two branches of biological study which we are contrasting cannot, however, be completely separated even by those whose studies are mostspecialised. In Bionomics it is necessary to distinguish the types whichare observed, and often even the species, as may be illustrated by thefact that controversies occasionally arise among amateur and evenprofessional fishermen on the question whether dog-fishes are viviparousor oviparous, the fact being that some species are the one and others theother, or the fact that the harmless slow-worm and ring-snake are dreadedand killed in the belief that they are venomous snakes. Taxonomics, on theother hand, must take account of the sex of its specimens, and the changesof structure that an individual undergoes in the course of its life, andof the different types that may be normally produced from the sameparents, otherwise absurd errors are perpetrated. The young, the male, andthe female of the same species have frequently been described underdifferent names as distinct species or even genera. For example, the larvaof marine crabs was formerly described as a distinct genus under the nameof _Zoaea_, and in the earlier part of the nineteenth century a livelycontroversy on the question was carried on between a retired naval surgeonwho hatched _Zoaea_ from the eggs of crabs, and an eminent authority whowas Professor at Oxford and a Fellow of the Royal Society, and whomaintained that _Zoaea_ was a mature and independent form. In the endtaxonomy had to be altered so as to conform with the fact of development, and the name _Zoaea_ disappeared altogether as that of an independentgenus, persisting only as a convenient term for an important larval stagein the development of crabs. 

These two kinds of study give us a knowledge of the animals now living. But we find it a universal rule that the individual animal is transitory, that the duration of life, though varying from a few weeks to more than acentury, is limited, and that new individuals arise by reproduction, andwe have no evidence that the series of successive generations has everbeen interrupted; that is to say, the series in any given individual orspecies may come to an end; species may be exterminated, but we know of noinstance of individuals coming into existence except by the process ofreproduction or generation from pre-existing individuals. Further, we knowfrom the evidence of fossil remains that the animals existing in formerperiods were very different from those existing now, and that many of theexisting forms, such as man, mammals, birds, bony fishes, can only betraced back in the succession of stratified rocks to the later strata orto those about the middle of the series, evidence of their existence inthe periods represented by the most ancient strata being entirely absent. Existing types then must have arisen by evolution, by changes occurring inthe succession of generations. 

These three facts--namely, the limited duration of individual life, theuninterrupted succession of generations, and the differences of theexisting animals and plants from those of former geological periods whoseremains are preserved in stratified rocks--are sufficient by themselves toprove that evolution has taken place, that the history of organisms hasbeen a process of descent with modification. If the animals and plantswhose remains are preserved as fossils, or at any rate forms closelyrelated to these, were not the ancestors of existing forms, there are onlytwo other possibilities: either the existing forms came into existence bynew creations after the older forms became extinct, or the ancestors ofexisting forms, although they coexisted with the older forms, never leftany fossil remains. Each of these suppositions is incredible. 

In view of these plain facts and their logical conclusion it is curious tonotice how Darwin in his _Origin of Species_ constantly mingles togetherarguments to prove the proposition that evolution has occurred, that thestructure and relations of existing animals can only be explained bydescent with modification, with arguments and evidence in favour ofnatural selection as the explanation and cause of evolution. In the greatcontroversy about evolution which his work aroused, the majority of theeducated public were ultimately convinced of the truth of evolution by thebelief that a sufficient cause of the process of change had beendiscovered, rather than by the logical conclusion that the organisms of alater period were the descendants of those of earlier periods. Even at thepresent day the theory of natural selection is constantly confused withthe doctrine of evolution. The fact is that the investigation of thecauses of evolution has been going on and has been making progress fromthe time of Darwin, and from times much earlier than his, down to thepresent day. 

Bionomics show that every type must be adapted in structure to maintainits life under the conditions in which it lives, the primary requirementsbeing food and oxygen. Every animal must be able to procure food either ofvarious kinds or some special kind--either plants or other animals; it maybe adapted to feed on plants or to catch insects or fish or animalssimilar to itself; its digestive organs must be adapted to the kind offood it takes; it must have respiratory organs adapted to breathe in airor water; it must produce eggs able to survive in particular conditions, and so on. 

One of the most interesting results of the study of the facts of evolutionis that each type of animal tends to multiply to such an extent as tooccupy the whole earth and adapt itself to all possible conditions. In theSecondary period reptiles so adapted themselves: there were oceanicreptiles, flying reptiles, herbivorous reptiles, carnivorous reptiles. Atthe present day the Chelonia alone include oceanic, fresh-water, andterrestrial forms. Birds again have adapted themselves to oceanicconditions, to forests, plains, deserts, fresh waters. Mammals haverepeated the process. The organs of locomotion in such cases show profoundmodifications, adapting them to their special functions. One thing to beexplained is the origin of adaptations. 

It is, however, necessary to distinguish between the adapted condition orstructure of an organ and the process by which it became adapted inevolution; two ideas which are often confused. The eye would he equallyadapted for seeing whether it had been created in its actual condition orgradually evolved. We have to distinguish here, as in other matters, between being and becoming, and, further, to distinguish between two kindsof becoming--namely, the development of the organ in the individual andits evolution in the course of descent. The word 'adaptation' is itselfthe cause of much fallacious reasoning and confusion of ideas, inasmuch asit suggests a process rather than a condition, and by biological writersis often used at one time to mean the former and at others the latter. Wemay take the mammary glands of mammals or organs adapted for the secretionof milk, whose only function is obviously the nourishment of theoffspring. Here the function is certain whatever view we take of theorigin of the organs, whether we believe they were created or evolved. Butif we consider the flipper or paddle of a whale, we see that it ishomologous with the fore-leg of a terrestrial mammal, and we are in thehabit of saying that in the whale the fore-limb is modified into a paddleand has become adapted for aquatic locomotion. This, of course, assumesthat it has become so adapted in the course of descent. But the pectoralfin of a fish is equally 'adapted' for aquatic locomotion, but it iscertainly not the fore-leg of a terrestrial mammal adapted for thatpurpose. The original meaning of adaptation in animals and plants, oforganic adaptation to use another term, is the relation of a mechanism toits action or of a tool to its work. A hammer is an adaptation forknocking in nails, and the woodpecker uses its head and beak in a similarway for making a hole in the bark of trees. The wings and the wholestructure of a bird's body form a mechanism for producing one of the mostdifficult of mechanical results, namely, flight. Then, again, there arestationary conditions, such as colour and patterns, or scales and armour, which may he useful in the life of an animal or flower, but are notmechanisms of moving parts like a bird's wing, or secreting organs likemammary glands. Unless we choose or invent some new term, we must defineadaptations apart from all questions of evolution as any structures orcharacters in an organism which can be shown either by their merepresence, or by their active function, to be either useful or necessary tothe animal's existence. We must be on our guard against assuming that theword 'adaptation' implies any particular theory or conclusion concerningthe method and process by which adaptations have arisen in the course ofevolution. It is that method and process which we have to investigate. 

On the other hand, when we look primarily at differences of structure wefind that not only are there wide and distinct gaps between the largercategories, such as mammals and birds, with few or no intermediate forms, but the actual individuals most closely similar to one another naturallyand inevitably fall into distinct groups which we call kinds or species. The conception of a species is difficult to define, and authorities arenot agreed about it. Some, like Professor Huxley, state that a species ispurely a mental conception, a generalised idea of a type to which actualindividuals more or less closely conform. According to Huxley, you cannotlock the species 'horse' in a stable. Others regard the matter moreobjectively, and regard the species merely as the total number ofindividuals which possess a certain degree of resemblance, including, asmentioned above, all the forms which may be produced by the same parents, or which are merely stages in the life of the individual. There are casesin which the limits of species or the boundaries between them areindistinct, where there is a graduated series of differences through awide range of structure, but these cases are the exception; usually thereare a vast majority of individuals which belong distinctly to one speciesor another, while intermediate forms are rare or absent. The problem thenis, How did these distinct species arise? How are we to explain theirrelations to one another in groups of species or genera; why are thegenera grouped into families, families into orders, orders into classes, and so on?

There are thus two main problems of evolution: first, how have animalsbecome adapted to their conditions of life, how have their organs becomeadapted to the functions and actions they have to perform, or, at least, which they do perform? The power of flight, for example, has been evolvedby somewhat different modifications in several different types of animalsnot closely related to one another: in reptiles, in birds, and in mammals. We have no reason to believe that this faculty was ever universal, or thatit existed in the original ancestors. How then was it evolved? The secondgreat problem is, How is it that existing animals, and, as the evidence ofthe remains of extinct animals shows, these that existed at former periodsof time also, are divided into the groups or types we call species, naturally classified into larger groups which are subdivisions of othersstill larger, and so on, in what we call the natural system ofclassification? The two problems which naturalists have to solve, andwhich for many recent generations they have been trying to solve, are theOrigin of Species and the Origin of Adaptations. 

Former generations of zoologists have assumed that these problems were thesame. Lamarck maintained that the peculiarities of different animals weredue to the fact that they had become adapted to modes of life different tothose of their ancestors, and to those in which allied forms lived, thechange of structure being due to the effect of the conditions of life andof the actions of the organs. He did not specially consider thedifferences of closely allied species, but the peculiarities of markedtypes such as the long neck of the giraffe, the antlers of stags, thetrunk of the elephant, and so on; but he considered that the action ofexternal conditions was the true cause of evolution, and assumed that incourse of time the effects became hereditary. 

Lamarck's views are expounded chiefly in his _Philosophie Zoologique_, first published in 1809, and an excellent edition of this work withbiographical and critical introduction was published by Charles Martins in1873. Although his conception of the mode in which structural changes wereproduced is of little importance to those now engaged in the investigationof the process of evolution, since it was naturally based on thephysiological ideas of his time, many of which are now obsolete, for thesake of accuracy it is worth while to cite his principal propositions inhis own words:--

'Il sera en effet évident que l'état où nous voyons tous les animaux, estd'une part, le produit de la composition croissante de l'organisation, quitend à former une gradation régulière, et de l'autre part qu'il est celuides influences d'une multitude de circonstances très différentes quitendent continuellement à détruire la régularité dans la gradation de lacomposition croissante de l'organisation. 

'Ici il devient nécessaire de m'expliquer sur le sens que j'attache à cesexpressions: Les circonstances influent sur la forme et l'organisation desanimaux, c'est-à-dire qu'en devenant très différentes elles changent avecle temps et cette forme et l'organisation elle-même par des modificationsproportionnées. 

'Assurément si l'on prenait ces expressions à la lettre, on m'attribueraitune erreur; car quelles que puissent être les circonstances ellesn'opèrent directement sur la forme et sur l'organisation des animauxaucune modification quelconque. Mais de grands changements dans lescirconstances amènent pour les animaux de grands changements dans leursbesoins et de pareils changements dans les besoins en amènentnécessairement dans les actions. Or, si les nouveaux besoins deviennentconstants ou très durables, les animaux prennent alors de nouvelleshabitudes qui sont aussi durables que les besoins qui les ont fait naître. Il en sera résulté l'emploi de telle partie par préférence à celui detelle autre, et dans certains cas le défaut total d'emploi de telle partiequi est devenue inutile. '

The supposed effect of these changes of habit is definitely stated in theform of two 'laws':--

PREMIÈRE LOI

'Dans tout animal qui n'a point dépassé le terme de ses développementsl'emploi plus fréquent et soutenu d'un organe quelconque, fortifie peu àpeu cet organe, le développe, l'agrandit et lui donne une puissanceproportionée à la durée de cet emploi; tandis que le défaut constantd'usage de tel organe Paffaiblit insensiblement, le détériore, diminueprogressivement ses facultés, et finit par le faire disparaître. 

DEUXIÈME LOI

'Tout ce que la nature a fait acquérir ou perdre aux individus parl'influence des circonstances ou leur race se trouve depuis longtempsexposée, et par conséquent, par l'influence de l'emploi prédominant de telorgane, ou par celle d'un défaut constant d'usage de telle partie, elle leconserve par la génération aux nouveaux individus qui en proviennent, pourvu que les changements acquis soient communs aux deux sexes, ou à ceuxqui ont produits ces nouveaux individus. '

It will be seen that this last condition excludes the question of theorigin of organs or characters confined to one sex, or secondary sexualcharacters. With regard to the expression 'emploi de telle partie, ' theexplanation which Lamarck gives of the evolution of horns and antlers iscurious. He does not attempt to show how the use or employment of the headleads to the development of these outgrowths of bone and epidermic horn, but attributes their development in stags and bulls to an 'interiorsentiment in their fits of anger, which directs the fluids more stronglytowards that part of their head. '

Lamarck's actual words (_Phil. Zool. , _ edit. 1873, p. 254) are: 'Dansleurs accès de colière qui sont fréquents surtout entre les mâles, leursentiment intérieurs par ses efforts dirige plus fortement les fluidesvers cette partie de leur tete, et il s'y fait une secrétion de matièrecornée dans les uns (_Bovidae_) et de matière osseuse mélangée de matièrecornée dans les autres (_Cervidae_), qui donne lieu à des protubérancessolides: de là l'origine des cornes, et des bois, dont la plupart de cesanimaux ont la tête armée. '

Darwin, on the other hand, definitely set before himself the problem ofthe origin of species, which the majority of naturalists, in spite ofLamarck and his predecessor Buffon, regarded as permanent and essentiallyimmutable types established by the Creator at the beginning of the world. This principle of the persistence and fundamentally unchangeable nature ofspecies was regarded as an article of religion, following necessarily fromthe divine inspiration of the Bible. This theological aspect of thesubject is sufficiently curious when we consider it in relation to thehistory of biological knowledge, for Linnaeus at the beginning of theeighteenth century was the first naturalist who made a systematic attemptto define and classify the species of the whole organic world, and thereare few species of which the limits and definition have not been alteredsince his time. In fact, at the present time there are very numerousgroups, both in animals and plants, on the species of which scarcelyany two experts are agreed. 

In many cases a Linnaean species has been split up till it became, first, a genus, then a family, and, in some cases, an order. What one naturalistconsiders a species is considered by another a genus containing severalspecies, and, vice versa, the species of one authority is described asmerely a variety by another. The older naturalists might have said withtruth: we do not know what the species are, but we are quite certain thatwhatever they are they have never undergone any change in theirdistinguishing characters. At the same time we know that whether we callrelated forms varieties or species or genera in different cases, we find, whatever organisms we study, whether plants or animals, definite typesdistinguished by special characters of form, colour, and structure, andthat individuals of one species or type never give rise by generation toindividuals of any other known species or type. We do not find wolvesproducing foxes, or bulldogs giving birth to greyhounds. As a generalrule the distinguishing characters are inherited, and it is by no meanseasy even in domesticated animals and plants to obtain an exact andcomplete record of the descent of a new variety from the original form. Among species in a state of nature it is the exception to find tworecognised species which can be crossed or hybridised. In the case of thehorse and the ass, although mules are the hybrid offspring of the two, themules themselves are sterile, and there are many similar cases, so thatsome naturalists have maintained that mutual infertility should berecognised as the test of separation in species. 

Darwin founded his theory on the assumption that differences of specieswere differences of adaptation. His theory of natural selection is atheory of the origin of adaptations, and only a theory of the origin ofspecies on the assumption that their distinguishing characters areadaptations to different modes and conditions of life, to differentrequirements. He pointed out that there is always a considerable range ofvariation in the specific characters, that, as a rule, no two individualsare exactly alike, even when produced by the same two parents. The centralprinciple of his theory was the survival of individuals possessing thosevariations which were most useful in the competition of species withspecies and of individual with individual. He thus explained adaptation tonew conditions and divergence of several species from a common ancestor. Characters which were not obviously adaptive were explained either bycorrelation or by the supposition that they had a utility of which wewere ignorant. Darwin also admitted the direct action of conditions as asubordinate factor. 

Weismannism not only retained the principle of utility and selection, butmade it the only principle, rejecting entirely the action of externalconditions as a cause of congenital modifications, _i. E. _ of characterswhose development is predetermined in the fertilised ovum. It is toWeismann that we owe precise and definite conceptions, if not of thenature of heredity, at least of the details of the process. From him welearned to think of the ova or sperms, of the reproductive cells or'gametes' of an individual, as cells which were from an early stage ofdevelopment distinguished from the cells forming the organs and tissues;to regard the organism as consisting of soma on the one hand and gameteson the other, both derived from the original zygote cell, not the gametesfrom the soma. Weismann saw no possibility of changes induced by any sortof stimulation in the soma affecting the gametes in such a way as to beredeveloped in the soma of the next generation. He attributed variationpartly to the union of gametes containing various determinants, which hetermed amphimixis: this, however, would introduce nothing new. Then heproposed his theory of germinal selection, determinants growing andmultiplying in competition, some perhaps disappearing altogether, thoughthis does not satisfactorily account for entirely new characters. WithWeismann, however, every species was a different adaptation, and naturalselection was the _deus ex machina_; to quote his own words, _Alles istangepasst_. 

Romanes and other writers, on the other hand, had always maintained thatin many cases the constant peculiarities of closely allied species had noknown utility whatever, so that the problem presented by these characterswas not explained by any theory of the origin of adaptations. 

Mendelism, since 1900, has studied experimentally the transmission ofdefinite characters, and maintains that the characters of species are ofthe same nature as the characters which segregate in Mendelianexperiments. Such characters are not in any way related to externalconditions, and cannot, therefore, be adaptive except by accident. Professor Bateson goes so far as to admit that such large variations ormutations offer more definite material to selection than minute variationstoo small to make any important difference in survival, but as regardsspecies the important factor is the occurrence of mutations which areinherited and at once form a distinct definite difference between alliedspecies which is not due to selection and has nothing to do withadaptation. 

In a book entitled _Problems of Genetics_, 1913, Bateson describes severalparticular cases which show how impossible it is to find any relation atall between the diagnostic characters of certain species or local formsand their mode of life. One of these cases is that of the species of_Colaptes_, a genus of Woodpeckers in North America, of which a detailedstudy was published in the _Bull. Am. Mus. Nat. Hist. _, 1892. The twoforms specially considered are named _C. Auratus_ and _C. Cafer_, and theydiffer in the following seven characters:--

 _C. Auratus. _ _C. Cafer. _

1. Quills yellow. 1. Quills red. 

2. Male with black cheek stripe. 2. Male with red cheek stripe. 

3. Adult female with no 3. Adult female with usually cheek stripe. Brown cheek stripe. 

4. A scarlet nuchal crescent 4. No nuchal crescent in in both sexes. Either sex. 

5. Throat and fore-neck brown. 5. Throat and fore-neck grey. 

6. Top of head and hind-neck grey. 6. Top of head and hind-neck brown. 

7. General tone of plumage 7. General tone of plumage olivaceous. Rufescent. 

_C. Auratus_ occurs all over Canada, and the United States, from the northto Galveston; westwards it extends to Alaska and the Pacific coast to thenorthern border of British Columbia. _C. Cafer_ in comparatively pure formoccupies Mexico, Arizona, California, part of Nevada, Utah, Oregon, and isbounded on the east by a line drawn from the Pacific south of WashingtonState, south and eastward through Colorado to the mouth of the Rio Grandeon the Gulf of Mexico. Between the two areas thus roughly defined is atract of country about 300 to 400 miles wide, which contains some normalbirds of each type, but chiefly birds exhibiting irregular mixtures of thecharacters of both. Bateson remarks that some naturalists may be disposedonce more to appeal to our ignorance, and suggest that if we only knewmore we should find that the yellow quills, the black 'moustache, ' and thered nuchal crescent specially adapt _auratus_ to the conditions of thenorthern and eastern region, while the red quills, red moustache, andabsence of crescent fit _cafer_ to the conditions of the more southern andwestern territory. But, as the author we are quoting points out, when wethink of the wide range of conditions in the country occupied by_auratus_, extending from Florida to the Arctic, it is impossible tobelieve that there is any common element in the conditions which demands ascarlet nuchal patch in _auratus_, while the equally varied conditions inthe _cafer_ area do not require that character. It may be added that thesame objection is equally valid whether we apply it to the utility of sucha character or to the supposition that the character has been caused byexternal conditions; in other words, whether we attempt to explain thefacts by selection or by the Lamarckian principle. 

Another case quoted by Bateson is that of the two common British Wasps, _Vespa vulgaris_ and _Vespa germanica_. Both usually make subterraneannests, but of somewhat different materials. That of _V. Vulgaris_ is of acharacteristic yellow colour, because made of rotten wood, while that of_V. Germanica_ is grey, from the weathered surface wood of palings orother exposed timber which is used in its construction. In characters thedifferences of the two forms are so slight as to be distinguishable onlyby the expert. _V. Vulgaris_ often has black spots on the tibiae, whichare wanting in _germanica_. A horizontal yellow stripe on the thorax isenlarged downwards in the middle in _germanica_, not in _vulgaris_. Thereare distinct though slight differences in the genital appendages of themales in the two species. Here there are differences of habit, and slightbut constant differences of structure; but it is impossible to find anyrelation between the former and the latter. 

Mendelism in itself affords no evidence of the origin of new characters, since it deals only with the heredity of the characters which it findsusually in the varieties of cultivated animals and plants. But indirectlyit draws the inference that new characters arose in the form in which theyare found to be inherited, as complete units, and not by gradual, continuous increase, that specific characters are due to mutations, andthat all evolution has been the result of similar hereditary factors, arising by some internal process in the divisions of reproductive cells, and not determined by external conditions. Some Mendelians maintain thatif the mutations are not compatible with the existing conditions of life, the organism must either die or find new conditions in which it can live. 

Bateson remarks (_Mendel's Principles of Heredity_, 1909, p. 288):'Mendelism provides no fresh clue to the problem of adaptation except inso far as it is easier to believe that a definite integral change inattributes can make a perceptible difference to the prospect of success, than that an indefinite and impalpable change should entail suchconsequences. ' Here the distinction between adaptive and non-adaptivecharacters is recognised, but both are emphatically attributed to the sameorigin. 

The American evolutionist, T. H. Morgan, also a specialist in Mendelism, goes further, and maintains, not merely that mutations which happened tomake a 'difference to the prospect of success' survived, or were selected, but that if a mutation arising from a change in the gametes was notcompatible with the conditions of the animal's life at the time, it eitherdied, or found other conditions, or adopted new habits which were adaptedto the new character or structure. He takes Flat-fishes as an example, andsuggests that having by mutation become asymmetrical, and having both eyeson one side, etc. , the fish adopted the habit of lying on the ground onone side of its body. This is, of course, the exact opposite of the olderconception: the structure of the animal has not been changed by new habitsor conditions, but new habits and conditions have been sought and found inorder to meet the requirements of the change of structure. 

The present writer, on the other hand, believes that not only are adaptivecharacters distinct from non-adaptive specific characters, and fromnon-adaptive diagnostic characters in general, but that their origin andevolution are entirely distinct and different. There are two separateproblems, the origin of adaptations and the origin of species, and theinvestigation of these two problems leads not to one explanation common toboth, but to two entirely different explanations, to two differentprocesses going on throughout the organic world and affecting everyindividual and every group in classification. 

The Flat-fishes, now regarded not as merely a family but a sub-order ofTeleosteans, afford a good example of the contrast between adaptive andnon-adaptive diagnostic characters. For the whole group the adaptivecharacters are diagnostic, distinguishing it from other sub-orders. It isconceivable that different phyletic groups of fishes, that is fishes ofdifferent descent, might have been modified in the same way, as, forinstance, grasshoppers and fleas have been adapted for leaping withoutbeing closely related to each other. It is generally held, however, thatthe Flat-fishes are of common descent. In this group the adaptivecharacters are diagnostic; that is to say, they distinguish the group fromother sub-orders, though there are other non-adaptive characters whichindicate the relationship to other groups and which are not adapted to thehorizontal position of the original median plane of symmetry. Theprincipal adaptive characters are: both eyes and the pigmentation on theside which is uppermost in the natural position, lower side without eyesand colourless; dorsal and ventral fins continuous and extending nearlythe whole length of the dorsal and ventral edges; dorsal fin extendingforwards on the head, not along the morphological median line, which isbetween the eyes, but between the more dorsal eye and the lower side ofthe body, in the same horizontal plane as the posterior part of the samefin. The 'adaptive' quality in these characters, as in other cases, doesnot necessarily consist in their utility to the animal, but in thedefinite relation between them and the external conditions. When therelation is one of function, the organ may be said to be useful: forexample, the position of the two eyes is adaptive because they are on theupper side where alone light can reach them, the other side resting on theground; and the adaptation is one of function, and therefore useful, because if the eyes were in their normal position, one of them would beuseless, being generally in contact with the ground or buried in it. Similarly with the extension of the dorsal and ventral fins, theundulations of which serve to move the fish gently along in a planeparallel to the ground. If the dorsal fin was not extended forward, the head would not be so well supported. But when we consider thepigmentation of the upper side and the normally white lower side, althoughthe adaptation is equally obvious, the utility is by no means certain. Toany naturalist who has observed these fishes in the living state theprotective resemblance of the pigmentation of the upper side is veryevident, especially because, as in many other fishes and amphibians, theintensity of the colour varies in harmony with the colour of the ground onwhich the fish rests. But the utility of the white lower side is not soeasy to prove. Would the fish be any worse off if the lower side werecoloured like the upper? Probably it would not, although it has beenmaintained that the white lower side serves to render the fish lessvisible when seen against the sky by an enemy below it. Ambicoloratespecimens occur, and there is no evidence that their lives are less securethan those of normal specimens. The essential and universal quality ofadaptation, then, is not utility, but relation to surroundings or tofunction or to habit. In this case colour is related to incidence oflight, absence of colour to absence of light. Position of eyes is alsorelated to light; they are situated where they can see, absent from theside which is shut off from light. The marginal fins are extended wheretheir movements best support and move the body. 

It is to be noted also that these adaptations of different organs of thebody, eyes, fins, colour, are entirely independent of each otherphysiologically. It may appear on first consideration that eyes andcolour, being both on the upper side, may have been somehow connected inthe constitution of the body, whereas the only connexion is external intheir common relation to light. This independence is well shown in themodification of the dorsal fin: if this were physiologically affected bythe change in the eyes, which is brought about by the twisting of theinterorbital region of the skull, the anterior end of the fin would bebetween the two eyes, since the morphological median line of the body isin that position. In fact, on the contrary, the attachment of the dorsalfin is continued forward where it is required for its mechanical function, regardless entirely of the morphology of the head. 

This is even more clearly evident in the structure of the jaws and teeth. These are entirely unaffected by the torsion of the interorbital part ofthe skull. In cases where the mouth is large and teeth are required onboth sides, the prey being active fish of other species, as in Turbot, Brill, and Halibut, the jaws and teeth are equally developed on the upperand lower sides, and there is almost complete symmetry in these parts ofthe skull. In Soles and Plaice, on the other hand, whose food consists ofworms, molluscs, etc. , living on or in the ground, the jaws of the lowerside are well developed and strong, those of the upper side diminished, and teeth are confined to the lower side. Here it is not a question of thejaws twisted, but simply unequally developed. There is no general andconstitutional asymmetry of head or body, but a modification of differentorgans independently of each other in relation to external conditions--light, food, movement. 

On the other hand, let us consider some of the diagnostic characters bywhich species and genera are distinguished in the Flat-fishes orPleuronectidae. The genus _Pleuronectes_ is distinguished by the followingcharacters: eyes on the right side, mouth terminal and rather small, teethmost developed on the blind (left) side. Of this genus there are fiveBritish species, namely:--

_P. Platessa_, the Plaice: scales small, mostly without spinules, reducedand not imbricated, imbedded in the skin; bony knobs on the head behindthe eyes, red spots on the upper side. 

_P. Flesus_, the Flounder: no ordinary scales; rough tuberoles along thebases of the marginal fins and along the lateral line; these are modifiedand enlarged scales; elsewhere scales of any kind are absent. 

In these two species the lateral line is nearly straight, having only aslignt curve above the pectoral fin. 

_P. Limanda_, the Dab: scales uniform all over the body, with spinules onthe projecting edges, making the skin rough; lateral line with asemicircular curve above the pectoral fin. 

_P. Microcephalus, _ the Lemon-dab: scales small, smooth, and imbedded;skin slimy, head and mouth very small, colour yellowish brown with largeround darker marks. 

_P. Cynoglossus, _ the Witch or Pole-dab: head and mouth smaller than inthe Plaice, eyes rather larger; scales all alike and uniformlydistributed, slightly spinulate on upper side, smooth on the lower;blister-like cavities beneath the skin of the head on the lower side. 

With regard to the generic characters, it is difficult to give any reasonwhy the mouth should be at the end of the head instead of behind the apexof the snout as in the genus _Solea, _ but, as we have seen already, thesmall size of the mouth and the greater development of teeth on the lowerside are adapted to the food and mode of feeding. It is impossible to saywhy one genus of Flat-fishes should have the right side uppermost andothers, _e. G. _ Sole and Turbot, the left; it would almost seem to havebeen a matter of chance at the commencement of the evolution: reversedspecimens occur as variations in most of the species. 

When we consider the specific differences, we find very definitecharacters in the structure and distribution of the scales, and noevidence has yet been discovered that these differences are related toexternal conditions. There are, of course, slight differences in habitsand habitat, but no constant relation between these and the structuraldifferences of the scales. Plaice and Dab are taken together on the sameground, and nothing has been discovered to indicate that the spinulatescales of the Dab are adapted to one peculiarity in habits or conditions, the spineless scales of the Plaice to another. In comparing certaingeographical races of Plaice and Flounder the facts seem to suggest thatdifferences of habitat may have something to do with the development ofthe scales. In the Baltic the Flounders are as large as those on our owncoasts, but the thorny tubercles are much more developed, nearly the wholeof the upper surface being covered with them. The Plaice, on the otherhand, are smaller than those of the North Sea, and the _males_ have thescales spinulate over a considerable portion of the upper side. The chiefdifference between the Baltic and the North Sea is the reduced salinity ofthe former, so that it might be supposed that fresher water caused thegreater development of the dermal skeleton. On the other hand, a speciesor geographical variety of the Plaice, whose proper is _P. Glacialis_, isfound on the Arctic coasts of Asia and America, on both sides of theextreme North Pacific, and on the east coast of North America. In thisform the bony tubercles on the head in the Plaice are replaced by acontinuous rough osseous ridge, and the scales are as much spinulated asin the Plaice of the Baltic. On the east coast of North America the malesin this form are more spinulated than the females; on the Alaskan coast, and apparently the Arctic coast, the females are spinulated, and thesexual difference in this respect is slight or absent. Lower salinitycannot be the cause of greater spinulation in this case, and thus it mightbe suggested that the condition was due to lower temperature. But we donot find that northern or Arctic species of fish in general have thescales more developed than southern species. 

The Dab, which occurs in the same waters as the Plaice, has the spinesmore spinulated than any of the forms of plaice above mentioned, thereforethe absence or slight development of spinules in the typical Plaice is notexplained by physical conditions alone. Freshness of water again will notexplain the difference of the structure and distribution of scales inFlounder and Plaice, considering the variety of squamation in fishesconfined to fresh water. Still less can we attribute any of thepeculiarities of scales to utility. We can discover no possible benefit ofthe condition in one species which would be absent in the case of otherspecies. We can go much further than this, and maintain that there is noreason to believe that scales in general in Teleosteans, or any of theirvarious modifications, are of special utility: they are not adaptivestructures at all, although of great importance as diagnostic characters. It may be urged that in some cases, such as the little _Agonuscataphractus_ or the Seahorse among the Syngnathidae, the body isprotected by a complete suit of bony armour; but accompanying these in thelittoral region are numerous other species such as the Gobies, and evenother species of Syngnathidae which have soft unprotected skins. 

Similarly with colour characters: the power of changing the colour so asto harmonize with the ground is obviously beneficial and adaptive, but ineach species there is a specific pattern or marking which remains constantthroughout life and has nothing to do with protective resemblance, variable or permanent. The red spots of the Plaice are specific anddiagnostic, but they confer no advantage over the Dab or the Lemon-dab, inwhich they are absent, nor can any relation be discovered between thesespots and mode of life or habits. 

The function of the lateral line organs is still somewhat obscure. Thetheory that they are sensitive to differences of hydrostatic pressure asthe fish moves from one depth to another rests on no foundation, since ithas yet to be shown how a change of pressure within the limits of theincompressibility of water can produce a sensation in an organ permeatedthroughout with water. It is more probable that the organs are affected byvibrations in the water, but we are unable to understand how a differencein the anterior curvature of the lateral line would make a difference inthe function in any way related to the difference in conditions of lifebetween Plaice and Dab. There is, however, reason to conclude that theorgans, especially on the head, are more important and larger in deeperwater, and thus the enlargement of the sensory canals in the head of theWitch, which lives in deeper water than other species, may be anadaptive character. 

Another genus of whose characters I once made a special study is thatnamed _Zeugopterus. _ The name was originally given by Gottsche to thelargest species _Z. Punctatus, _ from the fact that the pelvic fins areunited to the ventral, but this character does not occur in other speciesnow included in the genus. There are three species, occurring only inEuropean waters, which form this genus and agree in the followingcharacters. The outline of the body is more nearly rectangular than inother Flat-fishes from the obtuseness of the snout and caudal end, and thesomewhat uniform breadth of the body. The surface is rough from thepresence of long slender spines on the scales. There is a largeperforation in the septum between the gill cavities, but this occurs alsoin _Arnoglossus megastoma, _ which is placed in another genus. But thegeneric character of _Zeugopterus, _ which is most important for thepresent discussion, is the prolongation of the dorsal and ventral fins onto the lower of the body at the base of the tail, the attachments of theseaccessory portions being transverse to the axis of the body. These fisheshave the peculiar habit of adhering to the vertical surfaces of sides ofaquaria, even the smooth surfaces of slate or glass. In nature they aretaken occasionally on gravelly or sandy ground, but probably live alsoamong rocks and adhere to them in the same way as to vertical surfaces incaptivity. Many years ago (_Journ. Mar. Biol. Assn. _, vol. Iii 1893-95) Imade a careful investigation of the means by which these fishes were ableto adhere to a smooth surface, at least in the case of the largest andcommonest species _Z. Punctatus. _ It was observed that so long as the fishwas clinging to a vertical surface the posterior parts of the fins were inrhythmical motion, undulations passing along them in succession frombefore backwards, the edge of the body to which they were attached movingwith them. The effect of these movements was to pump out water backwardsfrom the space between the body and the surface it was clinging to, and tocause water to flow into this space at the anterior edges of the head. Thesubcaudal flaps were perfectly motionless and tightly pressed between thebase of the tail and the surface of support, so that any movement of themwas impossible. The question arose, however, whether the tail and theseflaps acted as a sucker which aided in the adhesion. The flaps weretherefore cut off with scissors--an operation which caused practically nopain or injury to the fish--and it adhered afterwards quite as well aswhen the fin-flaps were intact. The subcaudal prolongations of the finsare therefore not necessary to the adhesion, nor to the pumping action, ofthe muscles and fins, which went on as before. It seemed probable, therefore, that the pumping action was itself the cause of the adhesion. But the difficulty in accepting this conclusion was that there was adistinct though gentle respiratory movement of the jaws and opercula; andif the pumping of the water from beneath the body caused a negativepressure there, and a positive pressure on the outer side of the body, itseemed equally certain that the respiratory movement must force water intothe space beneath the body and so cause a positive pressure there whichwould tend to force the fish away from the surface with which it was incontact. Examination of the currents of water around the edges of thefish, by means of suspended carmine, showed that water passed in at themouth and out at the lower respiratory orifice, but also into the spacebelow the body at the upper and lower edges of the head, without passingthrough the respiratory channel. It was thus proved that the rate at whichwater was pumped out at the sides of the tail was greater than that atwhich it passed in by the respiratory movements, and consequently there aresultant negative pressure beneath the body. By means of a model made ofa thin flexible sheet of rubber, at each end of which on one side wasfastened a short piece of glass tube, I was able to imitate the physicalaction observed in the fish. A long piece of rubber tube was attached toone of the pieces of glass tube, and brought over the edge of the glassfront of an aquarium. The long rubber tube was set in action as a siphonand the sheet of rubber placed against the glass. As long as water wasrunning through the siphon the sheet of rubber remained pressed againstthe glass and supported. As soon as the current of water was stopped theapparatus fell to the bottom of the tank. In this model water passed outfrom beneath the rubber through the glass tube attached to the siphon andpassed in by the opposite glass tube, and at the sides of it. The lattertube represented the respiratory channel of the fish, and the spacebetween tube and rubber represented the spaces between the head of thefish and the vertical surface to which it clung. 

In the fish the marginal fins not only extend to the base of the tail, butare broader at the posterior end than elsewhere, whereas in otherFlat-fishes the posterior part of the marginal fins are the narrowestparts. The shape of the fins and the breadth of the body posteriorly, then, are adaptations which have a definite function, that of enabling thefish to adhere to vertical surfaces. But, on the other hand, the extensionof the marginal fins in a transverse direction beneath the tail has no usein the process of adhesion, nor has any other use been found for it. It isa generic character, so far as we know, without utility. On the otherhand, it is very probable that this subcaudal extension of the fins ismerely a result of the posterior extension and enlargement of these finswhich has taken place in the evolution of the adaptation. If theLamarckian explanation of adaptation were true, it would be possible tounderstand that the constant movements of the fins and muscles by whichthe adhesion was effected caused a longitudinal growth of the fins inexcess of the length actually required, and that this extra growthextended on to the body beneath the tail, although the small flaps on thelower side were not necessary to the new function which the finsperformed. 

When we consider such cases as this we are led to the conclusion that theusual conception of adaptation is not adequate. We require something morethan function or utility to express the difference between the two kindsof characters to be distinguished. For example, the absence ofpigmentation from the lower sides of Flat-fishes may have no utilitywhatever, but we see that it differs from the specific markings of theupper side in the fact that it shows a relation to or correspondence witha difference of external conditions--namely, the incidence of light, whilein such a case as the red spots of the Plaice we can discover no suchcorrespondence. 

We know that the American artist and naturalist Thayer has shown that thelighter colour of the ventral side of birds and other animals aids greatlyin reducing their visibility in their natural surroundings, the diminutionin coloration compensating for the diminution in the amount of lightfalling on the lower side, so that the upper and lower sides reflectapproximately the same amount of light, and contrast, which would beotherwise conspicuous, is avoided. But the white lower sides ofFlat-fishes are either not visible at all, or, if visible, are veryconspicuous, so that the utility of the character is very doubtful. 

We may distinguish then between characters which correspond to externalconditions, functions, or habits, and those which do not. The word'adaptation, ' which we have hitherto used, does not express satisfactorilythe peculiarities of all the characters in the former of these twodivisions. If we consider three examples--enlarged hind-legs for jumpingas in kangaroo or frog, absence of colour from the lower sides ofFlat-fishes, and, thirdly, the finlets on the lower side of_Zeugopterus_--we see that they represent three different kinds ofcharacters, all related to habits or external conditions. We may say thatthe third kind are correlated with some other character that has arelation to function or external conditions, as the extension of the finson the under side of _Zeugopterus_ is correlated with the enlargement ofthe fins, whose function is to cause the adhesion of the fish to avertical surface. 

With regard to the specific characters of the species of _Zeugopterus_nothing is known of peculiarities in mode of life which would give animportance in the struggle for existence to the concrescence of the pelvicfins with the ventral in _punctatus_, to the absence of this character andthe elongation of the first dorsal ray in _unimaculatus_, or to theabsence of both characters in _norvegicus_. No use is known for any of theother specific characters, which tend in each case to form a series. Thusin size _norvegicus_ is the smallest, _unimaculatus_ larger, and_punctatus_ largest, the last reaching a of 8-1/2 inches. The subcaudalfin-flaps are developed in _norvegicus_, most in _punctatus_; each hasfour rays in _norvegicus_ and _unimaculatus_, six in _punctatus_. Theshortening and spinulation of the scales are greatest in _punctatus_, least in _norvegicus_. In _punctatus_ there are teeth on the vomer, in _unimaculatus_ none, in _norvegicus_ they are very small. 

If we consider fishes in general, we see that there is no evidence of anyrelation between many of the most important taxonomic characters andfunction or external conditions. In the seas Elasmobranchs and Teleosteansexist in swarming numbers side by side, but it is impossible to say thatone type is more adapted to marine life than the other. There is goodreason to believe that bony fishes were evolved from Elasmobranchs infresh water which was shallow and foul, so that lungs were evolved forbreathing air, and that marine bony fishes are descended from fishes withlungs; but no reason has been given for the evolution of bone in place ofcartilage or for the various kinds of scales. Professor Houssaye, on theother hand, believes that the number and position of fins is adapted tothe shape and velocity of movement of each kind of fish. 

If we turn to other groups of animals we find everywhere similar evidenceof the distinction between adaptive and non-adaptive characters. Birds areadapted in their whole organization for flight, the structure of the wing, of the sternum, breast muscles, legs, etc. , are all co-ordinated for thisend. But how do we know that feathers in their origin were connected withflight? It seems equally probable that feathers arose as a mutation inplace of scales in a reptile, and the feathers were then adapted forflight. Nothing shows the distinction better than convergent adaptation. Owls resemble birds of prey in bill and claw and mode of life, yet theyare related to insect-eating swifts and goat-suckers and not to eagles andhawks. Swifts and swallows are similar in adaptive characters, but not inthose which show relationship. It may be said that the characters believedto show true affinities were originally adaptive, but we do not know this. Similarly, in reptiles the Chelonia are distinguished by the mostextraordinary union of skin-bones and internal skeleton enclosing the bodyin rigid armour: it may be said that the function of this is protection, that it is adaptation, and can be explained by natural selection, but theadaptation in this case is so indefinite that it is difficult to beconvinced of it. 

Systematists have always distinguished between adaptive characters andthose of taxonomic value--those which show the true affinities--and theyare perfectly right: also they have always distrusted and held aloof fromtheories of evolution which profess to explain all characters by oneuniversal formula. In my opinion, those who, like Weismann, consider alltaxonomic characters adaptive, are equally mistaken with Bateson and hisfollowers, who regard all characters as mutational. No system of evolutioncan be satisfactory unless it recognises that these two kinds ofcharacters are distinct and quite different in their nature. But it may beasked, What objection is there to the theory of natural selection as anexplanation of adaptations? The objection is that all the evidence goes toshow that the necessary variations only arose under the given conditions, and, further, that the actions of the conditions and the correspondingactions of the organism give rise to stimuli which would produce somaticmodifications in the same direction as the permanent modifications whichhave occurred. My view is, then, that specific characters are usually notadaptations, that other characters of taxonomic value are some adaptiveand some unrelated to conditions of life, and that while non-adaptivecharacters are due to spontaneous blastogenic variations or mutations, adaptive characters are due to the direct influence of stimuli, causingsomatic modifications which become hereditary, in other words, to theinheritance of acquired characters. It has become a familiar statementthat every individual is the result of its heredity and its environment. The thesis that I desire to establish is that the heredity of eachindividual and each type is compounded of variations or changes of twodistinct origins, one external and one internal; that is to say, ofvariations resulting from changes originating in the germ-cells orgametes, and of modifications produced originally in the soma by theaction of external stimuli, and subsequently affecting the gametes. 

When we study the characters of animals in relation to sex we find that inmany cases there are conspicuous organs or characters present in one sex, usually the male, which are absent or rudimentary in the other. Theconception of adaptation applies to these also, since we find thatcharacters consist often of weapons such as horns, antlers, and spurs, used in sexual combat, of copulatory or clasping organs such as the padson a frog's forefeet, of ornamental plumage like the peacock's tailserving to charm the female, or of special pouches as in species ofpipe-fish and frog for holding the eggs or young. Darwin attempted toexplain sexual adaptation by sexual selection. The selective process inthis case was supposed to be, not the survival of individuals best adaptedto secure food or shelter or to escape from enemies, but the success ofthose males which were victorious in combat, or which were most attractiveto the females, and therefore left the greater number of offspring whichinherited their variations. But, as Darwin himself admitted, this theoryof selection does not in any way explain the differences between thesexes--in other words, the limitation of the characters or organs to onesex--since there is no reason in the process of selection itself why thepeculiarity of a successful male should not be inherited by his femaleoffspring as well as by his male offspring. The real problem, then, is thesex-limited heredity, and we shall consider later whether in this kind ofheredity also there are characters of internal as well as external origin, blastogenic as well as somatogenic. 

CHAPTER II

 Mendelism And The Heredity Of Sex

We know that now individuals are developed from single cells which haveeither been formed by the union of two cells or which develop without suchunion, and that these reproductive cells are separated from pre-existingorganisms: the gametes or gonocytes are separated from the parents anddevelop into the offspring. The zygote has the power of developingparticular structures and characters in the complicated organisation ofthe adult, and we recognise that the characters are determined by theproperties and constitution of the zygote; that is to say, of one or bothof the gametes which unite to form the zygote. The distinction betweenpeculiarities or 'characters, ' determined in the ovum before development, and modifications due to influences acting on the individual during itsdevelopment or life, is often obvious enough. A child's health, size, modeof speech, and behaviour may be greatly influenced by feeding, training, and education, but the colour of his or her eyes and hair were determinedbefore birth. A human individual has, we know, a number of congenital orinnate characters, by which we mean characters which arise from theconstitution of the individual at the time of birth, and not frominfluences acting on him or her after birth. We have to remember, however, that modifications may be caused during development in the uterus, as, forexample, birth-marks on the skin, and these would not be due topeculiarities in the constitution of the ovum. Karl Pearson and otherdevotees of the cult of Eugenics have been lately impressing on the publicby pamphlets, lectures, and addresses the great importance of nature ascompared with nurture, maintaining that the latter is powerless tocounteract either the good or bad qualities of the former, and that theeffects of nurture are not transmitted to the next generation. 

We recognise that the characters of varieties of flowers, fruits, anddomesticated animals are not to be produced by any particular mode oftreatment. We see the various kinds of orchids or carnations in the samegreenhouse, of sweet peas and roses in the same garden. We go to a showand see the extraordinary variety of breeds of pigeons, rabbits, or fowls, and we know that these cannot be produced by treating the progeny ofindividuals of one kind in special ways, but are the progeny of parents ofthe same various races. If we want fowls of a particular breed we obtaineggs of that breed and hatch them with the certainty born of experiencethat we shall obtain chickens of that breed which will develop the colour, comb, size, and qualities proper to it. Similarly, in nature we recognisethat the 'characters' of species or varieties are not due to circumstancesacting on the individual during its development, but to the properties ofthe ova or seeds from which the individuals were developed. 

Formerly we regarded these congenital or innate characters as derived fromthe parents or inherited, and heredity was the transmission ofconstitutional characters from parent to offspring. Now that we fix ourattention on the fertilised ovum or the gametes by which it is formed wesee that the characters are determined by some properties in theconstitution of the gametes. What, then, is heredity? Clearly, it ismerely the development in the offspring of the same characters which werepresent in the ova from which the parents developed. When the characterspersist unchanged from generation to generation, we call the process bywhich they are continued heredity. When new characters appear, _i. E. _ newcharacters determined in the ovum not due to changes in the environment, we call them variations. When a fertilised ovum develops into a newindividual, it divides repeatedly to form a very large number of cellsunited into a single mass. Gradually the parts of this mass aredifferentiated to form the tissues and organs of the body or soma, butsome of the cells remain in their original condition and become thereproductive cells which will give rise to the next generation. Thereproductive cells also undergo division and increase in number, and whenthey separate from the new individual and unite in fertilisation theystill possess all the determinants of the fertilised ovum from which theyare descended. Heredity thus continues from gamete to gamete, not fromzygote to soma, and then from soma to gamete. 

Modern researches have shown that the nucleus, when the cell divides, assumes the form of a spindle of fibres, associated with which aredistinct bodies called chromosomes, that the number of these chromosomeswhere it can be counted is constant for all individuals of the samespecies, and that before the gametes are ready for fertilisation twocell-divisions take place, which result in the reduction of the number ofchromosomes to half the original number. When two gametes unite, thespecific number is restored. Since the male gamete is very small and seemsto contribute to the zygote almost nothing except the chromosomes, whichcarry with them all the characters of the male parent, it seems anecessary conclusion that the chromosomes alone determine the character ofthe adult. There are, however, facts which point to an oppositeconclusion. 

Hegner, [Footnote: R. W. Hegner, 'Experiments with Chrysomelid Beetles, 'III. , _Biological Bulletin_, vol. Xx. 1910-11. ] for example, found that inthe egg of the beetle _Leptinotarsa_, which is an elongated oval in shape, there is at the posterior end in the superficial cytoplasm a disc-shapedmass of darkly staining granules, while the fertilised nucleus is in themiddle of the egg. When the protoplasm containing these granules waskilled with a hot needle, development in some cases took place and anembryo was formed, but the embryo contained no germ cells. Here no injuryhad been done to the zygote nucleus, but these particular granules and theportion of protoplasm containing them were necessary for the formation ofgerm cells. In other experiments a large amount of protoplasm at theposterior end of the ovum was killed before the nucleus had begun tosegment, and the result was the development of an embryo consisting of thehead and part of the thorax, while the rest was wanting. The nucleussegmented and migrated into that part of the superficial cytoplasm whichremained alive, and this proceeded to develop that particular part of theembryo to which it would have given rise if the rest of the egg had notbeen killed. There was no regeneration of the part killed, no formation ofa complete embryo. It may be pointed out that segmentation in the insectegg is peculiar. The nuclei multiplied by segmentation migrate into thesuperficial cytoplasm surrounding the yolk, and then this cytoplasmsegments, and each part of the cytoplasm develops into a particular regionof the embryo. This, of course, does not prove that the nuclei or theirchromosomes do not determine the _characters_ of the parts of the embryodeveloped, but they show that the parts of the non-nucleated cytoplasmcorrespond to particular parts of the embryo. The most important object ofinvestigation at the present time is to find the origin of theseproperties of the chromosomes. We may say, using the word 'determinant' asa convenient term for that which determines the adult characters, that inorder to explain the origin of species or the origin of adaptations wemust discover the origin of determinants. Mendelism does not throw anydirect light on this question, but it certainly has shown how charactersmay be inherited as separate and independent units. When one differencebetween two breeds is considered, _e. G. _ rose comb and single in fowls, and individuals are crossed, we have the determinant for rose and thedeterminant for single in the same zygote. The result is that rosedevelops and single is not apparent. In the next generation rose andsingle appear, as at the beginning, in separate individuals. When two orthree or more differences are studied we find that they are usuallyinherited separately without connexion with each other, although in somecases they are connected or coupled. The facts of Mendelism are of greatinterest and importance, but we have to consider the general theory basedon them. This theory is that characters are generally separate units whichcan exist side by side, but do not mingle, and cannot be divided intoparts. When an apparently single character shows itself double or treble, it is concluded that it has not been really divided, but consists of twoor three units (Castle). Further, although Mendelism in itself shows noevidence of the origin of the characters, it assumes that they arose ascomplete units, and one suggestion is that a dominant factor might at someof the divisions in gametegenesis pass entirely into one daughter cell, and therefore be absent from the other, and thus individuals might bedeveloped in which a dominant character was absent. Bateson in hiswell-known books, _Mendel's _Principles of Heredity_, 1909, and _Problemsof Genetics_, 1913, discusses this question of the origin of the factorswhich are inherited independently. The difficulty that troubles him is theorigin of a dominant character. Naturally, if he persists in regarding thedeterminant factor as a unit which does not grow nor itself evolve in anyway, it is difficult to conceive where it came from. The dominant, according to Bateson, must be due to the presence of something which isabsent in the recessive. He gives as an instance the black pigment in theSilky fowl, which is present in the skin and connective tissues. In hisown experiments he found this was recessive to the white-skin character ofthe Brown Leghorn, and he assumes that the genetic properties of _Gallusbankiva_ with regard to skin pigment are similar to those of the BrownLeghorn. Therefore in order that this character could have arisen in theSilky, the pigment-producing factor _P_ must be added and the inhibitingfactor _D_ must drop out or be lost. He says we have no conception of theprocess by which these events took place. [Footnote: _Problems ofGenetics_, p. 85. ] Now my experiment in crossing Silky with _bankiva_shows that no inhibiting factor is present in the latter, so that only onechange, not two, was necessary to produce the Silky. Mendelians find itso difficult to conceive of the origin of a new dominant that they evensuggest that no such thing ever occurs: what appears as a new characterwas present from the beginning, but its development was prevented by aninhibiting factor: when this goes into one cell of a division and leavesthe other free, the suppressed character appears. This is the principleproposed to get over the difficulty of the origin of a new dominant. Allcharacters are due to factors, and all factors were present in theoriginal ancestor--say Amoeba. Evolution has been merely 'the rejection ofvarious factors from an original complex, and a reshuffling of those thatwere left. ' Professor Lotsy goes so far as to say that difference inspecies arose solely from crossing, that all domestic animals are of mixedstocks, and that it is easier to believe that a given race was derivedfrom some ancestor of which all trace has been lost than that all races offowls, for example, arose by variation from a single species, but theevidence that our varieties of pigeons have been derived from _C. Livia_, and of fowls from _G. Bankiva_, is too strong to be disregarded because itdoes not agree with theoretical conceptions. 

My own experiments in crossing Silky fowls with _Gallus bankiva_(_P. Z. S. _, 1919) show that the recessive is not always pure, thatsegregation is not in all cases complete. The colour of the _bankiva_ iswhat is called black-red, these being probably the actual pigmentspresent, mixed in some parts of the plumage, in separate areas in otherparts: the Silky is white. There are seven pairs of characters altogetherin which the Silky differs from the _bankiva_. Both the pigmented skin ofthe Silky and the colour in the plumage of the _bankiva_ are dominant, sothat all the offspring in _F1_ or the first generation are coloured fowlswith pigmented skins. But in later generations I found that with regard toskin pigment there were no pure recessives. Since the heterozygote in _F1_was deeply pigmented, it is certain that a bird with only a small amountof pigment in its skin was a recessive resulting from incompletesegregation of the pigmented character. The pigment occurred chiefly inthe skin of the abdomen and round the eyes, and also in the peritoneum andin the connective tissue of the abdominal wall. It varied in differentindividuals, but in some, at any rate, was greater in later generationsthan in the earlier. The condition bred true, as pure recessives do; andwhen such an impure recessive was mated with a heterozygote with blackskin, the offspring were half pigmented and half recessive, with somepigment on the abdomen of the latter. 

Still more striking was the incomplete segregation in the plumage colour. The white of the Silky was recessive, all the birds of the _F1_ generationbeing fully coloured. In the _F2_ generation there were two recessivewhite cocks which when mature showed slight yellow colour across theloins. These two were mated with coloured hens, and in later generationsall the recessives instead of being pure white, like the Silky, hadreddish-brown pigment distributed as in pile fowls. 

 [Illustration: PLATE I. Recessive Pile Fowls]

In the hens (Plate I. , fig. 1) it was chiefly confined to the breast andabdomen, and was well developed, not a mere tinge or trace, but a deepcoloration, extending on to the dorsal coverts at the lower edge of thefolded wings. The back and tail were white. In the cocks the colour wasmuch paler, and extended over the dorsal surface of the wings, where itwas darker than on the back and loins (Plate I. , fig. 2). Thesepile-coloured fowls when mated together bred true, with individualdifferences in the offspring. 

The pile fowl as recognised and described by fanciers is dominant incolour, not recessive as in the case above described. In fact, a recessivepile does not appear ever to have been mentioned before the publication ofthe results of my experiment. From the statements of John Douglas in_Wright's Book of Poultry_ (London, 1885), it appears that fanciers knewlong ago that the pile could be produced from a female of the black-redGame mated with a white Game-cock. It would seem, therefore, that the pileis the heterozygote of black-red and 'dominant' white. Bateson, however(_Principles of Heredity_, 1909, p. 120), writes that the whole problem ofthe pile is very obscure, and treats it as a case of peculiarity in thegenetics of yellow pigments. On p. 102 of the same volume he describes theresults of crossing White Leghorn with Indian Game or Brown Leghorn, the_F1_ being substantially white birds with specks of black and brown, though cocks have sometimes enough red in the wings to bring them intothe category known an pile. To test the matter I have crossed WhiteLeghorns with a pure-bred black-red Game-cock, and in the offspring out ofeight six were fairly good piles, but with not quite so much red on theback as in typical birds: one was a pile with yellow on the back insteadof red, and one was white with irregular specks. Of the hens, four were ofpile coloration with breast and abdomen of uniform reddish-brown colour, back, neck, and saddle hackles laced with pale brown, tail white. Theother four were white with black and brown specks. Whether these pileheterozygotes will breed true I do not yet know. 

These results tend to show that factors are not indivisible units, andsegregation is rather the difficulty of chromatin or germ plasm fromdifferent race uniting together. It must be remembered that the fertilisedovum which forms one individual gives rise also to dozens or hundreds orthousands or millions of gametes. If a given character is represented by aportion of the chromatin in the original ovum, this has to be divided somany times, and each time to grow to the same condition as before. How canwe suppose that the divisions shall be exactly equal or the growth alwaysthe same? It is inevitable that irregularities will occur, and if theoriginal chromatin produced a certain character, who shall say what moreor less of that chromatin will produce?

In the case of my recessive pile, my interpretation is that when thechromosomes corresponding to two distinct characters such as colour andabsence of colour are formed they do not separate from each othercompletely. Whether the mixture of the chromosomes occurs in every restingstage of the nucleus in the successive generations of the gametocytes, orwhether it occurs only in the synapsis stage preceding reduction division, it is not surprising that the colloid substance of the chromosomes shouldform a more or less complete intermixture, and that the two originalchromosomes should not be again separated in the pure condition in whichthey came into contact. A part, greater or less, of each may be left mixedwith the other. This is the probable explanation of the fact that therecessive white plumage has some of the pigment from the dominant form. Segregation, the repulsion between chromosomes, or chromatin, from gametesof different races may occur in different degrees from completesegregation to complete mixture. When the latter occurs there would beno segregation and the heterozygote would breed true. The most interestingfact is that a given factor in the cases I have described, namely, colourof plumage and pigmentation, of skin in the Jungle fowl and the Silky, isnot a permanent and indivisible unit, but is capable of subdivision in anyproportion. Bateson has already (in his Address to the Australian meetingof the British Association) expressed the same conclusion. He states thatalthough some Mendelians have spoken of genetic factors as permanent andindestructible, he is satisfied that they may occasionally undergo aquantitative disintegration, the results of which he calls subtraction orreduction stages. For example, the Picotee Sweet Pea with its purple edgescan be nothing but a condition produced by the factor which ordinarilymakes the fully purple flower, quantitatively diminished. He remarks alsothat these fractional degradations are, it may be inferred, theconsequences of irregularities in segregation. 

Bateson, however, proceeds to urge that the history of the Sweet Peabelies those ideas of a continuous evolution with which we had formerly tocontend. The big varieties came first, the little ones arose later byfractionation, although now the devotees of continuity could arrange themin a graduated series from white to deep purple. Now this may behistorically true of the Sweet Pea, but I would point out that once thedogma of the permanent indivisible unit or factor is abandoned, there isnothing in Mendelism inconsistent with the possibility of the gradualincrease or decrease of a character in evolution. I do not suggest thatthe colour and markings of a species or variety were, in all cases, due toexternal conditions, but if the effect of external stimuli can beinherited, can affect the chromosomes, then the evidence concerning unitfactors no longer contradicts the possibility of a character graduallyincreasing, under the influence of external stimuli acting on the somafrom zero to any degree whatever. 

 SEX AND SECONDARY SEXUAL CHARACTERS

The mystery of sex is hidden ultimately in the phenomenon of conjugation, that union of two cells which in general seems necessary to themaintenance of life, to be a process of rejuvenation. We know nothing ofthe nature of this process, or why in general it should produce areinvigoration of the cell resulting from it. We know little if anythingof the relation between the two conjugating cells or gametes, of the realnature of the attraction that causes them to approach each other andultimately unite together. We have, it is true, some evidence that onecell affects the other by some chemical action, as for instance in thefact that the mobile male gametes of a fern are attracted to a tubecontaining malic acid, but this may be merely an influence on thedirection of movement of the male gamete, while there are cases in whichneither cell is actively mobile. What we know in higher animals and plantsis that each gamete contains in its nucleus half the number of chromosomesfound in the other cells of the parent, and that in the fertilised ovumthe chromosomes of both gametes form the new nucleus, in which thereforethe original number of chromosomes is restored. 

The remarkable fact is that from this fertilised ovum or zygote isdeveloped usually an individual of one sex or the other, male or female, other cases being comparatively exceptional, although each act offertilisation is the union of the two sexes together. Various attemptshave been made to prove that the sex of the organism is determined byconditions affecting it during development subsequent to fertilisation, but now there is good reason to believe that generally the sex of theindividual is determined at fertilisation, though as we shall see there isevidence that it may in certain cases be changed at a laterstage. 

In Mendelian experiments, a heterozygote individual is one arising fromgametes containing opposite members of a pair of characters, in otherwords, from the union of a gamete carrying a dominant with anothercarrying a recessive. A pure recessive individual is one arising from theunion of two gametes both carrying recessives. If a heterozygote is bredwith a pure recessive the offspring are half heterozygote and halfrecessive. The heterozygote individual in typical cases shows the dominantcharacter. In the formation of its gametes when the reduction division ofthe chromosomes takes place, half of them receive the dominant character, half the recessive. When the division in the gametes of the recessiveindividual takes place its gametes all contain the recessive character. Thus, if we indicate the dominant character by _D_ and the recessiveby _d_, the constitution of the two individuals is

 _Dd_ and _dd_. 

The gametes they produce are

 _D+d_ and _d+d_, and the fertilisations are therefore

 _Dd_, _Dd_, _dd_, _dd_, or heterozygote dominants and pure recessives in equal numbers. 

It is evident that the reproduction of the sexes is very similar to this. One of the remarkable facts about sex is that, although the unitinggametes are male and female yet they give rise to males and females inequal numbers. If one sex were a dominant this would be in accordance withMendelian theory. In accordance with the view that the dominant issomething present which is absent in the recessive, the Mendelian theoryof sex assumes that femaleness is dominant, and that maleness is theabsence of femaleness, the absence of something which makes the individualfemale. If we represent the character of femaleness by _F_ and maleness orthe recessive by _f_, we have the ordinary sexual union represented by

 _Ff_x_ff_;

the gametes will then be

 _F_+_f_ and _f_+_f_

and the fertilisations

 _Ff_ and _ff_, or males and females in equal numbers, as they are, at leastapproximately, in fact. 

The close agreement of this theory with what actually happens is certainlyimportant and suggests that it contains some truth. But it cannot be saidto be a satisfactory explanation. It ignores the question of the nature ofsex. According to the theory the female character is entirely wanting inthe male. But what is sex but the difference between ovum andspermatozoon, between megagamete and microgamete? The theory then assertsthat an individual developed from a cell formed by the union of male andfemale gametes is entirely incapable of producing female gametes again. Every zygote after conjugation or fertilisation may be said to be bisexualor hermaphrodite. How comes it then that the female quality entirelydisappears? Whether the gametocytes are distinguishable at an early stagein the segmentation of the ovum, or only at a later stage of development, we know that the gametes ultimately formed have descended by a series ofcell-divisions from the fertilised ovum or zygote cell from whichdevelopment commenced. If segregation takes place at the reductiondivisions we might suppose that half the gametes formed are sperms andhalf are ova, and that in the male the latter do not survive but perishand disappear. But in this case it would be the whole of the chromosomescoming from the original female gamete which would disappear, and thespermatozoon would be incapable of transmitting characters derived fromthe female parent of the individual in which the spermatozoa were formed. An individual could never inherit character from its paternal grandmother. This, of course, is contrary to the results of ordinary Mendelianexperiments, for characters are inherited equally from individuals ofeither sex, except secondary sexual characters and sex-linked characterswhich we shall consider later. 

Similarly, if we suppose that segregation of ovum and sperm occurs in thefemale, the sperms must disappear and the ovum would contain no factorsderived from the male parent. But the theory supposes that the segregationof male and female does occur in the female, that half the ova are femaleand half are male. What meaning are we to attach to the words 'male ovum'or even 'male producing ovum'? It is a fundamental principle of Mendelismthat the soma does not influence the gametocytes or gametes; we havetherefore only to consider the sex of the gametes themselves, derived froma zygote which is formed by the union of two sexes. The quality ofmaleness consists only in the size, form, and mobility of the sperm in thehigher animals and of the microgamete in other cases. In what sense then, can an ovum be male? It may perhaps be said that though it is itselffemale, it has some property or factor which when united with a spermcauses the zygote to be capable of producing only sperms, and converselythe female ovum has a quality which causes the zygote to produce only ova. But since these qualities segregate in the reduction divisions, how is itthat the male quality in the _f_ ovum does not make it a sperm? We areasked to conceive a quality, or the absence of a factor, in an ovum whichis incapable of causing that ovum to be a sperm, but which, whensegregated in the gametes descended from that ovum, causes them all to besperms. It is impossible to conceive a single quality or factor which atdifferent times produces directly opposite effects. The Mendelian theoryis merely a theory in words, which have an apparent relation to the facts, but which when examined do not correspond to any real conceptions. 

However, we have to consider a number of remarkable facts concerning therelation of chromosomes to sex. In the ants, bees, and wasps theunfertilised ovum always develops into a male, the fertilised into afemale. The chromosomes of the ovum undergo reduction in the usual way, and are only half the number of those present in the nucleus beforereduction. We may call this reduced number _N_ and the full number _2N_. The ova developing by parthenogenesis and giving rise to males segment inthe usual way, and all the cells both of soma and gametocytes contain only_N_ chromosomes. In the maturation divisions reduction does not occur, _N_chromosomes passing to one gamete, none to the other, and the latterperishes so that the sperms all contain _N_ chromosomes. Whenfertilisation occurs the zygote therefore contains _2N_ chromosomes andbecomes female. Here then we have no segregation of _Fxf_ in the ova. Thedifference of sex merely corresponds to duplex and simplex conditions ofnucleus, but it is curious that the simplex condition in the gametesoccurs in both ova and sperms. 

In Daphnia and Rotifers the facts are different. Parthenogenesis occurswhen food supply is plentiful and temperature high. In this case reductionof the chromosomes does not occur at all, the eggs develop with _2N_chromosomes and all develop into females. Under unfavourable conditionsreduction or meiosis occurs, and two kinds of eggs larger and smaller areformed, both with _N_ chromosomes. The larger only develops whenfertilised and give rise to females with _2N_ chromosomes. The smallereggs develop without fertilisation, by parthenogenesis, and become males. Here then we have three kinds of gametes, large eggs, small eggs, andsperms, each with the same number of chromosomes. It is not the merenumber then which makes the difference, but we find a segregation in theova into what may for convenience be called female ova and male ova. 

In Aphidae or plant lice a third condition is found. Here againparthenogenesis continues for generation after generation so long asconditions are favourable, _i. E. _ in summer, and the eggs are in the samecondition as in Daphnia, etc. , that is to say, reduction does not occur, and the number of chromosomes is 2_N_. Under unfavourable conditions malesare developed as well as females by parthenogenesis, but the males arisefrom eggs which undergo partial reduction of chromosomes, only one or twobeing separated instead of half the whole number. The number then in anegg which develops into a male is 2_N_-1, while other eggs undergocomplete reduction and then have _N_ chromosomes. The latter, however, donot develop until they have been fertilised. In the males, when mature, reduction takes place in the gametes, so that two kinds of sperms areformed, those with _N_ chromosomes and those with _N_-l chromosomes. Thelatter degenerate and die, the former fertilise the ova, and thefertilised ova develop only into females. The chief difference in thiscase then is that the reduction in the male to the _N_ or simplexcondition takes place in two stages, one in the parthenogenetic ovum, onein the gametes of the mature male. In Hymenoptera and in Daphnia, etc. , the whole reduction takes place in the parthenogenetic ovum, and in themature male, though reduction divisions occur, no separation ofchromosomes takes place: at the first division one cell is formed with _N_chromosomes and one with none, and the latter perishes. 

In many insects and other Arthropods which are not parthenogenetic themale has been found to possess fewer chromosomes than the female. Thefemale forms, as in the above cases of parthenogenesis, only gametes ofone kind each with _N_ chromosomes, but the male forms gametes of twosorts, one with N chromosomes, the other with _N_-l or _N_-2 chromosomes. On fertilisation two kinds of zygotes are formed, female-producing eggswith 2_N_ chromosomes, and male-producing eggs with 2_N_-1 or 2_N_-2chromosomes. There is also evidence that in some cases, _e. G. _ thesea-urchin, the female is heterozygous, forming gametes, some with _N_ andsome with _N_+ chromosomes, while the male gametes are all _N_. Fertilisation then produces male-producing eggs with 2_N_ chromosomes, female-producing with 2_N_+. 

Such is the summary given by Castle in 1912. [Footnote: _Heredity andEugenics_, by Castle and Others. University of Chicago Press, 1912. ] Itwill be seen that he treats the differences as purely quantitative, meredifferences in the number of the chromosomes. Professor E. B. Wilson, however, who had contributed largely by his own researches to ourknowledge of sex from the cytological point of view, had alreadypublished, in 1910, [Footnote: '_The Determination of Sex_, ' _ScienceProgress_, April 1910. ] a very instructive _résumé_ of the facts observedup to that time. The important fact which is generally true for insects, according to Wilson, is that there is a special chromosome or chromosomeswhich can be distinguished from the others, and which is or are related tosex differentiation. This chromosome, to speak of it for convenience inthe singular, has been variously named by different investigators. Wilsoncalled it the 'X chromosome, ' McCluny the 'accessory chromosome, 'Montgomery the 'hetero-chromosome, ' while the names 'heterotropicchromosome' and idiochromosome have also been used. For the purpose of thepresent discussion we may conveniently name it the sex-chromosome. It isoften distinguished by its larger size and different shape. Wilsondescribes the following different cases:--

(1) The sex-chromosome in the male gametocytes is single and fails todivide with the others, but passes undivided to one pole. This may occurin the first reduction division (Orthoptera, Coleoptera, Diptera) or inthe second (many Hemiptera). But it is difficult to understand what ismeant by 'fails to divide. ' In one of the reduction divisions all thechromosomes divide as in ordinary or homotypic nucleus division, but inthe other the chromosomes simply separate into two equal groups withoutdivision. If there are an odd number of chromosomes, 2_N_-1, in all thegametocytes of the male, as stated in most accounts of the subject, thenif one chromosome fails to divide in the homotypic division, we shall have2_N_-2 in one spermatocyte and 2_N_-1 in the other. Then when theheterotypic division takes place and the number of chromosomes is halved, we shall have two spermatocytes with _N_-1 chromosomes from one of thefirst spermatocytes and one with _N_ and one with _N_-1 from the other. Thus there will be three spermatozoa with _N_-1 chromosomes and one with_N_ chromosomes, whereas we are supposed to find equal numbers with _N_and _N_-1 chromosomes. It is evident that what Dr. Wilson means isthat the sex-chromosome is unpaired, and that although it divideslike the others in the homotypic division, in the heterotypic divisionit has no mate and so passes with half the number of chromosomes to onepole of the division spindle, while the other group of chromosomes hasno sex-chromosome. Examples of this are the genera _Pyrrhocoris_ and_Protenor_ (Hemiptera) _Brachystola_ and many other Acrididae, _Anasa, Euthoetha, Narnia, Anax_. In a second class of cases the sex-chromosome isdouble, consisting of two components which pass together to one pole. Examples of this are _Syromaster, Phylloxera, Agalena_. In a third classthe sex-chromosome is accompanied by a fellow which is usually smaller, and the two separate at the differential division. The sizes of the twodiffer in different degrees, from cases as in many Coleoptera and Dipterain which the smaller chromosome is very minute, to those (_Benacus, Mineus_) in which it is almost as large as its fellow, and others(_Nezara, Oncopeltus_) in which the two are equal in size. Again, thereare cases in which one sex-chromosome, say _X_, is double, triple, or evenquadruple, while the other, say _Y_, is single. In all these cases thereare two _X_ chromosomes in the oocytes (and somatic cells) of the female, and after reduction the female gametes or unfertilised ova are all alike, having a single _X_ chromosome or group. On fertilisation half the zygoteshave _XX_ and half _XY_, whether _Y_ is absence of a sex-chromosome, or one of the other _Y_ forms above mentioned. The sex is thus determinedby the male gamete, the _X_ chromosome united with that of the femalegamete producing female individuals, while the _Y_ united with _X_produces male individuals. 

Professor T. H. Morgan has made numerous observations and experiments on asingle culture of the fruit-fly, _Drosophila ampelophila_, bred in bottlesin the laboratory for five or six years. He has not only studied thechromosomes in the gametes of this fly, and made Mendelian crosses withit, but has obtained numerous mutations, so that his work is a veryimportant contribution to the mutation doctrine. Drosophila in the handsof Professor Morgan and his students and colleagues has thus become asclassical a type as Oenothera in those of the botanical mutationists. Different branches of Morgan's work are discussed elsewhere in thisvolume, but here we are concerned only with its bearing on the question ofthe determination of sex. He describes [Footnote: _A Critique of theTheory of Evolution_. Princeton University Press and Oxford UniversityPress, 1916. ] the chromosomes of Drosophila as consisting in the diploidcondition of four pairs, that is to say, pairs which separate in thereduction division so that the gamete contains four single chromosomes, one of each pair. In two of these pairs the chromosomes are elongated andshaped like boomerangs, in the third they are small, round granules, andthe fourth pair are the sex-chromosomes: in the female these last arestraight rods, in the male one is straight as in the female, the other isbent. The straight ones are called the X chromosomes, the bent one the Ychromosome. The fertilisations are thus XX which develops into a femalefly, and XY which develops into a male. Drosophila therefore is an exampleof one of the cases described by Wilson. 

Dr. Wilson (_loc. Cit. _) discusses the question of how we are to interpretthese facts, in particular, the fact that the X chromosome infertilisation gives rise to females. He remarks that the X chromosome mustbe a male-determining factor since in many cases it is the onlysex-chromosome in the males, yet its introduction into the egg establishesthe _female_ condition. This is the same difficulty which I pointed outabove in connection with the Mendelian theory that the female washeterozygous and the male homozygous for sex. Dr. Wilson points out thatin the bee, where fertilised eggs develop into females and unfertilisedinto males, we should have to assume that the _X_ chromosome in the femalegamete is a female determiner which meets a recessive male determiner inthe _X_ chromosomes of the sperm. When reduction occurs, the _X_[female]must be eliminated since the reduced egg develops always into a male. Buton fertilisation, since the fertilised egg develops into a female, adominant _X_[female] must come from the sperm, so that our firstassumption contradicts itself. 

Dr. Wilson, T. H. Morgan, and Richard Hartwig have therefore suggestedthat the sex-difference as regards gametes is not a qualitative but aquantitative one. In certain cases there is no evident quantitativedifference of chromatin as a whole, but there may in all cases be adifference in the quantity of special sex-chromatin contained in the _X_element. The theory put forward by Wilson then is that a single _X_element means _per se_ the male condition, while the addition of a secondelement of the same kind produces the female condition. Such a theorymight apply even to cases where no sex-chromosomes can be distinguished bythe eye: the ova, in such cases (probably the majority), might also have adouble dose of sex-chromatin, the males a single dose. This theory, however, is still open to the objection that the female gametes beforefertilisation, and half the male gametes, have the half quantity ofsex-chromatin which by hypothesis determines the male condition, so thathere again we have the male condition as something which is distinct fromthe characteristics of the spermatozoon. But if this is the case, what isthe male condition? The parthenogenetic ovum of the bee is male, and yetit is an ovum capable only of producing spermatozoa. If the single Xchromosomes is the cause of the development of spermatozoa in the malebee, why does it not produce spermatozoa in the gametes of the female bee, since when reduction takes place all these gametes have a single Xchromosome?

In biology, as in every other science, we must admit facts even when wecannot explain them. The facts of what we call gravitation are obvious, and any attempt to disregard them would result in disaster, yet nosatisfactory explanation of gravitation has yet been discovered: manytheories have been suggested, but no theory has yet been proved to betrue. In the same way it may be necessary to admit that two X chromosomesresult in the development of a female, and one X, or XY chromosomes resultin the development of a male. But Mendelians have omitted to consider whatis meant by male and female. The soma with its male and female somaticcharacters has nothing to do with the question, since somaticsex-differences may be altogether wanting, and moreover, the essentialmale character, the formation of spermatozoa, is by the Mendelianhypothesis due to descent of the male gametes from the original fertilisedor unfertilised _ovum_. The Mendelian theory therefore is that when anovum has two X sex-chromosomes it can only after a number ofcell-divisions, at the following reduction division, give rise to ova, while an ovum containing one X sex-chromosome, or two different, XY, chromosomes, at the next reduction division gives rise to spermatozoa. TheX sex-chromosome is not in itself either female or male, since, as we haveseen, either ovum or spermatozoon may contain a single X chromosome. Theovum then with one X chromosome or one X and one Y changes its sex at thenext reduction division and becomes male. In parthenogenetic ova thishappens without conjugation with a spermatozoon at all: in other cases, since the zygote is compounded of spermatozoon and ovum, we can only saythat in the XX zygote, the ovum developing only ova, the female isdominant, in the X or XY zygote developing only spermatozoa the male isdominant. Hermaphrodite animals, as has been pointed out by Correns andWilson, cannot be brought under this scheme at all. In the earthworms, forinstance, we have, in every individual developed from a zygote, ova andspermatozoa developing in different gonads in different parts of the body. The differentiation here, therefore, must occur in some cell-divisionpreceding the reduction divisions. Every zygote must have the samecomposition, and yet give rise to two sexes in the same individual. 

Further light on the sex problem, as in many other problems in biology, can only be obtained by more knowledge of the physical and chemicalprocesses which take place in the chromosomes and in the relations ofthese structures to the rest of the cell. The recent advances in cytology, remarkable as they are, consist almost entirely of observations ofmicroscopic structure. They may be said to reveal the statics of the cellrather than its dynamics. Cytology is in fact a branch of anatomy, and inthe anatomy of the cell we have made some progress, but our knowledge ofthe physiology of the cell is still infinitesimal. The nucleus, andespecially the chromosomes, are supposed in some unknown way to influenceor govern the metabolism of the cytoplasm. From this point of view thehypothesis mentioned above that the sex-difference in the gametes is notqualitative but quantitative is probably nearer to the truth. Geddes andThomson and others have maintained that the sex-difference is one ofmetabolism, the ovum being more anabolic, the sperm more katabolic. Adouble quantity of special chromatin may be the cause of the greateranabolism of the ovum. In that case the difficulty indicated in a previouspart of this chapter, that the ovum after reduction resembles the sperm inhaving only one X chromosome, may be explained by the fact that the growthof the ovum and its accumulation of yolk substances has been alreadyaccomplished under the influence of the two chromosomes before reduction. Other difficulties previously discussed also appear to be diminished if weadopt this point of view. We need not regard maleness and femaleness asunit characters in heredity of the same kind as Mendelian characters ofthe soma. Instead of saying that the zygote composed of ovum andspermatozoon is incapable of giving rise in the male to ova, or in thefemale to sperms, we should hold that the gametocytes ultimately give riseto ova or to sperms according to the metabolic processes set up andmaintained in them through their successive cell-divisions under theinfluence of the double or single X chromosome. There still remains thedifficulty of explaining why the male gametocytes after reduction developinto similar sperms, with their heads and long flagella, although half ofthem possess one X chromosome each and the other half none. We can onlysuppose that the final development of the sperms is the result of thepresence of the single X chromosome in the successive generations of malegametocytes before the reduction divisions. 

The Mendelian theory of sex-heredity assumed that in the reductiondivisions the two sex-characters, maleness and femaleness, were segregatedin the same way as a pair of somatic allelomorphs, but the words malenessand femaleness expressed no real conceptions. The view above suggestedmerely attempts to bring our real knowledge of the difference between ovumand sperm into relation with our real knowledge of the sex-chromosomes andtheir behaviour in reduction and fertilisation. 

CHAPTER III

 Influence Of Hormones On Development Of Somatic Sex-Characters

We have next to consider what are commonly called secondary sexualcharacters. These are characters or organs more or less completely limitedto one sex. When we distinguish in the higher animals the generativeorgans or gonads on the one hand from the body or soma on the other, wesee that all differences between the sexes, except the gonads, aresomatic, and we may call them somatic sexual characters. The question atonce arises whether the soma itself is sexual, that is to say, whether onthe assumption that the sex of the zygote is already determined before itbegins to develop, the somatic cells as well as the gametocytes areindividually and collectively either male or female. In previousdiscussions of the subject I have urged that the only meaning of sex wasthe difference between the megagamete or ovum, and the microgamete orsperm. But if the zygote, although compounded of ovum and sperm, ispredestined to give rise in the gametes descended from it, either tosperms only or to ova only, it may be suggested that all the somatic cellsdescended from the zygote are likewise either male or female, althoughthey do not give rise to gametes. It is evident, however, that the somaticcells, organs, and characters do not differ necessarily or universally inthe two sexes. On the one hand, we have extraordinary and very conspicuouspeculiarities in the male, entirely absent in the female, such as theantlers of stags, and the vivid plumage of the gold pheasant; on the otherwe have the sexes externally alike and only distinguished by their sexualorgans, as in mouse, rabbit, hare, and many other Rodents, most Equidae, kingfisher, crows and rooks, many parrots, many Reptiles, Amphibia, Fishes, and invertebrate animals. In the majority of fishes, in whichfertilisation is external and no care is taken of the eggs or young, thereare no somatic sexual differences. Moreover, somatic sexual characterswhere they do occur have no common characteristics either in structure orposition in the body. It may be said that any part of the soma may indifferent cases present a sex-limited development. In the stag the malepeculiarity is an enormous development of bone on the head, in the peacockit is the enlargement of the feathers of the tail. In some birds there arespurs on the legs, in others spurs on the wings. It is no explanation, therefore, to say that these various organs and characters are theexpression of sex in the somatic cells. 

As I pointed out in my _Sexual Dimorphism_ (1900), the commoncharacteristic of somatic sexual characters is their adaptive relation tosome function in the sexual habits of the species in which they occur. There is no universal characteristic of sex except the difference betweenthe gametes and the reproductive organs (gonads) in which they areproduced. All other differences, therefore, including genital ducts andcopulatory or intromittent organs, are somatic. When we examine thesesomatic differences we find that they can be classified according to theirrelation to fertilisation and reproduction, including the care orprotection of the offspring. The precise classification is of no greatimportance, but we may distinguish the following kinds to show thechief functions to which the characters or organs are adapted. 

1. GENITAL DUCTS AND INTROMITTENT ORGANS. --According to the theory of thecoelom which we owe to Goodrich, in all the coelomata the coelom isprimarily the generative cavity, on the walls of which the gametocytes aresituated, and the coelomic ducts are the original genital ducts. InVertebrates we find two such ducts in both sexes in the embryo, originallyformed apparently by the splitting of a single duct. In the male one ofthese ducts becomes connected with the testis while the other degenerates:the one which degenerates in the male forms the oviduct in the female, while the one which is functional in the male degenerates in the female. 

Intromittent organs are formed in all sorts of different ways in differentanimals. In Elasmobranchs (sharks and skates) they are enlarged portionsof the pelvic fins, and therefore paired. In Lizards they are pouches ofthe skin at the sides of the cloacal opening. In Mammals the single penisis developed from the ventral wall of the cloaca. In Crustacea certainappendages are used for this function. There are a great many animals, from jelly-fishes to fishes and frogs, in which fertilisation is external, and there are no intromittent organs at all. 

2. ORGANS FOR, CAPTURING OR HOLDING THE FEMALE: for example, thethumb-pads of the frog, and a modification of the foot in a water-beetle. Certain organs on the head and pelvic fins of the Chimaeroid fishes arebelieved to be used for this purpose. 

3. WEAPONS. --Organs which are employed in combats between males for theexclusive possession of the females. For example, antlers of stags, hornsof other Ruminants, tusks of elephants, spurs of cocks and Phasiamidaegenerally, horns and outgrowths in males of Reptiles and many Beetles, probably used for this purpose. 

4. ALLUREMENTS. --Organs or characters used to attract or excite thefemale. These might be called the organs of courtship, such as thepeacock's tail, the plumes of the birds-of-paradise, and the brilliantplumage of humming birds and many others. The song of birds is anotherexample, and sound is produced in many Fishes for a similar purpose. 

5. ORGANS FOR THE BENEFIT OF THE OFFSPRING: for example, the extraordinarypouches in which the eggs are developed in certain Frogs. In the SouthAmerican species, _Rhinoderma darwinii_, the enlarged vocal sacs are usedfor this purpose. Pouches with the same function are developed in manyanimals, for instance in Pipe-fishes and Marsupials. Abdominal appendagesare enlarged in female Crustacea for the attachment of the eggs, theabdomen also being larger and broader. 

The argument in favour of the Lamarckian explanation of the evolution ofthese adaptive characters is the same as in the case of adaptations commonto both sexes, namely that in every case the function of the organs andcharacters involves special irritations or stimulations by externalphysical agents. Mechanical irritation, especially of the interruptedkind, repeated blows or friction causes hypertrophy of the epidermis andof superficial bone. I have stated this argument and the evidence for itin some detail in my volume on _Sexual Dimorphism_. It is one of the moststriking facts in support of this argument that the hypertrophied plumagewhich constitutes the somatic sexual character of the male in so manybirds is habitually erected by muscular action for the purpose of displayin the sexual excitement of courtship. I doubt if there is a singleinstance in which the male bird takes up a position to present hisornamental plumage to the sight of the female without a special erectionand movement of the feathers themselves. Such a stimulation must affectthe living epidermic cells of the feather papilla. Even supposing that thefeather is not growing at the time, it is probable, if not certain, thatthe stimulation will affect the papilla at the base of the featherfollicle, so as to cause increased growth of the succeeding feather. Butwe have no reason to believe that erection in display occurs only when thegrowth of the feathers is completed, still less that it did so always atthe beginning of the evolution. 

The antlers of stags are the best case in favour of the Lamarckian view ofthe evolution of somatic sexual characters. The shedding of the skin('velvet') followed by the death of the bone, and its ultimate separationfrom the skull, are so closely similar to the pathological processesoccurring in the injury of superficial bones, that it is impossible tobelieve that the resemblance is only apparent and deceptive. In anindividual man or mammal, if the periosteum of a bone is destroyed orremoved the bone dies, and is then either absorbed, or separated from theliving bone adjoining, by absorption of the connecting part. In the stagboth skin and periosteum are removed from the antler: probably they woulddie and shrivel of their own accord by hereditary development, but as amatter of fact the stag voluntarily removes them by rubbing the antleragainst tree trunks, etc. When the bone is dead the living cells at itsbase dissolve and absorb it, and when the base is dissolved the antlermust fall off. 

The adaptive relation is not the only common characteristic of thesesomatic sexual characters. Another most important fact is not only thatthey are fully developed in one sex, absent or rudimentary in the other, but that their development is connected with the functional maturity andactivity of the gonads. There is usually an early immature period of lifein which the male and female are similar, and then at the time of pubertythe somatic sexual characters of either sex, generally most marked in themale, develop. In some cases, where the activity of the gonads is limitedto a particular season of the year, the sexual characters or organs aredeveloped at this season, and then disappear again, so that there is aperiodic development corresponding to the periodic activity of the testesor ovaries. Stags have a limited breeding or 'rutting' season in autumn(in north temperate regions), and the antlers also are shed and developedannually. In this case we cannot assert that the development of the antlertakes place during the active state of the testes. The antlers are fullydeveloped and the velvet is shed at the commencement of the ruttingseason, and development of the antlers takes place between the beginningof the year and the month of August or September. In ducks and other birdsthere is a brilliant male-breeding plumage in the breeding season whichdisappears when breeding is over, so that the male becomes very similar tothe female. In the North American fresh-water crayfishes of the genusCambarus there are two forms of males, one of which has testes infunctional activity, while in the other these organs are small andquiescent: the one form changes into the other when the testes pass fromthe one condition to the other. 

It has long been known that the development of male sex-characters isprofoundly affected by the operation of castration. The removal of thetestes is most easily carried out in Mammals, in consequence of theexternal position of the organs in these animals, and the operation hasbeen practised on domesticated animals as well as on man himself from veryancient times. The effect is the more or less complete suppression of themale insignia, in man, for example, the beard fails to develop, the voicedoes not undergo the usual change to lower pitch which takes place atpuberty, and the eunuch therefore has much resemblance to the boy orwoman. Many careful experimental researches have been made on the subjectin recent years. The consideration of the subject involves two questions:(1) What are the exact effects of the removal of the gonads in male andfemale? (2) By what means are these effects brought about, what is thephysiological explanation of the influence of the gonads on the soma?

I have quoted the evidence concerning the effects of castration on stagsin my _Sexual Dimorphism_ and in my paper on the 'Heredity of SecondarySexual Characters. ' [Footnote: _Archiv für Entwicklungesmechanik_, 1908. ]When castration is performed soon after birth a minute, simple spikeantler is developed, only two to four inches in length: it remains coveredwith skin, is never shed, and develops no branches. When the operation isperformed on a mature stag with antlers, the latter are shed soon afterthe operation, whether they have lost their velvet or not. In thefollowing season new antlers develop, but these never lose their velvet orskin and are never shed. 

 CASTRATION IN FOWLS

The removal of the testes from young cocks has been commonly practised inmany countries, _e. G. _ France, capons, as such birds are called, beingfatter and more tender for the table than entire birds. The actual effect, however, on the secondary sexual characters has not in former times beenvery definitely described. The usual descriptions represent the castratedbirds as having rather fuller plumage than the entire birds; but the comband wattles are much smaller than in the latter, more similar to those ofa hen. It is stated that the capon will rear chickens, though he does notincubate, and that they are used in this way in France. 

The most precise of the statements on the subject by the earliernaturalists is that of William Yarrell [Footnote: _Proc. Linn. Soc. , 1857. ] (1857), who writes as follows:--

'The capon ceases to crow, the comb and gills do not attain the size ofthose parts in the perfect male, the spurs appear but remain short andblunt, and the hackle feathers of the neck and saddle instead of beinglong and narrow are short and broadly webbed. The capon will take to aclutch of chickens, attend them in their search for food, and brood themunder his wings when they are tired. '

It would naturally be expected, on the analogy of the case of stags, thatwhen a young cock was completely castrated all the male secondarycharacters would be suppressed, namely, the greater size of the comb andwattles in comparison with the hen, the long neck hackles, and saddlehackles, long tail feathers, especially the sickle-feathers, and thespurs. As a matter of fact, the castrated specimen usually shows only thefirst of these effects to any conspicuous degree. The comb and wattles ofthe capon are similar to those of the hen, but he still has the plumageand the spurs of the entire cock. Many investigators have made experimentsin relation to this subject, and most of them have found that completecastration is difficult, and that portions of the testes left in the birdduring the operation become grafted in some other position either on theparietal peritoneum, or on that covering the intestines, and producespermatozoa, which, of course hare no outlet. In such cases the secondarymale characters may fee more or less completely developed. Thus Shattockand Seligmann (1904) state that ligature of the vas deferens made nodifference to the male characters, and that after castration detachedfragments were often left in different positions as grafts, when thesecondary characters developed. In one particular case only a minutenodule of testicular tissue showing normal spermatogenesis was found onpost mortem examination attached to the intestine. In this bird there wasno male development of comb or wattles, a full development of neckhackles, a certain development of saddle hackles, a few straggling badlycurved feathers in the tail and short blunt spurs on the legs. Lode[Footnote: _Wiener klin. Wochenschr. _, 1895. ] (1895) found that testescould easily be transplanted into subcutaneous tissue and elsewhere, andthat the male characters then developed normally. Hanau [Footnote: _Arch. F. Ges. Physiologie_, 1896. ] (1896) obtained the same result. 

The question, however, to what degree the male characters of the cock aresuppressed after complete castration is not so definitely answered in theliterature of the subject. Shattock and Seligmann in their 1904 paper makeno definite statement on the subject. Rieger (1900), Selheim (1901), andFoges [Footnote: _Pfügers Archiv_, 1902. ] (1902) state that the true caponis characterised by shrivelling of the comb, wattles, _and spurs_; poordevelopment of the neck and tail feathers; hoarse voice and excessivedeposit of fat. Shattock and Seligmann, on the other hand, have placed inthe College of Surgeons Museum the head of a Plymouth Rock which wascastrated in 1901. It was hatched in the spring of that year. In December1901 the comb and wattles were very small, the spurs fairly welldeveloped, and the tail had a somewhat masculine appearance. In September1902, when the bird was killed, the comb and wattles were still poorlydeveloped, the neck hackles fairly well so; saddle hackles rather welldeveloped; the tail contained rather loosely-grouped long sickle feathers;the spurs stout. The description states that dissection showed no trace ofeither testicle, and I am informed by Mr. Shattock that there were nografts. The description ends with the conclusion that the growth of thespurs, and to a certain extent that of the long, curved sickle feathers, is not prevented by castration. With regard to the spurs this result doesnot agree with that of the German investigators, but it must be rememberedthat the latter speak only of the reduction of the spurs, not entireabsence. It is important in discussing the effects of castration in cocksto bear in mind the actual course of development of the secondary sexualcharacters. When the chicks are first hatched they are in the down:rudimentary combs are present, wattles can scarcely be distinguished, andthere is no external difference between the sexes. The ordinary plumagebegins to develop immediately after hatching, the primaries of the wingsbeing the first to appear. The feathers are completely developed in aboutfive weeks, and still there is no difference between the sexes. The firstsexual difference is the greater size of the combs in the males, and thisis quite distinct at the age of six weeks. At nine to ten weeks inblack-red fowls, in which the cocks have black breasts and red backs withyellow hackles, the black feathers on the breast and red on the back aregradually developing, both sexes previously having been a dull speckledbrown, closely similar to the adult hens. The spurs are the last of themale characters to develop, these at the age of four months being stillmere nodules, scarcely, if at all, larger than the rudiments visible inadult hens. This is the age at which castration is usually performed, asat an earlier age the birds are too small to operate on successfully. Itfollows, therefore, that the spurs develop after castration, and it wouldseem that their development does not depend upon the presence of thesexual organs. It is a question, however, whether castration in the cockis ever quite complete. In the original wild species and in the majorityof domesticated breeds the spurs are confined to the male sex, and aretypical secondary sex-characters, as much so as the antlers of stags orthe beard of man, yet the above discussion shows that there is some doubtwhether their development is prevented as much as in other cases by theabsence of the sexual organs. Even if it should be proved that in supposedcases of complete castration, such as that of Shattock and Seligmann, sometesticular tissue remained at the site of the testes, it would still betrue that the development of the comb and wattles is more affected by theremoval of the sexual organs than that of the spurs or tail feathers. 

My own experiments in castrating cocks were as follows: On August 20, 1910, I operated on a White Leghorn cock about five months old. One testiswas removed, with a small part of the end broken off, but the other, afterit was detached, was lost among the intestines. On the same day I operatedon another about thirteen weeks old, a speckled mongrel. In this case bothtestes were extracted but one was slightly broken at one end, although Iwas not sure that any of it was left in the body. An entire White Leghornof the same age as the first was kept as a control. On August 27 the twocastrated birds had recovered and were active. Their combs had diminishedin size and lost colour considerably, that of the White Leghorn wasscarcely more than half as large as that of the control. Such a rapiddiminution can scarcely he due to absorption of tissue, but shows that thesize of the normal cock's comb is largely due to distension with blood, which ceases when the sexual organs are removed. In the following January, the second cock, supposed to be completely castrated, was seen to make asexual gesture like a cock, though not a complete action like an entireanimal: this showed that the sexual instinct was not completelysuppressed. In February this same bird was seen to attempt to tread a hen, while the white one, supposed to be less perfectly emasculated, had nevershown such male instinct. 

The White Leghorn cock was killed and dissected on May 13, 1911, ninemonths after castration. I found an oval body of dark, dull brown colourloose among the intestines: this was evidently the left testis which wasseparated from its natural attachment and lost in the abdomen at the timeof the operation. I examined the natural sites of the testes: on the rightside there was a small testis of considerable size, about half an inch indiameter. When a portion of this was teased up and examined under themicroscope moving spermatozoa were seen, but they were not in swarms as ina normal testis, but scattered among numerous cells. On the left side wasa much smaller testis, in the tissue of which I with difficulty detected afew slowly moving spermatozoa. The vasa deferentia were seen as whiteconvoluted threads on the peritoneum, but contained no spermatozoa. 

On July 29, 1911, a little more than eleven months after the operation, I examined and killed the second of these castrated cocks, the speckledmongrel-bred bird. I measured the comb and wattles while it was alive, incase there might be reduction in the size of these appendages when thebird was killed. The comb was 1-1/3 inches high by 2-3/8 inches in length. The spurs were 1 inch long, curved and pointed. Saddle hackles short, hanging only a little below the end of the wing. Neck hackles welldeveloped, similar to those of an entire cock. Longest tail feather 15-5/8inches, blue-black in colour. 

I had no entire cock of same breed, but measured the entire White Leghornfor comparison. Comb 1-3/4 inches high by 3-3/4 inches in length. (It isto be remembered that the comb and wattles are especially large inLeghorns. ) Wattle 1-1/4 inches in vertical length. Spur 1 inch long, stouter and less pointed than in the capon. Longest tail feather 12 incheslong. 

When killed the capon was found to be very fat: there were masses of fataround the intestines and under the peritoneum, which made it impossibleto make out details such as ureter and vas deferens properly. I found awhite nodule about half an inch in diameter attached to mesentery. Theliquid pressed from this was swarming with spermatozoa in active motion. Two other masses about the same size or a little larger were found on thesites of the original testes. These also were full of mobile spermatozoa, and must have grown from portions of the testes left behind at castration. 

In ducks the sexual characters of the male differ from those in the fowl, especially in the fact that they almost completely disappear after thebreeding season and reappear in the following season. In the interval thedrake passes into a condition of plumage in which he resembles the female;and this condition is known as 'eclipse. ' The male plumage, therefore, inthe drake has a history somewhat similar to that of the antlers in deer. Two investigations of the effects of castration on ducks and drakes havebeen recorded. H. D. Goodale [Footnote: 'Castration of Drakes. ' _Biol. Bulletin_, Wood's Hole, Mass. , vol. Xx. , 1910] removed the generativeorgans from both drakes and ducks of the Rouen breed, which is stronglydimorphic in plumage. One drake was castrated in the early spring of 1909when a little less than a year old. This bird did not assume the summerplumage in 1909, that is, did not pass into eclipse. It was in the nuptialplumage when castrated. This breeding or nuptial plumage is well known: itincludes a white neck-ring, brilliant green feathers on the head, muchclaret on the breast, brilliant metallic blue on the wing, and two or moreupward curled feathers on the tail. The drake mentioned above wasaccidentally killed in the spring of 1910. Another drake was castrated onAugust 8, 1909: only the left testis was removed, the other beingligatured. At this time the bird would be in eclipse plumage. It appearsfrom the description that it assumed the nuptial plumage in the winter of1909, and did not pass into eclipse again in the summer of 1910. Thus indrakes the effect of castration is that the secondary sexual characterremains permanently instead of being lost and renewed annually. Goodale, however, does not describe the moults in detail. In the natural conditionthe drake must moult twice in the year, once when he sheds the nuptialplumage, and again when he drops the summer dress. Goodale insists, fromsome idea about secondary sexual characters which is not very obvious, that the eclipse or summer plumage is not the same as that of the female. He states that the male in summer plumage merely mimics the female butdoes not become entirely like her. In certain parts of the body there areno modifications toward the female type. In others, i. E. Head, breast, andkeel region, the feathers of the male become quite like those of thefemale. 'It can hardly be maintained that this is an example of assumptionby the male of the female's plumage, especially as the presence of thetestis is necessary for its appearance. ' The idea here seems to be thatsince the eclipse plumage is only assumed when the testis is present, therefore it must be a male character. 

Out of five females on which the operation was performed only two livedmore than a few days afterwards. One of these (a) was castrated in thespring of 1909 when a little less than a year old, the other (b) on August13 when twelve weeks old. In October 1909 they showed no markedmodifications. In July 1910 it was noticed that they had the male curledfeathers in the tail, and (a) had breast feathers similar to those of themale in summer plumage, (b) was rather more strongly modified: she had avery narrow white neck-ring, and breast feathers distinctly of male type. The next moult began in September, and in November was well advanced. Onthe whole (a) had made little advance towards the male type, but (b)closely resembled the male in nuptial plumage. It had brilliant greenfeathers on the head, a white neck-ring, much claret colour on the breast, and some feathers indistinguishable from those of the male, and also themale sex feathers on the tail. Goodale concludes that the female owes hernormal colour to the ovaries or something associated with them whichsuppresses the male characters and ensures the development of her owntype. He considers it is quite as conceivable that selection shouldoperate to pick out inconspicuously coloured females as that selection ofbrilliantly coloured males should bring about an addition to the femaletype. But as pointed out above, selection cannot explain the dimorphism ineither case. 

It may be mentioned here that owing to the fact that the single (left)ovary in birds is very closely attached to the peritoneum immediatelycovering the great post-caval vein, it is generally impossible to removethe whole of the ovary without cutting or tearing the wall of the vein andso causing fatal hemorrhage. The above results observed by Goodale aretherefore all the more remarkable, and it may be assumed that he removedat any rate nearly all the ovary. 

The research of Seligmann and Shattock [Footnote: Relation betweenSeasonal Assumption of the Eclipse Plumage in the Mallard _(Anas boscas_)and the Functions of the Testicle. ' _Proc. Zool. Soc. _ 1914. ] begins witha comparison between the stages of the development of the nuptial plumageand the stages of spermatogenesis. In the young pheasant the male plumageis fully developed in the autumn of its first year, but no pairing occursand no sexual instinct is exhibited till the following spring. The wildduck pairs in autumn or early winter, after the assumption of the nuptialplumage, but copulation does not occur till spring is advanced. Theinvestigation here considered was made upon specimens of semi-domesticated_Anas boscas_, such as are kept in London parks and supplied from gamefarms. The testes attain their maximum size during the breeding season--end of March or beginning of April. At this time each organ is almost aslarge as a pigeon's egg, is very soft, and the liquid exuding from it whencut is swarming with spermatozoa. The bird is of course in full nuptialplumage. By the end of May, although the plumage is unchanged, the testeshave diminished to the size of a haricot bean, and spermatogenesis hasceased. They diminish still further during June, July, and August, andacquire a yellow or brownish colour, while microscopically there is nosign of activity in the spermatic cells. The change from nuptial plumageto eclipse takes place between the beginning of June and the middle ofJuly. The reappearance of the nuptial plumage takes place in the month ofSeptember, and while this process takes place there is no sign of changeor renewed activity in the testes. During October and November, when thebrilliant plumage is fully developed, the testes increase slowly in sizebut remain yellow and firm and exude no liquid on incision. Spermatogenesis does not commence until the end of November or beginningof December. The testes increase greatly in size in January and February, and again reach their maximum size by the end of March. It is shown, therefore, that the loss of the nuptial plumage takes place in June whenspermatogenesis has ceased and the testes are diminishing in size, but theredevelopment of this plumage takes place in September without any renewedactivity of the testis and long before the beginning of spermatogenesis. The case of the antlers in the stag is probably very similar. 

The important statement is made with regard to castration (underanaesthetics, of course) that it was found impossible to extirpate thetestes completely. When the bird was killed some months after theoperation, a greater or lesser amount of regenerated testicular tissue wasfound either on the original site of the organs or engrafted uponneighbouring organs. This experience, it will be noted, agrees with my ownin the case of fowls. There were, however, reasons for believing that theresults observed within the first six or eight months after the operationare not much different from those which would follow complete castration. 

Castration carried out when the drake was in nuptial plumage produced thesame effect which was observed by Goodale, namely, delay, and imperfectionin the assumption of the eclipse condition, but the observations ofSeligmann and Shattock are more precise and detailed. One exampledescribed was castrated in full winter plumage in December 1906. On July11, when normally it would have been in eclipse, the nuptial plumage wasunmodified except for a diffuse light-brown coloration on the abdomen, which is stated to be due not to any growth of new feathers but topigmentary modification in the old. By September 1 this bird was almost ineclipse but not quite; curl feathers in the tail had disappeared, thebreast was almost in full eclipse, the white ring was slightly indicatedat the sides of the neck, the top of the head and the nape had still agood deal of gloss. After this the nuptial plumage developed again, and onNovember 12 the bird was in full nuptial plumage, with good curl feathersin the tail. The only trace of the eclipse was the presence of a few brownfeathers on the flanks. This bird was killed July 30, 1908, when the birdwas in eclipse, but not perfectly so, as there were vermiculated feathersmixed with eclipse feathers on the breast, abdomen, and flanks. Dissectionshowed on the right side a series of loosely attached nodular grafts oftesticular tissue, in total volume about the size of a haricot bean: onthe left side two small nodules, together about the size of a pea, and twoother grafts at the root of the liver and on the mesentery. Several othercases are described, and the general result was that the eclipse wasdelayed and never quite complete, while although the nuptial plumage wasalmost fully developed in the following winter, it retained some eclipsefeathers, and was also delayed and developed slowly. 

Several drakes were castrated in July when in the eclipse condition, andalthough the authors state, in their general conclusions, that this doesnot produce any constant appreciable effect upon the next passage of thebird into winter plumage, they describe one bird so treated which onNovember 18 retained many eclipse feathers: the general appearance of thechestnut area of the breast was eclipse. 

It must be remembered that not only was the castration in these casesincomplete, but also that it was performed on mature birds. Birds differfrom Mammals, firstly, in the difficulty of carrying out completecastration, and secondly, in the fact that the occurrence of puberty isnot so definite, and that immature birds are so small and delicate that itis almost impossible to operate upon them successfully. 

 ASSUMPTION OF MALE CHARACTERS BY THE FEMALE

That male somatic sexual characters are latent in the female is shown bythe frequent appearance of such characters in old age, or in individualcases. The development of hair on the face of women in old age, or afterthe child-bearing period, is a well-known fact. Rorig, [Footnote: 'UeberGeweihbildung und Geweihentwicklung. ' _Arch. Ent. -Mech. _ x. And xi. ] whocarefully studied the antlers of stags, states that old sterile females, and those with diseased ovaries, develop antlers to some degree. Cases ofcrowing hens, and female birds assuming male plumage have long been known, but the exact relation of the somatic changes to the condition of theovaries in these cases is worthy of consideration in view of the resultsobtained by Goodale after removal of the ovaries from ducks. Shattock andSeligmann [Footnote: 'True Hermaphroditism in Domestic Fowl, etc. ' _Trans. Path. Soc. _, Lond. , 57. 1, 1906. ] record the case of a gold pheasant henwhich assumed the full male plumage after the first moult: it had neverlaid eggs or shown any sexual instincts. The only male character which waswanting was that of the spurs. The ovary was represented by a smooth, slightly elevated deep black eminence 1 cm. In length and 1-5 mm. Inbreadth at its upper end. These authors also mention three ducks in maleplumage in which the ovary was similarly atrophied but not pigmented. Theyregard the condition of the ovary as insufficient to explain thedevelopment of the male characters, and suggest that such birds are reallyhermaphrodite, a male element being possibly concealed in a neighbouringorgan such as the adrenal or kidney. This hypothesis is not supported byobservation of testicular tissue in any such case, but by the conditionfound in a hermaphrodite specimen of the common fowl described in thepaper. This bird presented the fully developed comb and wattles and thespurs of the cock, but the tail was quite devoid of curved or sicklefeathers, and resembled that of the hen. Internally there were twooviducts, that of the left side normally developed, that of the rightdiminutive and less than half the full length. The gonad of the left sidehad the tubular structure of a testis, but showed no signs of activespermatogenesis, but in its lower part contained two ova. The organ of theright side was somewhat smaller, it had the same tubular structure, and inone small part the tubules were larger, showed division of nuclei (mitoticfigures), and one of them showed active spermatogenesis. 

In discussing Heredity and Sex in 1909, [Footnote: _Mendel's Principles ofHeredity_. Camb. Univ. Press, 1909. ] Bateson referred to the effects ofcastration as evidence that in different types sex may be differentlyconstituted. Castration, he urged, in the male vertebrate on the wholeleads merely to the non-appearance of male features, not to the assumptionof female characters, while injury or disease of the ovaries may lead tothe assumption of male characters by the female. This was supposed tosupport the view that the male is homozygous in sex, the femaleheterozygous in Vertebrates: that is to say, the female sex-character andthe female secondary sex-characters are entirely wanting in the male. Thisargument assumes that the secondary characters are essentially of sexualnature without inquiring how they came to be connected with sex, and itignores the fact that the influence of castration on such characters is aphenomenon entirely beyond the scope of Mendelian principles altogether. The fact that castration does affect, in many cases very profoundly, somatic characters confined to one sex, proves that Mendelian conceptions, however true up to a certain point, are by no means the whole truth aboutheredity and development. For it is the essence of Mendelism as ofWeismannism that not only sex but all other congenital characters aredetermined in the fertilised ovum or zygote. The meaning of a recessivecharacter in Mendelian terminology is one that is hidden by a dominantcharacter, and both of them are due to factors in the gametes, particularly in the chromosomes of the gametes which come together infertilisation. For example, in fowls rose comb is dominant over single. Adominant is something present which is absent in the recessive: the rosecomb is due to a factor which is absent from the single. The two segregatein the gametes of the hybrid or heterozygote, and if a recessive gamete isfertilised by another recessive gamete the single comb reappears. Butcastration shows that the antlers of stags and other such characters arenot determined in the zygote when the sex is determined, but owe theirdevelopment, partly at least, to the influence of another part of thebody, namely, the testes during the subsequent life of the individual. According to Mendelism the structure and development of each part of thesoma is due to the constitution of the chromosomes of the nuclei in thatpart. The effects of castration show that the development of certaincharacters is greatly influenced in some way by the presence of the testesin a distant part of the body. The Mendelians used to say it wasimpossible to believe in the heredity of somatic modifications due toexternal conditions, because it was impossible to conceive of any means bywhich such modifications could affect the constitution of the chromosomesin the gametes within the modified body. It would have been just aslogical to deny the proved effects of castration, because it wasimpossible to conceive of any means by which the testes could affect thedevelopment of a distant part of the body. 

But this is not all. The supposed fact that female secondary characters inVertebrates are absent in the male is completely disproved for Mammals bythe presence of rudimentary mammary glands in the male. It is true thatsecondary sex-characters are usually positive in the male, while those ofthe female are apparently negative, but in the case of the mammary glandsthe opposite is the case. There is no room for doubt that the mammaryglands are an essentially female somatic sex-character, not only in theirfunction but in the relation between the periodicity of that function andthose of the ovaries and uterus, and it is equally certain from theirpresence in rudimentary condition in the male that they are not absentfrom the male constitution. 

 INFLUENCE OF GONADS DUE TO HORMONES

The existence and the influence of hormones or internal secretions may besaid to have been first proved in the case of the testes, for Professor A. A. Berthold [Footnote: 'Transplantation der Hoden, ' _Archiv. F Anat. U. Phys. _, 1849. ] of Göttingen in 1849 was the first to make the experimentof removing the testicles from cocks and grafting them in another part ofthe body, and finding that the animals remained male in regard to voice, reproductive instinct, fighting spirit, and growth of comb and wattles. Healso drew the conclusion that the results were due to the effect of thetesticle upon the blood, and through the blood upon the organism. Littleattention was paid to Berthold's experiment at the time. The credit ofhaving been the first to formulate the doctrine of internal secretion isgenerally given to Claude Bernard. He discovered the glycogenic functionof the liver, and proved that in addition to secreting bile, that organstores up glycogen from the sugar absorbed in the stomach and intestines, and gives it out again as sugar to the blood. In 1855 he maintained thatevery organ of the body by a process of internal secretion gives upproducts to the blood. He did not, however, discover the action of suchproducts on other parts or functions of the body. Brown-Séquard, in hisaddress before the Medical Faculty of Paris in 1869, was the first tosuggest that glands, with or without ducts, supplied special substances tothe blood which were useful or necessary to the normal health, and in 1889at a meeting of the Société de Biologie he described the experiment he hadmade upon himself by the injection of testicular extract. This was thecommencement of organotherapy. Since that time investigation of the moreimportant organs of internal secretion--namely, the gonads, thyroid, thymus, suprarenals, pituitary, and pineal bodies--has been carried onboth by clinical observation and experiment by a great number ofphysiologists with very striking results, and new hormones have beendiscovered in the walls of the intestine and other organs. 

Here, however, we are more especially concerned with the gonads and otherreproductive organs. A great deal of evidence has now been obtained thatthe influence of the testes and ovaries on secondary sexual characters isdue to a hormone formed in the gonads and passing in the blood in thecourse of the circulation to the organs and tissues which constitute thosecharacters. The fact that transplanted portions of testes in birds (cocksand drakes) are sufficient to maintain the secondary characters in thesame condition as in normal individuals shows that the nexus between theprimary and somatic organs is of a liquid chemical nature and notanatomical, through the nervous system for example. Many physiologists inrecent years have maintained that the testicular hormone is not derivedfrom the male germ-cells or spermatocytes, but from certain cells betweenthe spermatic tubuli which are known as interstitial cells, orcollectively as the interstitial gland. 

The views of Ancel and Bouin, [Footnote: _C. R. Soc. Biol. , iv. _]published in 1903, may be described in large part as theory. They statethat the interstitial cells appear in the male embryo before thegametocytes present distinctive sex-characters. They conclude that theinterstitial cells supply a nutritive material (hormone?), which has aneffect on the sexual orientation of the primitive generative cells. Inaddition to this function, the interstitial cells by their hormone alsogive the sexual character to the soma. When castration is carried out atbirth the male somatic characters do not entirely disappear, because thehormone of the interstitial cells has acted during intrauterine life. Thefunctional independence between the interstitial cells and the seminaltubules is shown by the fact that if the vasa deferentia are closed theseminal gland (_i. E. _ tubules) degenerates while the interstitial cells donot. In the embryo the interstitial gland is large, in the adultproportionately small. 

There is complete disagreement between the results of Ancel and Bouin onthe one hand, and those of Shattock and Seligmann on the other, withregard to the effects of ligature of the vasa deferentia. The latterauthors, as mentioned above, found that after ligature not only thesomatic characters but the testis itself developed normally. Theexperiments were performed on Herdwick sheep and domestic fowls. Theystate that on examination the testes were found to be normally developed, and spermatogenesis was in progress. The experiments of Ancel and Bouinwere carried out on rabbits seven to eight weeks old, and consisted inremoving one testis, and ligaturing the vas deferens of the other. Aboutsix months after the operation the testis left _in situ_ was smaller, theseminal tubules contained few spermatogonia, though Sertoli's cells (cellson the walls of the tubules to which the true spermatic cells areattached) were unchanged; while the interstitial cells were enormouslydeveloped, by compensatory hypertrophy in consequence of the removal ofthe other testis. At the same time the male instincts and the othergenerative organs were unchanged. In a few cases, however, Ancel and Bouinobserved atrophy of the interstitial cells as well as the spermatic cells. They believe this is due to the nerves supplying the testis being includedin the ligature. This is rather a surprising conclusion in view of thefact that testicular grafts show active spermatogenesis. It is difficultto understand why nerve connection should be necessary for theinterstitial cells and not for the spermatic, and, moreover, if theinterstitial cells are really the source of the hormone on which thesomatic characters depend, they must be acting in the grafts in which thenerve connections have been all severed. 

The facts concerning cryptorchidism, that is to say, failure of thedescent of the testes in Mammals, seem to show that the hormone of thetestis is not derived from semen or spermatogenesis, for in the testeswhich have remained in the abdomen there is no spermatogenesis, while theinterstitial cells are present, and the animals in some cases exhibitnormal or even excessive sexual instinct, and all the male characteristicsare well marked. It may be remarked, however, in criticism of thisconclusion that the descent of the testes being itself a somatic sexualcharacter of the male, its failure when the interstitial cells are normaland the spermatic cells defective, would rather tend to prove that thedefect of the latter is itself the cause of cryptorchidism. 

Many investigators have found that the Röntgen rays destroy the spermaticcells of the testis in Mammals, leaving the cells of Sertoli, theinterstitial tissue, nerves, and vessels uninjured. Tandler and Gross[Footnote: _Wiener klinische Wochenschrift_, 1907. ] found that the antlersof roebuck were not affected after the testes had been submitted to theaction of the rays, showing that the interstitial cells were sufficient tomaintain the normal condition of the antlers. Simmonds, [Footnote:_Fortschr. A. D. G. D. Röntgenstr. _, xiv. , 1909-10. ] however, found thatisolated seminal tubules remained, and regeneration took place, andconcludes that both spermatic cells and interstitial cells take part inproducing the testis hormone. The conclusions of two other investigatorshave an important bearing on this question--namely, that of Miss Boring[Footnote: _Biol. Bull. _, xxiii. 1912. ] that there is no interstitialtissue in the bird's testis, and that of Miss Lane-Claypon, [Footnote:_Proc. Roy. Soc. _, 1905] that the interstitial cells of the ovary arisefrom the germinal epithelium, and are perfectly equipotential with thosewhich form the ova and Graafian follicles. It seems possible, although nosuch suggestion has been made, that the interstitial cells might eithernormally or exceptionally give rise to ova and spermatocytes. Theobservations of Seligmann and Shattock on the relation of spermatogenesisto the development of nuptial plumage in drakes probably receive theirexplanation from the above facts. Spermatogenesis is not the only sourceof the testicular hormone: changes in the secretory activity of theinterstitial cells or spermatocytes are sufficient to account for periodicdevelopment of somatic sex-characters, and the same reasoning applies tothe antlers of stags. 

 THE MAMMARY OR MILK GLANDS

The milk glands in Mammals constitute one of the most remarkable ofsecondary sexual characters. Except in their functional relations to theprimary organs, the ovaries, and to the uterus, there is nothing sexualabout them. They are parts of the skin, being nothing more or less thanenormous enlargements of dermal glands, either sebaceous or sudoriparous. Uterine and mammary functions are generally regarded as essentially femalecharacteristics, and are included in the popular idea of the sex of woman. Scientifically, of course, they are not at all necessary or universalfeatures of the female sex, but are peculiar to the mammalian class ofVertebrates in which they have been evolved. Milk glands, then, aresomatic sex-characters common to a whole class, instead of beingrestricted to a family like the antlers in Cervidae. There is not theslightest trace or rudiment of them in other classes of Vertebrates, suchas Birds or Reptiles. They are not actually sexual in their nature, sincetheir function is to supply food for the young, not to play a part in therelations of the sexes. What is sexual about them is--firstly, that theyare normally fully developed only in the female, rudimentary in the male;secondly, that their periodical development and functional activitydepends on the changes which take place in the ovary and uterus. Manyinvestigators have endeavoured to discover the nature of the nexus betweenthe latter organs and the milk glands. 

That this nexus is of the nature of a hormone is generally agreed, and maybe regarded as having been proved in 1874 when Goltz and Ewald [Footnote:_Pflügers Archiv, _ ix. , 1874. ] removed the whole of the lumbo-sacralportion of the spinal cord of a bitch and found that the mammae in theanimal developed and enlarged in the usual way during pregnancy andsecreted milk normally after parturition. Ribbert [Footnote: _Fortschritteder Medicin, _ Bd. 7. ] in 1898 transplanted a milk gland of a guinea-pig tothe neighbourhood of the ear, and found that its development and functionduring pregnancy and at parturition were unaffected. The effectivestimulus, therefore, is not conveyed through the nervous system, but mustbe a chemical stimulus passing through the vascular system. 

Physiologists, however, are not equally in agreement concerning the sourceof the hormone which regulates lactation. Starling and Miss Lane-Clayponconcluded from their experiments on rabbits that the hormone originated inthe foetuses themselves within the pregnant uterus. In virgin rabbits itis difficult to find the milk glands at all. When found the nipple isminute and sections through it show the gland to consist of only a fewducts a few millimetres in length. Five days after impregnation the glandis about 2 cm. In diameter. Nine days after impregnation the glands havegrown so much that the whole inner surface of the skin of the abdomen iscovered with a thin layer of gland tissue. In six cases by injectingsubcutaneously extracts of foetus tissue Starling and Lane-Clayponobtained a certain amount of growth of the milk glands. The hormone in thecase of the pregnant rabbit is of course acting continuously for the wholeperiod of pregnancy, while the artificial injection took place only oncein twenty-four hours, and the amount of hormone it contained may have beenabsorbed in a very short time. The amount of growth obtainedexperimentally in five weeks was less than that occurring in pregnancy innine days. Extracts of uterus, placenta, or ovary produced no growth, although the ovaries used were taken from rabbits in the middle ofpregnancy. In one experiment ovaries from a pregnant rabbit were implantedinto the peritoneum of a non-pregnant rabbit, but on post-mortemexamination of the latter eleven days later the implanted ovaries werefound to be necrosed and no proliferation of milk gland had taken place. 

The conclusions of Starling and Lane-Claypon were confirmed by Foa, [Footnote: _Archivo d. Fisiologia_, v. , 1909. ] and by Biedl andKönigstein, [Footnote: _Zeitschrift f. Exp. Path. Und Therap_. , 1910. ] Foastates that extracts of foetuses of cows produced swelling of the mammaein a virgin rabbit. 

O'Donoghue, however, concludes from a study of the Marsupial _Dasyurus_that the stimulus which upon the milk glands proceeds from the corporalutea in the ovary. In this animal changes in the pouch occur inpregnancy, which are doubtless also due to hormone stimulation, but whichwe will not consider here. The most important evidence in O'Donoghue'spaper [Footnote: _Quart. Journ. Mic. Sci_. , lvii. , 1911-12. ] is thatdevelopment of the milk glands takes place after ovulation not succeeded bypregnancy; that is to say, when corpora lutea are formed but no fertilisedova or foetus are present in the uterus. In one case eighteen days afterheat, the milk gland was in a condition resembling that found in thestages twenty-four and thirty-six hours after parturition. In anotherspecimen, twenty-one days after heat, the milk glands were still moreadvanced, with distended alveoli and enlarged ducts. The alveoli containeda secretion which was almost certainly milk, O'Donoghue states that theentire series of growth changes in these animals up to twenty-one daysafter heat in identical with that which occurs in normally pregnantanimals. 

O'Donoghue's conclusion is in agreement with that of Basch, [Footnote:_Monatesschr. F. Kinderh. V. _, No. Ix. , Dec. 1909. ] who states thatimplantation of the, ovaries from a pregnant bitch under the skin of theback of a one-year-old bitch that was not pregnant was followed byproliferation of the mammary glands of the latter. After six weeks theglands were considerably enlarged, and after eight weeks they were causedto secrete milk by the injection of extract of the placenta. It has to beremembered, however, that the milk glands undergo considerable growth, especially in the human species, at puberty and at every menstruation, orat oestrus in animals, which correspond to menstruation. In these casesthere is no question of any influence of the foetus, and experiment hasshown that if the ovaries are removed before puberty, the milk glands northe uterus undergo the normal development and menstruation does not occur. According to Marshall to Jolly [Footnote: _Quart. Journ. Exp. Phys. _, i. And ii. , 1906. ] the symptoms of oestrus in castrated bitches were found toresult from the implantation of ovaries from other individuals in thecondition of oestrus. 

Before considering further the question of the corpora lutea as organs ofinternal secretion, we may briefly refer to the origin and structure ofthese bodies and of other parts of the mammalian ovary. The maturefollicle containing the ovum differs from that of other Vertebrates in thefact that it is not completely filled by the ovum and the follicular cellssurrounding it, but there is a cell-free space of large size into whichthe ovum covered by follicular cells projects. In the wall of the follicletwo layers are distinguished, the theca externa, which is more fibrous, and the theca interna, which is more cellular. In the connective tissuestroma of the ovary between the follicles are scattered, or in some casesaggregated, epithelioid cells known as the interstitial cells, and it isstated that the cells of the theca interna are exactly similar to theinterstitial cells. According to Limon [Footnote: _Arch. D'Anat. Micr. _, v. , 1902. ] and Wallart [Footnote: _Arch. F. Gynock_, vi. 271. ] theinterstitial cells are actually derived from those of the theca interna ofthe follicles. Numbers of ova die without reaching maturity, thefollicular cells degenerate, and the follicle becomes filled with thecells of the theca interna, which have a resemblance to those of the truecorpus luteum. These degenerate follicles have been termed spuriouscorpora lutea, or atretic vesicles. The interstitial cells are the remainsof these atretic vesicles. The true corpora lutea arise from follicles inwhich the ova have become mature and from which they have escaped throughthe surface of the ovary. As a result of the escape of the ovum and thecontents of the cell-free space, the follicle contracts and the follicular(so-called granulosa) cells secrete a yellow substance, lutein, andenlarge. Buds from the theca interna invade the follicle and form theconnective tissue of the corpus luteum. 

Somewhat similar processes take place in the ovaries of Teleostean fishes, as I know from my own observations, but no corpora lutea are formed inthese, although the degenerating follicles in course of absorptioncorrespond to corpora lutea. The spawning of Fishes, usually annual, corresponds to ovulation in Mammals, and in the ovary after spawning thenumerous collapsed follicles containing the follicular cells may be seenin all stages of absorption. [Footnote: Cunningham, 'Ovaries ofTeleosteans. ' _Quart. Journ. Mic. Sci. _, vol. Xl. Pt. 1. , 1897. ] At othertimes of the year sections of the ovary show here and there ova whichafter developing to a certain stage die and undergo absorption with theirfollicles. 

In the higher Mammals (Eutheria) the corpora lutea show a special relationin their development to the occurrence of pregnancy, that is to say, theyhave a different history when ovulation is followed by pregnancy to thatwhich they have when the ova, from the escape of which they arise, are notfertilised. When fertilisation occurs the corpus luteum increases in sizeduring the first part of the period of gestation (four months, or nearly ahalf of the whole period in the human species). It then remains withoutmuch change till parturition, after which it shrinks and is absorbed. Whenpregnancy does not occur the corpus luteum is formed, but begins todiminish within ten or twelve days in the human species and is thengradually absorbed. According to O'Donoghue, in the Marsupial _Dasyurus_there seems to be no difference either in the development of the milkglands or of the corpora lutea between the pregnant and the non-pregnantanimal. Sandes [Footnote: _Proc. Lin. Soc. _, New South Wales, 1903. ]showed that in the same species the corpora lutea persisted not onlyduring the whole of pregnancy, which Professor J. P. Hill [Footnote:_Anat. Anz. _, xviii. , 1900. ] estimates at a little over eight days, butduring the greater part of the period of lactation, which according to thesame authority is about four months. In the specimens of _Dasyurus_described by O'Donoghue, in which the milk glands developed afterovulation without ensuing pregnancy, normally developed corpora lutea werepresent in the ovary. Of the five females which he mentions, the firstthree, one with unfertilised ova in the uteri, two five and six days afterheat, could not have been pregnant, but the other two killed eighteen andtwenty-one days after heat might, since pregnancy lasts only eight days, have been pregnant, the young having died at parturition or before. Tomake certain on this point it would have been necessary to examine theovaries and milk glands of females which had been kept separate from amale the whole time. There is no doubt, however, about the development ofthe milk glands in the first three specimens, which were certainly notpregnant. 

It is difficult to reconcile entirely the evidence described by O'Donoghuefrom _Dasyurus_, with that obtained from higher Mammals, although on thewhole there is reason to conclude that the corpora lutea have an importantinfluence on the development of the milk glands. According to Lane-Clayponand Starling, if the ovaries and uteri are removed from a pregnant rabbitbefore the fourteenth day the development of the mammary gland ceases, retrogression takes place, and no milk appears in the gland. If, on theother hand, the operation be performed after the fourteenth day, milkappears within two days after the operation. It is to be concluded fromthis that the cause of _secretion_ of milk is the withdrawal of a stimulusproceeding from ovary or uterus. But O'Donoghue believes that milk issecreted in _Dasyurus_ when no pregnancy has occurred. Ancel and Bouin[Footnote: _C. R. Soc. De Biol. _, t. Lxvii. , 1909. ] have shown that thegrowth of the mammary glands was produced in rabbits by the artificialrupture of egg follicles and consequent production of corpora lutea: thegrowth of the glands continued up to the fourteenth day, after whichregression set in. This shows that the development of the milk glands inrabbits is due to the corpora lutea. On the other hand, Lane-Claypon andStarling state that in rabbits the corpora lutea diminish after the firsthalf of pregnancy, while the growth of the milk glands is many timesgreater during the second half than during the first half of the period, and during the second half the ovaries may be removed entirely withoutinterfering with the course of pregnancy or the normal development of themilk glands. It is evident, therefore, that in rabbits, whatever influencethe corpora lutea may have in the first half of pregnancy, they have nonein the second half, and that at this period the essential hormone proceedsfrom the developing foetus or foetal placenta. Again, if it is thewithdrawal of a hormone stimulus which changes the milk gland from growthto secretion, it cannot be the corpora lutea which are exclusivelyconcerned even in _Dasyurus_, for they persist during lactation, whilesecretion begins shortly after parturition. 

Gustav Born suggested, and Fränkel tested the suggestion experimentally, that the corpus luteum of pregnancy is a gland of internal secretion whosefunction is to cause the attachment of the ovum in the uterus and thenormal development of uterus and placenta. Fränkel found that removal ofboth ovaries in rabbits between the first and sixth days afterfertilisation prevented pregnancy, and that the same result followed ifthe corpora lutea were merely destroyed _in situ_ by galvano-cautery. Either process carried out between the eighth and twentieth days ofpregnancy causes abortion. 

Lane-Claypon and Starling also found that removal of both ovaries in therabbit before the fifteenth day was apt to cause abortion, but at a laterstage the same operation could be performed without interfering with thecourse of pregnancy. According to these authors numberless instances provethat in women double ovariotomy does not necessarily interfere with thecourse of pregnancy or the development of the milk glands. Parturition maytake place and be followed by normal lactation. This shows that a hormonefrom the corpora lutea is not necessary either to the uterus or the milkglands, at any rate in the last third of pregnancy, though of course thisdoes not prove that such a hormone is not necessary for the earlier stagesboth of pregnancy and growth of the milk glands. 

The results of Steinach, if confirmed, would prove conclusively that theovaries and testes produce hormones which determine the development of allthe sexual characters, not merely physical but psychical. He adopts theview that the interstitial cells or gland are the source of the activehormone. He claims by transplantation of the gonads in young rats andguinea-pigs to have feminised males and masculised females. The femalesare smaller, and hare finer, softer hair than the males. The testes wereremoved and ovaries implanted in young males. The animals so treated grewless than the merely castrated specimens, and therefore when full-grownresembled females in size. In the young state both sexes have fine, softhair, the feminised males had the same character, like the normal females. They also developed teats and milk glands like the females, and weresought and treated as females by the normal males. When the implantedovaries are able to resist the influence of their new surroundings, thefemale interstitial gland, which Steinach calls the puberty gland, develops so much that an intensification of the female character takesplace: the animals are smaller than normal females, the milk glandsdevelop and secrete milk, which can be easily pressed out, and if youngare given to them they suckle them and show all the maternal instincts. 

Why the ovary in normal circumstances only when in the gravid conditioncalls forth this perfection of femaleness is to be shown in a laterpublication. By acting with Röntgen rays on the region where the ovarieslie, Steinach and his colleague Holzknecht brought about all the symptomsof pregnancy, development of teats and milk glands, secretion of milk, andgreat growth of the uterus in all its layers. 

Masculising of females was much more difficult than feminising of malesbecause the testicular tissue was less resistent, and could not be graftedso easily. When it succeeded, however, degeneration of the seminal tubulestook place, with increase of the interstitial or Leydig's cells. Thevaginal opening in rats disappeared, partly or completely. The sexualinstincts became male, the animals recognised a female in heat from onethat was not, and attempted to copulate. 

Steinach considers that he has proved from results that sex is not fixedor predetermined but dependent on the puberty gland. By sex here heobviously means the instincts and somatic characters, for sex in the firstinstance, as we have already pointed out, means the difference betweenovary and testis, between ova and spermatozoa. It is difficult to acceptall Steinach's results without confirmation, especially those which showthat the feminised male is more female than the normal female. Such aconclusion inevitably suggests that the investigator is proving too much. 

The subject of the influence of hormones from the gonads is mentioned, butnot fully discussed, in a volume by Dr. Jacques Loeb, entitles _TheOrganism as a Whole_. [Footnote: Putnam's Sons, 1916. ] Loeb entirely omitsthe problem of the _origin_ of somatic sex-characters, and fails toperceive that the fact that such characters are dependent to a markeddegree on hormones derived from the gonads, together with their relationto definite habits and functions connected with the behaviour of the sexesto each other, is proof are these characters are not gametogenic, but wereoriginally due to external stimulation of particular parts of the soma. 

CHAPTER IV

 Origin Of Somatic Sex-Characters In Evolution

In his _Mendel's Principles of Heredity_, 1909, Bateson does not discussthe nature of somatic sex-characters in general, but appears to regardthem as essential sex-features, as male or female respectively. Asmentioned above, he argues from the fact that injury or disease of theovaries may lead to the development of male characters in the female, thatthe female is heterozygous for sex, and from the supposed fact thatcastration of the male leads merely to the non-appearance of male somaticcharacters, that the female sex-factor is wanting in the male. He does notdistinguish somatic sex-characters from primary sex-factors, and discussescertain cases of heredity limited by sex as though they were examples ofthe same kind of phenomenon as somatic sex-characters in general. One ofthese cases is the crossing by Professor T. B. Wood of a breed of sheephorned in both sexes with another hornless in both sexes. In the _F1_generation the males were horned, the females hornless. Here, with regardto the horned character, both sexes were of the same genetic composition, _i. E. _ heterozygous, or if we represent the possession of horns by _H_, and their absence by _h_, both sexes were _Hh_. Thus _Hh[male]_ was hornedand _Hh[female]_ was hornless, or, as Bateson expresses it, the hornedcharacter was dominant in males, recessive in females. Bateson offers noexplanation of this, but it obviously suggests that some trace of theoriginal dimorphism of the sheep in this character was retained in bothhorned and hornless breeds. We may suppose that the factor for horns haddisappeared entirely from the hornless sheep by a mutation, but in thehorned breed another mutation had been a weakening of the influence of thesexual hormones on the development of the character, which, as in all suchcases, is really inherited in both sexes. In the _F1_, when the hornedcharacter in the female is only inherited from one side, the hereditarytendency is not enough to overcome the influence of the absence of thetestis hormone and presence of the ovarian hormone, and so the horns donot develop. The Mendelian merely sees a relation of the character to sex, but overlooks entirely the question of the dimorphism in the originalspecies from which the domesticated breeds are descended. Similarly, withregard to cattle where it has been found that hornlessness is dominant ornearly so in both sexes, no reference is made to the opposite fact thatwild cattle have horns in both sexes and are not dimorphic in thischaracter. 

Bateson proceeds to consider colour-blindness as though its heredity wereof similar kind. He refers to it as a male character latent in the female, remarks that we should expect that disease or removal of the ovaries mightlead to the occasional appearance of colour-blindness in females. He alsodiscusses the case of _Abraxas grossulariata_ and its variety_lacticolor_, and other cases of sex-linked heredity, apparently with theidea that all such cases are similar to those of sexual dimorphism. _A. Lacticolor_ occurs in nature only in the female sex, and when bred with_grossulariata_ [male] produces [male]'s and [female]'s all_grossulariata_, these of course being heterozygous. When the _F1grossulariata_ [male] was bred with the wild _lacticolor_ [female] itproduced both forms in both sexes, and thus _lacticolor_ [male] wasobtained for the first time. When this _lacticolor_ [male] was bred with_F1 grossulariata_[female] it produced all the [male]'s _grossulariata_and all the [female]'s _lacticolor_. Bateson's explanation is that thefemale, according to the Mendelian theory of sex, is heterozygous in sex, the male homozygous and recessive, and that _lacticolor_ is linked withthe female sex-character, _grossulariata_ being repelled by thatcharacter. Thus we have, the _lacticolor_ character being recessive, lact. Male, LL male male x F, gross. Female, GL female male Gametes L male + L male x G male + L female _____________________|______________________ | | GL male male LL male female gross. Male lact. Female

It will be seen that although in the progeny of this mating all the_grossulariata_ were males and all the _lacticolor_ females, yet this caseis by no means similar to that of sexual dimorphism in which thecharacters are normally always confined to the same sex. For the_lacticolor_ character in the parent was in the male, while in theoffspring it was in the female. We cannot say here that in the theoreticalfactors which are supposed to represent what happens, the _lacticolor_character is coupled with the female sex-factor, for we find it with themale sex-character in the _lacticolor_ [male]. It is so coupled only inthe heterozygous _grossulariata_ [female], and at the same time the_grossulariata_ character is repelled. 

According to Doncaster [Footnote: _Determination of Sex_, Camb. Univ. Press, 1914. ] sex-limited, or as it is now proposed to call it sex-linked, transmission in this case means that the female _grossulariata_ transmitsthe character to all her male offspring and to none of the female, while aheterozygous male _grossulariata_ mated with _lacticolor_ female transmitsthe character equally to both sexes: that is to say, the heredity iscompletely sex-limited in the female but not at all in the male. This isevidence that the female produces two kinds of eggs, one male producingand the other female producing. 

With regard to the ordinary form of colour-blindness, Bateson's firstexplanation was that it was like the horns in the cross-bred sheep, dominant in males, recessive in females. About 4 per cent. Of males inEuropean countries are colour-blind, but less than 1/2 per cent. Offemales. Affected males may transmit the defect to their sons but not totheir daughters: but daughters of affected persons transmit the defectfrequently to their sons. Bateson gives [Footnote: _Mendel's Principles ofHeredity_, 1909. ] a scheme of the transmission, but corrects this in anote stating that colour-blindness does not descend from father to son, unless the defect was introduced by the normal sighted mother also, _i. E. _was carried by her as a recessive. The fact that unaffected males do nottransmit the defect shows, according to Bateson, that it is due to theaddition of a factor to the normal, not to omission of a factor. 

According to later researches as quoted by Doncaster, colour-blindness isdue to the loss of some factor which is present in the normal individual. The normal male is heterozygous for this normal factor. If we denote thepresence of the normal factor by _N_ and its absence or recessive by _n_, then the male is _Nn_, while the female is homozygous or _NN_. But inaddition to this it is the male in this case which is heterozygousfor sex, and _n_ goes to the male-producing sperms, _N_ to thefemale-producing. Thus in the mating of normal man with normal woman thetransmission is as follows:--

 Nn (male) x NN (female) Gametes n (male) + N (female) x N + N

 n (male) + N N (female) + N | | Nn (male) NN (female)

That is all offspring normal, but the males again heterozygous. 

An affected male has the constitution _nn_, and if he marries a normalwoman the descent is as follows:--

 nn (male) x NN (female) Gametes n (male) + n (female) x N + N

 n (male) + N N (female) + N | | nN (male) nN (female)

When a normal male is mated with a heterozygous _nN_ female we get

 nN (male) x nN (female) Gametes n (male) + N (female) x n + N ______________________|______________________ | | | | nn (male) nN (male) nN (female) NN (female)

that is, half the sons are normal and half colour-blind, while half thefemales are homozygous and normal, and the other half heterozygous andnormal. 

T. H. Morgan [Footnote: _A Critique of the Theory of Evolution. _] hasobserved a number of cases of sex-linked inheritance in the mutationswhich occurred in his cultures of _Drosophila_. The eye of the wildoriginal fly is red, one of the mutants has a white eye, _i. E. _ the redcolour and its factor are absent. When a white-eyed male is mated to ared-eyed female all the offspring have red eyes. If these are bred _interse_, there are, as in ordinary Mendelian cases, three red-eyed to onewhite-eyed in the _F2_ generation, but white eyes occur only in the males, in other wards half the males are white-eyed. On the other hand, when awhite-eyed _female_ is mated to a red-eyed male all the daughters have redeyes, and all the sons white eyes. This has been termed crisscrossinheritance. If these are bred together the result in _F2_ is equalnumbers of red-eyed and white-eyed females, and equal numbers of red-eyedand white-eyed males. The ration of dominant to recessive is 2 to 2instead of the usual Mendelian ration of 3 to 1. 

According to Morgan the interpretation is as follows: In the nucleusof the female gametocytes there are two _X_ chromosomes related to sex, in those of the male there is one _X_ chromosome and one _Y_ chromosomeof slightly different shape. The factor for red eye occurs in thesex-chromosomes, that is to say, according to this theory, thesex-chromosome does not merely determine sex but carries other factorsas well, and this fact is the explanation of sex-linked inheritance. Thefactor for red eye then is present in both _X_ chromosomes of the wildfemale, absent from both _X_ and _Y_ chromosomes of the white-eyed male. The gametes of the female each carry one _X_ red chromosome, of those ofthe male half carry an _X_ white chromosome, and half the _Y_ whitechromosome. The fertilised female ova therefore carry an _X_ redchromosome + an _X_ white chromosome, the male producing ova one _X_ redchromosome and one _Y_ white chromosome. They are all therefore red-eyed, but heterozygous--that is, the red eye is due to one red-eye factor, nottwo. When the _F1_ are bred together, half the female gametes carry one_X_ red chromosome, the other half one _X_ white chromosome; half the malegametes carry one _X_ red chromosome, the other half one _Y_ whitechromosome. The fertilisations are therefore one _X_ red _X_ red, one _X_red _X_ white, one _X_ red _Y_ white, and one _X_ white _Y_ white. Theselast are the white-eyed males. The two different crosses are representeddiagrammatically below, the dark rod representing the _X_ red chromosome, the clear rod the _X_ white chromosome, and the bent clear rod the _Y_white chromosome. 

According to Morgan, the heredity of colour-blindness in man is to beexplained exactly in the same way as that of white eye in _Drosophila_. A colour-blind man married to a normal (homozygous) woman transmits thepeculiarity to half his grandsons and to none of his grand-daughters. Colour-blind women are rare, but in the few cases known where such womenhave married normal husbands the defect has appeared only in the sons, asin the second of the diagrams below. 

 Parents Red-eyed male White-eyed female XR XR x XW YW

 F1 Red-eyed male Red-eyed female XR XW XR YW

 F2 Red-eyed male Red-eyed male Red-eyed female White-eyed female XR XR XW XR XR YW XW YW Homozygous. Heterozygous. Heterozygous. Homozygous. 

 White-eyed male Red-eyed female XW XW x XR YW

 F1 Red-eyed male White-eyed female XW XR XW YW

 F2 White-eyed male Red-eyed male White-eyed female Red-eyed female XW XW XR XW XW YW XR YW Homozygous. Heterozygous. Homozygous. Heterozygous. 

It must be explained that according to this theory the normal male isalways heterozygous, because the _Y_ chromosome never carries any otherfactor except that for sex; it is thus of no more importance than theabsence of an _X_ chromosome which occurs in those cases where the malehas one sex-chromosome and the female two. According to the researches ofvon Winiwarter [Footnote: 'Spermatogénèse humaine, ' _Arch. De Biol. _, xxvii. , 1912. ] on spermatogenesis in man, the latter is actually the casein the human species. This investigator found that there were 48chromosomes in the female cell, 47 in the male; after the reductiondivisions the unfertilised ova had 24 chromosomes, half the spermatids 24and half 23, so that sex is determined in man by the spermatozoon. 

Morgan believes that the heredity of haemophilia (the constitutionaldefect which prevents the spontaneous cessation of bleeding) follows thesame scheme, and also at least some forms of stationary night-blindness--that is, the inability to see in twilight. 

We may mention a few other in animals, referring the reader for a fulleraccount to the works cited. One example in the barred character of thefeathers in the breed of fowls called Plymouth Rock. In this the female isheterozygous for sex as in _Abraxas grossulariata_, and the barredcharacter is sex-linked. When a barred hen is crossed with an unbarredcock all the male offspring are barred, all the females plain. On theother hand, if a barred cock is crossed with an unbarred hen, the barredcharacter appears in all the offspring, both and females. The female thustransmits the character only to her sons. If we represent the barredcharacter by _B_, and its absence by _b_, we can represent the heredity asfollows:--

 BARRED FEMALE WITH UNBARRED MALE

 B female b male X b male b male

 Bb male bb female

 Barred male. Unbarred female. Heterozygous. Homozygous. 

 B male B male X b female b male

 B male b female b male b male

 Barred female. Barred male. Heterozygous. Heterozygous. ]

This case is thus exactly similar to that of _Abraxas grossulariata_ and_A. Lacticolor_. The barred character like _grossulariata_ is dominant, the unbarred recessive, and to explain the results it is necessary toassume that the female is not only heterozygous for the barred character, but also for sex, with the female sex-factor dominant. The recessivecharacter in this case is linked to the female sex chromosome, or, as Bateson described it, the dominant character is repelled by thesex-factor. We may make a diagram of the kind given by Morgan if we usea rod of different shape for the female-producing sex-chromosome, and usethe black rod for the dominant character:--

 BARRED female x unbarred male BX uY uX uX | \/ | | /\ | BX uX uY uX BARRED male unbarred female Heterozygous Homozygous

 BARRED male x unbarred female BX BX uX uY | \/ | | /\ | BX uX BX uY BARRED male BARRED female Heterozygous Heterozygous

Another case is that of tortoise-shell, _i. E. _ black and yellow cats. Thetortoise-shell with very rare exceptions is female, the corresponding malebeing yellow, without any black colour. Doncaster found that a yellow malemated to a black female produced black male offspring and tortoise-shellfemales. When a black male is mated to a yellow female, the female kittensare tortoise-shell as before, but the males yellow. The Mendelianhypothesis which explains these results is that the male is alwaysheterozygous, or has only one colour factor whether yellow or black, andtransmits these colours only to his daughters, while the female has twocolour factors, either _BB_, _YY_, or _BY_. Thus the crosses are:--

 YELLOW male x BLACK female YO male BB female | \/ | | /\ | YB female BO male Tortoise-shell female BLACK male

 BLACK male x YELLOW female BO male YY female | \/ | | /\ | BY female YO male Tortoise-shell female YELLOW male

The sex must be determined therefore by the spermatozoa, as in the case ofcolour-blindness, etc. , in man, and the colour factor must always be inthe female-producing sperm. 

 SEXUAL DIMORPHISM

It is obvious from the above facts that however interesting and importantsex-linked heredity may be, it is not the same thing as the heredity ofsecondary sexual characters, and does not in the least explain sexualdimorphism. In the first place, the term sex-linked does not meanoccurring always exclusively in one sex, but the direct contrary--transmitted by one sex to the opposite sex--and in the second place thereis no suggestion that the development of the character is dependent in anyway on the presence or function of the gonad. The problem I am proposingto consider is what light the facts throw on the origin of the secondarysexual characters in evolution. In endeavouring to answer this questionthere are only two alternatives: either the characters are blastogenic--that is, they arise from some change in the gametocytes occurringsomewhere in the succession of cell-divisions of these cells--or theyarise in the soma and are impressed on the gametocytes by the influence ofthe soma within which these gametocytes are contained--that is to say, they are somatogenic. That characters do originate by the first of theseprocesses may be considered to be proved by recent researches, and suchcharacters are called mutations. There can be little doubt that the so-called sex-linked characters, of which examples have been given above, have originated in this way, and that their relation to sex is part of themutation. According to T. H. Morgan, it is simply due to the fact thatthe determinants for such characters are situated in the sex-chromosome. Morgan, however, also states that a case of true sexual dimorphism aroseas a mutation in his cultures of _Drosphilia_. The character was eosincolour in the eye instead of the red colour of the eye in the originalfly. In the female this was dark eosin colour, in the male yellowisheosin. But this case differs from the characters particularly underconsideration here in two points: (1) there is no suggestion that it wasadaptive, (2) or that it was influenced by hormones from the gonads. 

No character whose development is dependent in greater or less degree onthe stimulation of some substance derived from the gonads can haveoriginated as a mutation, because the term mutation means a new characterwhich develops in the soma as a result of the loss or gain of some factoror determinant in the chromosomes. To say that certain mutations consistof new factors which only the development of characters in the soma whenthe part of the soma concerned is stimulated by a hormone, is a mereassertion unsupported at present by any evidence. As an example of the wayin which Mendelians misunderstand the problem to be considered, I mayrefer to Doncaster's book, _The Determination of Sex_ [Footnote: Camb. Univ. 1914, p. 99. ] in which he remarks: 'It follows that the secondarysexual characters cannot arise simply from the action of hormones; theymust be due to differences in the tissues of the body, and the activity ofthe ovary or testis must be regarded rather as a stimulus to theirdevelopment than as their source of origin. ' This seems to imply a seriousmisunderstanding of the idea of the action of the hormones from the gonadsand of hormones in general. No one would suggest that the hormones fromthe testis should be regarded as in any sense the origin of the antlers ofa stag. If so, why should not antlers equally develop in the stallion orin the buck rabbit, or indeed in man? How far Doncaster is right inholding that the soma is different in the two sexes is a question alreadymentioned, but it is obvious that in each individual the somatic sexualcharacters proper to its species are present potentially in itsconstitution by heredity--in other words, as factors or determinants inthe chromosomes of the zygote from which it was developed; but the normaldevelopment of such characters in the individual soma is either entirelydependent on the stimulus of the hormone of the gonad or is profoundlyinfluenced by the presence or absence of that stimulus. The evidence, aswe have seen, proves that, at any rate in the large number of cases wherethis relation between somatic sex-characters and hormones produced by thereproductive organs exists, the characters are inherited by both sexes. Inone sex they are fully developed, in the other rudimentary or wanting. Butthe sex, usually the female, in which they are rudimentary or wanting iscapable of transmitting them to offspring, and also is capable ofdeveloping them more or less completely when the ovaries are removed, atrophied or diseased. If we state these facts in the terms of our presentconceptions of chromosomes and determinants or factors, we must say thatthe factors for these characters are present in the chromosomes of bothmale and female gametes. The question then is, how did these factorsarise? If they were mutations not caused by any influence from theexterior, what is the reason why these particular characters which alonehave an adaptive relation to the sexual or reproductive habits of theanimal are also the only characters which are influenced by the hormonesof the reproductive organs? The idea of mutations implies neither anexternal relation nor an internal relation in the organ or character; butthese characters have both, the external relation in the function theyperform in the sexual life of the individual, the internal relation in thefact that their development is affected by the sexual hormones. There isno more striking example of the inadequacy of the current conceptions ofMendelism and mutation to cover the of bionomics and evolution. 

The truth is that facts and experiments within a somewhat narrow fieldhave assumed too much importance in recent biological research. Noincrease in the number of facts or experimental results of a particularclass will compensate for the want of sound reasoning and a comprehensivegrasp of the phenomena to be explained. The coexistence of the externaland the internal relation in the characters we are considering suggeststhat one is the cause of the other, and as it is obvious that the relationfor instance of a stag's antlers to a testicular hormone could not verywell be the cause of the use of the antlers in fighting, the reasonablesuggestion is that the latter is the cause of the former. We have alreadyseen that the development and shedding of the antler are processes ofessentially the same kind physiologically, or pathologically, as thesewhich can be and are occasionally produced in the individual soma bymechanical stimulus and injury to the periosteum. The fact that a hormonefrom the testis affects the development of the antler, as well as ourknowledge of hormones in general, suggests a special theory of theheredity of somatic modifications due to external stimuli. Physiologistsare apt to look for a particular gland to produce every internalsecretion. But the fact that the wall of the intestine produces secretion, which carried by the blood causes the pancreas to secrete, shows that aparticular gland is not necessary. There is nothing improbable insupposing that a tissue stimulated to excessive growth by externalirritation would give off special substances to the blood. We know thatliving tissues give off products, and that these are not merely pure CO2and H2O, but complicated compounds. The theory proposed by me in 1908 wasthat we have within the gonads numerous gametocytes whose chromosomescontain factors corresponding to the different parts of the soma, and thatfactors or determinants might be stimulated by products circulating in theblood and derived from the parts of the soma corresponding to them. Thereis no reason to suppose that an exostosis formed on the frontal bone as aresult of repeated mechanical stimulation due to the butting of stagswould give off a special hormone which was never formed in the bodybefore, but it would probably in its increased growth give off anincreased quantity of intermediate waste products of the same kind as thetissues from which it arose gave off before. These products would act as ahormone on the gametocytes, stimulating the factors which in the nextgeneration would control the development of the frontal bone and adjacenttissues. 

The difficulty of this theory is one which has occurred to biologists whohave previously made suggestions of a connexion between hormones andheredity--namely, how hormones or waste products from one part of the bodycould differ from these from the same tissue in another part of the body. If there were no special relation, hypertrophy of bone on one part of thebody such as the head, would merely stimulate the factor for the wholeskeleton in the gametocytes, and the result would merely be an increaseddevelopment of the whole skeleton. On the other hand, we have the evidentfact that a number of chromosomes formed apparently of the same substance, by a series of equal chromosome divisions determine all the variousspecial parts of the complicated body. This is not more difficult tounderstand than that every part of the body should give off specialsubstances which would have a special effect on the corresponding parts ofthe chromosomes. We know that skin glands in different parts of the bodyproduce special odours, although all formed of the same tissue and allderived from the epidermis. It seems not impossible that bones ofdifferent parts of the body give off different hormones. If the factors inthe gametes were thus stimulated they would, when they developed in a newindividual, product a slightly increased development of the part which washypertrophied in the parent soma. No matter how slight the degree ofhereditary effect, if the stimulation was repeated in every generation, asin the case of such characters as we are considering it undoubtedly was, the hereditary effect would constantly increase until it was far greaterthan the direct effect of the stimulation. We may express the processmathematically in this way. Suppose the amount of hypertrophy in such acase as the antlers to be _x, _ and that some fraction of this isinherited. Then in the second generation the same amount of stimulationtogether with the inherited effect would produce a result equal to_x+x/n_. The latter fraction being already hereditary, a new fraction_x/n_ would be added to the heredity in each generation, so that after _m_generations the amount of hereditary development would be _x+mx/n_. If _n_were 1000, then after 1000 generations the inherited effect would be equalto _x_. This, it is true, would not be a very rapid increase. But it ispossible that the fraction _x/n_ would increase, for the heredity mightvery well consist not only in a growth independent of stimulation, but inan increasing response to stimulation, so that _x_ itself might beincreasing, and the fraction _x/n_ would become larger in each generation. The death and loss of the skin over the antler, originally duo to thelaceration of the skin in fighting, has also become hereditary, and it iscertainly difficult to conceive the action of hormones in this part of theprocess. All we can suggest is that the hormone from the rapidly growingantler, including the covering skin, is acting on the corresponding factorin the gametocytes for a certain part of every year, and then, when theskin is stripped off, the hormone disappears. The factor then may be saidto be stimulated for a time and then the stimulus suddenly ceases. Thebone also begins to die when the skin and periosteum is stripped off, andthe hormone from this also ceases to be produced. 

The annual shedding and recrescence of the antler, however, is only to beunderstood in connexion with the effect of the testicular hormone. According to my theory there are two hormone actions, the centripetal fromthe hypertrophied tissue to the corresponding factor in the gametocytes, and the centrifugal from the testis to the tissue of the antler or otherorgan concerned. The reason why the somatic sexual character does notdevelop until the time of puberty, and develops again each breeding seasonin such cases as antlers, is that the original hypertrophy due to externalstimulation occurred only when the testicular hormone was circulating inthe blood. The factor in the gametocytes then in each generation actedupon by both hormones, and we must suppose that in some way the result wasproduced that the hereditary development of the antler in the soma onlytook place when the testicular hormone was present. It is to be rememberedthat we are unable at present to form a clear conception of the processof development, to understand how the simple fertilised ovum is able bycell-division and differentiation to develop into a complicated organismwith organs and characters predetermined in the single cell whichconstitutes the ovum. If we accept the idea that characters arerepresented by particular parts of the chromosomes, according to Morgan'sscheme, our theory of development is the modern form of the theory ofpreformation. When in the course of development the cells of the head fromwhich the antlers arise are formed, each of these cells must be supposedto contain the same chromosomes as the original ovum from which the cellshave descended by repeated cell-division. The factors in these chromosomescorresponding to the forehead have been stimulated while in the parentanimal by hormones from the outgrowth of tissue produced by externalmechanical stimulation, while at the same time they were permeated by thetesticular hormone produced either by the gametocytes themselves or byinterstitial cells of the testis. When the head begins to form in theprocess of individual development, the factors, according to my theory, have a tendency to form the special growth of tissue of which theincipient antler consists, but part of the stimulus is wanting, and is notcompleted until the testicular hormone is produced and diffused into thecirculation--that is to say, when the testes are becoming mature andfunctional. 

I do not claim that this theory in complete--it is impossible tounderstand the process completely in the present state of knowledge--but Imaintain that it is the only theory which affords any explanation of theremarkable facts concerning the influence of the hormones from thereproductive organs on the development of secondary sexual characters, while at the same time explaining the adaptive relation of thesecharacters or organs to the sexual habits of the various species. On themutation hypothesis, adaptation is purely accidental. T. H. Morganconsiders that the appearance of two slightly different shades of eyecolour in male and female in a culture of a fruit-fly in a bottle issufficient to settle the whole problem of sexual dimorphism, and tosupersede Darwin's complicated theory of sexual selection. The possibilityof a Lamarckian explanation he does not even mention. He would doubtlessassume that the antlers of stags arose as a mutation, without explaininghow they came to be affected by the testicular hormone, and that when theyarose the stags found them convenient as fighting weapons. But thecomplicated adaptive relations are not to be disposed of by the simpleword mutation. The males have sexual instincts, themselves dependent onthe testicular hormone, which develop sexual jealousy and rivalry, and theRuminants fight by butting with their heads because they have no incisorteeth in the upper jaw, or tusks, which are used in fighting in otherspecies. Doubtless, mutations have occurred in antlers as in othercharacters; in fact all hereditary characters are subject to mutation. This in the most probable explanation, not only of the occasionaloccurrence of hornless individual stags, but of the differences betweenthe antlers of different species, for there is no reason to believe thatthe special character of the antler in each species is adapted to aspecial mode of fighting in each species. 

The different structure of the horns of the Bovine and Ovine Ruminants is, in my view, the result of a different mode of fighting. If we suppose thatthe fighting was slower and less fierce in the Bovidae, so that the skinover the exostosis was subject to friction but not lacerated, the resultwould be a thickening of the horny layer of the epidermis as we find it, and the fact that the skin and periosteum are not destroyed explains whythe horns are not shed but permanent. 

There is a tendency among Mendelians and mutationists to overestimate theimportance of experiments in comparison with reasoning, either inductiveor deductive. Bateson, however, has admitted that Mendelian experimentsand observations on mutation have not solved the problem of adaptation. Itseems to be demanded, nevertheless, that characters must be producedexperimentally and then inherited before the hereditary influence ofexternal stimuli can be accepted. Kammerer's experiments in this directionhave been sceptically criticised, and it must be granted that the evidencehe has published is not sufficient to produce complete conviction. Butexperiments of this kind are from the nature of the case difficult if notimpossible. There is, however, another method--namely, to take a characterwhich is certainly to some extent hereditary, and then to ascertain byexperiment if it is 'acquired. ' If it be proved that a hereditarycharacter was originally somatogenic, it follows that somatogeniccharacters in time become hereditary. This is the reasoning I have used inreference to my experiments on the production of pigment on the lowersides of Flat-fishes, and I obtained similar evidence with regard to theexcessive growth of the tail feathers in the Japanese Tosa-fowls, [Footnote: 'Observations and Experiments on Japanese Long-tailed Fowls, '_Proc. Zool. Soc. _, 1903. ] which is a modification of a secondary sexualcharacter. In these fowls the feathers of the tail in the hens are onlyslightly lengthened. 

I learned from Mr. John Sparks, who himself brought specimens of the breedfrom Japan, that the Japanese not only keep the birds separately on highperches in special cages, but pull the tail feathers gently every morningin order to cause them to grow longer. One question which I had toinvestigate on my specimens, hatched from eggs obtained from Mr. Sparks, was the relation of the growth of the feathers to the moult which occursin ordinary birds. My experiment consisted in keeping two cocks, A and B, the first of which was left to itself, while in the second the featherswere gently pulled by stroking between the finger and thumb from the baseoutwards. The feathers in the tail were seven pairs of rectrices, two rowsof tail coverts, anterior and posterior, four or five pairs in each row, anumber of transition feathers: all these were steel-blue, almost black; infront of them on the saddle were a number of reddish yellow, very slendersaddle hackles. 

In September 1901, when the birds ware just over three months old, theadult feathers of the tail were all growing. The growing condition can bedistinguished by the presence of a horny tubular sheath extending up thebase of the feather for about one inch. When growth ceases this sheath isshed. In cock A growth continued till the end of the following March, whenthe longest feathers, the central rectrices, 2 feet 4-1/2 inches long. Oneof the feathers--namely, one of the anterior tail coverts--wasaccidentally pulled out on 11th February 1902, when it was 15-1/4 incheslong and had nearly ceased to grow and formed its quill, and itimmediately began to grow again and continued to grow till the followingSeptember, when it was accidentally broken off at the base: it was then 18inches (44. 5 cm. ) long. 

The effect of stroking in cock B was to pull out from time to time one ofthe growing feathers. Of the original feathers, one, the left centralposterior covert, continued to grow till 13th July 1902, when it was 2feet 9-1/2 inches long without the part contained in the follicle. All thefeathers pulled out immediately commenced to grow again, except the lasttwo pulled out 27th May and 13th July, which did not grow again till thefollowing moulting season, in September. 

The first right central rectrix in cock B was accidentally pulled out on13th April 1902, when it was 2 feet 9-7/8 inches long. Its successor beganto grow immediately, and in course of time pieces of it were broken offaccidentally without injury to the base in the socket, which continued togrow until 16th June 1905, when it torn out of its socket. The totallength of the feather with the pieces previously broken off, which weremeasured and preserved, was 11 feet 5-1/2 inches. It therefore continuedto grow without interruption for three years and two months at an averagerate of 3. 6 inches per month. 

In cock A only four of the short outer rectrices were moulted in thebeginning of September 1902: the longer feathers--namely, centralrectrices and tail coverts--which ceased to grow naturally in the springof 1902, were not moulted till the beginning of October. This shows thegreat importance of pulling out the feathers as soon as they show signs ofceasing to grow, in order to obtain the abnormally long feathers. Thecentral rectrices continued to grow till the beginning of September 1903, when that of the left side was 3 feet 6 inches long, that of the rightabout an inch shorter. The coverts had ceased to grow of their own accordsome time before this, and the central ones of the posterior row wereabout 3 feet long. 

As it seemed possible that there was some natural congenital difference ingrowth of feathers between cocks A and B, I commenced early in March 1903to pull and stroke the feathers of the left side only in cock A, leavingthose of the right side untouched. On 30th July on the left side thecentral rectrix and the first and second posterior coverts were stillgrowing, on the right side the central rectrix was also growing, but thefirst and second posterior coverts had ceased growth and formed theirquills. The first posterior covert on the left or pulled side was 3 incheslonger than that of the right. The second posterior covert on the leftside was still longer. The first and second posterior coverts of left sidedid not cease growth till 26th August. On 2nd September the left centralrectrix was almost at the end of its growth, the right had ceased to growa little before. The left was about an inch longer than the right. Thusboth in length in duration of growth the feathers of the pulled side werelonger than those of the right, and this was the result of treatmentcontinued only six months, and commenced some months after the feathershad begun to grow. I have no doubt, however, that the pulling out of thefeather as soon as it shows signs of forming quill, so that its successorat once grows again, is even more important in producing the great lengthof feather than the stroking of the feather itself. 

In this case, then there is no doubt (_a_) that the long-tailed birds areartificially treated with the utmost care and ingenuity by the Japanese, who produced them; (_b_) that the mechanical stimulus in my experimentsdid cause the feathers to grow for a longer period and attain greaterlength; (_c_) that the tendency to longer growth is, even when notreatment is applied, distinctly inherited. It is a legitimate and logicalconclusion that the inherited tendency is the result of the artificialtreatment. No other breed of fowls shows such excessive growth of tailfeathers. It may be admitted that individuals differ considerably in theircongenital tendency to greater growth, _i. E. _ greater length of the tailfeathers, but according to my views this is not contradictory to the mainconclusion, for every hereditary character shows individual variation. 

It may be pointed out here that on the Lamarckian theory the conception ofadaptations is not teleological: they do not exist for a certain purpose, but are the result of external stimulations arising from the actions andhabits of the organism. The latter conception is the more general, forcases of somatic sexual characters exist which cannot be said to have ause or function. For example, the comb and wattles of _Gallus_ aresexually dimorphic, being in the original species larger in the cock thanin the hen. There is no convincing evidence that these appendages areeither for use or ornament. They are, in fact, a disadvantage to the bird, being used by his adversary to take hold of when he strikes. The firstthing that happens when cocks fight is the bleeding and laceration of thecomb, as they peck at each other's heads. This laceration of the skin is, in my view, the primary cause of the evolution of these structures, leading to hypertrophy. But in this, as in other cases, the hereditaryresult is regular, constant, and symmetrical, while the immediate effecton the individual is doubtless irregular. 

CHAPTER V

 Mammalian Sexual Characters Evidence Opposed To The Hormone Theory

Perhaps the most remarkable of all somatic sexual characters are thosewhich are almost universal in the whole class of Mammalia, the mammaryglands in the female, the scrotum in the male. We have considered theevidence concerning the relation of the development and functional actionof the milk glands to hormones arising in the ovary or uterus, now we haveto consider the origin of the glands and of their peculiar physiology inevolution. The obvious explanation from the Lamarckian point of view, andin my opinion the true one, is that they owed their origin at thebeginning to the same stimulation which is applied to them now in everyfemale mammal that bears young. There is, as we have seen, a difficulty inexplaining how the occurrence of parturition causes the secretion of milkto begin, but it is certain that the secretion soon stops if the milk isnot drawn from the glands by the sucking action of the offspring, or theartificial imitation of that action. A cow that is not milked or milkedincompletely ceases to give milk. When the stimulus ceases, lactationceases. The pressure of the secretion in the alveoli causes the cells tocease to secrete, much in the same way that pressure in the uretersinjures the secretory action of the renal epithelium. In the earliestMammals we may suppose that the young were born in a well-developedcondition, for at first the supply of milk would not have been enough tosustain them for a long time as their only food. We must also suppose thatthe mother began to cherish the young, keeping them in contact with herabdomen. Then being hungry they began to suck at her hair or fur. Theactual development of the milk glands in Marsupials has been described byBresslau [Footnote: Stuttgart, 1901. ] and by O'Donoghue. [Footnote:_Q. J. M. S. _, lvii. , 1911-12. ] The rudiment of the teat is a depression orinvagination of the epidermis from the bottom of which six stout hairsarise. The follicles of these hairs extend down into the derma, and fromthe upper end of the follicle, _i. E. _ near the aperture of theinvagination, a long cellular outgrowth extends down into the derma, branches at its end, and becomes hollow. These branches are the tubules ofthe future milk gland. Another outgrowth from the follicle forms asebaceous gland. Later on the hairs and the sebaceous glands entirelydisappear, and the milk gland alone is left with its tubules and ductsopening into the cavity of the teat. This is clear evidence that the milkgland was evolved in connexion with hairs, and was an enlargement ofglands opening into the hair follicle, but it is difficult to understandwhy a sebaceous gland is developed and afterwards disappears. This wouldseem to indicate that the milk gland was not a hypertrophied sebaceousgland, but a distinct outgrowth, which however had nothing to do withsweat glands. 

That the intra-uterine gestation, or its cessation, were not originallynecessary to determine the functional periodicity of the milk glands isproved by their presence in the Monotremes, which are oviparous. It isevident from the conditions in these mammals that both hair and milkglands were evolved before the placenta. 

It may also be pointed out here that, according to the evidence ofSteinach, in the milk glands at least among somatic sexual charactersthere is no difference between the male and female in the heredity of theorgans. The zygote therefore, whether the sex of it is determined as maleor female, has the same factor for the development of milk glands. On thechromosome theory as formulated by Morgan this factor must be in thesomatic chromosomes and not in the sex-chromosomes, and must be present inevery zygote. All the cells of the body, assuming that somatic segregationdoes not occur, must possess the same chromosomes as the zygote from whichit developed, and whether the sex chromosomes are _XX_ or _XY_ or _X_, there must be at any rate one chromosome bearing the factor for milkglands. The functional development of these depends normally, accordingto the evidence hitherto discovered, on the presence or absence ofhormones from the ovary or from the uterus. 

If we attribute, as in my opinion we must, the primary origin of the milkglands in evolution to the mechanical stimulus of sucking, we may attemptto reconstruct the stages of the evolution of the present relation of theglands to the other organs and processes of reproduction. In the earlieststage represented by the Monotremata or Prototheria, there was nointra-uterine development. We must suppose that in the beginning thesucking stimulus caused both growth and secretion, for at first there wasnothing but sebaceous or sweat glands, and although a mutation might besupposed to have produced larger glands, no mutation could explain theinfluence of hormones on the growth and function of such glands. Thenheredity of the effect of stimulus took place to some slight degree, andthis would occur, according to my theory, only in the presence of thehormone from the ovary in the same condition as that in which themodification was first caused. This would be of course after ovulation, and after hatching of the eggs. In the next stage, if we adopt the modernview that Marsupials are descended from Placental Mammals, the eggs wouldbe retained for increasing periods in the uteri, and would be born in awell-developed condition, since lactation would demand active suckingeffort on the part of the young. The early Placentalia would inherit fromthe Monotreme-like ancestors the development of the milk glands afterovulation, although no sucking was taking place while the young wereinside the uterus. It seems probable that the relation between parturitionand actual milk secretion originated with the sucking stimulus of theyoung after birth. 

There is good evidence that the secretion of milk may continue almostindefinitely under the stimulus of sucking or milking. Neithermenstruation nor gestation put an end to it. Cows may continue to givemilk until the next parturition, and if castrated during lactation willcontinue to yield milk for years. Women also may continue to produce milkas long as the child is allowed to suck, and this has been in some casestwo or three years or even more. Moreover, lactation may be induced by therepeated act of sucking without any gestation. This has happened in mares, virgin bitches, mules, virgin women, and in one woman lactation continueduninterruptedly for forty-seven years, to her eighty-first year, longafter the ovary had ceased to be functional. Lactation has also beeninduced in male animals, _e. G. _ in a bull, a male goat, male sheep, and inmen. [Footnote: Knott, 'Abnormal Lactation, ' _American Medicine_, vol. Ii(new series), 1907. ] We may conclude, therefore, that the secretion ofmilk normally begins by heredity after parturition, and this, inaccordance with what we have learned about hormones in connexion with thereproductive system, is probably the consequence of the withdrawal of thehormone absorbed from the foetus. I do not think it is necessary tosuppose, as do Lane-Claypon and Starling, that the hormone physiologicallyinhibits the dissimilative process and augments the assimilative, and thatthe withdrawal of the hormone at parturition therefore causes thedissimilative process, _i. E. _ secretion of milk. My conclusion is that theprocess of secretion set up by the mechanical stimulus of sucking isinherited as it was acquired, so that it only begins to take place in theindividual in the absence of the hormone from the foetus, which was absentwhen the process was acquired. The growth of the gland during gestationwould then be due to the postponement of the process of secretion inconsequence of the presence of the foetal hormone, and in this way thishormone has become in the course of evolution at once the stimulus togrowth and the cause of the inhibition of secretion. 

This interpretation does not, however, agree with the case of _Dasyurus_. If the foetal hormone is absorbed from the pouch, as I have suggested, inorder to explain the persistence of the corpora lutea during lactation, then the secretion of milk after parturition ought not to take place. Butin this case the sucking stimulus has been applied to the glands after avery short gestation, while the hormone from the foetus is being absorbedin the pouch, and therefore the hereditary correlation between secretionand absence of foetal hormone may be assumed to have been lost in thecourse of evolution. 

We have next to consider the question of the evolution of the corporalutea. If these bodies are formed only in Mammals which have uterinegestation, and not in Prototheria, they cannot be the only essentialsource of the hormone which stimulates the development of the milk glands, since the latter develop in Prototheria. Again it is difficult, it mightbe said impossible, to believe that an accidental mutation gave rise tocorpora lutea the secretion of which caused uterine gestation andultimately the formation of the placenta. It seems more probable that theretention of the originally yolked ova within the oviduct, however thisretention arose, was the essential cause of the formation of the placentaand all the changes which the uterus undergoes in gestation. Theabsorption of nutriment from the walls of the uterus, and the chemical andmechanical stimulation of those walls, might well be the cause of thediversion of nutrition from the ovary, leading gradually to the decline ofthe process of secretion of yolk in the ova. 

The conceptions and the mode of reasoning of the physiologist are verydifferent from those of the evolutionist. The former concludes fromcertain experiments that a given organ of internal secretion has a certainfunction. The corpora lutea, for example, according to one theory areductless glands, the function of whose secretion is to establish ova inthe uterus and promote their development. Another function suggested forthe secretion of the corpora lutea is to prevent further ovulation duringpregnancy. The evolutionist, on the other hand, asks what was the originof this corpora lutea, why should the ruptured ovarian follicles after theescape of the ova in Mammals undergo a progressive development and persistduring the greater part of the whole of pregnancy? It seems obvious thatthe corpora lutea in evolution were a consequence of intra-uterinegestation, for they occur only in association with this condition, and itis impossible to suppose that a mutation could arise accidentally by whichthe ruptured follicles should produce a secretion which would cause thefertilised ova to develop within the oviducts. The developing ovum withinthe uterus may, however, reasonably be supposed to give off somethingwhich is absorbed into the maternal blood, and this something would be ofthe same nature as that which was given off by the ovum while still withinthe ovarian follicle. The presence of this hormone might cause thefollicular cells to behave as though the ovum was still present in thefollicle, so that they would persist and not die and be absorbed. But thisleaves the question, what is lutein and why is it secreted? Lutein is acolouring matter sometimes found in blood-clots, and probably derived fromhaemoglobin. In the corpus luteum the lutein is contained in the cells, not in a blood-clot. 

Chemical investigation shows that the lutein of the corpus luteum isalmost if not quite identical with the colouring matter of the yolk inbirds and reptiles. Escher [Footnote: _Ztschr. F. Physiol. Chem. _, 83(1912). ] found that the lutein of the corpus luteum had the formulaC{40}H{56} and was apparently identical with the carotin of the carrot, while the lutein of egg-yolk was C{40}H{56}O{2} and more soluble inalcohol, less soluble in petroleum ether, than that of the corpus luteum. The difference, if it exists, is very slight, and it is evident that onecompound could easily be converted into the other. Moreover, thehypertrophied follicular cells which constitute the corpus luteum secretefat which is seen in them in globules. The similarity of their contentstherefore to yolk is very remarkable, and it may be suggested that thehormones absorbed from the ovum or embryo in the uterus acts upon thefollicular cells in such a way as to cause them to secrete substanceswhich in the ancestor were passed on to the ovum and formed the yolk. Itmay be urged that this idea is contradictory to the previous suggestionthat the absorption of nourishment by the intra-uterine embryo was thecause of the gradual decline of the process of yolk-secretion by the ovain the ovary, but it is not really so. Originally in the reptilianancestor, or in the Monotreme, the ovum in the follicle secretedyellow-coloured yolk. The materials for this, at any rate, passed throughthe follicle cells, and it is probable that these cells were not entirelypassive, but actively secretory in the process. Substances diffusing fromthe ovum would be present in the follicle cells during this process, andprobably act as a stimulus. The same substances diffusing from the ovumduring its development in the uterus would continue to stimulate thefollicle cells, and thus explain not merely their persistence, but theirsecretory activity. The ovum being no longer present in the ovary, thesecretions would remain in the follicular cells, and the corpus luteumwould be explained. 

If this theory is sound, it would follow that corpora lutea are not formedin cases where the ova are not retained in the oviduct during theirdevelopment. The essential process in the development of these structuresis the hypertrophy and, in some cases at least, multiplication of thefollicular cells in the ruptured follicle. I have already mentioned thatthis process does not occur in Teleosteans whose ovaries were studied byme. These were species of Teleosteans in which fertilisation is external. Marshall, in his _Physiology of Reproduction_, [Footnote: London, 1910, p. 151. ] quotes a number of authors who have published observations on thechanges occurring in the ruptured follicle in the lower Vertebrata, andalso in the Monotremes. According to Sandes, [Footnote: 'The Corpus Luteumof Dasyurus, ' _Proc. Lin. Soc. _, New South Wales, 1903. ] in the latterthere is a pronounced hypertrophy of the follicular epithelium afterovulation, but no ingrowth of connective tissue or blood-vessels from thefollicular wall. Marshall himself examined sections of the corpus luteumof _Ornithorhynchus_ and saw much hypertrophied and apparently fullydeveloped luteal cells, but no trace of any ingrowth from the wall of thefollicle. This fact would appear to be quite inconsistent with the theoryabove proposed, but it must be remembered that the ovum of Monotremes isknown to remain for a short period in the oviduct, or in other words topass through it very slowly, and to absorb fluid from its walls, as shownby the considerable increase in size which the ovarian ovum undergoesbefore it is laid. It would be interesting to know how long therudimentary corpus luteum persists in _Ornithorhynchus_: the period, according to my views, should be very short. It is remarkable that in theresults quoted by Marshall a well-developed corpus luteum was found andexclusively found in the lower Vertebrates which are viviparous. Forexample, among fishes in the Elasmobranchs _Myliobatis_ and _Spinax_; inTeleosteans, in _Zoarces_; in Reptiles, in _Anguis_ and _Seps_. Bühler onthe other hand, confirmed my own negative result with regard to oviparousTeleosteans, and also found no hypertrophy of the follicle in Cyclostomeswhich are also oviparous. In the viviparous forms mentioned there is yolkin the ovum which is retained in oviduct or ovary, but additionalnutriment is also absorbed from the uterine or ovarian walls. In thesecases there is no placenta and generally no adhesion of ovum or embryo towalls of oviduct or ovary. These facts alone would be sufficient todisprove the theory that the corpora lutea are organs producing asecretion whose function is to cause the attachment of the embryo to theuterine mucosa. It is also, in my opinion, unreasonable to suppose thatthe rudimentary corpora lutea of lower viviparous Vertebrates arose as amutation the result of which was to cause internal development of theovum. Habits might easily bring about retention of the fertilised ova forgradually increasing periods, [Footnote: According to Geddes and Thomson(_Evolution of Sex_, 1889), the common grass-snake has been induced underartificial conditions to bring forth its young alive. ] and the correlationbetween the retained developing ova and the hypertrophy of the rupturedfollicles is comprehensible on my theory of the influence of substancesabsorbed by the walls of oviduct or ovary from the developing ovum. 

The case of _Dasyurus_, however, seems inconsistent with this argument, for, as previously mentioned, Sandes found that in this Marsupial thecorpora lutea persisted during the greater part of the period oflactation, which continues for four months after parturition. During thewhole of this time there are no embryos in the uteri, and therefore itmight be urged absorption of hormones from the embryos cannot be the causeof the persistence of corpora lutea in pregnancy. But it seems to me thata complete answer to this objection is supplied by the peculiar relationsof the embryos to the pouch in _Dasyurus_ and other Marsupials. The skinof the pouch while the embryos are in it is very soft, congested, andglandular; at the same time the embryos when transferred to the pouch atparturition are very small, immature, and have a soft delicate skin. Therelation of embryos to pouch in _Dasyurus_, therefore, is closely similarto that of embryos to uterus after the first few days of pregnancy in theEutheria. It is true there is no placenta, but the mouths of the embryosare in very close contact with the teats, and both the skin of the embryosand that of the pouch are soft and moist. If any special substances aregiven off by the embryos in the uterus in ordinary gestation, the samesubstances would continue to be given off by the embryos in the marsupialpouch, and these must be absorbed by the skin of the pouch. In this way itseems to me we have a logical explanation of the fact that the corporalutea in the Marsupial are not absorbed at parturition as in Eutheria. AsSandes says the 'greater part of the period of lactation, ' it would appearthat absorption of the corpora lutea takes place when the young _Dasyurus_have grown to some size, become covered with hair, and are able to leavethe teats or even the pouch at will. Under these conditions it is obviousthat diffusion of chemical substances from the young through the walls ofthe pouch would come to an end. It would be interesting in this connexionto know more of the relation of egg and embryo to the pouch and to thecorpora lutea in _Echidna_. In _Ornithorhynchus_ the eggs are hatched in anest and there is no pouch. 

On this view that the corpora lutea are the result, not the cause, ofintra-uterine gestation, it would no longer be possible to maintain thetheory that the corpus luteum in the human species is the cause by itsinternal secretion of the phenomenon of menstruation. This was the theoryof Born and Fränkel. [Footnote: See Biedl, _Internal Secretory Organs_(Eng. Trans. ), 1912, p. 404. ] Biedl's conclusion is that the periodicdevelopment and disintegration of the uterine mucous membrane in themenstrual cycle is due to the hormone of the interstitial cells of theovary. Leopold and Ravana found that ovulation as a rule coincides withmenstruation, but may take place at any time. Here, again, the problemmust be considered from the point of view of evolution. It can scarcely bedoubted that the thickening and growth of the mucous membrane in themenstrual cycle is of the same nature as that which takes place inpregnancy. When the ovum or ova are not fertilised the development comesto an end after a certain time, differing in different species of Mammals, and the membrane sloughs, returns to its original, state, and then beginsthe same process of development again. 

Menstruation, then, must be interpreted as an abortive parturition, bothin woman and lower Mammals, though in the latter it is not usuallyaccompanied by hemorrhage, and is called pro-oestrus. The question then tobe considered is, what determines parturition and menstruation? Thepresence of the fertilised ovum must have been the original cause of thehypertrophy of the uterine mucous membrane, and in its congenital orhereditary development the chemical substances diffusing from the ova inthe uterus or even in the Fallopian tube may well be the stimulus startingthe hypertrophy. But what determines the end of the pregnancy? Is itmerely the increasing distension of the uterus by the developing foetus?This could scarcely be the case in the Marsupials in which the foetus whenborn is quite minute. Nor can we attribute parturition to renewedovulation, for this occurs in _Dasyurus_ only once a year. All we cansuggest at present is that a certain periodic development takes place byheredity in presence of the hormones exuded by the fertilised ovum and theembryo developed from it. When the ovum or ova, not being fertilised, diethe period of development is (usually) shortened and pro-oestrus ormenstruation occurs. In the dog, however, the period of the oestrus cycleis about the as that of gestation--namely, six months. 

The so-called descent of the testicles occurs exclusively in Mammals, inwhich with a few important exceptions it is universal. This is a veryremarkable case of the change of position of an organ in the course ofdevelopment. The original position of the testis on either side is quitesimilar to that of the same organ in birds or reptiles. The genital ridgeruns along the inner edge of the mesonephros, with which the testiculartubules become connected. The testis, with the mesonephros, forming theepididymis, closely attached to it, projects into the coelom, and withoutlosing its connexion with the peritoneum changes its position graduallyduring development, passing backwards and downwards until it comes to lieover the wall of the abdomen just in front of the pubic symphysis of thepelvic girdle. There the abdominal wall on either side of the middle linebecomes thin and distended to form a pouch, the scrotal sac, into whichthe testis passes, still remaining attached to the peritoneum which linesthe pouch, while the distal end of the vas deferens retains its originalconnexion with the urethra. The movement of the testis can thus beaccurately described as a transposition or dislocation. 

Various causes have been suggested for the formation of the scrotum, butno one has ever been able to suggest a use for it. It has always beenquite impossible to bring it within the scope of the theory of naturalselection. The evolution of it can only be explained either on the theoryof mutation or some Lamarckian hypothesis. The process of dislocation ofthe testis does not conform to the conception of mutation, nor agree withother cases of that phenomenon. A mutation is a change of structureaffecting more or less the whole soma, but showing itself especially insome particular organ or structure. But I know of no mutation occurringunder observation which consisted, not in a change of structure orfunction, but merely in a change of position of an organ from one part ofthe body to another, and moreover a change which takes place by acontinuous process in the course of development. If the testes weredeveloped from the beginning in a different part of the abdomen, theremight be some reason in calling the change a mutation. Moreover, if it isa mutation, why has it never occurred in any other class of Vertebratesexcept Mammals?

In 1903 Dr. W. Woodland published [Footnote: _Proc. Zool. Soc. _, 1903, Part 1. ] a Lamarckian theory of this mammalian feature, the probability ofwhich it seems to me has been increased rather than decreased by theprogress of research concerning heredity and evolution since that date. Dr. Woodland correlated the dislocation of the testes with the specialmechanical features of the mode of locomotion in Mammalia. His words are:'The theory here advocated is to the effect that the descent of the testesin the Mammalia has been produced by the action of mechanical strainscausing rupture of the mesorchial attachments, such strains being due tothe inertia of the organs reacting to the impulsiveness involved in theactivity of the animals composing the group. ' The 'impulsiveness' is thegalloping or leaping movement which is characteristic of most Mammals whenmoving at their utmost speed, as seen, for example, in horses, deer, antelopes, dogs, wolves, and other Ungulata and Carnivora. It is obviousthat when the body is descending to the ground after being hurled upwardsand forwards, the abdominal organs have acquired a rapid movementdownwards and forwards; when the body reaches the ground its movement isstopped suddenly, while the abdominal organs continue to move. The testestherefore are violently jerked downwards away from their attachments andat the same time forward. The check to the forward movement, however, ismomentary, while the body is immediately thrown again upwards andforwards, which by the law of inertia means that the testes are thrownstill more downwards and backwards. There is no reason to suppose, as Dr. Woodland suggests, that any rupture of the mesorchium was the usual resultof these strains, but a constant pull or tension was caused in thedirection in which the testes actually move during development. On thistheory we have to consider (1) how such strains could cause a shifting ofthe peritoneal attachment, (2) why the testes should be supposed to beparticularly affected more than other abdominal organs. The answer to thefirst question is that the strains would cause a growth of the connectingmembrane (mesorchium) at the posterior end, accompanied by an absorptionof it at the anterior end. The answer to the second question is that thetestes are at once the most compact and heaviest organs in the abdomen, and at the same time the most loosely attached. The latter statement doesnot apply to the mesonephros or epididymis which has moved with thetestis, but the latter cannot function without the former, and it may besupposed that the close attachment of the epididymis to the testis hadcome about in the early Mammalia before the change of position wasevolved. 

It is evident that the violent shocks of the galloping or leaping movementdo not occur in Birds, Reptiles, or Amphibia. Ostriches run very fast anddo not fly, but their progression is a stride with each foot alternately, not a gallop. The Anura among the Amphibia are saltatory, but their leapsare usually single, or repeated only a few times, not sustained gallops. The exceptions among the Mammalia still more tend to prove the closecorrespondence between the 'impulsive' mode of progression and thedislocation of the male gonads. In the Monotremata there is no scrotum, the testes are in a position similar to that which obtains in Reptiles, and they are the only Mammals in which these organs are anterior to thekidneys. In locomotion they are sluggish, there is no running or gallopingamong them. _Ornithorhynchus_ is aquatic in its habits, and _Echidna_ isnocturnal and moves very slowly. In Marsupials the scrotum is in front ofthe penis, but really in the same position as in other Mammals--that is, in front of the ventral part of the pelvic girdle. It is the penis whichis different, as the skin around the organ has not united in a ventralsuture below it, while the organ itself has not grown forward adnate tothe abdominal skin as in most other Mammals. The scrotum is alwaysanterior to the origin of the penis, although in the Eutheria apparentlybehind that organ. The larger Marsupials like the kangaroos are eminentlysaltatory, and the others are active in locomotion. The aquatic MammalsSirenia and Cetacea have no scrotum, the testes being abdominal. It isunnecessary to inquire whether this is the original position, or whetherthey are descended from ancestors which had a scrotum: in either case theposition of the testes corresponds to the absence of what Dr. Woodlandcalls impulsiveness in progression. The Fissipedia offer an instructiveexample, for while the Otariidae have the hind feet turned forward and canmove on land somewhat like ordinary Mammals, the Phocidae cannot movetheir hind legs independently or turn them forward, and can only dragthemselves about on land for short distances. In the former the testes aresituated in a well-defined scrotum, in the latter these organs areabdominal. The Phocidae are probably descended from Mammals of theterrestrial type with a scrotum, which has disappeared in the course ofevolution. Perhaps the most curious exception is that of the elephants, inwhich the testes are abdominal. Here, in consequence of their structureand massive shape, locomotion in usually a walk, and though they runoccasionally the gait is a trot, not a sustained gallop, and leaping isout of the question. Sloths which hang from branches upside down haveabdominal testes, but even here they are in a posterior position, between, the rectum and the bladder, so there has apparently been a degree ofdislocation, probably inherited from ancestors with more terrestrialhabits. 

The fact that the ovaries do not occupy normally a position similar tothat of the testes is in accordance with the theory, for they are verymuch smaller than the testes; and yet they have undergone some change ofposition, for they are posterior to the kidneys. 

The facts agree with the hormone theory, for it is to be noted thatalthough the development of the scrotum is confined to the males, the'descent' or dislocation takes place in the foetus, and not at the periodof puberty. This is in accordance with the fact that the mechanicalconditions to which the change is attributed are not related to sexualhabits, but to the general habits of life which begin soon after birth. The development, therefore, may be considered to be related to thepresence of a hormone derived from the normal testis, but not to a specialquantity or quality of hormone associated with maturity or the functionalactivity of the organ. In Rodents, however, there is a difference in theorgans, not only at maturity, but in every rutting season, at any rate inMuridae such as rats and others. In the rutting season the testes becomemuch larger and descend into the scrotal sacs, at other times of the yearbeing apparently more or less abdominal. In rabbits and hares, which havea much more impulsive progression, the organs seem to be always in thescrotal sacs. 

It might be thought that in this case, although the hormone theory ofheredity might be applied, there was no reason to suppose that a hormonederived from the testis in the individual development was necessary inorder that the hereditary change should take place. If the individual wasmale and therefore had a testis, this organ would by heredity go throughthe process of dislocation. But there is the curious fact that when thedescent is not normal and complete, in what is called cryptorchidism, theorgans are always sterile. The retention of the testes within the abdomenmay be regarded as a case of arrested development, like many otherabnormalities, but this does not explain why the retained testes shouldalways be sterile, without spermatogenesis. If the inherited or congenitalprocess of dislocation requires the presence of hormones produced by anormal testis, then we can understand why a defective testis does notdescend completely, because it does not produce the hormone which isnecessary to stimulate the hereditary mechanism to complete dislocation. It is often stated that in cryptorchidic individuals the sexual instinctsand somatic sexual characters are well developed, which would appearcontradictory to the above explanation, but according to Ancel and Bouinsuch individuals in the case of the pig show considerable differences inthe secondary signs of sex and in the external genital organs, presentingvariations which lie between the normal and the castrated animal. 

We have here, then, in the position of the testes in Mammalia a conditionwhich is not in the slightest degree 'adaptive' in the ordinary sense--that is, fulfilling any special function or utility. The condition must beregarded as distinctly disadvantageous, since the organs are more exposedto injury, and the abdominal wall is weakened, as we know from the risk ofscrotal hernia in man. But from the Lamarckian point of view the factssupport the conclusion that the condition is the effect of certainmechanical strains, and is of somatic origin, while the correlations herereviewed are entirely unexplained by any theory of mutation or blastogenicorigin. 

 OPPOSING EVIDENCE

We have now to review certain cases which seem to support conclusionscontrary to those which we have maintained in the preceding pages, and toconsider the evidence which has been published in support of othertheories. It must be admitted that the occurrence of male secondarycharacteristics on one side of the body, and female on the other, is inconsistent with the view that the development of such characters is due tothe stimulus of a hormone, since the idea of a hormone means somethingwhich diffuses by way of the blood-vessels, lymph-vessels, and intersticesof the tissues, throughout the body, and the hormone theory of secondarysexual characters assumes that these characters are potentially present byheredity in both sexes. The occurrence of male somatic characters on oneside or in some part of the body and female on the other, usuallyassociated with the corresponding gonads, has been termedgynandromorphism, and has long been known in insects. Cases of thiscondition have been observed, though much more rarely, in Vertebrates. Iam not aware of any authentic instances in Mammals, and the suppositionthat in stags reduction or abnormality of one antler may be the result ofremoval or injury to the testis of one side, or the opposite, have beencompletely disproved by experiments in which unilateral castration hasbeen carried out without any effect on the antlers at all. In birds, however, a few cases have been recorded by competent observers with adefiniteness of detail which leaves no possibility of doubt. One of themore recent of these is that of a pheasant of the white-ringed Formosanvariety, _P. Torquatus_, of the Chinese pheasant. [Footnote: C. J. Bond, 'Unilateral Development of Secondary Male Characters in a Pheasant, '_Journ. Of Genetics_, vol. Iii. , 1914. ] On the left side this bird showsthe plumage, colour, and the spur of the male; on the right leg there isno spur except the small rudiment normally occurring in the hen. Thedifference in plumage between the two sides, however, is not complete. Thewhite collar is strictly limited to the left side, but the iridescent bluegreen of head and neck is present on both sides, though more marked on theleft. Only a few male feathers appear in the wing coverts of the leftside. The breast feathers are rufous, especially on the left side. Thetail coverts show marked male characters, more especially on the leftside. In the tail, however, the barred character of the male is notpresent on one side, absent on the other, but in most of the feathers isconfined to one, the _outer_ side of each feather. With regard to thegonads, in this bird a single organ was found on the left side, _i. E. _ inthe position of the ovary in normal females, and there was no trace of agonad on the right side. The organ present was small, 3/4 inch long by 1/2inch broad, and microscopic sections showed in one part actively growingareas of tubular gland structure in some of which bodies like spermatozoacould be detected, while in another were fibrous tissue with degeneratingcysts. The latter appear to have been degenerating egg follicles. Theauthor concludes that the organ was originally a functional ovary, andthat the ovarian portion had atrophied while a male portion had becomefunctionally active. 

Another case in birds was described by Poll [Footnote: _B. B. Ges. Naturf. Freunde_, Berlin, 1909. ] and is mentioned by Doncaster. [Footnote:_Determination of Sex_, Cambridge, 1914. ] It is that of a Bullfinchwhich had the male and female plumage sharply separated on the two sidesof the body. The right side of the ventral surface was red like a normalmale, the left side grey like a normal female. In this case there was atestis on the right side, on the left an ovary as in normal females. 

A third case in birds, somewhat different from the two first mentioned, isthat of a domestic fowl described by Shattock and Seligmann. [Footnote:_Trans. Pathol. Soc. _ (London), vol. 57, Part i. , 1906. ] It was a bird ofthe Leghorn breed, two years old, and had the fully developed comb andwattles of the cock. Each leg bore a thick blunt spur, nearly an inch inlength, but in the Leghorn breed spurs are by no means uncommon in hens ofmature age, before they have ceased to lay eggs. In plumage the characterswere mainly female. The colour being white could not show sexualdifferences, the neck hackles were but moderately developed, saddlehackles practically absent, the tail resembled that of the hen. There wasa fully developed oviduct on the left side, on the right another less thanhalf the full length. There was also a vas deferens on each side. Therewas a gonad on each side, that of the right about one-fourth the size ofthat on the left. In microscopic structure the right gonad resembled atestis consisting entirely of tubuli lined by an epithelium consisting ofa single layer of cells. In one part of this organ the tubules were largerthan elsewhere, and one of them exhibited spermatogenesis in progress. Theleft and larger gonad had a quite similar structure, but at its lower endwere found two ova enclosed within a follicular epithelium. 

With regard to the last case it is to be remarked that though the gonad onthe right side was entirely male, there was no unilateral development ofmale characters. With regard to the other two cases it must be pointed out(1) that the difference between the two somatic sex-characters on the twosides is chiefly a difference of colour, except the difference in thespurs in Bond's pheasant; (2) that the evidence already cited shows thatin fowls castration does not prevent the development of the colour andform of the male plumage, nor of the spurs: that in drakes, althoughcastration does not seem to have been carried out on young specimensbefore the male plumage was developed, when performed on the mature birdit prevents the eclipse, and does not cause the male to resemble the hen. Castration, then, tends to prove that in Birds the development of the malecharacters is not so closely dependent on the stimulation of testicularhormone as in Mammals. The characters must therefore be developed byheredity in the soma, which implies that the soma must itself bedifferentiated in the two sexes. The development must therefore be morein the nature of gametic coupling. It does not follow that the primarysex-character or the somatic characters are exclusive in either sex. We may suppose that the zygote contains both sexes, one or other of whichis dominant, and that dominance of one primary sex involves dominance ofthe corresponding sexual characters. This does not, however, agree withthe result of removal of the ovaries in ducks, for this causes thecharacters of the male to appear, so that the dominance of the female isnot a permanent condition of the soma but is dependent on the ovarianhormone. 

In the hermaphrodite individuals mentioned above the difference ofdominance is on two sides of the body instead of two differentindividuals. It may also be remarked here that while it is very difficultto believe that spurs were not due in evolution to the mechanicalstimulation of striking with the legs in combat, and while speciallyenlarged feathers are erected in display, we cannot at present attributethe varied and brilliant _colour_ of male birds to the direct influence ofexternal stimuli. 

In Lepidoptera among insects the evidence concerning castration tends toprove that hormones from the gonads play no part at all in the developmentof somatic sexual characters. Kellog, an American zoologist, in 1905[Footnote: _Journ. Exper. Zool. _ (Baltimore), vol. I. , 1905. ] describedexperiments in which he destroyed by means of a hot needle the gonads insilkworm caterpillars (_Bombyx mori_), and found no difference in thesexual characters of the moths reared from such caterpillars. Oudemans hadpreviously obtained the same result in the Gipsy Moth, _Limantria dispar_. Meisenheimer [Footnote: _Experimentelle Studien zur Soma- undGeschlechtedifferenzierung_. Jena, 1909. ] made more extensiveexperiments on castration of caterpillars in the last-mentioned species, in which the male is dark in colour and has much-feathered antennae, whilethe female is very pale and has antennae only slightly feathered. In themoths developed from the castrated larvae there was no alteration in themale characters, and in the females the only difference was that some ofthem were slightly darker than the normal. Meisenheimer and Kopee afterhim claim to have grafted ovaries into males and testes into females, withthe result that the transplanted organs remained alive and grew, and insome cases at least became connected with the genital ducts. Even in thesecases the moth when developed showed the original characters of the sex towhich belonged the caterpillar from which it came, although it wascarrying a gonad of the opposite sex. It will be seen that these resultsare the direct opposite of those obtained by Steinach on Mammals. We haveno evidence that the darker colour of the normal male in this case isadaptive, or due to external stimuli, but the feathering of the antennaeis generally believed to constitute a greater development of the olfactorysense organs, and is therefore adaptive, enabling the male to find thefemale. This is therefore the kind of organ which would be expected to beaffected by hormones from the generative organs. It is stated that thesexual instincts were also unaltered, a male containing ovaries instead oftestes readily copulating with a normal female. 

These results, almost incredible as they appear, are in harmony with therelatively frequent occurrence of gynandromorphism in insects. [Footnote:See Doncaster, _Determination of Sex_ (Camb. Univ. Press, 1914), chap. Ix. ] One of the most remarkable cases of this is that of an ant(_Myrmica scabrinodis_) the left half of which is male, the right half notmerely female, but worker--that is, sterile female, without wing. Cases inLepidoptera, _e. G. Amphidasys betularia_, have frequently been recorded. Presumably not only the antennae and markings, but also the genitalappendages and the gonads themselves, are male and female on the twosides. On the view that both sexes and the somatic sex-characters of bothsexes are present in each zygote, and that the actual sex is due todominance, we must conclude that the male primary and secondary charactersare dominant on one side, and the female on the other, and it is evidentthat hormones diffusing throughout the body cannot determine thedevelopment of somatic sexual characters here. Various attempts have beenmade to explain gynandromorphism in insects in accordance with thechromosome theory of sex-determination. These are discussed by Doncasterin the volume already cited, but from the point of view of the presentwork the important question is that concerning the somatic sex-characters. According to Doncaster it has been found that in some Lepidoptera thedifferent sex-chromosomes occur in the female, not in the male as in otherinsects. Half the eggs, therefore, contain an X chromosome, and half a Y, while all the sperms contain an X chromosome. Doncaster has seen in_Abraxas grossulariata_ ova with two nuclei both undergoing maturation. If one of these in reduction expelled a Y chromosome, the other an X, then one would retain an X and the other a Y. Each was fertilised by asperm, one becoming therefore XX or male and the other XY or female. Itmay be supposed that as there was only the cytoplasm of one ovum, eachnucleus would determine the characters of half the individual developed. The question remains, therefore, where are the factors of the somaticsex-characters? One suggestion which might be made is that the femalecharacters are present in the _Y_, in this case female producingchromosome, or, if the female characters are merely negative, that themale characters are in the _X_ chromosome, but only show themselves in thehomozygous condition, thus:--

 FEMALE x MALE XY XX | \/ | | /\ | XX YX MALE FEMALE

The male characters in the male, _XX_, would appear because present in twochromosomes, but would be recessive in the female because present only inone chromosome. The validity of this scheme, however, is disproved by thefact that males can transmit the female characters of their race, as inthe case mentioned by Doncaster where a male _Nyssia zonaria_ when crossedtransmits the wingless character of its own female. 

Another, perhaps better, suggestion is that the somatic characters of bothsexes are present in each. Then as each somatic cell is descended withoutsegregation from the fertilised ovum, we may suppose that the presence ofthe sex-chromosomes in the somatic cells themselves in some way determineswhether male or female characters shall develop, without the aid of anyhormones from the gonads. This theory would be quite compatible withthe belief that adaptive somatic sex-characters may be due to externalstimulation, for supposing that the hypertrophy or modification isconveyed to the determinants in the gametocytes, and was confined toone sex, _e. G. _ the male, then these determinants would be modified inassociation with the sex-chromosomes of that sex, and thus thoughafter reduction and fertilisation they would be present in the femalezygote also, they would not develop in that sex. Thus supposing _M_ torepresent a modification acquired in the male and _m_ the absence ofthe modification, such as the feathered antenna of a moth, and thesex-chromosomes to be _X_ and _Y_, then we should have in thegametocytes--

 Male Female

 _MM mm_

 _XX XY_

 Gametes _MX, MX: mXmY_

 Zygotes _MmXX male, MmXY female_, and the character _M_ would only appear in the male because it onlydevelops in association with _XX_ in the somatic cells descended from themale zygote. This would be the result in the first generation in which asomatic modification affected the factors in the chromosomes. In the nextgeneration _m_ in the male would be affected, and the male for the sake ofsimplicity might be supposed to become _MMXX_. When the female gametessegregated, some would always be _mY_, and some zygotes therefore _MXmY_. Others might be _MMXY_. On this theory, therefore, there would always besome females heterozygous for the male character. 

Geoffrey Smith, one of the many promising young scientific investigatorswhose careers were cut short in the War, maintained views concerningsomatic sex-characters different from that which explains theirdevelopment as due to a hormone from the testis or ovary. Nussbaum in 1905[Footnote: 'Ergebuisse der Anat. Und Entwicklungsgesch. , ' Bd. Xv. ;_Pflügers Archiv_, Bd. Cxxvi, 1909. ] had recorded experiments on _Ranafusca_ (which is identical with the British species commonly called _R. Temporaria_) which appeared to prove that in the male frog aftercastration the annual development of the thumb-pad and the muscles of thefore-leg does not take place, and if these organs have begun to enlargebefore castration they atrophy again. When pieces of testis wereintroduced into the dorsal lymph-sac of a castrated frog the thumb-padsand muscles developed as in a normal frog. Geoffrey Smith and EdgarSchuster [Footnote: _Quart Journ. Mic. Sci_. , lvii, 1911-12. ] investigatedthe subject again with results contrary to those of Nussbaum. 

Smith and Schuster begin by describing the normal cycle of changes in thetestes on the one hand and the thumb-pad on the other. After the dischargeof the spermatozoa in March or April the testes are at their smallestsize. From this time onwards till August they steadily increase in size, attaining their maximum at the beginning of September. From then till thebreeding season no increase in size or alteration of cellular structureoccurs, the testes apparently remaining in a state of complete inactivityduring this period. With regard to internal development, after thedischarge of spermatozoa in the breeding season the spermatogonia divideand proliferate, forming groups of cells known as spermatocysts. In Juneand July spermatogenesis is active, and from August to October theformation of ripe spermatozoa is completed. 

The corresponding changes in the thumb-pads are as follows. Immediatelyafter the breeding season the horny epidermis of the pad with its deeplypigmented papillae is cast off, and the thumb remains comparatively smoothfrom April or May until August or September. When the large papillae areshed, smaller papillae remain beneath, and are gradually obliterated bythe epidermis growing up between them. The epidermis is therefore growingwhile the spermatogenesis is taking place. In August and September theepidermic papillae begin to be obvious, and from this time till February acontinuous increase in the papillae and their pigmentation occur. GeoffreySmith argues that the development of this somatic character occurs whilethe testes are inactive and unchanged. Considering that the testesthroughout the winter months are crammed with spermatozoa, which mustrequire some nourishment, and which may be giving off a hormone all thetime, the argument has very little weight. Smith and Schuster found thatovariotomy, with or without subsequent implantation of testes or injectionof testis extract, had no effect in causing the thumb of the female toassume any male characters. 

Castration during the breeding season causes the external pigmentedlayer with its papillae to be cast off very soon--that is to say, ithas the same effect as the normal discharge of the spermatozoa. Smithand Schuster found that castration at other seasons caused the pad toremain in the condition in which it was at the time, that there was noreduction or absorption as Nussbaum and Meisenheimer found, and thatallo-transplantation of testes--that is, the introduction of testes fromother frogs either into the dorsal lymph-sacs or into the abdominalcavity--or the injection of testis extract, had no effect in causinggrowth or development of the thumb-pad. 

There seems to be one defect in the papers of both Nussbaum and Smith andSchuster--namely, that neither of them mentions or apparently appreciatesthe fact that the thumb-pads, apart from the dermal glands, consist ofhorny epidermis developed from the living epidermis beneath. The hornylayer is not shown clearly in the figures of Smith and Schuster. It seemsimpossible that the horny layer or its papillae could atrophy inconsequence of castration, or be absorbed. The horny part of the frog'sthumb-pad is comparable with the horny sheath of the horns in themammalian Prong-buck (_Antilocapra_) which are shed after the breedingseason and annually redeveloped. Meisenheimer claims that he produceddevelopment of papillae on the thumb-pad, not only by implantation ofpieces of testis, but also by implantation of pieces of ovary. This seemsso very improbable that it suggests a doubt whether the same investigatorwas not mistaken with regard to the results of his experiments intransplanting gonads in Moths. 

Smith and Schuster conclude that the normal development of the thumb-paddepends on the presence of normal testes, but that there is no sufficientevidence that the effect is due to a hormone derived from the testis. Itis equally probable, according to Smith, that the testicular cells take upsome substance or substances from the blood, thus altering the compositionof the latter and perhaps stimulating the production of these substancesin some other organ of the body. These substances may be provisionallycalled sexual formative substances. Smith's theory therefore is that theaction of the testes in metabolism is rather to take something from theblood than to add something to it, and that it is this subtractive effectwhich influences the development of somatic sexual organs. 

Geoffrey Smith in fact, in the paper above considered, attempts to applyto the frog the views he put forward [Footnote: _Fauna und Flora desGolfes van Neapel_, 29 Monographie Rhizocephala. ] in relation to theeffect of the parasite _Sacculina_ on the sexual organs of crabs. Thespecies in which he made the most complete investigation of the influenceof the parasite was _Inachus scorpio_ (or _dorsettensis_). Figures showingthe changes in the abdomen produced by the presence of _Sacculina_ aregiven in Doncaster's _Determination of Sex_, Pl. Xv. _Sacculina_ is one ofthe Cirripedia, and therefore allied to the Barnacles. It penetrates intothe crab in its larval stage, and passes entirely into the crab's body, where it develops a system of branching root-like processes. When maturethe body of the _Sacculina_ containing its generative organs forms aprojection at the base of the abdomen of the crab on its ventral surface, and after this is formed the crab does not moult. Crabs so affected do notshow the usual somatic sexual characters, and at one time it was supposedthat only females were attacked. It is now known that both sexes of thehost may be infected by the parasite, but the presence of the lattercauses suppression of the somatic sex-differences. The entry of theparasite is effected when the crab is young and small, before the somaticsex-characters are fully developed. The gonads are not actuallypenetrated, at least in some cases, by the fibrous processes of theparasite, but nevertheless they are atrophied and almost disappear. In_Inachus_ the abdomen of the normal male is very narrow and has noappendages except two pairs of copulatory styles. The abdomen of thefemale is very broad, and has four pairs of biramous appendages coveredwith hairs, the normal function of which is to carry the eggs. The effectof the parasite in the male is that the abdomen is broader, the copulatorystyles reduced, and biramous hairy appendages are developed similar tothose of the female, but smaller. In the female the abdomen remains broad, but the appendages are much smaller than in the normal female, about equalin size to those of the 'sacculinised' male. Smith interpreted thealteration in the male as a development of female secondary characters, but it is obvious from the condition in Macrura or tailed Decapods, likethe lobster or crayfish, that the abdomen or tail of the male originallycarried appendages similar to those of the female, and that the malecharacter is a loss of these appendages. The absence of the male charactertherefore necessarily involves a development of these appendages, andthere is not much more reason for saying that the male under the influenceof the parasite develops female characters, than for saying that the malecharacter is absent. There is no evidence in the facts concerningparasitic castration for Geoffrey Smith's conclusion that the femalecharacters are latent in the male, but the male characters not latent inthe female: both return to a condition in which they resemble each other, and the primitive form from which they were differentiated. 

By his studies of parasitic castration Geoffrey Smith was led to formulatea theory for the explanation of somatic sex-characters different from thatof hormones. He found that in the normal female crab the blood containedfatty substances which were absorbed by the ovaries for the production ofthe yolk of the ova. When _Sacculina_ is present these substances areabsorbed by the parasite; the ovary is deprived of them, and thereforeatrophies. In the male the parasite requires similar substances, and itsdemand on the blood of the host stimulates the secretion of suchsubstances, so that the whole metabolism is altered and assimilated tothat of the female. It is this physiological change which causes thedevelopment of female secondary characters. He describes this change asthe production of a hermaphrodite sexual formative substance, on theground that in at least one case eggs were found in the testis of a male_Inachus_ which had been the host of a _Sacculina_, but had recovered. Itmust however be noted that the _Sacculina_ itself is hermaphrodite, withovaries much larger than the testes. It is possible that while theparasite prevents the development of testis or ovary in the host, it givesup to the body of the host a hormone from its own ovaries which tends todevelop the female secondary characters: for the parasite is itself aCrustacean, and therefore the hormone from its ovaries would not be of toodifferent a nature to act upon the tissues of the host. 

The observation of Geoffrey Smith that eggs may occur in the testis of acrab after recovery from the parasite appears of more importance than hispeculiar theoretical suggestions, for it tends to show that sex is notalways unalterably fixed at fertilisation. In this case the influence of aparasite predominantly female would seem to be the real cause of thedevelopment of eggs in the testis of the host. Geoffrey Smith does notdiscuss the origin of the somatic sexual characters in evolution, orattempt to show how his theories of sexual formative substance, and of theinfluence of the gonads by subtraction rather than addition, would bearupon the problem. 

CHAPTER VI

 Origin Of Non-Sexual Characters: The Phenomena Of Mutation

According to the theory here advocated, modifications produced by externalstimuli in the soma will also be inherited in some slight degree in eachgeneration when they have no relation to sex or reproduction. In this casethe habits and the stimuli which they involve will be common to bothsexes, and the hormones given off by the hypertrophied tissues will actupon the corresponding determinants in the gametocytes. The modificationsthus produced will therefore be related to habits, and the theory willinclude all adaptations of structure to function, but other characters mayalso be included which are the result of stimuli and yet have no functionor utility. 

The majority of evolutionists in recent years have taught that influencesexerted through the soma have no effect on the determinants in thechromosomes of the gametes, that all hereditary variations are gametogenicand none somatogenic. Mendelians believe that evolution has been due tothe appearance of characters or factors of the same kind as those whichdistinguish varieties in cultivated organisms, and which are the subjectof their experiments, but they have found a difficulty, as alreadymentioned in Chapter II, in forming any idea of the origin of a newdominant character. A recessive character is the absence of some positivecharacter, and if in the cell-divisions of gametogenesis the factor forthe positive character passes wholly into one cell, the other will bewithout it, will not 'carry' that factor. If such a gamete is fertilisedby a normal gamete the organism developed from the zygote will beheterozygous, and segregation will take place in its gametes between thechromosome carrying the factor and the other without it, so that therewill now be many gametes destitute of the factor in question. When twosuch gametes unite in fertilisation the resulting organism will be ahomozygous recessive, and the corresponding character will be absent. Inthis way we can conceive the origin of albino individuals from a colouredrace, supposing the colour was due to a single factor. 

In Bateson's opinion the origin of a new dominant is a much more difficultproblem. In 1913 he discussed the question in his Silliman Lectures. [Footnote: _Problems of Genetics_, Oxford Univ. Press, 1913. ] He considersthe difficulty is equally hopeless whether we imagine the dominants to bedue to some change internal to the organism or to the assumption ofsomething from without. Accounts of the origin of new dominants underobservation in plants usually prove to be open to the suspicion that theplant was introduced by some accident, or that it arose from a previouscross, or that it was due to the meeting of complementary factors. Inmedical literature, however, there are numerous records of the spontaneousorigin of various abnormalities which behave as dominants, such asbrachydactyly, and Bateson considers the authenticity of some of these tobe beyond doubt. He concludes that it is impossible in the present stateof knowledge to offer any explanation of the origin of dominantcharacters. In a note, however, he suggests the possibility that there areno such things as new dominants. Factors have been discovered which simplyinhibit or prevent the development of other characters. For example, thewhite of the plumage in the White Leghorn fowl is due to an inhibitingfactor which prevents the development of the colour factor which is alsopresent. Withdraw the dominant inhibiting factor, and the colour showsitself. This is shown by crossing the dominant white with a recessivewhite, when some birds of the F(2) generation are coloured. [Footnote:Bateson, _Principles of Heredity_, p. 104. ] Similarly, brachydactyly inman may be due to the loss of an inhibiting factor which prevents itappearing in normal persons. It is evident, however, that it is difficultto apply this suggestion to all cases. For example, the White Leghorn fowlmust have descended from a coloured form, probably from the wild species_Gallus bankiva_. If Bateson's suggestion were valid we should have tosuppose that the loss of the factor for colour caused the dominant whiteto appear, and then when this is withdrawn colour appears again, so thatthe colour factors and the inhibiting factors must lie over one another ina kind of stratified alternation. And then how should we account for therecessive white?

In his Presidential Address to the meeting of the British Association inAustralia, 1914, Bateson explains his suggestion somewhat more fully witha command of language which is scarcely less remarkable than the subjectmatter. The more true-breeding forms are studied the more difficult it isto understand how they can vary, how a variation can arise. When two formsof _Antirrhinum_ are crossed there is in the second generation such aprofusion of different combinations of the factors in the twograndparents, that Lotsy has suggested that all variations may be due tocrossing. Bateson does not agree with this. He believes that geneticfactors are not permanent and indestructible, but may undergo quantitativedisintegration or fractionation, producing subtraction or reductionstages, as in the Picotee Sweet Pea, or the Dutch Rabbit. Also variationmay take place by loss of factors as in the origin of the white Sweet Peafrom the coloured. But regarding a factor as something which, although itmay be divided, neither grows nor dwindles, neither develops nor decays, the Mendelian cannot conceive its beginning any more than we can conceivethe creation of something out of nothing. Bateson asks us to considertherefore whether all the divers types of life may not have been producedby the gradual unpacking of an original complexity in the primordial, probably unicellular forms, from which existing species and varieties havedescended. Such a suggestion in the present writer's opinion is in onesense a truism and in another an absurdity. That the potentiality of allthe characters of all the forms that have existed, pterodactyls, dinosaurs, butterflies, birds, etc. Etc. , including the characters of allthe varieties of the human race and of human individuals, must have beenpresent in the primordial ancestral protoplasm, is a truism, for if thepossibility of such evolution did not exist, evolution would not havetaken place. But that every distinct hereditary character of man wasactually present as a Mendelian factor in the ancestral _Amoeba_, and thatman is merely a group of the whole complex of characters allowed toproduce real effects by the removal of a host of inhibiting factors, isincredible. The truth is that biological processes are not within ourpowers of conception as those of physics and chemistry are, and Bateson'shypothesis is nothing but the old theory of preformation in ontogeny. Justas the old embryologists conceived the adult individual to be containedwith all its organs to the most minute details within the protoplasm ofthe fertilised ovum or one of the gametes, so the modern Mendelian, because he is unable to conceive or to obtain the evidence of the gradualdevelopment of a hereditary factor, conceives all the hereditary factorsof the whole animal kingdom packed in infinite complexity within theprotoplasm of the primordial living cells. That man is complex and_Amoeba_ simple is merely a delusion; the truth according to Mendelism isthat man is merely a fragment of the complexity of the original _Amoeba_. 

Mendelism studies especially the heredity of characters, and onlyincidentally deals with recorded instances of the appearance of new forms, such as the origin of a salmon-coloured variety of _Primula_ from acrimson variety. The occurrence of new characters, or mutations as theyare called, has been specially studied by other investigators, and Ipropose briefly to consider the two most important examples of suchresearch, namely, that by Professor T. H. Morgan, which deals with theAmerican fruit-fly _Drosophla_, and the other which concerns the mutationsof the genus of plants OEnothera, exemplified by our well-known EveningPrimrose. 

Professor T. H. Morgan informs us [Footnote: _A Critique of the Theory ofEvolution_ (Oxford Univ. Press, 1916), p. 60] that within five or sixyears in laboratory cultures of the fruit-fly, _Drosophila ampelophila_, arose over a hundred and twenty-five new types whose origin was completelyknown. The first of these which he mentions is that of eye colour, differing in the two sexes, in the female dark eosin, in the maleyellowish eosin. Another mutation was a change of the third segment of thethorax into a segment similar to the second. Normally the third segmentbears minute appendages which are the vestiges of the second pair ofwings; in the mutant the wings of the third segment are true wings thoughimperfectly developed. A factor has also occurred which causes duplicationof the legs. Another mutation is loss of the eyes, but in differentindividuals pieces of the eye may be present, and the variation is so widethat it ranges from eyes which until carefully examined appear normal, tothe total absence of eyes. Wingless flies also arose by a single mutation. These were found on mating with normal specimens to be all recessivecharacters, thus agreeing with Bateson's views. The next one described isdominant. A single male appeared with a narrow vertical red bar instead ofthe broad red normal eye. When this male was bred with normal females allthe eyes of the offspring were narrower than the normal eye, though not sonarrow as in the abnormal male parent. It may be pointed out that this isscarcely a sufficient proof of dominance. If the mutation were due to theloss of one factor affecting the eye, the heterozygote carrying the normalfactor from the mother only might very well develop a somewhat imperfecteye. 

Morgan arranges the numerous mutations observed in _Drosophila_ in fourgroups, corresponding in his opinion to the four pairs of chromosomesoccurring in the cells of the insect. After the meiotic or reductiondivisions each gamete of course contains in its nucleus four singlechromosomes. One of the four pairs consists of the sex-chromosomes. Allthe factors of one group are contained in one chromosome, and it is foundin experiments that the members of each group tend to be inheritedtogether--that is to say, if two or more enter a cross together, in otherwords, if a specimen possessing two or more mutations is crossed withanother in which they are absent, they tend to segregate as though theywere a single factor. This fact agrees with the hypothesis that thefactors in such a case are contained in a single chromosome whichsegregates from the fellow of its pair in the reduction divisions. Exceptions may occur, however, and these are explained by what is called'crossing over. ' When one chromosome of a pair, instead of being parallelto the other in the gametocyte, crosses it at a point of contact, thenwhen the chromosomes separate, part of one chromosome remains connectedwith the part of the other on the same side and the two parts separate asa new chromosome, so that two factors originally in the same chromosomemay thus come to lie in different chromosomes. In consequence of this, twoor more factors which are usually 'coupled' or inherited together may cometo appear in different individuals. 

Morgan emphasises the statement that a factor does not affect only oneparticular organ or part of the body. It may have a chief effect in onekind of organ, _e. G. _ the wings or eyes, but usually affects several partsof the body. Thus the factor that causes rudimentary wings also producessterility in females, general loss of vigour, and short hind legs. 

The facts to which I shall refer concerning _Oenothera_ are for the mostpart quoted on the authority of Dr. Ruggles Gates, and taken from his book_The Mutation Factor in Evolution_ (London, 1915). The occurrence ofmutations in _Oenothera_ was first noticed by De Vries, the Dutchbotanist, in the neighbourhood of Amsterdam in 1886. He found a largenumber of specimens of _Oenothera Lamarckiana_ growing in an abandonedpotato-field at Hilversum, and these plants showed an unusual amount ofvariation. He transplanted nine young plants to the Botanic Garden ofAmsterdam, and cultivated them and their descendants for seven generationsin one experiment. Similar experiments have been made by himself andothers. The large majority of the plants produced from the _Oe. Lamarckiana_ by self-fertilisation were of the same form with the samecharacters, but a certain percentage presented 'mutations'--that is, characters different from the parent form, and in some cases identicalwith those of plants occurring occasionally among those growing wild inthe field where the observations began. Nine of these mutants have beenrecognised and defined, and distinguished by different names. Thecharacters are precisely described and in many cases figured by Gates inthe volume cited above. The first mutant to be recognised--in 1887--wasone called _lata. _ It must be explained that the young plant of_Oenothera_ has practically no stem, but a number of leaves radiating inall directions from the growing point which is near the surface of thesoil. The plant is normally biennial, and in the first season theinternodes are not developed. This first stage is called the 'rosette. 'From the reduced stem are afterwards developed one or more long stems withelongated internodes, bearing leaves and flowers. In the mutation _lata_the rosette leaves are shorter and more crinkled than those of_Lamarckiana, _ and the tips of the leaves are very broad and rounded. The stems of the mature plant are short and usually more or less decumbentwith irregular branches. The flower-buds are peculiarly stout andbarrel-shaped, with a protrusion on one side. The seed-capsules areshort and thick, containing relatively few seeds, and the pollen iswholly or almost wholly sterile. 

It is to be noted here, a fact emphasised by DeVries in his earliestpublications on the subject, that in nearly all, if not all cases, amutation does not consist in a peculiarity of a single organ, but in analteration of the whole plant in every part. In this respect mutations asobserved in _Oenothera_ seem to be in striking contrast to the majority ofMendelian characters. Mutation in fact seems to be a case of what theearlier Darwinians called correlation, while Mendelian characters mayapparently be separated and rejoined in any combination. For example, inbreeds of fowls any colour or any type of plumage may be obtained withsingle comb or with rose comb. In my own experiments on fowls the loosekind of plumage first known in the Silky fowl, which is white, could becombined with the coloured plumage of the type known as black-red. At thesame time it must be borne in mind that since the factor, whether aportion of a chromosome or not, is transmitted in heredity as a part of asingle cell, the gamete, and since every cell of the developed individualis derived by division from the single zygote cell formed by the union ofthe two gametes, the factor or determinant must be contained in every cellof the soma, except in cases where differential division, or what iscalled somatic segregation, takes place. Thus the factor which causes thecomb to be a rose comb in a fowl must be present in the cells that producethe plumage or the toes or any other part of the body. Morgan, asmentioned above, finds in _Drosophila_ that factors do affect severalparts of the body. It is, however, curious to consider that the factorwhich produces intense pigmentation of the skin and all the connectivetissue in the Silky fowl has no effect on the colour of the plumage inthat breed, which is a recessive white. The plumage is an epidermicstructure, and therefore distinct from the connective tissue, but it isdifficult to understand why a pigment factor though present in every cellhas no effect on epidermic cells. 

The Mendelians, when the mutations of _Oenothera_ were first described, endeavoured to show that they were merely examples of the segregation offactors from a heterozygous combination. They suggested in fact that_Oenothera Lamarckiana_ was the result of a cross, or repeated crosses, between plants differing in many factors, that the numerous mutations weresimilar to the variety of different types which are produced by breedingtogether the grey mice arising from a cross between an albino and aJapanese waltzing mouse in Darbishire's experiment. Since that time, however, the natural distribution and the cultural history of _Oenothera_has been very thoroughly worked out. _Oenothera Lamarckiana_ is the commonEvening Primrose of English gardens. The species of the sub-genus _Onagra_to which _Lamarckiana_ belongs were originally confined to America(Canada, United States, and Mexico), but _Lamarckiana_ itself has neverbeen found there in a wild state. Attempts, however, to produce it bycrossing of other forms have not succeeded, and a specimen has beendiscovered at the Muséum d'Histoire Naturelle at Paris, collected byMichaux in North America about 1796, which agrees exactly with the_Oenothera Lamarckiana_ naturalised or cultivated in Europe. The plant wasfirst described by Lamarck from plants grown in the gardens of the Muséumd'Histoire Naturelle, under the name _OE. Grandiflora_, which had beenintroduced by Solander from Alabama, but Seringe subsequently decided thatLamarck's species was distinct from _grandiflora_, and named it_Lamarckiana_. Gates states that Michaux was in the habit of collectingseeds with his specimens, and that it is therefore highly probable thatLamarck's specimens were grown directly from seeds collected in America byMichaux. Gates considers that the suggestion of the hybrid origin of_Lamarckiana_ in culture is thus finally disposed of. By the year 1805, _Lamarckiana_ was apparently naturalised and flourishing on the coast ofLancashire, and in 1860 it was brought into commerce, probably from theseLancashire plants, by Messrs, Carter. The cultures of De Vries aredescended from these commercial seeds, but the Swedish race of_Lamarckiana_, as well as those of English gardens, differ in severalfeatures and must have come from another source or been modified bycrossing with _grandiflora_. This last remark is quoted from Gates, but itseems improbable that the Dutch plants should be derived from those ofLancashire, and those of English gardens from a different source. The factseems to be, according to other parts of Gates's volume, that there arevarious races of _Lamarckiana_ in English gardens and in the Isle ofWight, as well as in Sweden, etc. , and that these races differ from oneanother less than the mutants of De Vries and his followers. 

An important point about these mutations is that their production is aconstant feature of _Lamarckiana_. Whenever large numbers of the seeds ofthis plant are grown, a certain proportion of the plants developed presentthese _same_ mutations; not always all of them--some may be absent in oneculture, present in another, but four of them are fairly common and ofconstant occurrence. The total proportion of mutant plants compared withthe normal was 1. 55 per cent. In one family, 5. 8 per cent. In another. Itwould appear therefore, supposing that mutations arose subsequently in thesame determinate way from previous mutations, that evolution, though in anumber of divergent directions from one ancestral form, would proceedalong definite lines, and that there would be nothing accidental about it. We should thus arrive at a demonstration of what Eimer calledorthogenesis, or evolution in definite directions. 

The mutation _lata_ cannot be said to breed true, as the pollen is almostentirely sterile. It has therefore been propagated by crossing with_Lamarckiana_ pollen, with the result that both forms are obtainedwith _lata_ varying in proportion from 4 per cent. To 45 per cent. 

_Rubrinervis_ is a mutation from _Lamarckiana_, chiefly distinguished byred midribs in the leaves and red stripes on the sepals. When propagatedfrom self-fertilised seed it produced about 95 per cent. Of offspring withthe same characters, and the remaining 5 per cent. Mutants, one of whichwas _laevifolia_ which had been found by De Vries among plants growingwild at Hilversum. Gates obtained a single plant among offspring of_rubrinervis_ in which the sepals were red throughout, and to this he gavethe name _rubricalyx_. When selfed this plant gave rise to both_rubricalyx_ and _rubrinervis_, and in the second generation when the_rubricalyx_ was selfed again the numbers of the two were approximately 3to 1. _Rubricalyx_ is therefore a dominant heterozygote, and this fact wasfurther confirmed in the third generation when a selfed plant gave 200offspring all _rubricalyx_, the mother plant having evidently beenhomozygous for the red character. In this case, therefore, we have whatBateson was seeking, the origin of a new dominant character underobservation, the original mutation having arisen in a single gamete of thezygote which gave rise to the plant. It is claimed by mutationists thatmutations are not new combinations or separations of Mendelian unitcharacters already present, but are themselves new characters, though notalways necessarily, as in the case of _rubricalyx_, new unit characters inthe Mendelian sense. 

Perhaps the most interesting of the researches on the phenomena ofmutation are those concerning the relation of the characters to thechromosomes of the cell, in which Gates has been a pioneer and one ofthe most industrious and successful investigators. The behaviour ofthe chromosomes in meiosis or reduction division both in the pollenmother-cells and in the megaspore mother-cells which give rise to theso-called embryo-sac are fully described by Gates. Here it is onlynecessary to refer to the abnormalities in the reduction division whichare related to mutation, and the results of these abnormalities in thenumber of chromosomes. The original number of chromosomes in _OEnothera_is 14. In the mutation _lata_ this has become 15, and also in anothermutation called _semilata_. The chromosomes before the reduction divisionare arranged in pairs, each pair consisting, it is believed, of onepaternal and one maternal chromosome. One of each pair goes into onedaughter-cell and the other into the other, but not all maternal into oneand all paternal into the other. Thus each daughter-cell after the firstor heterotypic division in normal cases contains 7 chromosomes. A secondhomotypic division takes place in which each chromosome splits into two asin somatic divisions, and thus we have 4 gametes with 7 chromosomes each. Now when _lata_ is produced it is believed that in the heterotypicdivision one pair passes into one daughter-cell instead of one chromosomeof the pair into each daughter-cell, the other pairs segregating in theusual way. We thus have one daughter-cell with 8 chromosomes and the otherwith 6. This 6+8 distribution has actually been observed in the pollenmother-cell in _rubrinervis_. When a gamete with 8 chromosomes unites infertilisation with a normal gamete with 7 the zygote has 15. The _lata_mutants having an odd chromosome are almost completely male-sterile, andtheir seed production is also much reduced: but this partial sterilitycannot be attributed entirely to the odd chromosome because _semilata_, which has also 15 chromosomes, does not show the same degree of sterility. 

Other cases occur in which the number of chromosomes in the somatic cellsis double the ordinary number--namely, 28--and others in which the numberis 21. The normal number in the gamete, 7, is considered the simpleor haploid number, and therefore the number 28 is called tetraploid. This doubling of the somatic number of chromosomes is now known in anumber of plants and animals. It occurs in the _OEnothera_ mutant _gigas_. The origin of it has not been clearly made out, but it must result eitherfrom the splitting of each chromosome or from the omission of thechromosome reduction. In many cases the more numerous chromosomes areindividually as large as those in normal plants, and consequently thenucleus is larger, the cell is larger, and the whole plant is larger inevery part. But giantism may occur without tetraploidy, and vice versa. Inthe _OEnothera gigas_ the rosette leaves are broadly lanceolate withobtuse or rounded tips, more crinkled than in _Lamarckiana_, petiolesshorter. The stem-leaves are also larger, broader, thicker, more obtuse, and more crinkled than in _Lamarckiana_. The stem is much stouter, almostdouble as thick, but not taller because the upper internodes are shorterand less numerous. It is difficult to avoid the conclusion that thestouter character of the organs in this plant is causally connected withthe increased number of chromosomes. Where the number of cells formed isapproximately similar, as in two allied forms of plant in this case, thegreater size of the cells would naturally give a stouter habit, but it isclear that large cells do not necessarily mean greater size. The cells of_Salamander_ and _Proteus_ are the largest found among Vertebrates, butthose Amphibia are not the largest Vertebrates. It is curious to note howdifferent are these discoveries concerning differences in the _number_ ofchromosomes from the conception of Morgan that a mutation depends on afactor situated in a part of one chromosome. 

More copious details concerning mutations will be found in thepublications cited. The question to be considered here is how far theclaim is justified that the facts of this kind hitherto discovered affordan explanation of the process of evolution. It seems probable thatmutations are of different kinds, as exemplified in _Oenothera_ by _gigas_and _rubricalyx_ respectively, the former producing only sterile hybrids, the latter behaving exactly like a Mendelian unit. There can be littledoubt that, as Bateson states, numerous forms recognised as species orvarieties in nature differ in the same way as the races or breeds ofcultivated organisms which differ by factors independently inherited. There are facts, however, which prove that all species are not sterile_inter se_, and that their characters when they are hybridised do notalways segregate in Mendelian fashion. John C. Phillips, [Footnote:_Journ. Exper. Zool. _, vol. Xviii. , 1915. ] for example, crossed three wildspecies of duck, _Anas boscas_ (the Mallard) with _Dafila acuta_ (thePintail) and with _Anas tristis_. In the former cross he states thatexcept for one or two characters there seemed to be no more tendency tovariation in the _F2_ generation than in the _F1_. An _F1_ Pintail-Mallard[female] was mated with a wild Pintail [male]. According to Mendelianexpectation the offspring of this mating should have been half Pintail andhalf Pintail-Mallard hybrids, but Phillips states that on casualinspection the plumage of all the males appeared pure Pintail although theshape was distinctly Mallard-like. The statement is, however, open tocriticism. The question is, what were the unit characters in the parentspecies? If the unit characters were very small and numerous, anindividual in which all the characters of the Pintail existed togetheramong the offspring of the hybrid mated with pure Pintail would be rare inproportion to the individuals presenting other combinations. Of the _F2_'sobtained from crossing _Anas tristis_ [male] with _Anas boscas_ [female]Phillips obtained 23 females and 16 males. The females were all alike andsimilar to _F1_ females. Of the males one was a variate specially marked, about half-way between the _F1_ type and the Mallard parent. This, according to Phillips, was a segregate. The rest showed a range ofvariation but no distinct segregation. 

It is somewhat surprising that Mendelian experts, who seem to believe thatspecies are distinguished by Mendelian characters, have not madesystematic experiments on the crossing of species in order to prove ordisprove their belief. 

For my own part I cannot help thinking that the origin of varieties inspecies in a domesticated or cultivated state is in a sense pathological. Such variation doubtless occurs in nature, but not with such luxuriance. The breeds of domestic fowls differ so greatly that Bateson and othersrefuse to believe that they have all arisen from the single species_Gallus bankiva_. It seems to me from the evidence that there cannot beany doubt that they have so arisen. One fact that impresses my mind isthat if we consider colour variations in domesticated animals, we findthat a similar set of colours has arisen in the most diverse kinds ofanimals with sometimes certain markings or colours peculiar to one group, _e. G. _ dappling in horses, wing bars in pigeons. Thus in various kinds ofMammals and Birds we have white and black, red or yellow, chocolate withvarious degrees of dilution, and piebald combinations. Why should formsoriginally so different, as the cat with its striped markings and therabbit with no markings at all, give rise to the same colour varieties? Itseems probable that the reason is that the original form had the smallnumber of pigments which occur mixed together in very small particles, andthat in the descendants the single pigments have separated out, withincrease or decrease in different cases. It is true that historicalevidence tends to show that the greatest variations, such as albinism inone direction or excess of pigment in the other in the Sweet Pea, were thefirst to arise (see Bateson, Presidential Address to British Association, Australia, 1914, Part I. ), and the splitting appears often to beintentionally produced by crossing these extreme variations with theoriginal form, but the possibility remains that the conditions ofdomestication, abundant food, security and reduced activity, lead toirregularity in the process of heredity. In any case the mere separationamong different individuals of factors originally inherited together inone complex does not account for the origin of the complex or of thefactors. This is somewhat the same idea as that of Bateson when he statesthat it is easy to understand the origin of a recessive character butdifficult to conceive the origin of a dominant. 

The point, however, which I desire most to emphasise is that theinvestigations we have been discussing are concerned with variations whichhave no relation whatever to adaptation, and afford no explanation of theevolution of adaptations. These variations perform no function in the lifeof the individual, have no relation to external conditions, either in thesense of being caused by special conditions or fitting the individual tolive in special conditions. A still more important fact is that they donot explain the origin of metamorphosis. They do not arise by ametamorphosis: in the case of the rose comb of fowls the chick is nothatched with a single comb which gradually changes into a rose comb, butthe rose comb develops directly from the beginning. Mutationists andMendelians do not seem in the least to appreciate the importance ofmetamorphosis or of development generally in considering the relation ofthe mutations or factors which they study to evolution in general, becausethey have not grasped the fact that there are two kinds of characters tobe explained, adaptational and non-adaptational. T. H. Morgan, forexample, [Footnote: _A Critique of the Theory of Evolution_, p. 67(Princeton, U. S. A. , and London, 1916). ] describes a mutation in_Drosophila_ consisting in the loss of the eyes, and triumphantly remarks:'Formerly we were taught that eyeless animals arose in caves. This caseshows that they may also arise suddenly in glass milk-bottles by a changein a single factor. ' As it stands the statement is perfectly true, but itis obvious that the writer does not believe that the darkness of cavesever had anything to do with the loss of eyes. It is almost as though aman should discover that blindness in a certain case was due to acongenital, i. E. Gametic, defect, and should then scoff at the idea thatany person could become blind by disease. Some of those who specialise inthe investigation of genetics seem to give inadequate consideration toother branches of biology. It is a well-established fact that in the mole, in _Proteus_, and in _Ambtyopsis_ (the blind fish of the Kentucky caves), the eyes develop in the embryo up to a certain stage in a perfectly normalway and degenerate afterwards, and that they are much better developed inthe very young animal than in the adult. Does this metamorphosis takeplace in the blind _Drosophila_ of the milk-bottle? The larva of the flyis, I believe, eyeless like the larvae of other Diptera, but Morgan saysnothing of the eye being developed in the imago or pupa and thendegenerating. There is therefore no relation or connexion between themutation he describes and the evolution of blindness in cave animals. Itis a truth, too often insufficiently appreciated by biologists, that soundreasoning is quite as important in science as fact or experiment. Loeb[Footnote: _The Organism as a Whole_, p. 319 (New York and London, 1916). ]also endeavours to prove that the blindness of cave animals is no evidenceof the influence of darkness in causing degeneration of the eyes. Herefers to experiments by Uhlenhuth, who transplanted eyes of youngSalamanders into different parts of their bodies where they were no longerconnected with the optic nerves. These eyes underwent a degeneration whichwas followed by a complete regeneration. He showed that this regenerationtook place in complete darkness, and that the transplanted eyes remainednormal when the Salamanders were kept in the dark for fifteen months. Hence the development of the eyes does not depend on the influence oflight or on the functional action of the organs. But it must be obvious toany biologist who has thoroughly considered the problem, that thisexperiment has little to do with the question of the cause of blindness incave animals. No one ever supposed that cave fishes became blind infifteen months, or in fifteen years. The experiment cited merely provesthat in the individual the embryonic or young eye will continue developingby heredity even after it is transplanted and in the absence of light. Butthe eye of the Mammal normally develops in the uterus in the absence oflight. 

In his remarks concerning _Typhlogobius_, a blind fish on the coast ofsouthern California, Loeb seems to be mistaken with regard to the facts. He states that this fish lives 'in the open, in shallow water under rocks, in holes occupied by shrimps. ' According to Professor Eigenmann the samespecies of shrimp is found all over the Bay of San Diego, and isaccompanied by other genera of goby, such as _Clevelandia_ and_Gillichthys_, which have eyes; but these fishes live outside the holes, and only retreat into them when frightened, while the blind species isfound only at Point Loma, and never leaves the burrows of the shrimp. Itwould appear, therefore, that _Typhlogobius_ lives in almost if not quitecomplete darkness, instead of being, as Loeb states, 'blind in spite ofexposure to light, ' while the closely allied forms which are exposed tolight are not blind. 

Loeb states, on the authority of Eigenmann, that all those forms whichlive in caves were adapted to life in the dark before they entered thecave, because they are all negatively heliotropic and positivelystereotropic, and with these tropisms would be forced to enter a cavewhenever they were put at the entrance. Even those among the Amblyopsidaewhich live in the open have the tropisms of the cave dweller. But theselatter are not blind, and the argument only tends to show that the blindfish _Amblyopsis_ entered the caves before it was blind. Nocturnal animalsgenerally must be said to be negatively heliotropic, but these usuallyhave larger and more sensitive eyes than the diurnal. 

It is said, however, that _Chologaster agassizii_, which is not blind, lives in the underground streams of Kentucky and Tennessee, but I think itis open to doubt whether it is a species entirely confined to darkness. 

Another point which Loeb omits to mention is the absence of pigment incave animals, especially Vertebrates such as _Amblyopsis_ and _Proteus_. If absence of light is not the cause of blindness in these cases, how isit that the blindness is always associated with absence of pigment, sincewe know that the latter in Fishes and Amphibia is due to the absence oflight? It has been shown that _Proteus_ when kept in the light developssome amount of pigment, although it does not become pigmented to the samedegree as ordinary Amphibia. We have here, I think, an example of theessential difference between mutations and somatic modifications. Absenceof the gametic factor or factors for pigmentation results in albinism, andno amount of exposure to light produces pigmentation in albinos, _e. G. _albino Axolotls which are well known in captivity. Absence of light, onthe other hand, prevents the development of pigment. The questiontherefore is whether the somatic modification is inherited. The fact that_Proteus_ does not rapidly become as deeply coloured when exposed to lightas ordinary Amphibia shows that the gametic factors for pigmentation havebeen modified as well as the somatic tissues. 

Loeb attributes the blindness of cave fishes to a disturbance in thecirculation and mutation of the eyes originally occurring as a mutation. But how could an explanation of this kind be applied to the case of_Anableps tetrophthalmus_, in which each eye is divided by a partition ofthe cornea and lens into an upper half adapted for vision in air and alower half for vision in water? This fish lives in the smooth water ofestuaries in Central America, and swims habitually with the horizontalpartition of the lens level with the surface of the water. It isimpossible to understand in this case, firstly, how a mutation could causethe eyes to be divided and doubly adapted to two different opticconditions, and, secondly, how at the same time a convenient 'tropism'should occur which caused the animal to swim with its eyes half in andhalf out of water. Are we to suppose that the upper half of the body oreye had a positive heliotropism and the lower half a negativeheliotropism? The fact is that the fish swims at the surface in order towatch for and feed on floating particles. The tropism concerned is thefood tropism, but what is gained by calling the search for food common toall active animals a tropism, and how is the search for food before thefood is perceptible to the senses, before it can act as a stimulus on afood-sensitive substance in the body, to be compared to a tropism at all?

Loeb undertakes to prove that the organism as a whole acts automaticallyaccording to physicochemical laws. But he misses the question of evolutionaltogether. For example, he quotes Gudernatsch as having proved that legscan be induced to grow in tadpoles at any time, even in very youngspecimens, by feeding them with thyroid gland. Loeb writes: 'The earlierwriters explained the growth of the legs in the tadpole as a case of anadaptation to life on land. We know through Gudernatsch that the growth ofthe legs can be produced at any time by feeding the animal with thethyroid gland. ' Obviously he thinks that these two propositions arecontradictory to each other, whereas there is no contradiction, betweenthem at all. Loeb actually supposes that the thyroid is the cause of thedevelopment of the legs. Logically, if this were the case it would followthat if we fed an eel or a snake with thyroid it would develop legs likethose of a frog, and if a man were injected with extract of the testes ofa stag he would develop antlers on his forehead. It will be obvious tomost biologists that the thyroid, whether that of the tadpole itself orthat which is supplied as food, only causes the development of legsbecause the hereditary power to develop legs is already present. Thequestion is how this hereditary power was evolved. Legs _are_ anadaptation to life on land. What we have to consider and to investigate iswhether the legs arose as a gametic mutation or as a direct result oflocomotion on land. 

The general result of clinical and experimental evidence is to show thatthe hormone of the thyroid is necessary to normal development. The arrestof development in cretinous children is due to some deficiency of thyroidsecretion, and is counteracted by the administration of thyroid extract. Excess of the secretion produces a state of restlessness and excitementassociated with an abnormally rapid rate of metabolism and protrusion ofthe eye-balls (Graves' disease). The physiological text-books, however, say nothing of precocity of development in children as a result ofhyperthyroidism. This, however, is undoubtedly what occurs in the case oftadpoles. The legs would naturally develop at some time or other, after aprolonged period of larval life. Feeding with thyroid causes them todevelop at once. I have repeated Gudernatsch's experiment with thefollowing results:--

This year I had a considerable number of tadpoles of the common Englishfrog, which were hatched between March 26 and March 29. On April 12, when they had all passed the stage of external gills and developedinternal gills and opercula, I divided them into two lots, one in ashallow pie-dish, the other in a glass cylinder. To one lot I gave aportion of rabbit's thyroid, to the other a piece of rabbit's liver. Theyfed eagerly on both. Afterwards I obtained at intervals of a week or sothe thyroid of a sheep. I have seen no precise details of Gudernatsch'smethod of feeding tadpoles, but my own method was simply to put a piece ofthyroid into the water containing the tadpoles and leave it there forseveral days, then to take it out and put in another piece, changing thewater when it seemed to be getting foul. 

April 22. Noticed that the non-thyroid tadpoles were larger than those fedon thyroid. Changed the former into the pie-dish and the latter into theglass jar, to make sure that the difference in size was not due to largerspace. 

May 3. Only eighteen of the non-thyroid tadpoles surviving, owing to thewater having become foul, but these are three times as large as those fedon thyroid. In the latter no trace of hind-legs was visible, but theabdominal region was much emaciated and contracted, while the head regionwas broader. 

May 4. Noticed minute white buds of hind-legs in the thyroid-fed tadpoles. 

May 6. A number of the thyroid-fed were dying, and the skin and opercularmembranes were swollen out away from the tissues beneath. 

 Largest normal tadpole, 2. 7 cm. Long. Body, 1. 0 " tail, 1. 7 " Largest thyroid-fed tadpole, 1. 1 cm. Long. Body, 0. 5 " tail, 0. 6 "

May 10. A great number of the thyroid-fed dead and the rest dying, lyingat the bottom motionless. They now had the tail much shorter, and thefore-legs showing as well as the hind, but the latter not very long, andwithout joints or toes. 

Period from first feeding with thyroid, thirty days. I now decided to feedthe controls with thyroid, expecting that as they were large and vigorousthey would have strength enough to complete the metamorphosis and becomefrogs. 

May 15. Fed the controls with thyroid for first time. 

 The smallest of them was in total length 1. 7 cm. Body, 0. 7 " tail, 1. 0 " The largest measured was in total length 2. 2 " body, 0. 8 " tail, 1. 4 "

May 25. All but two of the tadpoles dead. The tails were only half theoriginal length, all had well-developed hind-legs, some with toes, but thefore-legs were beneath the opercula, not projecting from the surface. 

 Smallest total length, 1. 2 cm. Body, 0. 5 " tail, 0. 7 " Largest total length, 1. 8 " body, 0. 7 " tail, 1. 1 "

These last measurements were made after the tadpoles had been preserved inspirit, and were therefore doubtless somewhat less than in the freshcondition. Making allowance for this it is evident that the tails hadundergone reduction as part of the metamorphosis, but the body was alsoshorter. There is some reason therefore for concluding that actualreduction in size of body occurs as the result of metamorphosis induced bythyroid feeding. As in the other case the skin and opercular membraneswere distended by liquid beneath them. 

The total period of the change in this second experiment was ten days. 

I conclude that the amount of thyroid eaten was so excessive as to causepathological conditions as well as precocious metamorphosis, so that theanimals died without completing the process. 

On June 10 I still had four tadpoles which had never had thyroid, but onlypieces of meat, earthworm, or fish. These were very much larger than anyof the others, were active and vigorous, and the largest one showed smallrudiments of hind-legs, the others none at all. 

CHAPTER VII

 Metamorphosis And Recapitulation

As one of the most remarkable examples of metamorphosis and recapitulationin connexion with adaptation we will consider once more the case of theFlat-fishes which I have already mentioned in an earlier chapter. Thesefishes offer perhaps the best example of the difference betweengametogenic mutations and adaptive modifications. In several speciesspecimens occur occasionally in which the asymmetry is not fullydeveloped. [Footnote: See 'Coloration of Skins of Fishes, especially ofPleuronectidae, ' _Phil. Trans. Royal Soc_. , 1894. ] These abnormalities aremost frequent in the Turbot, Brill, Flounder, and Plaice. The chiefabnormal features are pigmentation of the lower side as well as of theupper, the eye of the lower side, left or right according to the species, on the edge of the head instead of the upper side, and the dorsal fin withits attachment ceasing behind this eye, the end of the fin projectingfreely forwards over the eye in the form of a hook. Such specimens havebeen called ambicolorate, but it is an important fact that they are alsoambiarmate--that is to say, the scales or tubercles which in the normalFlat-fish are considerably reduced or absent on the lower side, in theseabnormal specimens are developed on the lower side almost as much as onthe tipper. Minor degrees of the abnormality occur: in Turbot with thehook-like projection of the dorsal fin the lower side of the head is oftenwithout pigment, while the rest of the lower side is pigmented. Lessdegrees of pigmentation of the lower side occur without structuralabnormality of the eye and dorsal fin. 

There is no evidence that these abnormalities are due to abnormalconditions of life. One specimen of Plaice of this type was kept alive inthe aquarium, and it lay on its side, buried itself in the sand, and whendisturbed swam horizontally, like a normal specimen. The abnormalities areundoubtedly mutations of gametic origin. The development of one of theseabnormal specimens from the egg has not to my knowledge been traced, butthere is no reason to suppose that the fish develops first into the normalasymmetrical condition and then changes gradually to the abnormal conditiondescribed. On the contrary, everything points to the conclusion that theabnormality is an arrest or incomplete occurrence of the normal process ofdevelopment, _i. E. _ of the normal metamorphosis. T. H. Morgan, in a volumepublished some years ago, [Footnote: _Evolution and Adaptation_. ] putforward the extraordinary view that the Pleuronectidae arose fromsymmetrical fishes by a mutation which was entirely gametogenetic andentirely independent of habits or external conditions, and then findingitself with two eyes on one side of its head, and no air-bladder, adoptedthe new mode of life, the new habit of lying on the ground on one side inorder to make better use of its asymmetrically placed eyes. According tothis view habits have been adapted to structure, not structure to habits. We are thus to believe that Amphibia came out of the water and breathedair because by an accidental mutation they possessed lungs and a pulmonarycirculation capable of atmospheric respiration. Such is the result ofapplying conclusions derived from phenomena of one kind to phenomena of atotally different kind. One of the chief differences between structuralfeatures and correlations which are adaptive from those which are not isthe process of metamorphosis, where we see the structure changing inthe individual life history as the mode of life changes. The egg of theFlat-fish develops into a symmetrical pelagic larva similar to that ofmany other marine fishes. The larva has an eye on each side of its headand swims with its plane of symmetry in a vertical position: it has alsocolour on both sides equally. When the skeleton begins to develop thetransformation takes place: the eye of one side, left in some species, right in others, moves gradually to the edge of the head and then on tothe other side. The dorsal fin extends forward, preserving its originaldirection, and so passes between the eye that has changed its position andthe lower side of the fish, on which that eye was originally situated. Insome cases this extension of the fin takes place earlier and the eyepasses beneath the base of the fin to reach the other side. Any one whotakes the trouble to make himself acquainted with the facts will see thatthe three chief features of the Pleuronectid--namely, the position of theeyes, the extension of the dorsal and ventral fins, and the absence ofpigment from the lower side--are not structurally correlated with oneanother at all as changes in different parts of the organism in a mutationare said to be, but are all closely related to their functions in the newposition of the body. A mutation consisting in general asymmetry would becomprehensible, but the head of the Pleuronectid is not asymmetrical in ageneral sense, but only so far as to allow of the changed position of theeyes. The posterior end of the skull is as symmetrical as in any otherfish, and in some cases the mouth and jaws are also symmetrical, entirelyunaffected by the change in the position of the eyes. In other cases thejaws are asymmetrical in a direction opposite to that of the eyes, thereis no change of position but a much greater development of the lower halfof the jaws, reduction, with absence of teeth, of the upper half. In thelatter case the fish feeds on worms and molluscs living on the ground andseized with the lower half of the jaws, in the former the food consists ofsmall fish swimming above the Flat-fish and seized with the whole of thejaws (Turbot, Halibut, etc. ). 

I contend, then, that the mode in which the normal Flat-fish develops isquite different from that in which mutations arise. T. H. Morgan[Footnote: _A Critique of the Theory of Evolution_ (1916), p. 18. ] statesthat a variation arising in the germ-plasm, no matter what its cause, mayaffect any stage in the development of the next individuals that arisefrom it. In certain cases this is true, that is to say, when there arevery distinct stages already. For example, a green caterpillar becomes awhite butterfly with black spots. A mutation might affect the black spots, an individual might be produced which had two spots on each wing insteadof one, and no sign of this mutation would be evident in the caterpillar. But my contention is that when this mutation occurred, the originalcondition of one spot would not be first developed and then graduallysplit into two. Morgan proceeds to state clearly what I wish to insistupon concerning mutations. He writes that in recent times the idea thatvariations are discontinuous has become current. Actual experience, hetells us, shows that new characters do not add themselves to the line ofexisting characters, but if they affect the adult characters, they changethem without as it were passing through and beyond them. 

Now in the case of the ancestors of the Flat-fish the adult and the larvamust have had the same symmetry with regard to eyes and colour and thedorsal fin terminated behind the level of the eyes. Thus the variationswhich gave rise to the Flat-fish were not discontinuous but continuous. Ineach individual development now, not merely hypothetically in theancestor, the condition of the adult arises by an absolutely continuouschange of the eyes, fins, and colour. Such a continuous change cannot beexplained by a discontinuous variation, _i. E. _ a mutation. Theabnormalities above mentioned on the other hand, although they doubtlessarise from the same kind of symmetrical larva as the normal Flat-fish, anddevelop by a gradual and continuous process, do not presumably passthrough the condition of the normal adult Flat-fish and then changegradually into the condition we find in them. As compared with the normalFlat-fish they arise by a discontinuous variation, they are mutations, whereas the normal Flat-fish as compared with its symmetrical ancestorarises by a continuous change. 

In order to make my meaning clear I must point out that I have been usingthe word continuous in a different sense from that in which it is used byother biologists, Bateson for example. The word has been appliedpreviously to variations which form a continuous series in a large numberof individuals, each of which differs only slightly from those mostsimilar to it. No two individuals are exactly alike, and thus suchcontinuous variations are universal. According to the theory of naturalselection the course of evolutionary change in any organ or characterwould form a similar continuous series, the mean of each generationdiffering only by a small difference from that of the preceding. Accordingto the modern mutationists such small differences are to be calledfluctuations, and have no effect on evolution at all, are not evenhereditary, are not due to genetic factors in the gametes. Discontinuousvariations, on the other hand, are as a rule differences in an individualfrom the normal type and from its parents of considerable degree, and areconspicuous: these are what are called mutations. 

The mutationists and Mendelians have not shown how the essentialcharacteristics of mutations are to be reconciled with the facts ofmetamorphosis, or with recapitulation in development which is so oftenassociated with metamorphosis. T. H. Morgan is the only mutationist, sofar as my reading has gone, who has attempted to do this, and he seems tome to have failed to understand the difficulties or even the nature of theproblem. He points out that the embryos of Birds and Mammals have gillslits representing the same structures as those of the adult Fish, but theyoung stage of the Fish also possessed gill slits, therefore it is 'moreprobable that the Mammal and Bird possess this stage in their developmentsimply because it has never been lost. ' He concludes therefore that thegill slits of the embryo Bird represent the gill slits of the embryo Fish, and not the adult gill slits of the Fish, which have been in somemysterious way pushed back into the embryo of the Bird. 

Morgan evidently does not realise that the Birds and Reptiles must havebeen derived from Amphibia, and that the embryo Reptile or Bird with gillslits and gill arches is merely a tadpole enclosed in an egg shell. TheFrog in its adult state differs much from a Fish, while the larva in itsgill arches and gill slits resembles a Fish. Morgan contends that the newcharacters do not add themselves to the end of the line of alreadyexisting characters. But in the case of the Frog this is exactly what theyhave done. The existing characters were in this case the gill arches andslits. Those who believe in recapitulation do not suppose that the animalhad to live a second life added on to the life of its ancestors and thatthe new characters appeared in the second life. They believe that in theancestor a certain character or general structure of body when developedpersisted without change throughout life like the gill arches and slits ina Fish. At some stage of life before maturity this character underwent achange, and in the descendants the development of the original characterand the change were repeated by heredity. There is no 'mysterious pushingback of adult characters into the embryo, ' although it is possible or evenprobable that in some cases the change gradually became earlier in thelife history: it is the new character which is pushed back, not the adultcharacter of the ancestor. 

It is perfectly true, as Morgan says, that new characters which arise asdiscontinuous variations--in other words, those kinds of variation whichare called mutations--do not add themselves to the line of alreadyexisting characters, but 'change the adult characters without as it werepassing through and beyond them. ' The mutations which Morgan describes inhis own experiments on _Drosophila_ illustrate this in every case. In nocase is the original organ or character, _e. G. _ wings, of the normal Flyfirst developed and then changed by a gradual continuous process into thenew character. It might perhaps be said that this took place in the pupa, but that seems impossible, for the complete wing is not fully developed inthe pupa. The same truth is equally apparent in the mutations describedin _OEnothera_. It follows, therefore, that none of the evolutionarychanges which have produced what are called recapitulations can have beendue to changes of that kind which is known as mutation. 

The abnormalities in Pleuronectidae to which I have referred are of thekind usually regarded as due to arrested development. But closerconsideration gives rise to doubt concerning the validity of thisexplanation. It might be supposed that the attached base of the dorsal finis unable to extend forward because the eye on the edge of the head is inthe way, but if the metamorphosis is arrested, why should the fin growforward in a free projection? I have described a very abnormal specimen ofTurbot in a paper communicated to the Zoological Society of London, [Footnote: _Proc. Zool Soc. _, 1907. ] and in that paper have discussedother possible explanations of these mutations. In the specimen to which Irefer the pigmentation instead of being present on both sides wasreversed: the lower side was pigmented from the posterior end to the edgeof the operculum (Plate II, fig. 2), while the upper side was unpigmentedexcepting a scattering of minute black specks and a little pigment on thehead (Plate II. , fig. 1). 

 [Illustration: PLATE II, Fig. 1 and Fig. 2, Abnormal Specimen Of Turbot]

I have suggested that the explanation here is that in the zygote theprimordia of a normal body and a reversed head have been united together. We may suppose that different parts of the body are represented in thegametes by different determinants or factors, and therefore it is possiblethat these factors may be separated. In the specimen we are consideringthe body is normal or nearly so, with the pigmentation on the left side, which is normal for the Turbot, while the head has both eyes with somepigment on the right side and the left side unpigmented. Reversedspecimens occasionally occur in many species of Pleuronectidae, and if thedeterminants for a reversed head and a normal body were united in onezygote, the curious abnormality observed might be the result. It is just apossibility that if this fish which was only 4. 4 cm. Long had lived toadult size, the upper side would have become pigmented under the influenceof light, while the strong hereditary influence would have prevented thedisappearance of the pigment from the lower side. In that case the adultcondition would have been similar to that of ordinary ambicoloratespecimens, but reversed, with eyes on the right side instead of the left. Other explanations of the more frequent ambicolorate mutation arepossible: the body may consist of two left sides instead of a left andright, joined on to a normal head. But the first suggestion seems the moreprobable, as two rights or two lefts would not be symmetrical. Supposingthe head and body not properly to belong to each other, one being reversedand one normal, we can in a way understand why the dorsal fin does notform the usual connexion with the edge of the head, because thedeterminants would not be in the normal intimate relation to each other. In thus writing of reversed and normal it must be understood that theformer word does not mean merely turned over, for in that case right sideof the body would be joined to the left side of the head, and the dorsalfin would be next to the ventral side of the head, which is not the case. What is meant is that a left side of the body which is normally pigmentedis joined to a left side of the head which instead of having both eyes hasneither, the two eyes being on the right side of the head which is joinedto the right side of the body, and this is normal and unpigmented. Thedorsal fin belonging to the normal sinistral body would therefore have acongenital tendency in the metamorphosis to unite with the head on theouter side of the original lower or right eye after it has moved to theleft side. Actually, however, in this abnormal specimen it finds itself onthe outer side of the left eye which has passed to the right side, and ithas no tendency to unite with this part of the head. At the same time ithas no tendency to bend over at an angle to reach the outer side of theright eye, and therefore it grows directly forward without attachment tothe head at all. 

It will be seen, therefore, that what is changed in relative position inthese mutations is not the actual parts of the body, but merely the_characters_ of those parts. In a sinistral Flat-fish, whether it isnormally sinistral like the Turbot or abnormally like a 'reversed'Flounder, the viscera are in the same position as in a dextral specimen:the liver is on the left side, the coils of the intestine on the right. Thus in a reversed or sinistral Flounder, which is normally dextral, theleft side which is uppermost is still the left side, but it has colour andtwo eyes, whereas in the normal specimen the right side has thesecharacters and not the left. Thus we are forced to conceive of thedeterminants in the chromosomes of the fertilised ovum which correspond tothe two sides of the body, as entirely distinct from the determinantswhich cause the condition or 'characters' of the two sides, unless indeedwe suppose that determinants of right side with eyes and colour occur insome gametes and of right side without eyes and colour in others, and viceversa, and that homozygous and heterozygous combinations occur infertilisation. On this last hypothesis the mutation here considered mightbe a heterozygous specimen, with the dextral condition dominant in thehead and the sinistral in the body. Or it might be somehow due to whatMorgan and his colleagues have called crossing over in the segregation ofheterozygous chromosomes, so that a part corresponding to a sinistral bodyis united with a part corresponding to a dextral head. 

My conclusion from the evidence is that any process of congenitaldevelopment may in particular zygotes exhibit a mutation, a departure fromthe normal. We need not use the term heredity at all, or if we do, mustremember that in the present argument it does not refer to anytransmission from the parent. The factors in the gametes of the normalFlat-fish egg cause the normal metamorphosis to take place after thelarval symmetry has lasted a certain time. In occasional individuals thefactors whatever they are, portions of the chromosomes or arrangement ofthe chromosomes or anything else, are different from those of the normalegg, and in consequence the abnormalities above described are developed. But the chief fact which I cannot too strongly emphasise is that thedevelopment of the abnormality from the symmetrical larva is direct, whether it is merely an arrest of development or an abnormal combinationof reversed and normal parts. The abnormal development is not due to achange occurring _after_ the normal asymmetry has been developed. Theseabnormalities are true mutations. 

The evolution of the normal Flat-fish, on the other hand, was obviouslydue to a change of a different kind. Here we are dealing with the changefrom a symmetrical fish to the asymmetrical. Judging from what takes placein other mutations, it was quite possible for asymmetry to have developeddirectly from the egg, in consequence of some difference in thechromosomes of the nucleus. It has been shown that placing a fish egg fora short time in MgCl[2] [Footnote: Stockard, _Arch. Eut. Mech. _, xxiii. (1907). ] causes a cyclopean monstrosity to be developed in which the twoeyes are united into one: but the two eyes do not develop separately firstand then gradually approach each other and unite, the development of theoptic cups is different from the first. In the normal Flat-fish theevolution that has occurred is the original development of the symmetricalfish, and the subsequent _continuous gradual_ change in eyes, fin, andcolour to the adult Flat-fish as we see it. All the evidence accumulatedby the experiments and observations of mutationists and Mendelians goes toprove that this change is of an entirely different kind from thosevariations which are described as mutations, or as loss or addition ofgenetic factors. 

This being the case, we have to inquire what is the explanation of theevolution of the normal metamorphosis. 

The important fact is that the original symmetrical structure of the larvaand the asymmetrical structure of the adult Flat-fish correspond to thedifferent positions of the body of the fish in relation to the vertical, the horizontal ground at the bottom of the water, and incidence of light. The larva swims with its plane of symmetry vertical like most otherfishes; its locomotion requires symmetrical development of muscles andfins; the two sides being equally exposed to light, it requires an eye oneach side, and the pigment on each side is also related to the equalexposure to light. The adult lying with one side on the ground has itsoriginal plane of symmetry horizontal and parallel to the ground, and onlythe other side exposed to light, and on this side only eyes and colour, _i. E. _ pigment. The change of structure corresponds with the change ofhabit. It consists in the change of position of the lower eye, theextension of the dorsal fin forwards, and the disappearance of pigmentfrom the lower side. In the actual metamorphosis these changes take placeas the skeleton develops, before the hard bones are fully formed, whilethe fish is still small, but the young Turbot reaches a much larger sizebefore metamorphosis is complete, namely, about one inch in length, thanthe young Plaice or Flounder. It is of little importance to considerwhether at the beginning of the evolution the change of position occurredlate or early in life. It may have become earlier in the course of theevolution. The important matter is to consider the evidence in support ofthe conclusion that the relation to external conditions has been the causeof the evolutionary change. We have already seen that the nature of thechange and the relation of the change of structure to the change ofconditions necessarily tend to the inference that the latter is the causeof the former. But we have to consider the particular changes in detail. 

To take first the loss of pigmentation from the lower side. I have shownexperimentally that exposure of the lower sides of Flounders to lightreflected upwards from below causes development of pigment on the lowerside. At the same time the experiments proved that the loss of pigment inthe fish in the natural state and the development of it under exposure tolight were not merely direct results of the presence or absence of lightin the individual, for in some cases the young fish were placed in theapparatus before the pigment had entirely disappeared from the lower side, and the metamorphosis went on, the lower side becoming quite white, andthe pigment only developed gradually after long exposure to the light. Inthe principal experiment four specimens were placed in the apparatus onSeptember 17, 1890, when about six months old and 7 to 9 cm. In length. One of these died on July 1, 1891, and had no pigment on the lower side. The other three all developed pigment on that side. In one it was firstnoticed in April 1891, and in the following November the fish was 22 cm. Long and had pigmentation over the greater part of the lower side (PlateIII. ). Microscopically examined, the pigmentation was found to consist ofblack and orange chromatophores exactly similar to those of the upperside. Some hundreds of young Flounders were reared at the same time underordinary conditions and none of them developed pigment. 

It is clear, therefore, that exposure of the lower side to light andreduction of the amount of light falling on the upper side (for the topsof the aquaria used were covered with opaque material) does not cause thetwo sides to behave in the same way in respect of pigment, as they wouldif the normal condition of the fish was merely due to the difference inthe exposure to light of the two sides in the individual life. There is avery strong congenital or hereditary tendency to the disappearance ofpigment from the lower side, and this is only overcome after long exposureto the light. On the other hand, if the disappearance of the pigment weredue to a mutation, were gametogenic and entirely independent of externalconditions, there would be no development of pigment after the longestexposure. To prove that an inherited character is an acquired character isquite as good evidence as to show that an acquired character is inherited. The latter kind of evidence is very difficult to get, for the effect ofconditions in a single lifetime is but slight, and is not likely to show aperceptible inherited effect. The theory that adaptations are due to theheredity of the effects of stimulation assumes that the same stimulus hasbeen acting for many generations. 

 [Illustration: PLATE III - Flounder, Showing Pigmentation Of Lower SideAfter Exposure To Light]

It is necessary, however, to consider how far the conclusions drawn fromthese experiments are contradicted by the mutations occurring in nature, some of which have already been mentioned. We will consider firstambicolorate specimens. If the absence of pigment from the lower side innormal Flat-fishes is due to the absence of light, how is it that thepigmentation persists on the lower side of ambicolorate specimens, whichis no more exposed to light than in normal specimens? The answer is thatin the mutants the determinants for pigmentation are united with thedeterminants for the lower side of the fish. My view is that thedifferentiation of these determinants for the two sides was due in thecourse of evolution to the different exposure to light, was of somaticorigin, but once the congenital factors or determinants were in existencethey were liable to mutation, and thus in the ambicolorate specimens thereis a congenital tendency to pigmentation on the lower side, which wouldonly be overcome by exclusion of light for another series of generations. 

Mutations also occur in which part or whole of the upper side is white andunpigmented. Several such specimens are mentioned in the memoir by myselfand Dr. MacMunn in the _Phil. Trans. _ already cited, one being a Solewhich was entirely white on the lower side, and also on the upper, whichwas pigmented only over the head region from the free edge of theoperculum forwards. Since the upper sides in these specimens are fullyexposed to light in the natural state and yet remain unpigmented, it wouldappear impossible to believe that the action of light was the cause of thedevelopment of pigment on the lower sides of normal specimens in myexperiments. To some it may be so, but in my own opinion the one fact isas certain as the other. I believe the two facts can be reconciled. I hadone specimen of Plaice in the living condition which had the middle thirdof its upper surface white, and the whole of the lower side white asusual. This specimen was kept for 4-1/2 months with its _lower_ surfaceexposed to light and the upper side shaded. At the end of that periodthere were numerous small patches of pigment scattered over the lower sideprincipally in the regions of the interspinous bones, above and below thelateral line. In the area of the upper side, which was originallyunpigmented, there were also numerous small pigment spots. I believe, therefore, that in this case there were determinants for absence ofpigment not only on the lower side but on part of the upper side also, andthat so long as light was excluded from the lower side the patch on theupper side remained unpigmented in sympathy. When the congenital tendencyof the determinants on the lower side was overcome by the action of light, the white patch on the upper side also began to develop pigment. 

Lastly, I may refer again to the specially abnormal Turbot mentionedabove. In this case the lower side was over the greater part pigmented andthe upper side white, and this would appear to contradict the conclusionjust drawn concerning the piebald Plaice. But this Turbot was only 4. 4 cm. Long, and is the only case known to me where so much of the lower side waspigmented with the upper side almost entirely white. The theory ofsympathy or correlation might apply here since the lower side of the headwas unpigmented, but from the small size of the specimen and the amount ofpigment on the lower side, it seems to me most probable that if thespecimen had lived to be adult the upper side would have developed pigmentunder the action of light and the specimen would have become ambicolorate. 

When we compare the results reached by the mutationists with thoseobtained by the Mendelians we find that they tend to two differentconceptions of the relation between the gametes and the organism developedfrom them. The effect of a change in the determinants of the gametesaccording to the mutationists is evident in every part of the plant. Afactor in Mendelian experiments usually affects only one organ or one partof the organism. The factor for double hallux in fowls, for instance, maycoexist with single comb or rose comb. The general impression produced onthe mind by study of Mendelian phenomena is that the organism is a mosaicof which every element corresponds to a separate element in thechromosomes. Thus we know that what we call a single factor may cause thewhole plumage of a fowl to have the detached barbs, which constitutes theSilky character, but we also know that an animal may be piebald, stronglypigmented in one part and white or unpigmented in another. So we find inthese Flat-fish mutations mosaic-like forms which evidently result frommosaic-like factors in the gametes, or in the chromosomes of the gametes. 

Experimental evidence concerning the movement of the lower eye to theupper side and of the forward extension of the dorsal fin has not beenobtained, though years ago I made some attempts, at the suggestion of Mr. G. J. Romanes, to obtain such evidence with regard to the eye by keepingyoung Flounders, already partially metamorphosed, in a reversed position. I did not succeed in devising apparatus which would keep the young fishalive in the reversed position for a sufficiently long time. We can onlyconsider, therefore, whether those other changes can reasonably beattributed to the conditions of life. Anatomical investigation shows thatthe bony interorbital septum composed principally of the frontal bones, which in symmetrical fish passes between the eyes, is still between theeyes in the Flat-fish, but has been bent round through an angle of 90degrees on the upper side, while in the lower side a new bony connexionhas been formed on the outer side of the eye which has moved from thelower side. This connexion is due to a growth from the prefrontalbackwards to join a process of the frontal, and is entirely absent insymmetrical fishes. It is along this bony bridge that the dorsal finextends. The origin of the eye muscles and of the optic nerves ismorphologically the same as in symmetrical fishes. On the theory ofmodification by external stimuli we must naturally attribute thedislocation of the eye of the lower side to the muscular effort of thefish to direct this eye to the dorsal edge, but something may also be dueto the pressure of the flat ground on the eye-ball. There is littledifficulty in attributing the bending of the interorbitl septum topressure of the lower eye-ball against it, pressure which is probably duepartly if not chiefly to the action of the eye muscles. The formation ofthe bony bridge outside the dislocated eye is more difficult to explain, as I have never had the opportunity to study the relation of this bridgeto the muscles. It is worth mentioning that in the actual development ofTurbot and Brill the metamorphosis takes place to a considerable degreewhile the young fish is pelagic, before the habit of lying on the groundis assumed, but of course this is no evidence that the change was notoriginally caused by the habit of lying on the ground. 

With regard to the extension of the dorsal fin there is no difficulty indiscovering a stimulus which would account for it. Symmetrical fishespropel themselves chiefly by the tail; in shuffling over the ground orswimming a little above it. Flat-fishes move by means of undulations ofthe dorsal and ventral fins. Increased movement produces hypertrophy, andaccording to the theory here maintained, not merely enlargement of partsexisting, but phylogenetic increase in the number of such parts, here finrays and their muscles. In Flat-fishes the dorsal and ventral fins extendalong the whole length of the dorsal and ventral edges: the dorsal fromthe head, in some cases from a point anterior to the eyes, to the base ofthe tail, the ventral from the anus, which is pushed very far forward, tothe base of the tail, and in some species of Solidae these fins areconfluent with the caudal fin. 

Formerly it was dogmatically maintained that the effect of an externalstimulus on somatic organs or tissues could have no influence on thedeterminants in the chromosomes of the gametes to which the hereditarycharacters of the organism were due. As we have tried to show, this dogmais no longer credible in face of the discoveries concerning hormones. Thehormone theory supposes that the somatic modifications due to externalstimuli--in the case of the Flat-fish the disappearance of pigment fromthe lower side, the torsion of the orbital region of the skull, and theextension of the dorsal fin--modify the hormones given off by these parts, increasing some and decreasing others, and that these changes in thehormones affect the determinants, whatever they are, in the gametocyteswithin the body. 

Here arises an interesting question--namely, how does the hormone theoryexplain the phenomenon of metamorphosis any better than the mutationtheory? It might be agreed that if the determinants are stimulated ordeprived of stimulation, the effect of the change should logically showitself from the beginning of development, and that therefore the processof metamorphosis or indirect development does not support the hormonetheory any more than the theory of gametogenic mutations. This objectionmay be answered in the following way. The reason why the determinants giverise to the original structure first and then change it into the newstructure is probably the same as that which causes secondary sexualcharacters to develop only at the stage of puberty. By the hypothesis thenew habits and new stimuli begin to act at some stage after the completedevelopment of the original structure of the body. The differences in theoriginal hormones of the modified parts are therefore actingsimultaneously with the hormones, that is, the chemical substances derivedfrom all other parts of the body in its fully developed condition. It isvery probable that in the early stages of development the metabolism ofthe body would be considerably different from that of the adult stage, andthe same combination of hormones would not be present. We may suppose, therefore, that the determinants of the zygote have acquired a tendencyto produce the increases and decreases of tissue which constitute acertain modification, _e. G. _ the change in the position of the eyes in aFlat-fish, but the stimulus which caused this tendency has always actedwhen the adult combination of hormones was present. In consequence of thisthe developed tissues do not undergo the inherited modification until theadult combination is again present. In this way we can form a definiteconception of the reason why an adaptive modification is inherited at thesame stage in which it was produced, just as the antlers of a stag areonly developed when the hormone of the mature testis is present. At thesame time it is probable that the age at which the inherited developmenttakes place tends to become earlier in later generations, to occur in factas soon as the necessary hormone medium is present. 

The diagnostic characters, of some of the species of Pleuronectidae havebeen mentioned in an earlier part of this volume, in order to point outthat they have no relation to differences of habit or external conditions. Here it is to be pointed out that there is no evidence that they arise bymetamorphosis. The scales, for example, afford distinct and constantdiagnostic characters both of species and genera, but their peculiaritieshave not been found to arise by modification of a primitive form. Therough tubercles of the Flounder, and the scattered thornlike tubercles ofthe Turbot, develop directly, not by the continuous modification ofimbricated scales. There is, however, one scale-character among thePleuronectidae which appears to stand in direct contradiction to theconclusions drawn by me concerning scales in general. It not only developsby a gradual change, but it is a secondary sexual character developing inthe males only at maturity. The character was described by E. W. L. Holtin specimens of the Baltic variety of the Plaice, _Pleuronectes platessa_, [Footnote: _Journ. Mar. Biol. Assn. _, vol iii. (Plymouth, 1893-95. )] andconsists in the spinulation of the posterior edges of the scales, especially on the upper side, in mature males. The same condition, but toa much slighter degree, was afterwards shown by myself to occur constantlyin Plaice from the English Channel and North Sea. [Footnote: _Ibid. _, vol. Iv. P. 323. ] It occurs also in _P. Glacialis_, the representative of thePlaice in more northern seas. I have shown that the spinules develop inthe mature males not as a modification of the scale, but as separatecalcareous deposits the bases of which afterwards become united to thescale. It would seem that the development of this character is dependenton the hormone from the mature testis, and in order to conform with thearguments used by me in other cases, the spinulation should have somedefinite function in relation to the habits of the sexes, and thisfunction should involve some kind of external stimulation restricted tothe mature male. So far, however, no evidence whatever of such function orsuch stimulation has been discovered. It is possible that the case differsfrom other secondary sexual characters as the antlers of stags in onerespect, namely, that the Dab (_P. Limanda_), the Sole, and other speciesof _Solea. _ and several other Pleuronectidae have what are called etenoidscales--that is, scales furnished with spines on the posterior edge--andsince the ordinary scales of the Plaice are reduced, the spinulation ofscales in the mature male Plaice is not a new character but the retentionof a primitive character. Then the question would remain why the scales inthe mature female and immature male have degenerated, or rather why theprimitive character develops only in the mature stage of the male. 

There is one point in which this sexual dimorphism in the Plaice appearsto differ from typical cases, and which suggests that the greaterspinulation of scales in the males has no function at all in the relationsof the sexes, and is therefore not subject to and external stimulation. This point is the remarkable way in which the degree of development ofspiny armature differs in different regions and in local races, and seemsto correspond to different climatic conditions. Both Plaice and Floundersin the Baltic are much more spiny than in the North Sea, although in theFlounder no sexual difference in this respect has been noted. On the eastcoast of North America occurs _P. Glacialis_, in which the scales of themale are strongly spinulate and those of the female smooth. On the coastof Alaska females of this species seem to be more spinulate thanelsewhere. The Flounder does not occur in the Arctic, but on the westcoast of North America occurs a local form called _P. Stellatus_, scarcely distinct as a species, which has a strong development of spinytubercles all over the upper side. The Flounders of the Mediterranean aremuch less spinous than those of the North Sea or Channel. The Dab (_P. Limanda_) occurs on the American coast in a local form called _Limandaferruginea_, and in the North Pacific there is a rougher form called _L. Aspera_. In these three species therefore, apart from mutations, thenorthern forms all show a greater development of spines on the scales. Whether this is an effect of colder temperature it is difficult to say. Itis possible that the difference is due to external conditions, of whichlower temperature of the water is the most obvious, and it may be thatthese conditions have a greater effect on the male than on the female inthe Plaice. 

Sexual differences in scales, which have a function in the relations ofthe sexes, occur in a few other fishes, and these can be attributed withgood reason to mechanical stimulation. For example, in the Rajidae amongElasmobranchs the males possess on each 'wing' or pectoral two series oflarge, recurved, hooked spines. It has been stated, [Footnote: Darwin, _Descent of Man_ (2nd edit. , 1885), p. 331. ] apparently by Yarrell, thatthese spines are developed only in the breeding season. It is doubtful ifthere is any marked breeding season in these fishes, but it is probablethat the spines are absent in the immature male, as it is known that in_Raia clavata_ the adult male has sharp pointed teeth, while the youngmale and the female at all ages have broad flat teeth. It is supposed thatthe spines and perhaps the sharp teeth are used for holding the female, but it seems equally probable that these structures are really used by themales in fighting with each other. The habits of these marine fish havenot been much observed, but there is little reason to doubt that thesedifferences in scales and teeth correspond with differences of mechanicalstimulation. This does not at all imply that the scales and teeththemselves have been produced by mechanical stimulation, or that thedifference between the dermal denticles of Elasmobranchs and the scales ofTeleosteans correspond to differences of stimulation. But the degree ofdevelopment of a structure whose presence is due to gametic factors mayvery probably be modified by external stimulation, and the modificationmay become hereditary. If the views here advocated are true, the twoprocesses mutation and modification must be always acting together andaffecting the development not only of the individual but of any organ orstructure. Thus the peculiarities of antlers in stags, it seems to me, prove that the mechanical stimulation due to fighting was the cause of theevolution of antlers, that without the habit of fighting in the malesantlers would not exist. At the same time each species of the _Cervidae_has its special characters in the antlers, in shape and branching, and itwould be impossible to attribute these to differences in mode of fighting:they are due to mutation. 

In connexion with the metamorphosis of Amphibia the case of the Axolotlhas always been of very great interest. In the few small lakes near thecity of Mexico where it occurs it has never been known to undergometamorphosis but is aquatic throughout its life and breeds in thatcondition. Yet in captivity by reducing the quantity of water in which itis placed the young Axolotl can be forced to breathe air, and then itundergoes complete metamorphosis to the abranchiate condition. The samespecies in other parts of North America normally goes through themetamorphosis, like other species of the Urodela. It is evident, therefore, that the Mexican Axolotls, although they have beenperennibranchiate for a great number of generations, have not lost thehereditary tendency to the metamorphosis which changes the larvae of_Amblystoma_ elsewhere into an air-breathing terrestrial animal. This maybe regarded as evidence that the conditions of life which prevent themetamorphosis in the Mexican Axolotl have produced no hereditary effect. The fact, however, that Axolotls require special treatment to inducemetamorphosis seems to show that they have distinctly less congenitaltendency to metamorphosis than larvae of the same species, _Amblystomatigrinum_, in other parts of North America, and this difference must beattributed to the inherited effect of the conditions. The most importantof these conditions seems to be abundance of oxygen in solution in thewater, and the next in importance abundance of food in the water. Recentlyit has been shown that the metamorphosis may be induced by feedingAxolotls on thyroid gland. But there is no reason to suppose that acongenital defect of thyroid arising as a mutation was the original causeof the neoteny, _i. E. _ the peisistence of the larval or aquatic, branchiate condition. Such a supposition would imply that the associationbetween Axolotls and the peculiar Mexican lakes, supplied with oxygenatedwater by springs at the bottom, was purely accidental. Moreover, there isno evidence that there is any deficiency of thyroid in the Axolotl. Thesecretion of the thyroid gland is necessary for the normal growth anddevelopment of all Vertebrates, and we are only beginning to understandthe effects of defect or excess of this secretion. There is nothing verysurprising in the fact that excess in the case of the Axolotl causes theoccurrence of the metamorphosis which had already in numerous experimentsbeen produced by forcing the animals to breathe air. 

Metamorphosis, as in the development of gill arches and gill slits in theembryos of Birds, Reptiles, and Mammals, exhibits a recapitulation of thestages of evolution of certain organs. But in the case of other organs theabsence of recapitulation is remarkable by contrast. If, as I believe, thedevelopment of lungs and disappearance of gills was directly due to thenecessity of breathing air, it is difficult to avoid the conclusion thatthe terrestrial legs were originally evolved from some type of fishes'fins by the use of the fins for terrestrial locomotion. Yet neither theamphibian larva nor the embryo of higher Vertebrates develops anythingclosely similar to a fin. There is no gradual change of a fin-like limbinto a leg, but the leg develops directly from a simple bud of tissue. Thelarva of the Urodela is probably more primitive than the tadpole of theFrogs and Toads, and in the former the legs develop while the externalgills are still large, long before the animal leaves the water. 

It is possible that the limbs were transformed to the terrestrial typebefore the animal itself became terrestrial, the habit of swimming havingbeen partly abandoned for that of crawling or walking at the bottom of thewater, and the tail being used merely for swimming to the surface toobtain air. But the condition of the Dipnoi, which possess lungs but donot walk on land, does not support this supposition, for they possess finswhich are either filamentous or fin-like, having a central axis with rayson each side. There can be little doubt that the digits of the terrestriallimb are homologous with endoskeletal fin-rays, but the evolution of theaxis of the limb is not to be ascertained either from development orpalaeontology. The absence of metamorphosis here may perhaps be due to thefact that the lateral fins ceased to function in the earlier aquaticstages, only the caudal fin being used for swimming. If this were the casethe absence of metamorphosis in the legs is itself an adaptation, thedisuse of the paired limbs in the larva having caused the earlier fin-likestages of these limbs to disappear, while the terrestrial leg wasdeveloped later by heredity, just as the legs have disappeared in thelarvae of many insects, though fully developed in the adult. 

Metamorphosis of structure in Amphibia and in Flat-fishes corresponds tothe change of conditions of life in the free-living animal. In the case ofthe eyes of the Cave-fishes the conditions in respect of absence of lightare constant throughout life, and we find only an embryonic development ofthe eye taking place by heredity. The question arises whether, when thereis no embryonic recapitulation, it must be concluded that apparentadaptations are due to mutation and not to function or externalconditions. One case of this kind is that of the limbs of Snakes, where, if we except the vestiges of hind limbs in the Pythons, there is no traceof limbs either in the embryo or after hatching. There are several similarcases among Reptiles and Amphibia. The Slow-worm (_Anguis fragilis_) islimbless, and so are the members of the sub-class Apoda among theAmphibia. In these also rudiments of limbs are entirely absent in theembryos or larval stages. Considering the recent evolution of Snakes ascompared with the origin of lungs and loss of gills and gill slits interrestrial Vertebrates in general, we have here a remarkable contrastwhich shows in the first place the difference resulting when the change inhabits and conditions in the one case takes place from one stage of lifeto another, and in the other case the new habits are constant throughoutlife from the moment of hatching. It seems to me that in the present stateof our knowledge we cannot form a decisive opinion on the question whetherthe absence of limbs in such cases is the result of mutation or ofdisuse--that is, absence of functional stimulation. 

The power of flight is an excellent example of adaptation. It has beenevolved independently in Pterodactyls, Bats, and Birds. In the two firstgroups, and to a slight degree in the third, the expanse of the wing isformed by an extension of the skin into a thin membrane, supported by thefore-limbs. It is not necessary to argue in detail that the evolution ofthis membrane and of the modifications of bones and muscles by which it issupported and moved, can be satisfactorily explained on the theory thatmodifications due to mechanical and functional stimulation are ultimatelyinherited. In birds, however, the surface of the wing is supplied chieflyby feathers, and consideration of the matter affords no reason forsupposing that the evolution of feathers was due to any external orfunctional stimulation. It is often stated that the feathers of birds area modification of the epidermic scales of reptiles, but investigation doesnot fully confirm this statement. The reptilian scales are retained on thetarso-metatarsal region of the leg in the majority of birds, and it wouldbe expected, if the view just quoted were correct, that a transition fromscales to feathers would be visible at the ankle-joint. This, however, isnot the case. In fowls some breeds have scaly shanks and others feathered. In those with scaly legs I have found cases in winch, in the chicks, therewere two or three very minute feathers, and I have examined thesemicroscopically by means of sections of the skin. The result was to showthat the minute feathers were not a prolongation of the tips or edges ofthe scales, but arose from follicles between the scales. The scale is flatand is a fold of the epidermis not arising from an invaginated follicle. The feather, on the other hand, is a tubular structure arising from apapilla at the base of a deep follicle extending inwards from the surfaceof the skin. As the feather grows the papilla grows with it. This papillaconsists of vascular dermal, _i. E. _ mesodermic tissue, and if the featheris pulled out during growth bleeding occurs. The epidermic horny tubesplits posteriorly towards the apex of the feather, and is divided intorachis and barbs, and thus the dermal tissue within, by this time dead anddry, is exposed and is shed. Every feather is in fact an open wound, andis perhaps the only other case, in addition to that of the antlers ofstags, in which vascular mesodermic tissue is normally shed in suchconsiderable quantities. When the development of the feather is complete, growth gradually ceases, the proximal part of the feather remains tubularand does not split, and the vascular tissue within dies, shrivels, anddries up, forming the pith of the quill When the papilla recommences togrow the old feather is pushed out, and this process causes the moult. Itwould appear, therefore, that the feather must have been evolved, not by acontinuous modification from the scale but by a development of a new kindbetween the scales. I have been unable to discover hitherto any evidencesuggesting an external stimulus which could cause this remarkable processof development in feathers, or indicating that the function of flightwould involve such a stimulus. For the present, therefore, we mustconclude that feathers are not an adaptation, and not due to somatogenicmodification, but must be result of a gametogenic mutation. 

Feathers, having been evolved, served in the wings and tail as importantorgans of flight. There is reason to believe that, once present, thegrowth of feathers was modified greatly by the degree of stimulationapplied to the papillae at roots by the movement and bending strain of thefeathers. The modification of the hones and of the wing, shoulders, andsternum by the functional stimuli involved in flying are obviouslyadaptations, and in my opinion are only to be explained as the hereditaryeffects of functional stimulation, like all skeleto-muscular adaptations. The strains produced in bones by muscular contraction produce hypertrophyof the part of the bone to which the muscles are attached and thus we canunderstand the origin of the carina of the sternum in flying birds, andits absence in flightless forms. In bats and in pterodactyls also thesternum is produced into a carina along the median line. The reduction ofthe digits of the wing in birds to three, with the bones firmly unitedtogether, would follow from their use in flight and their disuse asdigits, and it would seem, from the fact that the flight-feathers musthave been always on the posterior edge of the wing, and that the ulna islarger than the radius, that the three digits which have persisted are the3rd, 4th, and 5th, and not the 1st, 2nd, and 3rd as usually taught. Acomparison of the hind-limbs of birds with those of bats and pterodactylssuggests strongly that the patagium flyers have arisen from arboreal orclimbing animals, while the birds arose from terrestrial forms whichacquired the bipedal habit, as certain reptiles have. An arboreal animalwould necessarily use all four limbs, as climbing animals actually do. Thewings of birds, on the other hand, would have arisen, from the endeavourto increase speed by movements of the fore-limbs. The perching birds wouldtherefore have arisen by later adaptations after the power of flight hadbeen evolved. 

Complete recapitulation does not occur in the development of the digits ofthe wing. Only a rudiment of a fourth digit has been found in theembryonic wing, not, as might be expected, rudiments of five digits ofwhich two disappear. The metacarpals are free, not united as in the adult, and there are separate distal carpals, which in the adult are united withthe metacarpals. In other respects the modifications of wings and sternumare so obviously adaptive that it is difficult to believe that thereduction of digits was not due to disuse. This is another of those casesin which the function to which structure is adapted is constant from thebeginning of independent life to the end, and there is some ground forbelieving that in course of time in such cases embryonic recapitulationmay be much diminished or disappear. The period of time since birds werefirst evolved is in all probability immensely greater than that which haselapsed since the blind fish, _Amblyoysis_, was modified by cave-life, sothat we can understand why the eye is developed to a certain stage in theembryo of the blind fish, although it lives in darkness all its life, while embryonic recapitulation in the wing of the bird is very incomplete. 

In another class of adaptations the embryonic or larval stage is adaptedto new conditions, while the adult condition is either less changed or notchanged at all. One of the most obvious examples of this is the allantoisin the Amniota. The embryos of Reptiles, Birds, and Mammals all developtwo embryonic or foetal membranes, the amnion and the allantois. Of thefunction or origin of the amnion little is known: to state that it isprotective affords little explanation. It seems possible that it is merelythe mechanical result of the weight of the embryo and the development ofthe allantois. The latter is a precocious hypertrophy of the cloacalbladder found in Amphibia, with the function of embryonic respiration. Inthe water the amphibian larva respires by means of gills and gill slits. In adaptation to terrestrial life it is necessary, if the free aquaticlarval stage is to be eliminated, that the embryo should be able tobreathe air before hatching. Various Amphibia show how this requirementwas met in various ways. In the South American tree-frogs of the genus_Nototrema_ the eggs are developed in a dorsal pouch of the skin of thefemale, and within this pouch the respiration of the embryo is carried onby a membranous expansion of the second and third external gills on eachside. In the Reptilia the bladder is expanded for the same function, andabsorbs oxygen and gives off carbon dioxide through the pores of theshell. It is impossible to reconcile the conception of mutation with theadaptive relation between this allantois and the expulsion of the eggenclosed in a shell on land. The transition probably came about graduallyfrom the deposition of the eggs in moist places but not in water. In themidwife toad (_Alytes obstetricans_) the male carries the eggs aboutattached to his legs, respiration is effected by enlarged external gills, and the larvae are hatched in water. In the ancestral reptiles externalgills may have helped at first, until by the enlargement of the bladderthey were rendered unnecessary. In all such cases the absorption of oxygenmust be regarded as the stimulus which caused the enlargement of therespiratory membrane. As the allantois could not be absorbed or retractedagain into the abdomen, the umbilicus was evolved--that is to say, thescar formed by the union of the folded edge between the body wall andamnion surrounding the stalk of the allantois. It would he difficult for amutationist to explain how a mutation should affect the development of thecloacal bladder to such an enormous degree, just when it was required forembryonic respiration, and cause the sides of the body to unite ventrallyat the time of hatching, cutting off the allantois and the amnion. 

T. H. Morgan [Footnote: _A Critique of the Theory of Evolution_, p. 18. ]states that a mutation of gametic origin may affect any stage in thedevelopment of the individual. This may be true when there are alreadydistinct stages in the life history. The more important question iswhether distinct stages can be caused by mutation. It is true that inheterozygous individuals characters may develop more fully in the adultstage than in the young. But when we find different stages evidentlyadapted to different modes of life, it is impossible to explain them bymutations affecting different stages of life. In such cases as the larvalstages of Insects we find the larvae have become adapted to new habitswhile the adults have remained unchanged, or have evolved quiteindependent adaptations. For example, the adults in the chief orders ofInsects have the typical three pairs of legs, while the maggots or grubsof the Diptera or Hymenoptera have no legs at all, the caterpillars ofLepidoptera have evolved pseudo-legs on the abdomen, and the larvae ofColeoptera have the ordinary legs and no more. This is the reverse ofrecapitulation: in the case of legless maggots, and caterpillars withpro-legs, the adult is more similar to the ancestor than the larva. Butthe same principle holds, that where functions and habits are different, there organs are different. No mutationist has yet produced by breedingexperiments a caterpillar without the three pairs of thoracic legs and yetdeveloping into a moth that had normal three pairs. Morgan, with all hismutations of the adult _Drosophila_, says nothing of mutants possessinglegs. The only rational conclusion is that legless larvae have lost thedisuse, since those larvae which are destitute of legs do not go in searchof food but either live in the midst of it or are fed by others, and thatthe pro-legs of the caterpillar have been developed by the muscular actionof the insect in clinging to leaves. Here again the hormone theory, although we cannot pretend to understand the matter completely, helps usto form a conception of the process of heredity and evolution. The disuseof legs in the larva affects the determinants, so that they remaininactive in the presence of the hormones produced in the body generally inthis stage. In the adult stage activity of the legs produces hormoneswhich influence the same determinants in the gametes to develop legs, butagain in the presence of the different hormones which are present in thebody generally in the adult stage. As the habits of larva and adult becamemore specialised and contrasted, the change became less and less gradual, and the intermediate stage, not being adapted to any transitional mode oflife, became an inactive pupa in which the adult organs develop. 

In conclusion I will briefly consider the attempts which have been made toprove the influence of somatic modifications or characters on the gametesby direct experiment. The method of Kammerer of inducing changes of habitor structure by conditions, and then showing that the change is in somedegree inherited, has already been mentioned. One obvious criticism ofthis evidence is that it seems to prove too much, for it is difficult tobelieve that a change produced in individuals would show so muchhereditary effect in their immediate offspring. Two other methods areconceivable by which the influence of somatic hormones might be evident. One of these is to graft ovaries or testes from one animal into anotherwhich possesses a certain somatic character, and then to see if theoffspring produced from these gonads shows any trace of the character ofthe foreign soma in which it was nourished. C. C. Guthrie [Footnote:_Journ. Exper. Zool. _ (1908), v. ] claimed to have done this in hisexperiments on hens. He grafted the ovaries of two Black Leghorn pulletsinto two White pullets of the same breed, and vice versa. The black andthe white birds bred true when mated to cocks of their own colour. Theblack hen with white ovary mated with black cock produced four blackchicks and two black chicks with white legs, the white hen with blackovary mated with white cock produced some white chicks, some black andsome white with black spots. This is held to prove that the transplantedovaries were functional, because they produced evidence of the characteroriginally belonging to them. On the other hand, the black hen with whiteovary mated with white cock produced nine white chicks, and eleven chickswhich were white spotted with black, and the white hen with black ovarymated with black cock produced not black chicks but white chicks spottedwith black. This was held to prove that the somatic characters of the"foster mothers" were transmitted. 

Davenport repeated Guthrie's experiments on different fowls, grafting theovary from a cinnamon-coloured hen into a white hen, and mating her with acinnamon-coloured cock. The chicks were exactly similar to those obtainedfrom crossing such a cock with a normal white hen, and Davenport concludesthat the engrafted ovary was not functional but had degenerated. It isknown to be almost if not quite impossible to remove the ovary completelyfrom a hen, owing to its close attachment over the great post-caval vein. At the same time it is difficult to see how Guthrie could have obtainedblack and spotted chicks from a white hen mated with, a white cock if thegrafted ovary from a black hen had not been functional. One point whichGuthrie does not mention, and of which apparently he was not aware, isthat the white of the White Leghorn is dominant to colour, theheterozygotes not being pure white but white with spots. Thus when hemated a black cock with a white hen with grafted ovary and obtainedspotted chicks, this would have been the result if the original whiteovary was functional. None of his results prove conclusively the influenceof the soma of the hen into which ovaries were grafted, but would all beexplained if some eggs were derived from the part of the original ovarynot removed in the operation, and others from the grafted ovary. 

The grafting of ovaries in Mammals has often been tried, but very rarelywith success. The introduced ovary usually dies and is absorbed. C. Foa[Footnote: _Arch. Ital. De Bid. _ (1901), Tome xxxv. ] states that he madebilateral grafts of ovaries from newborn rabbits into adult rabbits, andtwo months after the operation one of the operated females was fecundatedand produced five normal young. In other cases he placed ovaries fromnew-born young in positions far from the normal position, such as thespace between the uterus and bladder, and in one case the female sotreated became pregnant, and when killed had a single embryo in one uterusand no trace of the original ovaries in the normal position. But Foa wasnot investigating the influence of somatic characters on ova in thegrafted ovaries, and does not even mention the characters or breed of therabbits he used or of the young which were produced from the graftedovaries. Castle [Footnote: W. E, Castle and J. C. Phillips, _On GerminalTransplantation in Vertebrates_, Pub. Carnegie Institution in Washington(1911), No. 144. ] carried out seventy-four transplantations of ovariesprincipally in guinea-pigs. Out of all these only one grafted femaleproduced young. In this case the ovaries of two different blackguinea-pigs about one month old were grafted into an albino female aboutfive months old. After recovery the grafted female was kept with an albinomale. She produced six young in three pregnancies, first two, then one, and lastly died with three foetus in the uteri. All these were black, withsome red hairs among the black. One of the first two young had a whiteforefoot. In this case black is dominant, and therefore there is nothingextraordinary in the offspring from a black grafted ovary being black. Thepresence of red hairs and a white foot is no evidence of the influence ofthe foster soma, but is due to imperfect dominance. When the same male wasmated with a normal black female the offspring were black with red hairsinterspersed. 

All these experiments are open to the following criticism. It has been themain argument of this volume that there are two distinct kinds ofcharacters in all organisms--namely, those of somatogenic origin and thoseof gametogenic origin. Theory supposes that somatic modifications by meansof hormones affect the determinants in the gametes. But it is obvious thatthe black and white of Leghorn fowls and of guinea-pigs are gametogeniccharacters, and are strongly established in the gametes of theirrespective varieties. It is not even certain that the black or white hairor feathers are giving off special hormones which would or could influencethe gametes. The hormone theory only postulates such influence fromhormones issuing from tissues modified by external stimuli. It is quitecertain that the black colour in Leghorns or guinea-pigs is not due to anyexternal stimulus or influence. The experiments therefore are entirelyirrelevant to what has been called the inheritance of acquired characters. All that they can be said to prove is that an albino soma does not convertingrafted ova of black race into ova carrying the albino character. 

It is probably impossible to prove experimentally the influence of amodified soma in one generation. I have endeavoured to find a case whichwould not be open to the above criticism--that is, to find a characterwhich could be considered somatogenic and which was absent in a closelyallied variety. Most of the characters in domesticated varieties areobviously gametogenic mutations, but the lop-ear in rabbits may be, partlyat least, somatogenic. Since many breeds have upright ears, we cannot saythat disuse of the external ear has produced lop-ears in domesticatedrabbits generally, but in lop-eared breeds the ears are much enlarged; andthough this may be gametogenic, the increased weight may have been thecause of the loss of the power to erect the ears. I therefore triedgrafting ovaries from straight-eared females into lop-eared individuals. The operation was perfectly successful in seven specimens--that is to say, they recovered completely and lived for many months, up to a year or moreafterwards, but none of them became pregnant. When killed no trace ofovary was in any of them; in every case it had been completely absorbed, and the uteri and vagina were diminished in size and anaemic. For graftingI used ovaries from young rabbits of various ages from seven days to sixweeks or more, but all were equally unsuccessful. Satisfactory evidence bydirect experiment of the inheritance of somatogenic modifications due toexternal stimuli cannot be said to have been yet produced, and, as I haveshown, such evidence from the nature of the case must be very difficult toobtain. The indirect evidence, however, which has been considered in thisvolume is too strong to be ignored--namely, the case of Japaneselong-tailed fowls, that of colour on the lower sides of Flat-fishes, andthe similarity of the congenital development of the antlers in stags, tothe generally admitted effects of mechanical stimulation and injury on theskin and superficial bones of Mammals. 

The general conclusions which are logically to be drawn from our presentknowledge with regard to the problems of heredity and evolution in animalsare in my opinion as follows:--

1. All attempts to explain adaptation by gametogenic mutations, or changesin gametic factors or 'genes, ' have completely failed, as Bateson himselfhas admitted. 

2. The facts discovered concerning mutations and Mendelian heredityharmonize with the nature of the majority of specific and varietalcharacters, and with the diagnostic characters of many larger divisions inclassification. 

3. Some of the most striking cases of adaptation, such as the organs ofrespiration and circulation in terrestrial Vertebrates, and the asymmetryof Flat-fishes, are developed in the individual by a metamorphosis whichis generally regarded as a recapitulation of the ancestral evolution. Nocases of mutation or gametogenic variation hitherto described exhibit asimilar metamorphosis or recapitulation. 

4. Secondary sexual characters, usually in the male sex, correspond intheir development with the development of maturity and functional activityin the gonads, and it has been proved that the latter influence the formerby means of 'hormones' or internal secretions. The evidence concerning sexand sex-linked characters and the localisation of their factors in thechromosomes of the gametes has no bearing on the action of hormones. 

5. The facts concerning the action of hormones are beyond the scope ofcurrent conceptions of the action of factors or genes localised in thegametes and particularly in the chromosomes. According to theseconceptions, characters are determined entirely by the genes in thechromosomes, whereas in certain cases the development of organs orcharacters depends on a chemical substance secreted in some distant partof the body. 

6. It was formerly stated that no process was known or could be conceivedby which modifications produced in the soma by external stimuli couldaffect the determinants in the gametes in such a way that themodifications would be inherited. The knowledge now obtained concerningthe nature and action of hormones shows that such a process actuallyexists, and in modern theory real substances of the nature of specialchemical compounds take the place of the imaginary gemmules of Darwin'stheory of pangenesis or the 'constitutional units' of Spencer. 

7. The theory of the heredity of somatogenic modifications by means ofhormones harmonises with and goes far to explain the facts ofmetamorphosis and recapitulation in adaptive characters, and also theorigin of secondary sexual characters, their correlation with theperiodical changes in the gonads and the effects of castration. At thesame time there are some somatic sex-characters, _e. G. _ in insects andbirds, which do not appear to be correlated with changes in the gonads, and which are probably gametogenic, not somatogenic in origin. 

8. The theory of the heredity of somatogenic modifications is not inopposition to the mutation theory. The author's view is that are two kindsof variation in evolution, one somatogenic and due to external stimuli, acting either directly on passive tissues or indirectly through function, and the other gametogenic and due to changes in the chromosomes of thegametes which are spontaneous and not in any way due to modifications ofthe soma. Adaptations are due to somatogenic modifications, non-adaptivediagnostic characters to gametogenic mutations. It is a mistake to attemptto explain all the results of evolution by a principle. There are twokinds of congenital, constitutional or hereditary characters in allorganisms, namely, the adaptive and the non-adaptive, and every distincttype in classification exhibits a combination of the two. To assert thatall characters are adaptive is as erroneous as to state that allcharacters are blastogenic mutations, and therefore in their originnon-adaptive. 

9. Finally it may be urged, although the question has not been directlydiscussed in this volume, that no biologist is justified in the presentstate of knowledge in dogmatically teaching the lay public thatgametogenic characters are alone worthy of attention in questions ofeugenics and sociology. Hereditary or constitutional factors are of courseof the highest importance, but there exists very good evidence thatmodifications due to external stimulus do not perish with the individual, but are in some degree handed on to succeeding generations, and that goodqualities and improvement of the race are not exclusively due to mutationswhich are entirely independent of external stimuli and functionalactivity. It is important to produce good stock, but it is also necessaryto exercise and develop the moral, mental, and physical qualities of thatstock, not merely for the benefit of the individual, but for the benefitof succeeding generations and to prevent degeneration. 

INDEX

_Abraxas groussularioun_ and _lacticolor_Adaptations, origin of; evolution of_Agonus entaphractus_AlbinismAllantoisAllurements_Alytes obstetricans__Amblyopsis_, eyes of_Amblystoma tigrinum_Amnion_Anableps tetrophthalmus__Anas boscas_, crosses of_Anas tristis_, crossesAncel and Bouin_Anguis fragilis__Antilocapra__Antirrhinum_, crossing ofAntlers of stagsAnts, heredity of sex inAphidae, heredity of sex inApodaAxolotl, albino; metamorphosis; influence of thyroid feeding

Barred plumage in fowlsBasohBatesonBees, heredity of sex inBernard, ClaudeBerthold, A. A. Biedl and KonigsteinBionomiesBlindness in cave animals_Bombyx mori_Boring, MissBorn and FränkelBrachydactylyBresslauBrown-SéquardBühler

_Cambarus_, males ofCaponsCastle, experiments in grafting; on sexCastration; in ducks; of frog; of LepidopteraCats, heredity of colour inCave animals, absence of pigmentCephalopodaCetacea, absence of scrotumChelonia_Chologaster agassixii_Chromosomes; in mutations_Clevelandia__Colaptes_Colour-blindness; heredity ofColours, origin of, in domesticated breedsComb of fowls, uselessness ofCorpora lutea, evolution of; in viviparous lower vertebrates; origin of_Corystes cassivelaunus_Courtship, organs ofCriss-cross inheritanceCrossing overCryptorchidismCuttle-fishesCyclostomes, absence of corpora lutea inCytologyCytoplasm, in heredity

_Dafila acuta_ crosses_Daphnia_, heredity of sex inDarwin_Dasyurus_; corpora lutea; lactationDavenportDeterminantsDetermination of sexDipnoi, finsDog-fishes, oviparous and viviparousDominant characters, origin ofDoncaster; on heredity in cats_Drosophila_, blind mutation, heredity of sex, mutationsDucks, crosses ofDutch rabbit

Earthworms, sex inEclipse plumageEigenmannEimerElasmobranchs; corpus luteum inElephants, testesEugenicsEunuchEvolution, evidence of

Factors, origin ofFeathers, evolution ofFlat-fishes, mutations ofFlight, evolution ofFlounderFoa, on lactation; on graftingovariesFogesFowls, castration of; origin of breedsFractionation of Mendelian factorsFränkelFrog, thumb-pad

_Gallus bankiva_Gates, Dr. R. RugglesGeddes and ThomsonGemmulesGenital ducts_Gigas, Oenothera__GillichthysGipsy mothGoltz and EwaldGonads, hormones ofGoodale, H. D. Grafting, of ovaries or testesGraves' diseaseGudernatschGuthrie, C. C. Gynandromorphism

HaemophiliaHanauHegnerHerdwick sheep, castration inHeredity; and sexHermaphroditismHill, J. P. HornsHoussaye

_Inachus scorpio_Insects, heredity of sex inInterstitial cellsIntromittent organs

Japanese long-tailed fowls; artificial treatment of

KammererKellogKopec

Lactation, dependence on stimulation, in males; regulation of_Laevifolia, Oenothera_LamarckLamarckian theoryLane-Claypon, Miss; and Starling, on ovaries of rabbitLarvae of insects_Lata, Cenothera_Leghorn, WhiteLemon-dabLeopold and RavanaLepidoptera, castration in_Leptinotarsa__Limantria dispar_LimonLinnæusLodeLoeb, on "blind fish; on blindness in cave animals; on tadpoles and thyroidLop-eared rabbits, grafting experimentsLotsy, Professor; on crossingLutein, of corpora lutes

Male characters in femaleMallard crossesMammary glands; origin of rudimentary in maleMarshall; and JollyMarsupials, relation of foetus to pouch; scrotum ofMasked crabMeisenheimer; thumb-pad of frog_Mendel's Principles of Heredity_Mendelism; and castrationMenstruationMetamorphosis; in Flat-fishes; causes of; and hormones; and diagnostic charactersMichaux, Midwife toad, Milk glands, Mole, eyes of, Monotremata, origin of milk glands, Morgan, T. H. , on blindness in cave animals, on mutations, on sex:, on sex-linked heredity, on sexual dimorphism in _Drosophila_, on variation, Mutations, in antlers, Natural selection, Nuptial plumage, Nussbaum, _Nyssia zonaria_

O'Donoghue, development of milk glands, _OEnothera_, mutations, _grandiflora_, lata_, _Lamarckiana_, Onagra, species of, _Origin of Species_, Darwin's, _Ornithorhyncus_, corpus luteumOrthogenesis, Otariidae, scrotum, Ovaries, position of, Ovary, in birds, Ovulation, Pangenesis, Parthenogenesis, Parturition, Pearson, Karl, Pheasant, male, gynandramorphism inPhillips, John C. , _Philosophie Zoologique_Phoeidae, testes, _Physiology of Reproduction_, Picotee Sweet Pea, Pigeons, Pigment, absence in cave animals, Pile fowls, Pintail duck, crosses, Plaice, _Pleuronectes flesus_, _glacialis_, _platesca_, Plymouth Rock fowl, Pole-dab, Poll, Preformation, _Problems of Genetics_, Prong buck, Pro-oestrus, _Proteus_, eyes of, Prototheria, milk glands in, Rabbits, lactation in, Recapitulation, absence of, and mutations, Reptiles, corpora lutea in, Reversal, in Flat-fishes, _Rhinoderma darwinii_, Ribbert, Rieger, Rodents, testes, Romanes, GJRöntgen rays, effect on testes, Rose comb, in fowls, Rotifers, heredity of sex in, _Rubricalyx, Oenothera_, _Rubrinervis, Oenothera_, _Sacculina_, Salamanders, transplantation of eye, Sandes, Schuster, Edgar, Scrotum, origin, of, Sea-horse, Secondary sexual characters, Selheim, _Semilata, Oenothera_, Sertoli's cells, Sex, chromosomes; Mendelian theory of, Sex-Linked heredity, _Sexual Dimorphism_, Sexual dimorphism, in Rajidae, in Plaice, Shattock and Seligmann, Silkworm, Silky fowl, plumage of, Sirenia, absence of scrotum, Slow-worm, Smith, Geoffrey, Snakes, absence of limbs, Sociology, Somatic sexual characters, Species, conception of, origin of, characters of, sterility and hybridism, Spermatogenesis, in man, Starling and Lane-Claypon, on lactation, Steinach, heredity of milk glands, Sternum, carina of, Swallows, Sweet Pea, Swifts, Tadpoles, effect of thyroid inTandler and GrossTaxonomiesTeleosteans; corpora lutea in; ovarian folliclesTestes, descent ofTetraploidyThayerThumb-pad of frogThyroid-gland feedingTortoise-shell colour in catsTosa fowls, JapaneseTransplantation of gonads_Typhiogobius_

UhlenhuthUrodela, larva

Variations_Vespa vulgaris_; _germanica_Vries, De

WallartWasps; heredity of sex inWeapons, organs used asWeismannWhale, paddle ofWhite Leghorn, crossesWilson, E. B. Wing, development ofWiniwarter, vonWitchWood, T. B. , on crossing of sheepWoodland, W. Woodpecker

X chromosome

_Zeugopterus__Zoaea_