[Illustration: Frontispiece-Charles Darwin] DARWIN, AND AFTER DARWIN _AN EXPOSITION OF THE DARWINIAN THEORYAND A DISCUSSION OFPOST-DARWINIAN QUESTIONS_ BY GEORGE JOHN ROMANES, M. A. , LL. D. , F. R. S. _Honorary Fellow of Gonville and Caius College, Cambridge_ ITHE DARWINIAN THEORY FOURTH EDITION ChicagoTHE OPEN COURT PUBLISHING COMPANY1910 The Illustrations of this book (with the exception of the Frontispieceand the colored plate facing page 332) are copyrighted under the title"Darwinism Illustrated. " THE OPEN COURT PUBLISHING CO. PRESS OF THEBLAKELY-OSWALD PRINTING CO. CHICAGO [Illustration: Letter to Mr. Hegeler Transcription: Ch. Ch. Oxford: March 15th 1892. My dear Sir, As we have now agreed that the Open Court Publishing Company is to undertake the American edition of my work entitled "Darwin and after Darwin, " I have much pleasure in transferring to you the copyright thereof, with all that this includes. Thanking you very much for the kindness and liberality which have marked your conduct of these negotiations, I remain, Yours very faithfully, George J. Romanes To Edward C. Hegeler Esq. La Salle, Ill. U. S. ] PREFACE Several years ago Lord Rosebery founded, in the University of Edinburgh, a lectureship on "The Philosophy of Natural History, " and I was invitedby the Senatus to deliver the lectures. This invitation I accepted, andsubsequently constituted the material of my lectures the foundation ofanother course, which was given in the Royal Institution, under thetitle "Before and after Darwin. " Here the course extended over threeyears--namely from 1888 to 1890. The lectures for 1888 were devoted tothe history of biology from the earliest recorded times till thepublication of the "Origin of Species" in 1859; the lectures for 1889dealt with the theory of organic evolution up to the date of Mr. Darwin's death, in 1882; while those of the third year discussed thefurther developments of this theory from that date till the close of thecourse in 1890. It is from these two courses--which resembled each other in comprisingbetween thirty and forty lectures, but differed largely in otherrespects--that the present treatise has grown. Seeing, however, that ithas grown much beyond the bulk of the original lectures, I have thoughtit desirable to publish the whole in the form of three separate works. Of these the first--or that which deals with the purely historical sideof biological science--may be allowed to stand over for an indefinitetime. The second is the one which is now brought out and which, as itssub-title signifies, is devoted to the general theory of organicevolution as this was left by the stupendous labours of Darwin. As soonas the translations shall have been completed, the third portion willfollow (probably in the Autumn season), under the sub-title, "Post-Darwinian Questions. " As the present volume is thus intended to be merely a systematicexposition of what may be termed the Darwinism of Darwin, and as on thisaccount it is likely to prove of more service to general readers than toprofessed naturalists, I have been everywhere careful to avoid assumingeven the most elementary knowledge of natural science on the part ofthose to whom the exposition is addressed. The case, however, will bedifferent as regards the next volume, where I shall have to deal withthe important questions touching Heredity, Utility, Isolation, &c. , which have been raised since the death of Mr. Darwin, and which are nowbeing debated with such salutary vehemence by the best naturalists ofour time. My obligations to the Senatus of the University of Edinburgh, and to theBoard of Management of the Royal Institution, have already beenvirtually expressed; but I should like to take this opportunity of alsoexpressing my obligations to the students who attended the lectures inthe University of Edinburgh. For alike in respect of their largenumbers, their keen intelligence, and their generous sympathy, themembers of that voluntary class yielded a degree of stimulatingencouragement, without which the labour of preparing the originallectures could not have been attended with the interest and thesatisfaction that I found in it. My thanks are also due to Mr. R. E. Holding for the painstaking manner in which he has assisted me inexecuting most of the original drawings with which this volume isillustrated; and likewise to Messrs. Macmillan and Co. For kindlyallowing me to reprint--without special acknowledgment in everycase--certain passages from an essay which they published for me manyyears ago, under the title "Scientific Evidences of Organic Evolution. "Lastly, I must mention that I am indebted to the same firm forpermission to reproduce an excellent portrait of Mr. Darwin, whichconstitutes the frontispiece. G. J. R. CHRIST CHURCH, OXFORD, _April 19th, 1892. _ CONTENTS PAGE CHAPTER I. INTRODUCTORY 1 CHAPTER II. CLASSIFICATION 23 CHAPTER III. MORPHOLOGY 50 CHAPTER IV. EMBRYOLOGY 98 CHAPTER V. PALÆONTOLOGY 156 CHAPTER VI. GEOGRAPHICAL DISTRIBUTION 204 CHAPTER VII. THE THEORY OF NATURAL SELECTION 251 CHAPTER VIII. EVIDENCES OF THE THEORY OF NATURAL SELECTION 285 CHAPTER IX. CRITICISMS OF THE THEORY OF NATURAL SELECTION 333 CHAPTER X. THE THEORY OF SEXUAL SELECTION, AND CONCLUDING REMARKS 379 * * * * * APPENDIX TO CHAPTER V. 421 NOTE A TO PAGE 257 443 NOTE B TO PAGE 295 445 NOTE C TO PAGE 394 448 INDEX 451 LIST OF ILLUSTRATIONS FIG. PAGE 1. Successive forms of Paludina, from the Tertiary deposits of Slavonia 19 2. Skeleton of Seal 52 3. Skeleton of Greenland Whale 53 4. Paddle of Whale compared with Hand of Man 54 5. Wing of Reptile, Mammal, and Bird 56 6. Skeleton of _Dinornis gravis_ 61 7. Hermit crabs compared with the cocoa-nut crab 64 8. Rudimentary or vestigial hind-limbs of Python 67 9. _Apteryx Australis_ 69 10. Illustrations of the nictitating membrane in various animals named 75 11. Rudimentary, or vestigial and useless, muscles of the human ear 76 12. Portrait of a young male gorilla 78 13. Portrait of a young male child 79 14. An infant, three weeks old, supporting its own weight 81 15. Sacrum of Gorilla compared with that of Man, showing the rudimentary tail-bones of each 82 16. Diagrammatic outline of the human embryo when about seven weeks old 83 17. Front and back view of adult human sacrum 84 18. _Appendix vermiformis_ in Orang and in Man 85 19. The same, showing variation in the Orang 85 20. Human ear 86 21. Foetus of an Orang 87 22. Vestigial characters of human ears 88 23. Hair-tracts on the arms and hands of Man, as compared with those on the arms and hands of Chimpanzee 90 24. Molar teeth of lower jaw in Gorilla, Orang, and Man 93 25. Perforation of the humerus (supra-condyloid foramen) in three species of Quadrumana where it normally occurs, and in Man, where it does not normally occur 95 26. Antlers of stag, showing successive addition of branches in successive years 100 27. Fission of a Protozoön 107 28. _Hydra viridis_, partly in section 111 29. Successive stages in the division of the ovum, or egg-cell, of a worm 113 30. Ovarian ovum of a Mammal 121 31. Amoeboid movements of young egg-cells 122 32. Human ovum, mature and greatly magnified 123 33. Stages in the formation of the polar bodies in the ovum of a star-fish 125 34. Fertilization of the ovum of an echinoderm 126 35. Fertilization of the ovum of a star-fish 127 36. Karyokinesis of a typical tissue-cell (epithelium of Salamander) 129 37. Study of successive changes taking place in the nucleus of an epithelium-cell, preparatory to division of the cell 131 38. Formation and conjugation of the pronuclei in _Ascaris megalocephala_ 132, 133 39. Segmentation of ovum 135 40. The contents of an ovum in an advanced stage of segmentation, drawn in perspective 135 41. Formation of the gastrula of _Amphioxus_ 137 42. Gastrulation 138 43. Gastrula of a Chalk Sponge 139 44. _Prophysema primordiale_, an extant gastræa-form 140 45. Ideal primitive vertebrate, seen from the left side 143 46. The same in transverse section through the ovaries 144 47. _Amphioxus lanceolatus_ 145 48. _Balanoglossus_ 148 49. A large Sea-lamprey (_Petromyzon marinus_) 148 50. Adult Shark (_Carcharias melanopterus_) 149 51. Diagram of heart and gill-arches of a fish 150 52. One gill-arch, with branchial fringe attached 150 53. Diagram of heart and gill-arches in a lizard 150 54. Ideal diagram of primitive gill-or aortic-arches 151 55. The same, modified for a bird 151 56. The same, modified for a mammal 151 57. A series of embryos at three comparable and progressive stages of development, representing each of the classes of vertebrated animals below the Mammalia 152 58. Another series of embryos, also at three comparable and progressive stages of development, representing four different divisions of the class Mammalia 153 59. Diagram of geological succession of the classes of the Animal Kingdom 165 60. Skull of _Oreodon Culbertsoni_ 167 61, 62. Horns of _Cervus dicrocerus_ 168 63. Horns of _C. Matheronis_ 168 64. Horns of _C. Pardinensis_ 168 65. Horns of _C. Issiodorensis_ 168 66. Horns of _C. Sedgwickii_ 168 67. Successive stages in the development of an existing Deer's Antlers 169 68. Homocercal tail 169 69. Heterocercal tail 170 70. Vertebrated but symmetrical fin (diphycercal) 170 71. Tail of _Archæopteryx_ 171 72. Tail of modern Bird 171 73. _Archæopteryx macura_, restored 172 74. Skeleton of Polar Bear 174 75. Skeleton of Lion 175 76. Anterior limb of Man, Dog, Hog, Sheep, and Horse 176 77. Posterior limb of Man, Monkey, Dog, Sheep, and Horse 177 78. Posterior limb of _Baptanodon discus_, and anterior limb of _Chelydra serpentina_ 179 79. Paddle of a Whale 180 80. Fossil skeleton of _Phenacodus primævus_ 184 81. Bones of the foot of four different forms of the perissodactyl type 186 82. Bones of the foot of four different forms of the artiodactyl type 187 83. Feet and teeth In fossil pedigree of the Horse 189 84. _Palæotherium_. (Lower Tertiary of Paris Basin) 190 85. _Hipparion_. (New World Pliocene) 192 86. Comparative series of Brains 194 87. Ideal section through all the above stages 195 88. Skulls of Canadian Stag, _Cervalces Americanus_, and Elk 198 89. Transmutations of _Planorbis_ 200 90. Transformation of _Strombus_ 202 91. Pigeons. Drawn from life 298 92. Pigeons (_continued_). Drawn from life 299 93. Fowls. Drawn from life 300 94. Fowls (_continued_). Drawn from life 301 95. Pair of Japanese Fowls, long-tailed breed 302 96. Canaries. Drawn from life 303 97. Sebastopol, or Frizzled Goose 304 98. The Dingo, or wild dog of Australia 304 99. Dogs. Drawn from life 305 100. Dogs (_continued_). Drawn from life 306 101. The Hairless Dog of Japan 307 102. The skull of a Bull-dog compared with that of a Deer-hound 307 103. Rabbits. Drawn from life 308 104. Horses. Drawn from life 309 105. Sheep. Drawn from life 310 106. Cattle. Drawn from life 311 107. Wild Boar contrasted with a modern Domesticated Pig 312 108. Seasonal changes of colour in Ptarmigan (_Lagopus mutus_) 317 109. _Oedicneus crepitans_, showing the instinctive attitude of concealment 320 110. Imitative forms and colours in insects 322 111. The larva of Puss Moth (_Cerura vinula_) 325 112. The larva of Puss Moth in disturbed attitude 326 113. Three cases of mimicry 328 114. Two further cases of mimicry; flies resembling a wasp in the one and a bee in the other 329 115. A case of mimicry where a non-venomous species of snake resembles a venomous one 330 116. A case of mimicry where a homopterous resembles a leaf-cutting ant 332 117. Feather-footed pigeon 359 118. _Raia radiata_ 368 119. Electric organ of the Skate 369 120. Electric cells of _Raia radiata_ 370 121. The Garden Bower-bird (_Amblyornis inornata_) 382 122. Courtship of Spiders 388 123. Courtship of Spiders (_continued_) 389 124. The Bell-bird (_Chasmorhynchus niveus_) 396 125. _C. Tricarunculatus_ 397 SECTION I _EVOLUTION_ CHAPTER I. INTRODUCTORY. Among the many and unprecedented changes that have been wrought by Mr. Darwin's work on the _Origin of Species_, there is one which, althoughsecond in importance to no other, has not received the attention whichit deserves. I allude to the profound modification which that work hasproduced on the ideas of naturalists with regard to method. Having had occasion of late years somewhat closely to follow the historyof biological science, I have everywhere observed that progress is notso much marked by the march of discovery _per se_, as by the alteredviews of method which the march has involved. If we except whatAristotle called "the first start" in himself, I think one may fairlysay that from the rejuvenescence of biology in the sixteenth century tothe stage of growth which it has now reached in the nineteenth, there isa direct proportion to be found between the value of work done and thedegree in which the worker has thereby advanced the true conception ofscientific working. Of course, up to a certain point, it is notoriousthat the revolt against the purely "subjective methods" in the sixteenthcentury revived the spirit of _inductive_ research as this had been leftby the Greeks; but even with regard to this revolt there are two thingswhich I should like to observe. In the first place, it seems to me, an altogether disproportionate valuehas been assigned to Bacon's share in the movement. At most, I think, hedeserves to be regarded but as a literary exponent of the _Zeitgeist_ ofhis century. Himself a philosopher, as distinguished from a man ofscience, whatever influence his preaching may have had upon the generalpublic, it seems little short of absurd to suppose that it could haveproduced any considerable effect upon men who were engaged in thepractical work of research. And those who read the _Novum Organon_ witha first-hand knowledge of what is required for such research canscarcely fail to agree with his great contemporary Harvey, that he wroteupon science like a Lord Chancellor. The second thing I should like to observe is, that as the revolt againstthe purely subjective methods grew in extent and influence it passed tothe opposite extreme, which eventually became only less deleterious tothe interests of science than was the bondage of authority, andaddiction to _a priori_ methods, from which the revolt had set her free. For, without here waiting to trace the history of this matter in detail, I think it ought now to be manifest to everyone who studies it, that upto the commencement of the present century the progress of science ingeneral, and of natural history in particular, was seriously retarded bywhat may be termed the Bugbear of Speculation. Fully awakened to thedangers of web-spinning from the ever-fertile resources of their owninner consciousness, naturalists became more and more abandoned to theidea that their science ought to consist in a mere observation of facts, or tabulation of phenomena, without attempt at theorizing upon theirphilosophical import. If the facts and phenomena presented any suchimport, that was an affair for men of letters to deal with; but, as menof science, it was _their_ duty to avoid the seductive temptations ofthe world, the flesh, and the devil, in the form of speculation, deduction, and generalization. I do not allege that this ideal of natural history was either absoluteor universal; but there can be no question that it was both orthodox andgeneral. Even Linnæus was express in his limitations of true scientificwork in natural history to the collecting and arranging of species ofplants and animals. In accordance with this view, the _status_ of abotanist or a zoologist was estimated by the number of specific names, natural habitats, &c. , which he could retain in his memory, rather thanby any evidences which he might give of intellectual powers in the wayof constructive thought. At the most these powers might legitimatelyexercise themselves only in the direction of taxonomic work; and if aHales, a Haller, or a Hunter obtained any brilliant results in the wayof observation and experiment, their merit was taken to consist in thediscovery of facts _per se_: not in any endeavours they might make inthe way of combining their facts under general principles. Even as latein the day as Cuvier this ideal was upheld as the strictly legitimateone for a naturalist to follow; and although Cuvier himself was far frombeing always loyal to it, he leaves no doubt regarding the estimate inwhich he held the still greater deviations of his colleagues, St. Hilaire and Lamarck. Now, these traditional notions touching the severance between the factsof natural history and the philosophy of it, continued more or less todominate the minds of naturalists until the publication of the _Originof Species_, in 1859. Then it was that an epoch was marked in thisrespect, as in so many other respects where natural history isconcerned. For, looking to the enormous results which followed from adeliberate disregard of such traditional canons by Darwin, it has longsince become impossible for naturalists, even of the strictest sect, notto perceive that their previous bondage to the law of a mere ritual hasbeen for ever superseded by what verily deserves to be regarded as a newdispensation. Yet it cannot be said, or even so much as suspected, thatDarwin's method in any way resembled that of pre-scientific days, therevolt against which led to the straight-laced--and for a long time mostsalutary--conceptions of method that we have just been noticing. Where, then, is the difference? To me it seems that the difference is asfollows; and, if so, that not the least of our many obligations toDarwin as the great organizer of biological science arises from hishaving clearly displayed the true principle which ought to governbiological research. To begin with, he nowhere loses sight of the primary distinction betweenfact and theory; so that, thus far, he loyally follows the spirit ofrevolt against subjective methods. But, while always holding thisdistinction clearly in view, his idea of the scientific use of facts isplainly that of furnishing legitimate material for the construction oftheories. Natural history is not to him an affair of the herbarium orthe cabinet. The collectors and the species-framers are, as it were, hisdiggers of clay and makers of bricks: even the skilled observers and thetrained experimentalists are his mechanics. Valuable as the work of allthese men is in itself, its principal value, as he has finallydemonstrated, is that which it acquires in rendering possible the workof the architect. Therefore, although he has toiled in all the tradeswith his own hands, and in each has accomplished some of the best workthat has ever been done, the great difference between him and most ofhis predecessors consists in this, --that while to them the discovery oraccumulation of facts was an end, to him it is the means. In their eyesit was enough that the facts should be discovered and recorded. In hiseyes the value of facts is due to their power of guiding the mind to afurther discovery of principles. And the extraordinary success whichattended his work in this respect of _generalization_ immediatelybrought natural history into line with the other inductive sciences, behind which, in this most important of all respects, she has soseriously fallen. For it was the _Origin of Species_ which first clearlyrevealed to naturalists as a class, that it was the duty of theirscience to take as its motto, what is really the motto of naturalscience in general, Felix qui potuit rerum cognoscere causas. Not facts, then, or phenomena, but causes or principles, are theultimate objects of scientific quest. It remains to ask, How ought thisquest to be prosecuted? Well, in the second place, Darwin has shown that next only to theimportance of clearly distinguishing between facts and theories on theone hand, and of clearly recognising the relation between them on theother, is the importance of not being scared by the Bugbear ofSpeculation. The spirit of speculation is the same as the spirit ofscience, namely, as we have just seen, a desire to know the causes ofthings. The _hypotheses non fingo_ of Newton, if taken to mean what itis often understood as meaning, would express precisely the oppositespirit from that in which all scientific research must necessarily takeits origin. For if it be causes or principles, as distinguished fromfacts or phenomena, that constitute the final aim of scientificresearch, obviously the advancement of such research can be attainedonly by the framing of hypotheses. And to frame hypotheses is tospeculate. Therefore, the difference between science and speculation is not adifference of spirit; nor, thus far, is it a difference of method. Theonly difference between them is in the subsequent process of verifyinghypotheses. For while speculation, in its purest form, is satisfied totest her explanations only by the degree in which they accord with oursubjective ideas of probability--or with the "Illative Sense" ofCardinal Newman, --science is not satisfied to rest in any explanation asfinal until it shall have been fully verified by an appeal to objectiveproof. This distinction is now so well and so generally appreciated thatI need not dwell upon it. Nor need I wait to go into any details withregard to the so-called canons of verification. My only object is tomake perfectly clear, first, that in order to have any question to putto the test of objective verification, science must already have so faremployed the method of speculation as to have framed a question to betested; and, secondly, that the point where science parts company withspeculation is the point where this testing process begins. Now, if these things are so, there can be no doubt that Darwin wasfollowing the truest method of inductive research in allowing any amountof latitude to his speculative thought in the direction of scientifictheorizing. For it follows from the above distinctions that the dangerof speculation does not reside in the width of its range, or even in theimpetuosity of its vehemence. Indeed, the wider its reach, and thegreater its energy, the better will it be for the interests of science. The only danger of speculation consists in its momentum being apt tocarry away the mind from the more laborious work of adequateverification; and therefore a true scientific judgment consists ingiving a free rein to speculation on the one hand, while holding readythe break of verification with the other. Now, it is just because Darwindid both these things with so admirable a judgment, that he gave theworld of natural history so good a lesson as to the most effectual wayof driving the chariot of science. This lesson we have now all more or less learnt to profit by. Yet noother naturalist has proved himself so proficient in holding the balancetrue. For the most part, indeed, they have now all ceased to confoundthe process of speculation _per se_ with the danger of inadequateverification; and therefore the old ideal of natural history asconcerned merely with collecting species, classifying affinities, and, in general, tabulating facts, has been well-nigh universallysuperseded. But this great gain has been attended by some measure ofloss. For while not a few naturalists have since erred on the side ofinsufficiently distinguishing between fully verified principles ofevolution and merely speculative deductions therefrom, a still largernumber have formed for themselves a Darwinian creed, and regard anyfurther theorizing on the subject of evolution as _ipso facto_unorthodox. Having occupied the best years of my life in closely studying theliterature of Darwinism, I shall endeavour throughout the followingpages to avoid both these extremes. No one in this generation is able toimitate Darwin, either as an observer or a generalizer. But this doesnot hinder that we should all so far endeavour to follow his _method_, as always to draw a clear distinction, not merely between observationand deduction, but also between degrees of verification. At all events, my own aim will everywhere be to avoid dogmatism on the one hand, andundue timidity as regards general reasoning on the other. For everythingthat is said justification will be given; and, as far as prolongeddeliberation has enabled me to do so, the exact value of suchjustification will be rendered by a statement of at least the maingrounds on which it rests. The somewhat extensive range of the presenttreatise, however, will not admit of my rendering more than a smallpercentage of the facts which in each case go to corroborate theconclusion. But although a great deal must thus be necessarily lost onthe one side, I am disposed to think that more will be gained on theother, by presenting, in a terser form than would otherwise bepossible, the whole theory of organic evolution as I believe that itwill eventually stand. My endeavour, therefore, will be to exhibit thegeneral structure of this theory in what I take to be its strictlylogical form, rather than to encumber any of its parts by a lengthycitation of facts. Following this method, I shall in each case give onlywhat I consider the main facts for and against the positions which haveto be argued; and in most cases I shall arrange the facts in twodivisions, namely, first those of largest generality, and next a few ofthe most special character that can be found. As explained in the Preface, the present instalment of the treatise isconcerned with the theory of evolution, from the appearance of the_Origin of Species_ in 1859, to the death of its author in 1882; whilethe second part will be devoted to the sundry post-Darwinian questionswhich have arisen in the subsequent decade. To the possible criticismthat a disproportionate amount of space will thus be allotted to aconsideration of these post-Darwinian questions, I may furnish inadvance the following reply. In the first place, besides the works of Darwin himself, there are anumber of others which have already and very admirably expounded theevidences, both of organic evolution as a fact, and of natural selectionas a cause. Therefore, in the present treatise it seemed needless to gobeyond the ground which was covered by my original lectures, namely, acondensed and connected, while at the same time a critical statement ofthe main evidences, and the main objections, which have thus far beenpublished with reference to the distinctively Darwinian theory. Indeedwhile re-casting this portion of my lectures for the presentpublication, I have felt that criticism might be more justly urged fromthe side of impatience at a reiteration of facts and arguments alreadyso well known. But while endeavouring, as much as possible, to avoidoverlapping the previous expositions, I have not carried this attempt tothe extent of damaging my own, by omitting any of the more importantheads of evidence; and I have sought to invest the latter with somemeasure of novelty by making good what appears to me a deficiency whichhas hitherto obtained in the matter of pictorial illustration. Inparticular, there will be found a tolerably extensive series ofwoodcuts, serving to represent the more important products of artificialselection. These, like all the other original illustrations, have beendrawn either direct from nature or from a comparative study of the bestauthorities. Nevertheless, I desire it to be understood that the firstpart of this treatise is intended to retain its original character, as amerely educational exposition of Darwinian teaching--an exposition, therefore, which, in its present form, may be regarded as a compendium, or hand-book, adapted to the requirements of a general reader, orbiological student as distinguished from those of a professednaturalist. The case, however, is different with the second instalment, which willbe published at no very distant date. Here I have not followed withnearly so much closeness the material of my original lectures. On thecontrary, I have had in view a special class of readers; and, although Ihave tried not altogether to sacrifice the more general class, I shalldesire it to be understood that I am there appealing to naturalists whoare specialists in Darwinism. One must say advisedly, naturalists whoare specialists in Darwinism, because, while the literature of Darwinismhas become a department of science in itself, there are nowadays manynaturalists who, without having paid any close attention to the subject, deem themselves entitled to hold authoritative opinions with regard toit. These men may have done admirable work in other departments ofnatural history, and yet their opinions on such matters as we shallhereafter have to consider may be destitute of value. As there is nonecessary relation between erudition in one department of science andsoundness of judgment in another, the mere fact that a man isdistinguished as a botanist or zoologist does not in itself qualify himas a critic where specially Darwinian questions are concerned. Thus ithappens now, as it happened thirty years ago, that highly distinguishedbotanists and zoologists prove themselves incapable as judges of generalreasoning. It was Darwin's complaint that for many years nearly all hisscientific critics either could not, or would not, understand what hehad written--and this even as regarded the fundamental principles of histheory, which with the utmost clearness he had over and over againrepeated. Now the only difference between such naturalists and theirsuccessors of the present day is, that the latter have grown up in aDarwinian environment, and so, as already remarked, have more or lessthoughtlessly adopted some form of Darwinian creed. But this scientificcreed is not a whit less dogmatic and intolerant than was the moretheological one which it has supplanted; and while it usuallyincorporates the main elements of Darwin's teaching, it still moreusually comprises gross perversions of their consequences. All this Ishall have occasion more fully to show in subsequent parts of thepresent work; and allusion is made to the matter here merely for thesake of observing that in future I shall not pay attention tounsupported expressions of opinion from any quarter: I shall consideronly such as are accompanied with some statement of the grounds uponwhich the opinion is held. And, even as thus limited, I do not think itwill be found that the following exposition devotes any disproportionalamount of attention to the contemporary movements of Darwinian thought, seeing, as we shall see, how active scientific speculation has been inthe field of Darwinism since the death of Mr. Darwin. * * * * * Leaving, then, these post-Darwinian questions to be dealt withsubsequently, I shall now begin a systematic _résumé_ of the evidencesin favour of the Darwinian theory, as this was left to the world byDarwin himself. There is a great distinction to be drawn between the fact of evolutionand the manner of it, or between the evidence of evolution as havingtaken place somehow, and the evidence of the causes which have beenconcerned in the process. This most important distinction is frequentlydisregarded by popular writers on Darwinism; and, therefore, in order tomark it as strongly as possible, I will effect a complete separationbetween the evidence which we have of evolution as a fact, and theevidence which we have as to its method. In other words, not until Ishall have fully considered the evidence of organic evolution as aprocess which somehow or another _has_ taken place, will I proceed toconsider _how_ it has taken place, or the causes which Darwin and othershave suggested as having probably been concerned in this process. Confining, then, our attention in the first instance to a proof ofevolution considered as a fact, without any reference at all to itsmethod, let us begin by considering the antecedent standing of thematter. * * * * * First of all we must clearly recognise that there are only twohypotheses in the field whereby it is possible so much as to suggest anexplanation of the origin of species. Either all the species of plantsand animals must have been supernaturally created, or else they musthave been naturally evolved. There is no third hypothesis possible; forno one can rationally suggest that species have been eternal. Next, be it observed, that the theory of a continuous transmutation ofspecies is not logically bound to furnish a full explanation of _all_the natural causes which it may suppose to have been at work. Theradical distinction between the two theories consists in the oneassuming an immediate action of some supernatural or inscrutable cause, while the other assumes the immediate action of natural--and thereforeof possibly discoverable--causes. But in order to sustain this latterassumption, the theory of descent is under no logical necessity tofurnish a full proof of _all_ the natural causes which may have beenconcerned in working out the observed results. We do not know thenatural causes of many diseases; but yet no one nowadays thinks ofreverting to any hypothesis of a supernatural cause, in order to explainthe occurrence of any disease the natural causation of which is obscure. The science of medicine being in so many cases able to explain theoccurrence of disease by its hypothesis of natural causes, medical mennow feel that they are entitled to assume, on the basis of a wideanalogy, and therefore on the basis of a strong antecedent presumption, that all diseases are due to natural causes, whether or not inparticular cases such causes happen to have been discovered. And fromthis position it follows that medical men are not logically bound toentertain any supernatural theory of an obscure disease, merely becauseas yet they have failed to find a natural theory. And so it is withbiologists and their theory of descent. Even if it be fully proved tothem that the causes which they have hitherto discovered, or suggested, are inadequate to account for all the facts of organic nature, thiswould in no wise logically compel them to vacate their theory ofevolution, in favour of the theory of creation. All that it would socompel them to do would be to search with yet greater diligence for thenatural causes still undiscovered, but in the existence of which theyare, by their independent evidence in favour of the theory, bound tobelieve. In short, the issue is not between the theory of a supernatural causeand the theory of any one particular natural cause, or set ofcauses--such as natural selection, use, disuse, and so forth. The issuethus far--or where only the _fact_ of evolution is concerned--is betweenthe theory of a supernatural cause as operating immediately innumberless acts of special creation, and the theory of natural causes asa whole, whether these happen, or do not happen, to have been hithertodiscovered. This much by way of preliminaries being understood, we have next tonotice that whichever of the two rival theories we choose to entertain, we are not here concerned with any question touching the origin of life. We are concerned only with the origin of particular forms of life--thatis to say, with the origin of species. The theory of descent starts fromlife as a _datum_ already granted. How life itself came to be, thetheory of descent, as such, is not concerned to show. Therefore, in thepresent discussion, I will take the existence of life as a fact whichdoes not fall within the range of our present discussion. No doubt thequestion as to the origin of life is in itself a deeply interestingquestion, and although in the opinion of most biologists it is aquestion which we may well hope will some day fall within the range ofscience to answer, at present, it must be confessed, science is not in aposition to furnish so much as any suggestion upon the subject; andtherefore our wisdom as men of science is frankly to acknowledge thatsuch is the case. * * * * * We are now in a position to observe that the theory of organic evolutionis strongly recommended to our acceptance on merely antecedent grounds, by the fact that it is in full accordance with what is known as theprinciple of continuity. By the principle of continuity is meant theuniformity of nature, in virtue of which the many and varied processesgoing on in nature are due to the same kind of method, i. E. The methodof natural causation. This conception of the uniformity of nature isone that has only been arrived at step by step through a long andarduous course of human experience in the explanation of naturalphenomena. The explanations of such phenomena which are first given arealways of the supernatural kind; it is not until investigation hasrevealed the natural causes which are concerned that the hypotheses ofsuperstition give way to those of science. Thus it follows that thehypotheses of superstition which are the latest in yielding to theexplanations of science, are those which refer to the more reconditecases of natural causation; for here it is that methodical investigationis longest in discovering the natural causes. Thus it is only by degreesthat fetishism is superseded by what now appears a common-senseinterpretation of physical phenomena; that exorcism gives place tomedicine; alchemy to chemistry; astrology to astronomy; and so forth. Everywhere the miraculous is progressively banished from the field ofexplanation by the advance of scientific discovery; and the places whereit is left longest in occupation are those where the natural causes aremost intricate or obscure, and thus present the greatest difficulty tothe advancing explanations of science. Now, in our own day there are butvery few of these strongholds of the miraculous left. Nearly the wholefield of explanation is occupied by naturalism, so that no one everthinks of resorting to supernaturalism except in the comparatively fewcases where science has not yet been able to explore the most obscureregions of causation. One of these cases is the origin of life; and, until quite recently, another of these cases was the origin of species. But now that a very reasonable explanation of the origin of species hasbeen offered by science, it is but in accordance with all previoushistorical analogies that many minds should prove themselves unable allat once to adjust themselves to the new ideas, and thus still lingerabout the more venerable ideas of supernaturalism. But we are now inpossession of so many of these historical analogies, that all minds withany instincts of science in their composition have grown to distrust, onmerely antecedent grounds, any explanation which embodies a miraculouselement. Such minds have grown to regard all these explanations as mereexpressions of our own ignorance of natural causation; or, in otherwords, they have come to regard it as an _a priori_ truth that nature iseverywhere uniform in respect of method or causation; that the reign oflaw universal; the principle of continuity ubiquitous. Now, it must be obvious to any mind which has adopted this attitude ofthought, that the scientific theory of natural descent is recommended byan overwhelming weight of antecedent presumption, as against thedogmatic theory of supernatural design. To begin with, we must remember that the fact of evolution--or, which isthe same thing, the fact of continuity in natural causation--has nowbeen unquestionably proved in so many other and analogous departments ofnature, that to suppose any interruption of this method as betweenspecies and species becomes, on grounds of such analogy alone, well-nighincredible. For example, it is now a matter of demonstrated fact thatthroughout the range of _inorganic_ nature the principles of evolutionhave obtained. It is no longer possible for any one to believe with ourforefathers that the earth's surface has always existed as it nowexists. For the science of geology has proved to demonstration that seasand lands are perpetually undergoing gradual changes of relativepositions--continents and oceans supplanting each other in the course ofages, mountain-chains being slowly uplifted, again as slowly denuded, and so forth. Moreover, and as a closer analogy, within the limits ofanimate nature we know it is the universal law that every individuallife undergoes a process of gradual development; and that breeds, races, or strains, may be brought into existence by the intentional use ofnatural processes--the results bearing an unmistakeable resemblance towhat we know as natural species. Again, even in the case of naturalspecies themselves, there are two considerations which present enormousforce from an antecedent point of view. The first is that organic formsare only then recognised as species when intermediate forms are absent. If the intermediate forms are actually living, or admit of being foundin the fossil state, naturalists forthwith regard the whole series asvarieties, and name all the members of it as belonging to the samespecies. Consequently it becomes obvious that naturalists, in their workof naming species, may only have been marking out the cases whereintermediate or connecting forms have been lost to observation. Forexample, here we have a diagram representing a very unusually completeseries of fossil shells, which within the last few years has beenunearthed from the Tertiary lake basins of Slavonia. Before the serieswas completed, some six or eight of the then disconnected forms weredescribed as distinct species; but as soon as the connecting forms werefound--showing a progressive modification from the older to the newerbeds, --the whole were included as varieties of one species. [Illustration: FIG. 1. --Successive forms of Paludina, from the Tertiary deposits of Slavonia (after Neumayr). ] Of course, other cases of the same kind might be adduced, and therefore, as just remarked, in their work of naming species naturalists may onlyhave been marking out the cases where intermediate forms have been lostto observation. And this possibility becomes little less than acertainty when we note the next consideration which I have to adduce, namely, that in all their systematic divisions of plants and animals ingroups higher than species--such as genera, families, orders, and therest--naturalists have at all times recognised the fact that the oneshades off into the other by such imperceptible gradations, that it isimpossible to regard such divisions as other than conventional. It isimportant to remember that this fact was fully recognised before thedays of Darwin. In those days the scientifically orthodox doctrine was, that although species were to be regarded as fixed units, bearing thestamp of a special creation, all the higher taxonomic divisions wereto be considered as what may be termed the artificial creation ofnaturalists themselves. In other words, it was believed, and in manycases known, that if we could go far enough back in the history of theearth, we should everywhere find a tendency to mutual approximationbetween allied _groups of species_; so that, for instance, birds andreptiles would be found to be drawing nearer and nearer together, untileventually they would seem to become fused in a single type; that theexisting distinctions between herbivorous and carnivorous mammalswould be found to do likewise; and so on with all the largergroup-distinctions, at any rate within the limits of the samesub-kingdoms. But although naturalists recognised this even in thepre-Darwinian days, they stoutly believed that a great exception was tobe made in the case of species. These, the lowest or initial members oftheir taxonomic series, they supposed to be permanent--the miraculouslycreated units of organic nature. Now, all that I have at present toremark is, that this pre-Darwinian exception which was made in favour ofspecies to the otherwise recognised principle of gradual change, was anexception which can at no time have been recommended by any antecedentconsiderations. At all times it stood out of analogy with the principleof continuity; and, as we shall fully find in subsequent chapters, it isnow directly contradicted by all the facts of biological science. There remains one other fact of high generality to which prominentattention should be drawn from the present, or merely antecedent, pointof view. On the theory of special creation no reason can be assigned whydistinct specific types should present any correlation, either in timeor in space, with their nearest allies; for there is evidently noconceivable reason why any given species, A, should have been speciallycreated on the same area and at about the same time as its nearestrepresentative, B, --still less, of course, that such should be a generalrule throughout all the thousands and millions of species which haveever inhabited the earth. But, equally of course, on the theory of anatural evolution this is so necessary a consequence, that if nocorrelation of such a two-fold kind were observable, the theory would benegatived. Thus the question whether there be any indication of such atwo-fold correlation may be regarded as a test-question as between thetwo theories; for although the vast majority of extinct species havebeen lost to science, there are a countless number of existing specieswhich furnish ample material for answering the question. And the answeris so unequivocal that Mr. Wallace, who is one of our greatestauthorities on geographical distribution, has laid it down as a generallaw, applicable to all the departments of organic nature, that, so faras observation can extend, "every species has come into existencecoincident both in space and time with a pre-existing and closely alliedspecies. " As it appears to me that the significance of these wordscannot be increased by any comment upon them, I will here bring thisintroductory chapter to a close. CHAPTER II. CLASSIFICATION. The first line of direct evidence in favour of organic evolution which Ishall open is that which may be termed the argument from Classification. It is a matter of observable fact that different forms of plants andanimals present among themselves more or less pronounced resemblances. From the earliest times, therefore, it has been the aim of philosophicalnaturalists to classify plants and animals in accordance with theseresemblances. Of course the earliest attempts at such classificationwere extremely crude. The oldest of these attempts with which we areacquainted--namely, that which is presented in the books of Genesis andLeviticus--arranges the whole vegetable kingdom in three simpledivisions of Grass, Herbs, and Trees; while the animal kingdom isarranged with almost equal simplicity with reference, first to habitatsin water, earth, or air, and next as to modes of progression. These, ofcourse, were what may be termed common-sense classifications, havingreference merely to external appearances and habits of life. But whenAristotle laboriously investigated the comparative anatomy of animals, he could not fail to perceive that their entire structures had to betaken into account in order to classify them scientifically; and, also, that for this purpose the internal parts were of quite as muchimportance as the external. Indeed, he perceived that they were ofgreatly more importance in this respect, inasmuch as they presented somany more points for comparison; and, in the result, he furnished anastonishingly comprehensive, as well as an astonishingly accurateclassification of the larger groups of the animal kingdom. On the otherhand, classification of the vegetable kingdom continued pretty much asit had been left by the book of Genesis--all plants being divided intothree groups, Herbs, Shrubs, and Trees. Nor was this primitive state ofmatters improved upon till the sixteenth century, when Gesner(1516-1565), and still more Cæsalpino (1519-1603), laid the foundationsof systematic botany. But the more that naturalists prosecuted their studies on the anatomy ofplants and animals, the more enormously complex did they find theproblem of classification become. Therefore they began by forming whatare called artificial systems, in contradistinction to natural systems. An artificial system of classification is a system based on the more orless arbitrary selection of some one part, or set of parts; while anatural classification is one that is based upon a complete knowledge ofall the structures of all the organisms which are classified. Thus, the object of classification has been that of arranging organismsin accordance with their natural affinities, by comparing organism withorganism, for the purpose of ascertaining which of the constituentorgans are of the most invariable occurrence, and therefore of the mosttypical signification. A porpoise, for instance, has a large number ofteeth, and in this feature resembles most fish, while it differs fromall mammals. But it also gives suck to its young. Now, looking to thesetwo features alone, should we say that a porpoise ought to be classed asa fish or as a mammal? Assuredly as a mammal; because the number ofteeth is a very variable feature both in fish and mammals, whereas thegiving of suck is an invariable feature among mammals, and occursnowhere else in the animal kingdom. This, of course, is chosen as a verysimple illustration. Were all cases as obvious, there would be butlittle distinction between natural and artificial systems ofclassification. But it is because the lines of natural affinity are, asit were, so interwoven throughout the organic world, and because thereis, in consequence, so much difficulty in following them, thatartificial systems have to be made in the first instance as feelerstowards eventual discovery of the natural system. In other words, whileforming their artificial systems of classification, it has always beenthe aim of naturalists--whether consciously or unconsciously--to admitas the bases of their systems those characters which, in the then stateof their knowledge, seemed most calculated to play an important part inthe eventual construction of the natural system. If we were dealing withthe history of classification, it would here be interesting to note howthe course of it has been marked by gradual change in the principleswhich naturalists adopted as guides to the selection of characters onwhich to found their attempts at a natural classification. Some of thesechanges, indeed, I shall have to mention later on; but at present whathas to be specially noted is, that through all these changes of theoryor principle, and through all the ever-advancing construction of theirtaxonomic science, naturalists themselves were unable to give anyintelligible reason for the faith that was in them--or the faith thatover and above the artificial classifications which were made for themere purpose of cataloguing the living library of organic nature, therewas deeply hidden in nature itself a truly natural classification, forthe eventual discovery of which artificial systems might prove to be ofmore or less assistance. Linnæus, for example, expressly says--"You ask me for the characters ofthe natural orders; I confess that I cannot give them. " Yet he maintainsthat, although he cannot define the characters, he knows, by a sort ofnaturalist's instinct, what in a general way will subsequently be foundto be the organs of most importance in the eventual grouping of plantsunder a natural system. "I will not give my reasons for the distributionof the natural orders which I have published, " he said: "you, or someother person, after twenty or after fifty years, will discover them, andsee that I was right. " Thus we perceive that in forming their provisional or artificialclassifications, naturalists have been guided by an instinctive beliefin some general principle of natural affinity, the character of whichthey have not been able to define; and that the structures which theyselected as the bases of their classifications when these wereconsciously artificial, were selected because it seemed that they werethe structures most likely to prove of use in subsequent attempts atworking out the natural system. This general principle of natural affinity, of which all naturalistshave seen more or less well-marked evidence in organic nature, and afterwhich they have all been feeling, has sometimes been regarded asnatural, but more often as supernatural. Those who regarded it assupernatural took it to consist in a divine ideal of creation accordingto types, so that the structural affinities of organisms were to themexpressions of an archetypal plan, which might be revealed in itsentirety when all organisms on the face of the earth should have beenexamined. Those, on the other hand, who regarded the general principleof affinity as depending on some natural causes, for the most partconcluded that these must have been utilitarian causes; or, in otherwords, that the fundamental affinities of structure must have dependedupon fundamental requirements of function. According to this view, thenatural classification would eventually be found to stand upon a basisof physiology. Therefore all the systems of classification up to theearlier part of the present century went upon the apparent axiom, thatcharacters which are of most importance to the organisms presenting themmust be characters most indicative of natural affinities. But the truthof the matter was eventually found to be otherwise. For it waseventually found that there is absolutely no correlation between thesetwo things; that, therefore, it is a mere chance whether or not organswhich are of importance to organisms are likewise of importance asguides to classification; and, in point of fact, that the generaltendency in this matter is towards an inverse instead of a directproportion. More often than not, the greater the value of a structurefor the purpose of indicating natural affinities, the less is its valueto the creatures presenting it. Enough has now been said to show three things. First, that long beforethe theory of descent was entertained by naturalists, naturalistsperceived the fact of natural affinities, and did their best toconstruct a natural system of classification for the purpose ofexpressing such affinities. Second, that naturalists had a kind ofinstinctive belief in some one principle running through the wholeorganic world, which thus served to bind together organisms in groupssubordinate to groups--that is, into species, genera, orders, families, classes, sub-kingdoms, and kingdoms. Third, that they were not able togive any very intelligible reason for this faith that was in them;sometimes supposing the principle in question to be that of asupernatural plan of organization, sometimes regarding it as dependenton conditions of physiology, and sometimes not attempting to account forit at all. Of course it is obvious that the theory of descent furnishes theexplanation which is required. For it is now evident to evolutionists, that although these older naturalists did not know what they were doingwhen they were tracing these lines of natural affinity, and thus helpingto construct a natural classification--I say it is now evident toevolutionists that these naturalists were simply tracing the lines ofgenetic relationship. The great principle pervading organic nature, which was seen so mysteriously to bind the whole creation together as ina nexus of organic affinity, is now easily understood as nothing more orless than the principle of Heredity. Let us, therefore, look a littlemore closely at the character of this network, in order to see how farit lends itself to this new interpretation. The first thing that we have to observe about the nexus is, that it is anexus--not a single line, or even a series of parallel lines. In otherwords, some time before the theory of descent was seriously entertained, naturalists for the most part had fully recognised that it wasimpossible to arrange either plants or animals, with respect to theirmutual affinities, in a ladder-like series (as was supposed to be thetype of classification by the earlier systematists), or even in map-likegroups (as was supposed to be the type by Linnæus). And similarly, also, with respect to grades of organization. In the case of the largergroups, indeed, it is usually possible to say that the members of thisgroup as a whole are more highly organized than the members of thatgroup as a whole; so that, for instance, we have no hesitation inregarding the Vertebrata as more highly organized than the Invertebrata, Birds than Reptiles, and so on. But when we proceed to smallersubdivisions, such as genera and species, it is usually impossible tosay that the one type is more highly organized than another type. Ahorse, for instance, cannot be said to be more highly organized than azebra or an ass; although the entire horse-genus is clearly a morehighly organized type than any genus of animal which is not a mammal. In view of these facts, therefore, the system of classification whichwas eventually arrived at before the days of Darwin, was the systemwhich naturalists likened to a tree; and this is the system which allnaturalists now agreed upon as the true one. According to this system, ashort trunk may be taken to represent the lowest organisms which cannotproperly be termed either plants or animals. This short trunk soonseparates into two large trunks, one of which represents the vegetableand the other the animal kingdom. Each of these trunks then gives offlarge branches signifying classes, and these give off smaller, but morenumerous branches, signifying families, which ramify again into orders, genera, and finally into the leaves, which may be taken to representspecies. Now, in such a representative tree of life, the height of anybranch from the ground may be taken to indicate the grade oforganization which the leaves, or species, present; so that, if wepicture to ourselves such a tree, we may understand that while there isa general advance of organization from below upwards, there are manydeviations in this respect. Sometimes leaves growing on the same branchare growing at a different level--especially, of course, if the branchbe a large one, corresponding to a class or sub-kingdom. And sometimesleaves growing on different branches are growing at the same level: thatis to say, although they represent species belonging to widely divergentfamilies, orders, or even classes, it cannot be said that the onespecies is more highly organized than the other. Now, this tree-like arrangement of species in nature is an arrangementfor which Darwin is not responsible. For, as we have seen, the detectingof it has been due to the progressive work of naturalists for centuriespast; and even when it was detected, at about the commencement of thepresent century, naturalists were confessedly unable to explain thereason of it, or what was the underlying principle that they wereengaged in tracing when they proceeded ever more and more accurately todefine these ramifications of natural affinity. But now, as justremarked, we can clearly perceive that this underlying principle wasnone other than Heredity as expressed in family likeness, --likeness, therefore, growing progressively more unlike with remoteness ofancestral relationship. For thus only can we obtain any explanation ofthe sundry puzzles and apparent paradoxes, which a working out of theirnatural classifications revealed to botanists and zoologists during thefirst half of the present century. It will now be my endeavour to showhow these puzzles and paradoxes are all explained by the theory thatnatural affinities are merely the expression of genetic affinities. First of all, and from the most general point of view, it is obviousthat the tree-like system of classification, which Darwin found alreadyand empirically worked out by the labours of his predecessors, is assuggestive as anything could well be of the fact of geneticrelationship. For this is the form that every tabulation of familypedigree must assume; and therefore the mere fact that a scientifictabulation of natural affinities was eventually found to take the formof a tree, is in itself highly suggestive of the inference that such atabulation represents a _family_ tree. If all species were separatelycreated, there can be no assignable reason why the ideas of earliernaturalists touching the form which a natural classification wouldeventually assume should not have represented the truth--why, forexample, it should not have assumed the form of a ladder (as wasanticipated in the seventeenth century), or of a map (as was anticipatedin the eighteenth), or, again, of a number of wholly unrelated lines, circles, &c. (as certain speculative writers of the present century haveimagined). But, on the other hand, if all species were separately andindependently created, it becomes virtually incredible that we shouldeverywhere observe this progressive arborescence of characters common tolarger groups into more and more numerous, and more and more delicate, ramifications of characters distinctive only of smaller and smallergroups. A man would be deemed insane if he were to attribute the originof every branch and every twig of a real tree to a separate act ofspecial creation; and although we have not been able to witness thegrowth of what we may term in a new sense the Tree of Life, thestructural relations which are now apparent between its innumerableramifications bear quite as strong a testimony to the fact of theirhaving been due to an organic growth, as is the testimony furnished bythe branches of an actual tree. Or, to take another illustration. Classification of organic forms, asDarwin, Lyell, and Häckel have pointed out, strongly resembles theclassification of languages. In the case of languages, as in the case ofspecies, we have genetic affinities strongly marked; so that it ispossible to some extent to construct a Language-tree, the branches ofwhich shall indicate, in a diagrammatic form, the progressive divergenceof a large group of languages from a common stock. For instance, Latinmay be regarded as a fossil language, which has given rise to a group ofliving languages--Italian, Spanish, French, and, to a large extent, English. Now what would be thought of a philologist who should maintainthat English, French, Spanish, and Italian were all specially createdlanguages--or languages separately constructed by the Deity, and by asmany separate acts of inspiration communicated to the nations which nowspeak them--and that their resemblance to the fossil form, Latin, mustbe attributed to special design? Yet the evidence of the naturaltransmutation of species is in one respect much stronger than that ofthe natural transmutation of languages--in respect, namely, of therebeing a vastly greater number of cases all bearing testimony to the factof genetic relationship. But, quitting now this most general point of view--or the suggestivefact that what we have before us is a _tree_--let us next approach thistree for the purpose of examining its structure more in detail. When wedo this, the fact of next greatest generality which we find is asfollows. In cases where a very old form of life has continued to existunmodified, so that by investigation of its anatomy we are brought backto a more primitive type of structure than that of the newer formsgrowing higher up _upon the same branch_, two things are observable. Inthe first place, the old form is less differentiated than the newerones; and, in the next place, it is seen much more closely to resembletypes of structure belonging to some of the other and larger branches ofthe tree. The organization of the older form is not only _simpler_; butit is, as naturalists say, more _generalized_. It comprises withinitself characters belonging to its own branch, and also charactersbelonging to neighbouring branches, or to the trunk from which alliedbranches spring. Hence it becomes a general rule of classification, thatit is by the lowest, or by the oldest, forms of any two natural groupsthat the affinities between the two groups admit of being best detected. And it is obvious that this is just what ought to be the case on thetheory of descent with divergent modification; while, upon thealternative theory of special creation, no reason can be assigned whythe lowest or the oldest types should thus combine the characters whichafterwards become severally distinctive of higher or newer types. Again, I have already alluded to the remarkable fact that there is nocorrelation between the value of structures to the organisms whichpresent them, and their value to the naturalist for the purpose oftracing natural affinity; and I have remarked that up to the close ofthe last century it was regarded as an axiom of taxonomic science, thatstructures which are of most importance to the animals or plantspossessing them must likewise prove of most importance in any naturalsystem of classification. On this account, all attempts to discover thenatural classification went upon the supposition that such a directproportion must obtain--with the result that organs of mostphysiological importance were chosen as the bases of systematic work. And when, in the earlier part of the present century, De Candolle foundthat instead of a direct there was usually an inverse proportion betweenthe functional and the taxonomic value of a structure, he was unable tosuggest any reason for this apparently paradoxical fact. For, upon thetheory of special creation, no reason can be assigned why organs ofleast importance to organisms should prove of most importance as marksof natural affinity. But on the theory of descent with progressivemodification the apparent paradox is at once explained. For it isevident that organs of functional importance are, other things equal, the organs which are most likely to undergo different modifications indifferent lines of family descent, and therefore in time to have theirgenetic relationships in these different lines obscured. On the otherhand, organs or structures which are of no functional importance arenever called upon to change in response to any change of habit, or toany change in the conditions of life. They may, therefore, continue tobe inherited through many different lines of family descent, and thusafford evidence of genetic relationship where such evidence fails to begiven by any of the structures of vital importance, which in the courseof many generations have been required to change in many ways accordingto the varied experiences of different branches of the same family. Here, then, we have an empirically discovered rule in the science ofclassification, the _raison d'être_ of which we are at once able toappreciate upon the theory of evolution, whereas no possible explanationof why it should ever have become a rule could be furnished upon thetheory of special creation. Here, again, is another empirically determined rule. The larger the_number_, as distinguished from the _importance_, of structures whichare found common to different groups, the greater becomes their value asguides to the determination of natural affinity. Or, as Darwin puts it, "the value of an aggregate of characters, even when none are important, alone explains the aphorism enunciated by Linnæus, namely, that thecharacters do not give the genus, but the genus gives the characters;for this seems founded on the appreciation of many trifling points ofresemblance, too slight to be defined[1]. " [1] _Origin of Species_, p. 367. Now it is evident, without comment, of how much value aggregates ofcharacters ought to be in classification, if the ultimate meaning ofclassification be that of tracing lines of pedigree; whereas, if thisultimate meaning were that of tracing divine ideals manifested inspecial creation, we can see no reason why single characters are notsuch sure tokens of a natural arrangement as are aggregates ofcharacters, even though the latter be in every other respectunimportant. For, on the special creation theory, we cannot explain whyan assemblage, say of four or five trifling characters, should have beenchosen to mark some unity of plan, rather than some one character offunctional importance, which would have served at least equally well anysuch hypothetical purpose. On the other hand, as Darwin remarks, "wecare not how trifling a character may be--let it be the mere inflectionof the angle of the jaw, the manner in which an insect's wing is folded, whether the skin be covered with hair or feathers--if it prevailthroughout many and different species, especially those having verydifferent habits of life, it assumes high value; for we can account forits presence in so many forms, with such different habits, only byinheritance from a common parent. We may err in this respect in regardto single points of structure, but when several characters, let them beever so trifling, concur throughout a large group of beings havingdifferent habits, we may feel almost sure, on the theory of descent, that these characters have been inherited from a common ancestor; andwe know that such aggregated characters have especial value inclassification[2]. " [2] _Origin of Species_, p. 372. It is true that even a single character, if found common to a largenumber of forms, while uniformly absent from others, is also regarded bynaturalists as of importance for purposes of classification, althoughthey recognise it as of a value subordinate to that of aggregates ofcharacters. But this also is what we should expect on the theory ofdescent. If even any one structure be found to run through a number ofanimals presenting different habits of life, the readiest explanation ofthe fact is to be found in the theory of descent; but this does nothinder that if several such characters always occur together, theinference of genetic relationship is correspondingly confirmed. And thefact that before this inference was ever drawn, naturalists recognisedthe value of single characters in proportion to their constancy, and theyet higher value of aggregates of characters in proportion to theirnumber--this fact shows that in their work of classification naturalistsempirically observed the effects of a cause which we have nowdiscovered, to wit, hereditary transmission of characters throughever-widening groups of changing species. There is another argument which appears to tell strongly in favour ofthe theory of descent. We have just seen that non-adaptive structures, not being required to change in response to change of habits orconditions of life, are allowed to persist unchanged through manygenerations, and thus furnish exceptionally good guides in the scienceof classification--or, according to our theory, in the work of tracinglines of pedigree. But now, the converse of this statement holds equallytrue. For it often happens that adaptive structures are required tochange in different lines of descent in analogous ways, in order to meetanalogous needs; and, when such is the case, the structures concernedhave to assume more or less close resemblances to one another, eventhough they have severally descended from quite different ancestors. Thepaddles of a whale, for instance, most strikingly resemble the fins of afish as to their outward form and movements; yet, on the theory ofdescent, they must be held to have had a widely different parentage. Now, in all such cases where there is thus what is called an analogous(or adaptive) resemblance, as distinguished from what is called anhomologous (or anatomical) resemblance--in all such cases it isobservable that the similarities do not extend further into thestructure of the parts than it is necessary that they should extend, inorder that the structures should both perform the same functions. Thewhole anatomy of the paddles of a whale is quite unlike that of the finsof a fish--being, in fact, that of the fore-limb of a mammal. Thechange, therefore, which the fore-limb has here undergone to suit it tothe aquatic habits of this mammal, is no greater than was required forthat purpose: the change has not extended to any one feature of_anatomical_ significance. This, of course, is what we should expect onthe theory of descent with modification of ancestral characters; but onthe theory of special creation it is not intelligible why there shouldalways be so marked a distinction between resemblances as analogical oradaptive, and resemblances as homological or of meaning in reference toa natural classification. To take another and more detailed instance, the Tasmanian wolf is an animal separated from true wolves in a naturalsystem of classification. Yet its jaws and teeth bear a strong generalresemblance to those of all the dog tribe, although there aredifferences of anatomical detail. In particular, while the dogs all haveon each side of the upper jaw four pre-molars and two molars, theTasmanian wolf has three pre-molars and four molars. Now there is noreason, so far as their common function of dealing with flesh isconcerned, why the teeth of the Tasmanian wolf should not have resembledhomologically as well as analogically the teeth of a true wolf; andtherefore we cannot assign any intelligible reason why, if all thespecies of the dog genus were separately created with one pattern ofteeth, the unallied Tasmanian wolf should have been furnished with whatis practically the same pattern from a functional point of view, whilediffering from a structural point of view. But, of course, on the theoryof descent with modification, we can well understand why similarities ofhabit should have led to similarities of structural appearance of anadaptive kind in different lines of descent, without there being anytrace of such real or anatomical similarities as could possibly point togenetic relationship. Lastly, to adduce the only remaining argument from classification whichI regard as of any considerable weight, naturalists have found itnecessary, while constructing their natural classifications, to setgreat store on what Mr. Darwin calls "chains of affinities. " Thus, forinstance, "nothing can be easier than to define a number of characterscommon to all birds; but with crustaceans any such definition hashitherto been found impossible. There are crustaceans at the oppositeends of the series, which have hardly a character in common; yet thespecies at both ends, from being plainly allied to others, and these toothers, and so onwards, can be recognised as unequivocally belonging tothis, and to no other class of the articulata[3]. " Now it is evidentthat this progressive modification of specific types--where it cannot besaid that the continuity of resemblance is anywhere broken, and yetterminates in modification so great that but for the connecting links noone could divine a natural relationship between the extreme members ofthe series, --it is evident that such chains of affinity speak moststrongly in favour of a transmutation of the species concerned, while itis impossible to suggest any explanation of the fact in terms of therival theory. For if all the links of such a chain were separatelyforged by as many acts of special creation, we can see no reason why Bshould resemble A, C resemble B, and so on, but with ever slight thoughaccumulating differences, until there is no resemblance at all between Aand Z. [3] _Origin of Species_, pp. 368-9. * * * * * I hope enough has now been said to show that all the general principlesand particular facts appertaining to the natural classification ofplants and animals, are precisely what they ought to be according to thetheory of genetic descent; while no one of them is such as mightbe--and, indeed, used to be--expected upon the theory of specialcreation. Therefore, the only possible way in which all this uniformbody of direct evidence can be met by a supporter of the latter theory, is by falling back upon the argument from ignorance. We do not know, itmay be said, what hidden reasons there may have been for following allthese general principles in the separate creation of specific types. Now, it is evident that this is a form of argument which admits of beingbrought against all the actual--and even all the possible--lines ofevidence in favour of evolution. Therefore I deem it desirable thusearly in our proceedings to place this argument from ignorance on itsproper logical footing. If there were any independent evidence in favour of special creation asa _fact_, then indeed the argument from ignorance might be fairly usedagainst any sceptical cavils regarding the _method_. In this way, forexample, Bishop Butler made a legitimate use of the argument fromignorance when he urged that it is no reasonable objection against arevelation, _otherwise accredited_, to show that it has been rendered ina form, or after a method, which we should not have antecedentlyexpected. But he could not have legitimately employed this argument, except on the supposition that he had some independent evidence infavour of the revelation; for, in the absence of any such independentevidence, appeal to the argument from ignorance would have become a merebegging of the question, by simply assuming that a revelation had beenmade. And thus it is in the present case. A man, of course, may quitelegitimately say, _Assuming that the theory of special creation istrue_, it is not for us to anticipate the form or method of theprocess. But where the question is as to whether or not the theory _is_true, it becomes a mere begging of this question to take refuge in theargument from ignorance, or to represent in effect that there is noquestion to be discussed. And if, when the form or method isinvestigated, it be found everywhere charged with evidence in favour ofthe theory of descent, the case becomes the same as that of a supposedrevelation, which has been discredited by finding that all availableevidence points to a natural growth. In short, the argument fromignorance is in any case available only as a negative foil againstdestructive criticism: in no case has it any positive value, or value ofa constructive kind. Therefore, if a theory on any subject is destituteof positive evidence, while some alternative theory is in possession ofsuch evidence, the argument from ignorance can be of no logical use tothe former, even though it maybe of such use to the latter. For it isonly the possession of positive evidence which can furnish a logicaljustification of the argument from ignorance: in the absence of suchevidence, even the negative value of the argument disappears, and itthen implies nothing more than the gratuitous assumption of a theory. * * * * * I will now sum up the various considerations which have occupied usduring the present chapter. First of all we must take note that the classification of plants andanimals in groups subordinate to groups is not merely arbitrary, orundertaken only for a matter of convenience and nomenclature--such, forinstance, as the classification of stars in constellations. On thecontrary, the classification of a naturalist differs from that of anastronomer, in that the objects which he has to classify presentstructural resemblances and structural differences in numberlessdegrees; and it is the object of his classification to present a tabularstatement of these facts. Now, long before the theory of evolution wasentertained, naturalists became fully aware that these facts ofstructural resemblances running through groups subordinate to groupswere really facts of nature, and not merely poetic imaginations of themind. No one could dissect a number of fishes without perceiving thatthey were all constructed on one anatomical pattern, which differedconsiderably from the equally uniform pattern on which all mammals wereconstructed, even although some mammals bore an extraordinaryresemblance to fish in external form and habits of life. And similarlywith all the smaller divisions of the animal and vegetable kingdoms. Everywhere investigation revealed the bonds of close structuralresemblances between species of the same genus, resemblance less closebetween genera of the same family, resemblance still less close betweenfamilies of the same order, resemblance yet more remote between ordersof the same class, and resemblance only in fundamental features betweenclasses of the same sub-kingdom, beyond which limit all anatomicalresemblance was found to disappear--the different sub-kingdoms beingformed on wholly different patterns. Furthermore, in tracing all thesegrades of structural relationship, naturalists were slowly led torecognise that the form which a natural classification must eventuallyassume would be that of a tree, wherein the constituent branches woulddisplay a progressive advance of organization from below upwards. Now we have seen that although this tree-like arrangement of naturalgroups was as suggestive as anything could well be of all the forms o£life being bound together by the ties of genetic relationship, such wasnot the inference which was drawn from it. Dominated by the theory ofspecial creation, naturalists either regarded the resemblance of typesubordinate to type as expressive of divine ideals manifested in suchcreation, or else contented themselves with investigating the factswithout venturing to speculate upon their philosophical import. But eventhose naturalists who abstained from committing themselves to any theoryof archetypal plans, did not doubt that facts so innumerable and souniversal must have been due to some one co-ordinating principle--that, even though they were not able to suggest what it was, there must havebeen some hidden bond of connexion running through the whole of organicnature. Now, as we have seen, it is manifest to evolutionists that thishidden bond can be nothing else than heredity; and, therefore, thatthese earlier naturalists, although they did not know what they weredoing, were really tracing the lines of genetic descent as revealed bydegrees of structural resemblance, --that the arborescent grouping oforganic forms which their labours led them to begin, and in largemeasure to execute, was in fact a family tree of life. Here, then, is the substance of the argument from classification. Themere fact that all organic nature thus incontestably lends itself to anatural arrangement of group subordinate to group, when due regard ispaid to degrees of anatomical resemblance--this mere fact of itselftells so weightily in favour of descent with progressive modification indifferent lines, that even if it stood alone it would be entitled torank as one of our strongest pieces of evidence. But, as we have seen, it does not stand alone. When we look beyond this large and general factof all the innumerable forms of life being thus united in a tree-likesystem by an unquestionable relationship of some kind, to those smallerdetails in the science of classification which have been found mostuseful as guides for this kind of research, then we find that all thesedetails, or empirically discovered rules, are exactly what we shouldhave expected them to be, supposing the real meaning of classificationto have been that of tracing lines of pedigree. In particular, we have seen that the most archaic types are both simplerin their organization and more generalized in their characters than arethe more recent types--a fact of which no explanation can be given onthe theory of special creation. But, upon the theory of naturalevolution, we can without difficulty understand why the earlier formsshould have been the simpler forms, and also why they should have beenthe most generalized. For it is out of the older forms that the newermust have grown; and, as they multiplied, they must have become more andmore differentiated. Again, we have seen that there is no correlation between the importanceof any structure from a classificatory point of view, and the importanceof that structure to the organism which presents it. On the contrary, it is a general rule that "the less any part of the organization isconcerned with special habits, the more important it becomes forclassification. " Now, from the point of view of special creation it isunintelligible why unity of ideal should be most manifested by leastimportant structures, whereas from the point of view of evolution it isto be expected that these life-serving structures should have been mostliable to divergent modification in divergent lines of descent, or inadaptation to different conditions of life, while the trivial or lessimportant characters should have been allowed to remain unmodified. Thuswe can now understand why all primitive classifications were wrong inprinciple when they went upon the assumption that divine ideals werebest exhibited by resemblances between life-serving (and thereforeadaptive) structures, with the result that whales were classed withfishes, birds with bats, and so on. Nevertheless, these primitivenaturalists were quite logical; for, from the premises furnished by thetheory of special creation, it is much more reasonable to expect thatunity of ideal should be shown in plainly adaptive characters than intrivial and more or less hidden anatomical characters. Moreover, longafter biological science had ceased consciously to follow anytheological theory, the apparent axiom continued to be entertained, thatstructures of most importance to organisms must also be structures ofmost importance to systematists. And when at last, in the presentcentury, this was found not to be the case, no reason could be suggestedwhy it was not the case. But now we are able fully to explain thisapparent anomaly. Once more, we have seen that aggregates of characters presentingresemblances to one another have always been found to be of specialimportance as guides to classification. This, of course, is what weshould have expected, if the real meaning of classification be that oftracing lines of pedigree; but on the theory of special creation noreason can be assigned why single characters are not such sure tokens ofa natural arrangement as are aggregates of characters, however trivialthe latter may be. For it is obvious that unity of ideal might have beeneven better displayed by everywhere maintaining the pattern of some oneimportant structure, than by doing so in the case of several unimportantstructures. Take an analogous instance from human contrivances. Unity ofideal in the case of gun-making would be shown by the same principles ofmechanism running through all the different sizes and shapes ofgun-locks, rather than by the ornamental patterns engraved upon theoutside. Yet it must be supposed that in the mechanisms assumed to havebeen constructed by special creation, it was the trivial details ratherthan the fundamental principles of these mechanisms which were chosen bythe Divinity to display his ideals. And this leads us to the next consideration--namely, that when in twodifferent lines of descent animals happen to adopt similar habits oflife, the modifications which they undergo in order to fit them forthese habits often induces striking resemblances of structure betweenthe two animals, as in the case of whales and fish. But in all suchinstances it is invariably found that the resemblance is onlysuperficial and apparent: not anatomical or real. In other words, theresemblance does not extend further than it is necessary that itshould, if both sets of organs are to be adapted to perform the samefunctions. Now this, again, is just what one would expect to find as theuniversal rule on the theory of descent, with modification of ancestralcharacters. But, on the opposite theory of special creation, I know nothow it is to be explained that among so many instances of closesuperficial resemblance between creatures belonging to differentbranches of the tree of life, there are no instances of any real oranatomical resemblance. So far as their structures are adapted toperform a common function, there is in all such cases what may be termeda deceptive appearance of some unity of ideal; but, when carefullyexamined, it is always found that two apparently identical structuresoccurring on different branches of the classificatory tree are in factfundamentally different in respect of their structural plan. Lastly, we have seen that one of the guiding principles ofclassification has been empirically found to consist in setting a highvalue on "chains of affinities. " That is to say, naturalists notunfrequently meet with a long series of progressive modifications oftype, which, although it cannot be said that the continuity is anywherebroken, at last leads to so much divergence of character that, but forthe intermediate links, the members at each end of the chain could notbe suspected of being in any way related. Well, such cases of chains ofaffinity obviously tell most strongly in favour of descent withcontinuous modification; while it is impossible to suggest why, if allthe links were separately forged by as many acts of special creation, there should have been this gradual transmutation of characters carriedto the point where the original creative ideal has been so completelytransformed that, but for the accident of the chain being stillcomplete, no one of nature's interpreters could possibly have discoveredthe connexion. For, as we have seen, this is not a case in which anyappeal can be logically made to the argument from ignorance of divinemethod, unless some independent evidence could be adduced in favour ofspecial creation. And that no such independent evidence exists, it willbe the object of future chapters to show. CHAPTER III. MORPHOLOGY. The theory of evolution supposes that hereditary characters admit ofbeing slowly modified wherever their modification will render anorganism better suited to a change in its conditions of life. Let us, then, observe the evidence which we have of such adaptive modificationsof structure, in cases where the need of such modification is apparent. We may begin by again taking the case of the whales and porpoises. Thetheory of evolution infers, from the whole structure of these animals, that their progenitors must have been terrestrial quadrupeds of somekind, which gradually became more and more aquatic in their habits. Nowthe change in the conditions of their life thus brought about would haverendered desirable great modifications of structure. These changes wouldhave begun by affecting the least typical--that is, the least stronglyinherited--structures, such as the skin, claws, and teeth. But, as timewent on, the adaptation would have extended to more typical structures, until the shape of the body would have become affected by the bones andmuscles required for terrestrial locomotion becoming better adapted foraquatic locomotion, and the whole outline of the animal more fish-likein shape. This is the stage which we actually observe in the seals, where the hind legs, although retaining all their typical bones, havebecome shortened up almost to rudiments, and directed backwards, so asto be of no use for walking, while serving to complete the fish-liketaper of the body. (Fig. 2. ) But in the whales the modification has gonefurther than this so that the hind legs have ceased to be apparentexternally, and are only represented internally--and even this only insome species--by remnants so rudimentary that it is difficult to makeout with certainty the homologies of the bones; moreover, the head andthe whole body have become completely fish-like in shape. (Fig. 3. ) Butprofound as are these alterations, they affect only those parts of theorganism which it was for the benefit of the organism to have altered, so that it might be adapted to an aquatic mode of existence. Thus thearm, which is used as a fin, still retains the bones of the shoulder, fore-arm, wrist, and fingers, although they are all enclosed in afin-shaped sack, so as to render them useless for any purpose other thanswimming (Fig. 4. ) Similarly, the head, although it so closely resemblesthe head of a fish in shape, still retains the bones of the mammalianskull in their proper anatomical relations to one another; but modifiedin form so as to offer the least possible resistance to the water. Inshort, it may be said that all the modifications have been effected withthe least possible divergence from the typical mammalian type, which iscompatible with securing so perfect an adaptation to a purely aquaticmode of life. [Illustration: FIG. 2. --Skeleton of Seal, 1/8 nat. Size. Drawn from nature (_R. Coll. Surg. Mus. _). ] [Illustration: FIG. 3. --Skeleton of Greenland Whale, 1/100 nat. Size. The rudimentary bones of the pelvis are shown on a larger scale in the upper drawing. (From Prof. Flower. )] [Illustration: FIG. 4. --Paddle of Whale compared with Hand of Man. Drawn from nature (_R. Coll. Surg. Mus. _). ] Now I have chosen the case of the whale and porpoise group, because theyoffer so extreme an example of profound modification of structure inadaptation to changed conditions of life. But the same thing may be seenin hundreds and hundreds of other cases. For instance, to confine ourattention to the arm, not only is the limb modified in the whale forswimming, but in another mammal--the bat--it is modified for flying, byhaving the fingers enormously elongated and overspread with a membranousweb. In birds, again, the arm is modified for flight in a wholly differentway--the fingers here being very short and all run together, while thechief expanse of the wing is composed of the shoulder and fore-arm. Infrogs and lizards, again, we find hands more like our own; but in anextinct species of flying reptile the modification was extreme, the winghaving been formed by a prodigious elongation of the fifth finger, and amembrane spread over it and the rest of the hand. (Fig. 5. ) Lastly, inserpents the hand and arm have disappeared altogether. [Illustration: FIG. 5. --Wing of Reptile, Mammal, and Bird. Drawn from nature (_Brit. Mus. _). ] Thus, even if we confine our attention to a single organ, how wonderfulare the modifications which it is seen to undergo, although never losingits typical character. Everywhere we find the distinction betweenhomology and analogy which was explained in the last chapter--thedistinction, that is, between correspondence of structure andcorrespondence of function. On the one hand, we meet with structureswhich are perfectly homologous and yet in no way analogous: thestructural elements remain, but are profoundly modified so as to performwholly different functions. On the other hand, we meet with structureswhich are perfectly analogous, and yet in no way homologous: totallydifferent structures are modified to perform the same functions. How, then, are we to explain these things? By design manifested in specialcreation, or by descent with adaptive modification? If it is said bydesign manifested in special creation, we must suppose that the Deityformed an archetypal plan of certain structures, and that he determinedto adhere to this plan through all the modifications which thosestructures exhibit. But, if so, why is it that some structures areselected as typical and not others? Why should the vertebral skeleton, for instance, be tortured into every conceivable variety of modificationin order to subserve as great a variety of functions; while anotherstructure, such as the eye, is made in different sub-kingdoms onfundamentally different plans, notwithstanding that it has throughout toperform the same function? Will any one have the hardihood to assertthat in the case of the skeleton the Deity has endeavoured to show his_ingenuity_, by the manifold functions to which he has made the samestructure subservient; while in the case of the eye he has endeavouredto show his _resources_, by the manifold structures which he has adaptedto serve the same function? If so, it becomes a most unfortunatecircumstance that, throughout both the vegetable and animal kingdoms, all cases which can be pointed to as showing ingenious adaptation of thesame typical structure to the performance of widely differentfunctions--or cases of homology without analogy, --are cases which comewithin the limits of the same natural group of plants and animals, andtherefore admit of being equally well explained by descent from a commonancestry; while all cases of widely different structures performing thesame function--or cases of analogy without homology, --are to be found indifferent groups of plants or animals, and are therefore suggestive ofindependent variations arising in the different lines of hereditarydescent. To take a specific illustration. The octopus, or devil-fish, belongs toa widely different class of animals from a true fish; and yet its eye, in general appearance, looks wonderfully like the eye of a true fish. Now, Mr. Mivart pointed to this fact as a great difficulty in the way ofthe theory of evolution by natural selection, because it must clearly bea most improbable thing that so complicated a structure as the eye of afish should happen to be arrived at through each of two totallydifferent lines of descent. And this difficulty would, indeed, be aformidable one to the theory of evolution, if the similarity were notonly analogical but homological. Unfortunately for the objection, however, Darwin clearly showed in his reply that in no one anatomical orhomologous feature do the two structures resemble one another; so that, in point of fact, the two organs do not resemble one another in anyparticular further than it is necessary that they should, if both are tobe analogous, or to serve the same function as organs of sight. But now, suppose that this had not been the case, and that the two structures, besides presenting the necessary superficial or analogical resemblance, had also presented an anatomical or homologous resemblance, with whatforce might it have then been urged, --Your hypothesis of hereditarydescent with progressive modification being here excluded by the factthat the animals compared belong to two widely different branches of thetree of life, how are we to explain the identity of type manifested bythese two complicated organs of vision? The only hypothesis open to usis intelligent adherence to an ideal plan or mechanism. But as thiscannot now be urged in any comparable case throughout the whole organicworld, we may on the other hand present it as a most significant fact, that while within the limits of the same large branch of the tree oflife we constantly find the same typical structures modified so as toperform very different functions, we never find any of these particulartypes of structure in other large branches of the tree. That is to say, we never find typical structures appearing except in cases where theirpresence may be explained by the hypothesis of hereditary descent; whilein thousands of such cases we find these structures undergoing everyconceivable variety of adaptive modification. Consequently, special creationists must fall back upon another positionand say, --Well, but it may have pleased the Deity to form a certainnumber of ideal types, and never to have allowed the structuresoccurring in one type to appear in any of the others. Weanswer, --Undoubtedly such may have been the case; but, if so, it is amost unfortunate thing for your theory, because the fact implies thatthe Deity has planned his types in such a way as to suggest thecounter-theory of descent. For instance, it would seem most capriciouson the part of the Deity to have made the eyes of an innumerable numberof fish on exactly the same ideal type, and then to have made the eye ofthe octopus so exactly like these other eyes in superficial appearanceas to deceive so accomplished a naturalist as Mr. Mivart, and yet tohave taken scrupulous care that in no one ideal particular should theone type resemble the other. However, adopting for the sake of argumentthis great assumption, let us suppose that God did lay down thesearbitrary rules for his own guidance in creation, and then let us see towhat the assumption leads. If the Deity formed a certain number of idealtypes, and determined that on no account should he allow any part of onetype to appear in any part of another, surely we should expect thatwithin the limits of the same type the same typical structures shouldalways be present. Thus, remember what efforts, so to speak, have beenmade to maintain the uniformity of type in the case of the fore-limb aspreviously explained, and should we not expect that in other and similarcases a similar method should have been followed? Yet we repeatedly findthat this is not the case. Even in the whale, as we have seen, thehind-limbs are either altogether absent or dwindled almost to nothing;and it is impossible to see in what respect the hind-limbs are of anyless ideal value than the fore-limbs--which are carefully preserved inall vertebrated animals except the snakes, and the extinct _Dinornis_, where again we meet in this particular with a sudden and sublimeindifference to the maintenance of a typical structure. (Fig. 6. )[4] NowI say that if the theory of ideal types is true, we have in these factsevidence of a most unreasonable inconsistency. But the theory of descentwith continued adaptive modification fully explains all the known cases;for in every case the degree of divergence from the typical structurewhich an organism presents corresponds, in a general way, with thelength of time during which the divergence has been going on. Thus wescarcely ever meet with any great departure from the typical form withrespect to one of the organs, without some of the other organs being sofar modified as of themselves to indicate, on the supposition ofdescent with modification, that the animal or plant must have beensubject to the modifying influences for an enormously long series ofgenerations. And this combined testimony of a number of organs in thesame organism is what the theory of descent would lead us to expect, while the rival theory of design can offer no explanation of the fact, that when one organ shows a conspicuous departure from the supposedideal type, some of the other organs in the same organism should tend tokeep it company by doing likewise. [4] It is, however, probable that all species of the genus retained a tiny rudiment of wings in greatly dwindled scapulo-coracoid bones. And Mr. H. O. Forbes has detected, in a recently exhumed specimen of the latter, an indication of the glenoid cavity, for the articulation of an extremely aborted humerus. (See _Nature_, Jan. 14th, 1892. ) [Illustration: FIG. 6. --Skeleton of _Dinornis gravis_, 1/16 nat. Size. Drawn from nature (_Brit. Mus. _). As separate cuts on a larger scale are shown, 1st, the sternum, as this appears in mounted skeletons, and, 2nd, the same in profile, with its (hypothetical) scapulo-coracoid attached. ] As an illustration both of this and of other points which have beenmentioned, I may draw attention to what seems to me a particularlysuggestive case. So-called soldier-or hermit-crabs, are crabs which haveadopted the habit of appropriating the empty shells of mollusks. Inassociation with this peculiar habit, the structure of these animalsdiffers very greatly from that of all other crabs. In particular, thehinder part of the body, which occupies the mollusk-shell, and whichtherefore has ceased to require any hard covering of its own, has beensuffered to lose its calcareous integument, and presents a soft fleshycharacter, quite unlike that of the more exposed parts of the animal. Moreover, this soft fleshy part of the creature is specially adapted tothe particular requirements of the creature by having its lateralappendages--i. E. Appendages which in other crustacea perform thefunction of legs--modified so as to act as claspers to the inside of themollusk-shell; while the tail-end of the part in question is twistedinto the form of a spiral, which fits into the spiral of themollusk-shell. Now, in Keeling Island there is a large kind of crabcalled _Birgus latro_, which lives upon land and there feeds uponcocoa-nuts. The whole structure of this crab, it seems to me, unmistakeably resembles the structure of a hermit-crab (see drawings onthe next page, Fig. 7). Yet this crab neither lives in the shell of amollusk, nor is the hinder part of its body in the soft and fleshycondition just described: on the contrary, it is covered with a hardintegument like all the other parts of the animal. Consequently, I thinkwe may infer that the ancestors of _Birgus_ were hermit-crabs living inmollusk-shells; but that their descendants gradually relinquished thishabit as they gradually became more and more terrestrial, while, concurrently with these changes in habit, the originally soft posteriorparts acquired a hard protective covering to take the place of thatwhich was formerly supplied by the mollusk-shell. So that, if so, we nowhave, within the limits of a single organism, evidence of a whole seriesof morphological changes in the past history of its species. First, there must have been the great change from an ordinary crab to ahermit-crab in all the respects previously pointed out. Next, there musthave been the change back again from a hermit-crab to an ordinary crab, so far as living without the necessity of a mollusk-shell is concerned. From an evolutionary point of view, therefore, we appear to have in theexisting structure of _Birgus_ a morphological record of all thesechanges, and one which gives us a reasonable explanation of why theanimal presents the extraordinary appearance which it does. But, on thetheory of special creation, it is inexplicable why this land-crab shouldhave been formed on the pattern of a hermit-crab, when it never has needto enter the shell of a mollusk. In other words, its peculiarstructure is not specially in keeping with its present habits, althoughso curiously allied to the similar structure of certain other crabs oftotally different habits, in relation to which the peculiarities are ofplain and obvious significance. [Illustration: FIG. 7. --Hermit-crabs compared with the cocoa-nut crab. On the left of the illustration one hermit-crab is represented as occupying a mollusk-shell, and another (larger specimen) as it appears when withdrawn from such a shell. On the right of the illustration the cocoa-nut crab is represented in its natural habitat on land. When full-grown, however, it is much larger than our hermit-crabs. The latter are drawn from life, natural size, the former from a specimen in the British Museum, 1/6 natural size. ] * * * * * I will devote the remainder of this chapter to considering anotherbranch of the argument from morphology, to which the case of _Birgus_serves as a suitable introduction: I mean the argument from rudimentarystructures. Throughout both the animal and vegetable kingdoms we constantly meetwith dwarfed and useless representatives of organs, which in other andallied kinds of animals and plants are of large size and functionalutility. Thus, for instance, the unborn whale has rudimentary teeth, which are never destined to cut the gums; and throughout its life thisanimal retains, in a similarly rudimentary condition, a number of organswhich never could have been of use to any kind of creature save aterrestrial quadruped. The whole anatomy of its internal ear, forexample, has reference to hearing in air--or, as Hunter long agoremarked, "is constructed upon the same principle as in the quadruped";yet, as Owen says, "the outer opening and passage leading therefrom tothe tympanum can rarely be affected by sonorous vibrations of theatmosphere, and indeed they are reduced, or have degenerated, to adegree which makes it difficult to conceive how such vibrations can bepropagated to the ear-drum during the brief moments in which the openingmay be raised above the water. " [Illustration: FIG. 8. --Rudimentary or vestigial hind-limbs of Python, as exhibited in the skeleton and on the external surface of the animal. Drawn from nature, 1/4 nat. Size (_Zoological Gardens_). ] Now, rudimentary organs of this kind are of such frequent occurrence, that almost every species presents one or more of them--usually, indeed, a considerable number. How, then, are they to be accounted for? ofcourse the theory of descent with adaptive modification has a simpleanswer to supply--namely, that when, from changed conditions of life, anorgan which was previously useful becomes useless, it will be sufferedto dwindle away in successive generations, under the influence ofcertain natural causes which we shall have to consider in futurechapters. On the other hand, the theory of special creation can onlymaintain that these rudiments are formed for the sake of adhering to anideal type. Now, here again the former theory appears to be triumphantover the latter; for, without waiting to dispute the wisdom of makingdwarfed and useless structures merely for the whimsical motive assigned, surely if such a method were adopted in so many cases, we should expectthat in consistency it would be adopted in all cases. This reasonableexpectation, however, is far from being realized. We have already seenthat in numberless cases, such as that of the fore-limbs of serpents, novestige of a rudiment is present. But the vacillating policy in thematter of rudiments does not end here; for it is shown in a still moreaggravated form where within the limits of the same natural group oforganisms a rudiment is sometimes present and sometimes absent. Forinstance, although in nearly all the numerous species of snakes thereare no vestiges of limbs, in the python we find very tiny rudiments ofthe hind-limbs. (Fig. 8. ) Now, is it a worthy conception of deity that, while neglecting to maintain his unity of ideal in the case of nearlyall the numerous species of snakes, he should have added a tiny rudimentin the case of the python--and even in that case should have maintainedhis ideal very inefficiently, inasmuch as only two limbs, instead offour, are represented? how much more reasonable is the naturalisticinterpretation; for here the very irregularity of their appearance indifferent species, which constitutes rudimentary structures one of thecrowning difficulties to the theory of special design, furnishes thebest possible evidence in favour of hereditary descent; seeing that thisirregularity then becomes what may be termed the anticipated expressionof progressive dwindling due to inutility. Thus, for example, to returnto the case of wings, we have already seen that in an extinct genus ofbird, _dinornis_, these organs were reduced to such an extent as toleave it still doubtful whether so much as the tiny rudimenthypothetically supplied to fig. 6 (p. 61) was present in all thespecies. And here is another well-known case of another genus of stillexisting bird, which, as was the case with _dinornis_, occurs only innew zealand. (Fig. 9. ) Upon this island there are no four-footedenemies--either existing or extinct--to escape from which the wings ofbirds would be of any service. Consequently we can understand why onthis island we should meet with such a remarkable dwindling away ofwings. [Illustration: FIG. 9. --_Apteryx Australis. _ Drawn from life in the Zoological Gardens, 1/8 nat. Size. The external wing is drawn to a scale in the upper part of the cut. The surroundings are supplied from the most recent descriptions. ] Similarly, the logger-headed duck of South America can only flap alongthe surface of the water, having its wings considerably reduced thoughless so than the _Apteryx_ of New Zealand. But here the interesting factis that the young birds are able to fly perfectly well. Now, inaccordance with a general law to be considered in a future chapter, thelife-history of an individual organism is a kind of condensedrecapitulation of the life-history of its species. Consequently, we canunderstand why the little chickens of the logger-headed duck are able tofly like all other ducks, while their parents are only able to flapalong the surface of the water. Facts analogous to this reduction of wings in birds which have nofurther use for them, are to be met with also in insects under similarcircumstances. Thus, there are on the island of Madeira somewherebetween 500 and 600 species of beetles, which are in large partpeculiar to that island, though related to other--and thereforepresumably parent--species on the neighbouring continent. Now, no lessthan 200 species--or nearly half the whole number--are so far deficientin wings that they cannot fly. And, if we disregard the species whichare not peculiar to the island--that is to say, all the species whichlikewise occur on the neighbouring continent, and therefore, asevolutionists conclude, have but _recently_ migrated to the island, --wefind this very remarkable proportion. There are altogether 29 peculiargenera, and out of these no less than 23 have _all_ their species inthis condition. Similar facts have been recently observed by the Rev. A. E. Eaton withrespect to insects inhabiting Kerguelen Island. All the species which hefound on the island--viz. A moth, several flies, and numerousbeetles--he found to be incapable of flight; and therefore, as Wallaceobserves, "as these insects could hardly have reached the islands in awingless state, even if there were any other known land inhabited bythem, which there is not, we must assume that, like the Madeiraninsects, they were originally winged, and lost their power of flightbecause its possession was injurious to them"--Kerguelen Island being"one of the stormiest places on the globe, " and therefore a place whereinsects could rarely afford to fly without incurring the danger of beingblown out to sea. Here is another and perhaps an even more suggestive class of facts. It is now many years ago since the editors of _Silliman's Journal_requested the late Professor Agassiz to give them his opinion on thefollowing question. In a certain dark subterranean cave, called theMammoth cave, there are found some peculiar species of blind fishes. Nowthe editors of _Silliman's Journal_ wished to know whether Prof. Agassizwould hold that these fish had been specially created in these caves, and purposely devoided of eyes which could never be of any use to them;or whether he would allow that these fish had probably descended fromother species, but, having got into the dark cave, gradually lost theireyes through disuse. Prof. Agassiz, who was a believer in specialcreation, allowed that this ought to constitute a crucial test asbetween the two theories of special design and hereditary descent. "Ifphysical circumstances, " he said, "ever modified organized beings, itshould be easily ascertained here. " And eventually he gave it as hisopinion, that these fish "were created under the circumstances in whichthey now live, within the limits over which they now range, and with thestructural peculiarities which now characterise them. " Since then a great deal of attention has been paid to the fauna of thisMammoth cave, and also to the faunas of other dark caverns, not only inthe New, but also in the Old World. In the result, the following generalfacts have been fully established. (1) Not only fish, but many representatives of other classes, have beenfound in dark caves. (2) Wherever the caves are totally dark, all the animals are blind. (3) If the animals live near enough to the entrance to receive somedegree of light, they may have large and lustrous eyes. (4) In all cases the species of blind animals are closely allied tospecies inhabiting the district where the caves occur; so that the blindspecies inhabiting American caves are closely allied to Americanspecies, while those inhabiting European caves are closely allied toEuropean species. (5) In nearly all cases structural remnants of eyes admit of beingdetected, in various degrees of obsolescence. In the case of some of thecrustaceans of the Mammoth cave the foot-stalks of the eyes are present, although the eyes themselves are entirely absent. Now, it is evident that all these general facts are in full agreementwith the theory of evolution, while they offer serious difficulties tothe theory of special creation. As Darwin remarks, it is hard to imagineconditions of life more similar than those furnished by deep limestonecaverns under nearly the same climate in the two continents of Americaand Europe; so that, in accordance with the theory of special creation, very close similarity in the organizations of the two sets of faunasmight have been expected. But, instead of this, the affinities of thesetwo sets of faunas are with those of their respective continents--as ofcourse they ought to be on the theory of evolution. Again, what wouldhave been the sense of creating useless foot-stalks for the imaginarysupport of absent eyes, not to mention all the other various grades ofdegeneration in other cases? So that, upon the whole, if we agree withthe late Prof. Agassiz in regarding these cave animals as furnishing acrucial test between the rival theories of creation and evolution, wemust further conclude that the whole body of evidence which they nowfurnish is weighing on the side of evolution. So much, then, for a few special instances of what Darwin calledrudimentary structures, but what may be more descriptivelydesignated--in accordance with the theory of descent--obsolescent orvestigial structures. It is, however, of great importance to add thatthese structures are of such general occurrence throughout both thevegetable and animal kingdoms, that, as Darwin has observed, it isalmost impossible to point to a single species which does not presentone or more of them. In other words, it is almost impossible to find asingle species which does not in this way bear some record of its owndescent from other species; and the more closely the structure of anyspecies is examined anatomically, the more numerous are such recordsfound to be. Thus, for example, of all organisms that of man has beenmost minutely investigated by anatomists; and therefore I think it willbe instructive to conclude this chapter by giving a list of the morenoteworthy vestigial structures which are known to occur in the humanbody. I will take only those which are found in adult man, reserving forthe next chapter those which occur in a transitory manner during earlierperiods of his life. But, even as thus restricted, the number ofobsolescent structures which we all present in our own persons is soremarkable, that their combined testimony to our descent from aquadrumanous ancestry appears to me in itself conclusive. I mean, thateven if these structures stood alone, or apart from any more generalevidences of our family relationships, they would be sufficient to proveour parentage. Nevertheless, it is desirable to remark that of coursethese special evidences which I am about to detail do not stand alone. Not only is there the general analogy furnished by the general proof ofevolution elsewhere, but there is likewise the more specialcorrespondence between the whole of our anatomy and that of our nearestzoological allies. Now the force of this latter consideration is soenormous, that no one who has not studied human anatomy can be in aposition to appreciate it. For without special study it is impossible toform any adequate idea of the intricacy of structure which is presentedby the human form. Yet it is found that this enormously intricateorganization is repeated in all its details in the bodies of the higherapes. There is no bone, muscle, nerve, or vessel of any importance inthe one which is not answered to by the other. Hence there are hundredsof thousands of instances of the most detailed correspondence, withoutthere being any instances to the contrary, if we pay due regard tovestigial characters. The entire corporeal structure of man is an exactanatomical copy of that which we find in the ape. My object, then, here is to limit attention to those features of ourcorporeal structure which, having become useless on account of ourchange in attitude and habits, are in process of becoming obsolete, andtherefore occur as mere vestigial records of a former state of things. For example, throughout the vertebrated series, from fish to mammals, there occurs in the inner corner of the eye a semi-transparent eye-lid, which is called the nictitating membrane. The object of this structureis to sweep rapidly, every now and then, over the external surface ofthe eye, apparently in order to keep the surface clean. But although themembrane occurs in all classes of the sub-kingdom, it is more prevalentin some than in others--e. G. In birds than in mammals. Even, however, where it does not occur of a size and mobility to be of any use, it isusually represented, in animals above fishes, by a functionlessrudiment, as here depicted in the case of man. (Fig. 10. ) [Illustration: FIG. 10. --Illustrations of the nictitating membrane in the various animals named drawn from nature. The letter N indicates the membrane in each case. In man it is called the _plica semilunaris_, and is represented in the two lower drawings under this name. In the case of the shark (_Galeus_) the muscular mechanism is shown as dissected. ] Now the organization of man presents so many vestigial structures thusreferring to various stages of his long ancestral history, that it wouldbe tedious so much as to enumerate them. Therefore I will yet furtherlimit the list of vestigial structures to be given as examples, by notonly restricting these to cases which occur in our own organization; butof them I shall mention only such as refer us to the very last stage ofour ancestral history--viz. Structures which have become obsolescentsince the time when our distinctively human branch of the family treediverged from that of our immediate forefathers, the Quadrumana. (1) _Muscles of the external ear. _--These, which are of large size andfunctional use in quadrupeds, we retain in a dwindled and uselesscondition (Fig. 11). This is likewise the case in anthropoid apes; butin not a few other Quadrumana (e. G. Baboons, macacus, magots, &c. )degeneration has not proceeded so far, and the ears are voluntarilymoveable. [Illustration: FIG. 11. --Rudimentary, or vestigial and useless, muscles of the human ear. (From _Gray's Anatomy_. )] (2) _Panniculus carnosis. _--A large number of the mammalia are able tomove their skin by means of sub-cutaneous muscle--as we see, forinstance, in a horse, when thus protecting himself against the suckingof flies. We, in common with the Quadrumana, possess an active remnantof such a muscle in the skin of the forehead, whereby we draw up theeyebrows; but we are no longer able to use other considerable remnantsof it, in the scalp and elsewhere, --or, more correctly, it is rarelythat we meet with persons who can. But most of the Quadrumana (includingthe anthropoids) are still able to do so. There are also many othervestigial muscles, which occur only in a small percentage of humanbeings, but which, when they do occur, present unmistakeable homologieswith normal muscles in some of the Quadrumana and still loweranimals[5]. [5] See especially Mr. John Wood's papers, _Proc. R. S. _, xiii to xvi, and xviii; also _Journ. Anat. _, i and iii. In this connexion Darwin refers to M. Richard, _Annls. D. Sc. Nat. Zoolg. _, tom. Xviii, p. 13, 1852. (3) _Feet. _--It is observable that in the infant the feet have a strongdeflection inwards, so that the soles in considerable measure face oneanother. This peculiarity, which is even more marked in the embryo thanin the infant (see p. 153), and which becomes gradually less and lessconspicuous even before the child begins to walk, appears to me a highlysuggestive peculiarity. For it plainly refers to the condition ofthings in the Quadrumana, seeing that in all these animals the feet aresimilarly curved inwards, to facilitate the grasping of branches. Andeven when walking on the ground apes and monkeys employ to a greatextent the outside edges of their feet, as does also a child whenlearning to walk. The feet of a young child are also extraordinarilymobile in all directions, as are those of apes. In order to show thesepoints, I here introduce comparative drawings of a young ape and theportrait of a young male child. These drawings, moreover, serve at thesame time to illustrate two other vestigial characters, which haveoften been previously noticed with regard to the infant's foot. I alludeto the incurved form of the legs, and the lateral extension of the greattoe, whereby it approaches the thumb-like character of this organ in theQuadrumana. As in the case of the incurved position of the legs andfeet, so in this case of the lateral extensibility of the great toe, thepeculiarity is even more marked in embryonic than in infant life. For, as Prof. Wyman has remarked with regard to the foetus when about an inchin length, "The great toe is shorter than the others; and, instead ofbeing parallel to them, is projected at an angle from the side of thefoot, thus corresponding with the permanent condition of this part inthe Quadrumana[6]. " So that this organ, which, according to Owen, "isperhaps the most characteristic peculiarity in the human structure, "when traced back to the early stages of its development, is found topresent a notably less degree of peculiarity. [6] _Proc. Nat. Hist. Soc. _, Boston, 1863. [Illustration: FIG. 12. --Portrait of a young male gorilla (after Hartmann). ] [Illustration: FIG. 13. --Portrait of a young male child. Photographed from life, when the mobile feet were for a short time at rest in a position of extreme inflection. ] (4) _Hands. _--Dr. Louis Robinson has recently observed that the graspingpower of the whole human hand is so surprisingly great at birth, andduring the first few weeks of infancy, as to be far in excess of presentrequirements on the part of a young child. Hence he concludes that itrefers us to our quadrumanous ancestry--the young of anthropoid apesbeing endowed with similar powers of grasping, in order to hold on tothe hair of the mother when she is using her arms for the purposes oflocomotion. This inference appears to me justifiable, inasmuch as noother explanation can be given of the comparatively inordinate muscularforce of an infant's grip. For experiments showed that very young babiesare able to support their own weight, by holding on to a horizontal bar, for a period varying from one half to more than two minutes[7]. With hiskind permission I here reproduce one of Dr. Robinson's instantaneous, and hitherto unpublished, photographs of a very young infant. Thisphotograph was taken after the above paragraph (3) was written, and Iintroduce it here because it serves to show incidentally--and perhapseven better than the preceding figure--the points there mentioned withregard to the feet and great toes. Again, as Dr. Robinson observes, theattitude, and the disproportionately large development of the arms ascompared with the legs, give all the photographs a striking resemblanceto a picture of the chimpanzee "Sally" at the Zoological Gardens. For"invariably the thighs are bent nearly at right angles to the body, andin no case did the lower limbs hang down and take the attitude of theerect position. " He adds, "In many cases no sign of distress is evinced, and no cry uttered, until the grasp begins to give way. " [7] _Nineteenth Century_, November, 1891. [Illustration: FIG. 14. --An infant, three weeks old, supporting its own weight for over two minutes. The attitude of the lower limbs, feet, and toes, is strikingly simian. Reproduced from an instantaneous photograph, kindly given for the purpose by Dr. L. Robinson. ] (5) _Tail. _--The absence of a tail in man is popularly supposed toconstitute a difficulty against the doctrine of his quadrumanousdescent. As a matter of fact, however, the absence of an external tailin man is precisely what this doctrine would expect, seeing that thenearest allies of man in the quadrumanous series are likewise destituteof an external tail. Far, then, from this deficiency in man constitutingany difficulty to be accounted for, if the case were not so--i. E. Ifman _did_ possess an external tail, --the difficulty would be tounderstand how he had managed to retain an organ which had beenrenounced by his most recent ancestors. Nevertheless, as the anthropoidapes continue to present the rudimentary vestiges of a tail in a fewcaudal vertebræ below the integuments, we might well expect to find asimilar state of matters in the case of man. And this is just what we dofind, as a glance at these two comparative illustrations will show. (Fig. 15. ) Moreover, during embryonic life, both of the anthropoid apesand of man, the tail much more closely resembles that of the lower kindsof quadrumanous animals from which these higher representatives of thegroup have descended. For at a certain stage of embryonic life the tail, both of apes and of human beings, is actually longer than the legs (seeFig. 16). And at this stage of development, also, the tail admits ofbeing moved by muscles which later on dwindle away. Occasionally, however, these muscles persist, and are then described by anatomists asabnormalities. The following illustrations serve to show the muscles inquestion, when thus found in adult man. [Illustration: FIG. 15. --Sacrum of Gorilla compared with that of Man, showing the rudimentary tail-bones of each. Drawn from nature (_R. Coll. Surg. Mus. _). ] [Illustration: FIG. 16. --Diagrammatic outline of the human embryo when about seven weeks old, showing the relations of the limbs and tail to the trunk (after Allen Thomson), _r_, the radial, and _u_, the ulnar, border of the hand and fore-arm; _t_, the tibial, and _f_, the fibular, border of the foot and lower leg; _au_, ear; _s_, spinal cord; _v_, umbilical cord; _b_, branchial gill-slits; _c_, tail. ] [Illustration: FIG. 17. --Front and back view of adult human sacrum, showing abnormal persistence of vestigial tail-muscles. (The first drawing is copied from Prof. Watson's paper in _Journl. Anat. And Physiol. _, vol. 79: the second is compiled from different specimens. )] (6) _Vermiform Appendix of the Cæcum. _--This is of large size andfunctional use in the process of digestion among many herbivorousanimals; while in man it is not only too small to serve any suchpurpose, but is even a source of danger to life--many persons dyingevery year from inflammation set up by the lodgement in this blind tubeof fruit-stones, &c. In the orang it is longer than in man (Fig. 18), as it is also in thehuman foetus proportionally compared with the adult. (Fig. 19. ) Insome of the lower herbivorous animals it is longer than the entire body. Like vestigial structures in general, however, this one is highlyvariable. Thus the above cut (Fig. 19) serves to show that it maysometimes be almost as short in the orang as it normally is in man--boththe human subjects of this illustration having been normal. [Illustration: FIG. 18. --_Appendix vermiformis_ in Orang and in Man. Drawn from dried inflated specimens in the Cambridge Museum by Mr. J. J. Lister. _Il_, ilium; _Co_, colon; _C_, cæcum; W, a window cut in the wall of the cæcum; X X X, the appendix. ] [Illustration: FIG. 19. --The same, showing variation in the Orang. Drawn from a specimen in the Museum of the Royal College of Surgeons. ] (7) _Ear. _--Mr. Darwin writes:-- The celebrated sculptor, Mr. Woolner, informs me of one little peculiarity in the external ear, which he has often observed both in men and women.... The peculiarity consists in a little blunt point, projecting from the inwardly folded margin, or helix. When present, it is developed at birth, and, according to Prof. Ludwig Meyer, more frequently in man than in woman. Mr. Woolner made an exact model of one such case, and sent me the accompanying drawing.... The helix obviously consists of the extreme margin of the ear folded inwards; and the folding appears to be in some manner connected with the whole external ear being permanently pressed backwards. In many monkeys, which do not stand high in the order, as baboons and some species of macacus, the upper portion of the ear is slightly pointed, and the margin is not at all folded inwards; but if the margin were to be thus folded, a slight point would necessarily project towards the centre.... The following wood-cut is an accurate copy of a photograph of the foetus of an orang (kindly sent me by Dr. Nitsche), in which it may be seen how different the pointed outline of the ear is at this period from its adult condition, when it bears a close general resemblance to that of man [including even the occasional appearance of the projecting point shown in the preceding woodcut]. It is evident that the folding over of the tip of such an ear, unless it changed greatly during its further development, would give rise to a point projecting inwards[8]. [8] _Descent of Man_, 2nd ed. , pp. 15-16. [Illustration: FIG. 20. --Human ear, modelled and drawn by Mr. Woolner. _a_, the projecting point. ] [Illustration: FIG. 21. --Foetus of an Orang. Exact copy of a photograph, showing the form of the ear at this early stage. ] The following woodcut serves still further to show vestigialresemblances between the human ear and that of apes. The last twofigures illustrate the general resemblance between the normal ear offoetal man and the ear of an adult orang-outang. The other two figureson the lower line are intended to exhibit occasional modifications ofthe adult human ear, which approximate simian characters somewhat moreclosely than does the normal type. It will be observed that in theircomparatively small lobes these ears resemble those of all the apes; andthat while the outer margin of one is not unlike that of the Barbaryape, the outer margin of the other follows those of the chimpanzee andorang. Of course it would be easy to select individual human ears whichpresent either of these characters in a more pronounced degree; butthese ears have been chosen as models because they present bothcharacters in conjunction. The upper row of figures likewise shows theclose similarity of hair-tracts, and the direction of growth on the partof the hair itself, in cases where the human ear happens to be of anabnormally hirsute character. But this particular instance (which I donot think has been previously noticed) introduces us to the subject ofhair, and hair-growth, in general. [Illustration: FIG. 22. --Vestigial characters of human ears. Drawn from nature. ] (8) _Hair. _--Adult man presents rudimentary hair over most parts of thebody. Wallace has sought to draw a refined distinction between thisvestigial coating and the useful coating of quadrumanous animals, in theabsence of the former from the human back. But even this refineddistinction does not hold. On the one hand, the comparatively hairlesschimpanzee which died last year in the Zoological Gardens (_T. Calvus_)was remarkably denuded over the back; and, on the other hand, men whopresent a considerable development of hair over the rest of their bodiespresent it also on their backs and shoulders. Again, in all men therudimentary hair on the upper and lower arm is directed towards theelbow--a peculiarity which occurs nowhere else in the animal kingdom, with the exception of the anthropoid apes and a few American monkeys, where it presumably has to do with arboreal habits. For, when sitting intrees, the orang, as observed by Mr. Wallace, places its hands above itshead with its elbows pointing downwards: the disposition of hair onthe arms and fore-arms then has the effect of thatch in turning therain. Again, I find that in all species of apes, monkeys, and baboonswhich I have examined (and they have been numerous), the hair on thebacks of the hands and feet is continued as far as the first row ofphalanges; but becomes scanty, or disappears altogether, on the secondrow; while it is invariably absent on the terminal row. I also find thatthe same peculiarity occurs in man. We all have rudimentary hair on thefirst row of phalanges, both of hands and feet: when present at all, itis more scanty on the second row; and in no case have I been able tofind any on the terminal row. In all cases these peculiarities arecongenital, and the total absence or partial presence of hair on thesecond phalanges is constant in different species of Quadrumana. Forinstance, it is entirely absent in all the chimpanzees, which I haveexamined, while scantily present in all the orangs. As in man, it occursin a patch midway between the joints. [Illustration: FIG. 23. --Hair-tracts on the arms and hands of Man, as compared with those on the arms and hands of Chimpanzee. Drawn from life. ] Besides showing these two features with regard to the disposition ofhair on the human arm and hand, the above woodcut illustrates a third. By looking closely at the arm of the very hairy man from whom thedrawing was taken, it could be seen that there was a strong tendencytowards a whorled arrangement of the hairs on the backs of the wrists. This is likewise, as a general rule, a marked feature in the arrangementof hair on the same places in the gorilla, orang, and chimpanzee. In thespecimen of the latter, however, from which the drawing was taken, thischaracteristic was not well marked. The downward direction of the hairon the backs of the hands is exactly the same in man as it is in allthe anthropoid apes. Again, with regard to hair, Darwin notices thatoccasionally there appears in man a few hairs in the eyebrows muchlonger than the others; and that they seem to be representative ofsimilarly long and scattered hairs which occur in the chimpanzee, macacus, and baboons. Lastly, it may be here more conveniently observed than in the nextchapter on Embryology, that at about the sixth month the human foetusis often thickly coated with somewhat long dark hair over the entirebody, except the soles of the feet and palms of the hands, which arelikewise bare in all quadrumanous animals. This covering, which iscalled the lanugo, and sometimes extends even to the whole forehead, ears, and face, is shed before birth. So that it appears to be uselessfor any purpose other than that of emphatically declaring man a child ofthe monkey. (9) _Teeth. _--Darwin writes:-- It appears as if the posterior molar or wisdom-teeth were tending to become rudimentary in the more civilized races of man. These teeth are rather smaller than the other molars, as is likewise the case with the corresponding teeth in the chimpanzee and orang; and they have only two separate fangs.... They are also much more liable to vary, both in structure and in the period of their development, than the other teeth. In the Melanian races, on the other hand, the wisdom-teeth are usually furnished with three separate fangs, and are usually sound [i. E. Not specially liable to decay]; they also differ from the other molars in size, less than in the Caucasian races. Now, in addition to these there are other respects in which thedwindling condition of wisdom-teeth is manifested--particularly withregard to the pattern of their crowns. Indeed, in this respect it wouldseem that even in the anthropoid apes there is the beginning of atendency to degeneration of the molar teeth from behind forwards. For ifwe compare the three molars in the lower jaw of the gorilla, orang, andchimpanzee, we find that the gorilla has five well-marked cusps on allthree of them; but that in the orang the cusps are not so pronounced, while in the chimpanzee there are only four of them on the third molar. Now in man it is only the first of these three teeth which normallypresents five cusps, both the others presenting only four. So that, comparing all these genera together, it appears that the number ofcusps is being reduced from behind forwards; the chimpanzee having lostone of them from the third molar, while man has not only lost this, butalso one from the second molar, --and, it may be added, likewisepartially (or even totally) from the first molar, as a frequentvariation among civilized races. But, on the other hand, variations areoften met with in the opposite direction, where the second or the thirdmolar of man presents five cusps--in the one case following thechimpanzee, in the other the gorilla. These latter variations, therefore, may fairly be regarded as reversionary. For these facts I amindebted to the kindness of Mr. C. S. Tomes. [Illustration: FIG. 24. --Molar teeth of lower jaw in Gorilla, Orang, and Man. Drawn from nature, nat. Size (_R. Mus. Coll. Surg. _). ] (10) _Perforations of the humerus. _--The peculiarities which we have tonotice under this heading are two in number. First, the supra condyloidforamen is a normal feature in some of the lower Quadrumana (Fig. 25), where it gives passage to the great nerve of the fore-arm, and oftenalso to the great artery. In man, however, it is not a normal feature. Yet it occurs in a small percentage of cases--viz. , according to Sir W. Turner, in about one per cent. , and therefore is regarded by Darwin as avestigial character. Secondly, there is inter-condyloid foramen, whichis also situated near the lower end of the humerus, but more in themiddle of the bone. This occurs, but not constantly, in apes, and alsoin the human species. From the fact that it does so much more frequentlyin the bones of ancient--and also of some savage--races of mankind (viz. In 20 to 30 per cent. Of cases), Darwin is disposed to regard it also asa vestigial feature. On the other hand, Prof. Flower tells me that inhis opinion it is but an expression of impoverished nutrition duringthe growth of the bone. [Illustration: FIG. 25. --Perforation of the humerus (supra-condyloid foramen) in three species of Quadrumana where it normally occurs, and in Man, where it does not normally occur. Drawn from nature (_R. Coll. Surg. Mus. _). ] (11) _Flattening of tibia. _--In some very ancient human skeletons, therehas also been found a lateral flattening of the tibia, which rarelyoccurs in any existing human beings, but which appears to have beenusual among the earliest races of mankind hitherto discovered. Accordingto Broca, the measurements of these fossil human tibiæ resemble those ofapes. Moreover, the bone is bent and strongly convex forwards, while itsangles are so rounded as to present the nearly oval section seen inapes. It is in association with these ape-like human tibiæ thatperforated humeri of man are found in greatest abundance. On the other hand, however, there is reason to doubt whether this formof tibia in man is really a survival from his quadrumanous ancestry. For, as Boyd-Dawkins and Hartmann have pointed out, the degree offlattening presented by some of these ancient human bones is _greater_than that which occurs in any existing species of anthropoid ape. Ofcourse the possibility remains that the unknown species of ape fromwhich man descended may have had its tibia more flattened than is nowobservable in any of the existing species. Nevertheless, as some doubtattaches to this particular case, I do not press it--and, indeed, onlymention it at all in order that the doubt may be expressed. Similarly, I will conclude by remarking that several other instances ofthe survival of vestigial structures in man have been alleged, which areof a still more doubtful character. Of such, for example, are thesupposed absence of the genial tubercle in the case of a very ancientjaw-bone of man, and the disposition of valves in human veins. From theformer it was argued that the possessor of this very ancient jaw-bonewas probably speechless, inasmuch as the tubercle in existing man givesattachment to muscles of the tongue. From the latter it has been arguedthat all the valves in the veins of the human body have reference, intheir disposition, to the incidence of blood-pressure when the attitudeof the body is horizontal, or quadrupedal. Now, the former case hasalready broken down, and I find that the latter does not hold. But wecan well afford to lose such doubtful and spurious cases, in view of allthe foregoing unquestionable and genuine cases of vestigial structureswhich are to be met with even within the limits of our ownorganization--and even when these limits are still further limited byselecting only those instances which refer to the very latest chapter ofour long ancestral history. CHAPTER IV. EMBRYOLOGY. We will next consider what of late years has become the most importantof the lines of evidence, not only in favour of the general fact ofevolution, but also of its history: I mean the evidence which has beenyielded by the newest of the sciences, the science of Embryology. Buthere, as in the analogous case of adult morphology, in order to dojustice to the mass of evidence which has now been accumulated, a wholevolume would be necessary. As in that previous case, therefore, I mustrestrict myself to giving an outline sketch of the main facts. First I will display what in the language of Paley we may call "thestate of the argument. " It is an observable fact that there is often a close correspondencebetween developmental changes as revealed by any chronological series offossils which may happen to have been preserved, and developmentalchanges which may be observed during the life-history of now existingindividuals belonging to the same group of animals. For instance, thesuccessive development of prongs in the horns of deer-like animals, which is so clearly shown in the geological history of this tribe, isclosely reproduced in the life-history of existing deer. Or, in otherwords, the antlers of an existing deer furnish in their development akind of _résumé_, or recapitulation, of the successive phases wherebythe primitive horn was gradually superseded by horns presenting agreater and greater number of prongs in successive species of extinctdeer (Fig. 26). Now it must be obvious that such a recapitulation in thelife-history of an existing animal of developmental changes successivelydistinctive of sundry allied, though now extinct species, speaksstrongly in favour of evolution. For as it is of the essence of thistheory that new forms arise from older forms by way of _hereditary_descent, we should antecedently expect, if the theory is true, that thephases of development presented by the individual organism would follow, in their main outlines, those phases of development through which theirlong line of ancestors had passed. The only alternative view is that asspecies of deer, for instance, were separately created, additionalprongs were successively added to their antlers; and yet that, in orderto be so added to successive species every individual deer belonging tolater species was required to repeat in his own lifetime the process ofsuccessive additions which had previously taken place in a remote seriesof extinct species. Now I do not deny that this view is a possible view;but I do deny that it is a probable one. According to the evolutionaryinterpretation of such facts, we can see a very good _reason_ why thelife-history of the individual is thus a condensed _résumé_ of thelife-history of its ancestral species. But according to the oppositeview no reason can be assigned why such should be the case. In aprevious chapter--the chapter on Classification--we have seen that ifeach species were created separately, no reason can be assigned why theyshould all have been turned out upon structural patterns so stronglysuggestive of hereditary descent with gradual modifications, or slowdivergence--the result being group subordinated to group, with the mostgeneralized (or least developed) forms at the bottom, and the highestproducts of organization at the top. And now we see--or shallimmediately see--that this consideration admits of being greatlyfortified by a study of the developmental history of every individualorganism. If it would be an unaccountable fact that every separatelycreated species should have been created with close structuralresemblances to a certain limited number of other species, less closeresemblances to certain further species, and so backwards; assuredly itwould be a still more unaccountable fact that every individual of everyspecies should exhibit in its own person a history of developmentalchange, every term of which corresponds with the structuralpeculiarities of its now extinct predecessors--and this in the exacthistorical order of their succession in geological time. The more thatwe think about this antithesis between the naturalistic and thenon-naturalistic interpretations, the greater must we feel the contrastin respect of rationality to become; and, therefore, I need not spendtime by saying anything further upon the antecedent standing of the twotheories in this respect. The evidence, then, which I am about to adducefrom the study of development in the life-histories of individualorganisms, will be regarded by me as so much unquestionable evidence infavour of similar processes of development in the life-histories oftheir respective species--in so far, I mean, as the two sets of changesadmit of being proved parallel. [Illustration: FIG. 26. --Antlers of Stag, showing successive addition of branches in successive years. Drawn from nature (_Brit. Mus. _). ] In the only illustration hitherto adduced--viz. That of deers'horns--the series of changes from a one-pronged horn to a fullydeveloped arborescent antler, is a series which takes place during theadult life of the animal; for it is only when the breeding age has beenattained that horns are required to appear. But seeing that every animalpasses through most of the phases of its development, not only beforethe breeding age has been attained, but even before the time of its ownbirth, clearly the largest field for the study of individual developmentis furnished by embryology. For instance, there is a salamander whichdiffers from most other salamanders in being exclusively terrestrial inits habits. Now, the young of this salamander before their birth arefound to be furnished with gills, which, however, they are neverdestined to use. Yet these gills are so perfectly formed, that if theyoung salamanders be removed from the body of their mother shortlybefore birth, and be then immediately placed in water, the littleanimals show themselves quite capable of aquatic respiration, and willmerrily swim about in a medium which would quickly drown their ownparent. Here, then, we have both morphological and physiologicalevidence pointing to the possession of gills by the ancestors of theland salamander. It would be easy to devote the whole of the present chapter to anenumeration of special instances of the kinds thus chosen for purposesof illustration; but as it is desirable to take a deeper, and thereforea more general view of the whole subject, I will begin at thefoundation, and gradually work up from the earliest stages ofdevelopment to the latest. Before starting, however, I ask the reader tobear in mind one consideration, which must reasonably prevent ouranticipating that in _every case_ the life-history of an individualorganism should present a _full_ recapitulation of the life-history ofits ancestral line of species. Supposing the theory of evolution to betrue, it must follow that in many cases it would have been more or lessdisadvantageous to a developing type that it should have been obliged toreproduce in its individual representatives all the phases ofdevelopment previously undergone by its ancestry--even within the limitsof the same family. We can easily understand, for example, that thewaste of material required for building up the useless gills of theembryonic salamanders is a waste which, sooner or later, is likely to bedone away with; so that the fact of its occurring at all is in itselfenough to show that the change from aquatic to terrestrial habits on thepart of this species must have been one of comparatively recentoccurrence. Now, in as far as it is detrimental to a developing typethat it should pass through any particular ancestral phases ofdevelopment, we may be sure that natural selection--or whatever otheradjustive causes we may suppose to have been at work in the adaptationof organisms to their surroundings--will constantly seek to get rid ofthis necessity, with the result, when successful, of dropping out thedetrimental phases. Thus the foreshortening of developmental historywhich takes place in the individual lifetime may be expected often totake place, not only in the way of condensation, but also in the way ofexcision. Many pages of ancestral history may be recapitulated in theparagraphs of embryonic development, while others may not be so much asmentioned. And that this is the true explanation of what embryologiststerm "direct" development--or of a more or less sudden leap from onephase to another, without any appearance of intermediate phases--isproved by the fact that in some cases both direct and indirectdevelopment occur within the same group of organisms, some genera orfamilies having dropped out the intermediate phases which other generaor families retain. * * * * * The argument from embryology must be taken to begin with the firstbeginning of individual life in the ovum. And, in order to understandthe bearings of the argument in this its first stage, we must considerthe phenomena of reproduction in the simplest form which these phenomenaare known to present. The whole of the animal kingdom is divided into two great groups, whichare called the Protozoa and the Metazoa. Similarly, the whole of thevegetable kingdom is divided into the Protophyta and the Metaphyta. Thecharacteristic feature of all the Protozoa and Protophyta is that theorganism consists of a single physiological cell, while thecharacteristic of all the Metazoa and Metaphyta is that the organismconsists of a plurality of physiological cells, variously modified tosubserve different functions in the economy of the animal or plant, asthe case may be. For the sake of brevity, I shall hereafter deal onlywith the case of animals (Protozoa and Metazoa); but it may throughoutbe understood that everything which is said applies also to the case ofplants (Protophyta and Metaphyta). A Protozoön (like a Protophyton) is a solitary cell, or a "unicellularorganism, " while a Metazoön (like a Metaphyton) is a society of cells, or a "multicellular organism. " Now, it is only in the multicellularorganisms that there is any observable distinction of sex. In all theunicellular organisms the phenomena of reproduction appear to be more orless identical with those of growth. Nevertheless, as these phenomenaare here in some cases suggestively peculiar, I will consider them morein detail. A Protozoön is a single corpuscle of protoplasm which in differentspecies of Protozoa varies in size from more than one inch to less than1/1000 of an inch in diameter. In some species there is an envelopingcortical substance; in other species no such substance can be detected. Again, in most species there is a nucleus, while in other species nosuch differentiation of structure has hitherto been observed. Nevertheless, from the fact that the nucleus occurs in the majority ofProtozoa, coupled with the fact that the demonstration of this body isoften a matter of extreme difficulty, not only in some of the Protozoawhere it has been but recently detected, but also in the case of certainphysiological cells elsewhere, --from these facts it is not unreasonableto suppose that all the Protozoa possess a nucleus, whether or not itadmits of being rendered visible by histological methods thus far at ourdisposal. If this is the case, we should be justified in saying, as Ihave said, that a Protozoön is an isolated physiological cell, and, likecells in general, multiplies by means of what Spencer and Häckel haveaptly called a process of discontinuous growth. That is to say, when acell reaches maturity, further growth takes place in the direction of aseverance of its substance--the separated portion thus starting anew asa distinct physiological unit. But, notwithstanding the complex changeswhich have been more recently observed to take place in the nucleus ofsome Protozoa prior to their division, the process of multiplication bydivision may still be regarded as a process of growth, which differsfrom the previous growth of the individual cell in being attended by aseverance of continuity. If we take a suspended drop of gum, andgradually add to its size by allowing more and more gum to flow into it, a point will eventually be reached at which the force of gravity willovercome that of cohesion, and a portion of the drop will fall away fromthe remainder. Here we have a rough physical simile, although of courseno true analogy. In virtue of a continuous assimilation of nutriment, the protoplasm of a cell increases in mass, until it reaches the size atwhich the forces of disruption overcome those of cohesion--or, in otherwords, the point at which increase of size is no longer compatible withcontinuity of substance. Nevertheless, it must not be supposed that theprocess is thus merely a physical one. The phenomena which occur even inthe simplest--or so-called "direct"--cell-division, are of themselvesenough to prove that the process is vital, or physiological; and this ina high degree of specialization. But so, likewise, are all processes ofgrowth in organic structures; and therefore the simile of the drop ofgum is not to be regarded as a true analogy: it serves only to indicatethe fact that when cell-growth proceeds beyond a certain pointcell-division ensues. The size to which cells may grow before they thusdivide is very variable in different kinds of cells; for while some maynormally attain a length of ten or twelve inches, others divide beforethey measure 1/1000 of an inch. This, however, is a matter of detail, and does not affect the general physiological principles on which we areat present engaged. Now, as we have seen, a Protozoön is a single cell; for even although insome of the higher forms of protozoal life a colony of cells may bebound together in organic connexion, each of these cells is in itself an"individual, " capable of self-nourishment, reproduction, and, generally, of independent existence. Consequently, when the growth of a Protozoönends in a division of its substance, the two parts wander away from eachother as separate organisms. (Fig. 27. ) [Illustration: FIG. 27. --Fission of a Protozoön. In the left-hand drawing the process is represented as having advanced sufficiently far to have caused a division and segregation both of the nucleus and the vesicle. In the right-hand drawing the process is represented as complete. _n_, N, severed nucleus; _vc_, severed vesicle; _ps_, pseudopodia; _f_, ingested food. ] The next point we have to observe is, that in all cases where a cell ora Protozoön multiplies by way of fissiparous division, the processbegins in the nucleus. If the nucleus divides into two parts, the wholecell will eventually divide into two parts, each of which retains aportion of the original nucleus, as represented in the above figure. Ifthe nucleus divides into three, four, or even, as happens in thedevelopment of some embryonic tissues, into as many as six parts, thecell will subdivide into a corresponding number, each retaining aportion of the nucleus. Therefore, in all cases of fissiparous division, the seat or origin of the process is the nucleus. Thus far, then, the phenomena of multiplication are identical in all thelowest or unicellular organisms, and in the constituent cells of all thehigher or multicellular. And this is the first point which I desire tomake apparent. For where the object is to prove a continuity between thephenomena of growth and reproduction, it is of primary importance toshow--1st, that there is such a continuity in the case of all theunicellular organisms, and, 2nd, that there are all the above points ofresemblance between the multiplication of cells in the unicellular andin the multicellular organisms. It remains to consider the points of difference, and, if possible, toshow that these do not go to disprove the doctrine of continuity whichthe points of resemblance so forcibly indicate. The first point of difference obviously is, that in the case of all themulticellular organisms the two or more "daughter-cells, " which areproduced by division of the "mother-cell, " do not wander away from oneanother; but, as a rule, they continue to be held in more or less closeapposition by means of other cells and binding membranes, --with theresult of giving rise to those various "tissues, " which in turn go toconstitute the material of "organs. " I cannot suppose, however, that anyadvocate of discontinuity will care to take his stand at this point. But, if any one were so foolish as to do so, it would be easy todislodge him by describing the state of matters in some of the Protozoawhere a number of unicellular "individuals" are organically united so asto form a "colony. " These cases serve to bridge this distinction betweenProtozoa and Metazoa, of which therefore we may now take leave. In the second place, there is the no less obvious distinction that theresult of cell-division in the Metazoa is not merely to multiply cellsall of the same kind: on the contrary, the process here gives rise to asmany different kinds of cells as there are different kinds of tissuecomposing the adult organism. But no one, I should think, is likely tooppose the doctrine of continuity on the ground of this distinction. Forthe distinction is clearly one which must necessarily arise, if thedoctrine of continuity between unicellular and multicellular organismsbe true. In other words, it is a distinction which the theory ofevolution itself must necessarily pre-suppose, and therefore it is noobjection to the theory that its pre-supposition is realized. Moreover, as we shall see better presently, there is no difficulty inunderstanding why this distinction should have arisen, so soon as itbecame necessary (or desirable) that individual cells, when composing a"colony, " should conform to the economic principle of the division oflabour--a principle, indeed, which is already foreshadowed in theconstituent parts of a single cell, since the nucleus has one set offunctions and its surrounding protoplasm another. But now, in the third place, we arrive at a more important distinction, and one which lies at the root of the others still remaining to beconsidered. I refer to sexual propagation. For it is a peculiarity ofthe multicellular organisms that, although many of them may likewisepropagate themselves by other means (Fig. 28), they all propagatethemselves by means of sexual congress. Now, in its essence, sexualcongress consists in the fusion of two specialized cells (or, as nowseems almost certain, of the nuclei thereof), so that it is out of sucha combination that the new individual arises by means of successivecell-divisions, which, beginning in the fertilized ovum, eventuallybuild up all the tissues and organs of the body. [Illustration: FIG. 28. --_Hydra viridis_, partly in section. M, mouth; O, ovary, or bud containing female reproductive cells; T, testis, or bud containing male reproductive cells. In addition to these buds containing germinal elements alone, there is another which illustrates the process of "gemmation"--i. E. The direct out-growth of a fully formed offspring. ] This process clearly indicates very high specialization on the part ofgerm-cells. For we see by it that although these cells when youngresemble all other cells in being capable of self-multiplication bybinary division (thus reproducing cells exactly like themselves), whenolder they lose this power; but, at the same time, they acquire anentirely new and very remarkable power of giving rise to a vastsuccession of many different kinds of cells, all of which are mutuallycorrelated as to their several functions, so as to constitute ahierarchy of cells--or, to speak literally, a multicellular_co-organization_. Here it is that we touch the really importantdistinction between the Protozoa and the Metazoa; for although I havesaid that some of the higher Protozoa foreshadow this state of mattersin forming cell-colonies, it must now be noted that the cells composingsuch colonies are all of the same kind; and, therefore, that theprinciple of producing different kinds of cells which, by mutualco-adaptation of functions, shall be capable of constructing amulticellular Metazoön, --this great principle of _co-organization_ isbut dimly nascent in the cell-colonies of Protozoa. And its marvellousdevelopment in the Metazoa appears ultimately to depend upon the highlyspecialized character of germ-cells. Even in cases where multicellularorganisms are capable of reproducing their kind without the need of anypreceding process of fertilization (parthenogenesis), and even in thestill more numerous cases where complete organisms are budded forth fromany part of their parent organism (gemmation, Fig. 28), there is nowvery good reason to conclude that these powers of a-sexual reproductionon the part of multicellular organisms are all ultimately due to thespecialized character of their germ-cells. For in all these cases thetissues of the parent, from which the budding takes place, wereultimately derived from germ-cells--no matter how many generations ofbudded organisms may have intervened. And that propagation by budding, &c, in multicellular organisms is thus ultimately due to theirpropagation by sexual methods, seems to be further shown by certainfacts which will have to be discussed at some length in my next volume. Here, therefore, I will mention only one of them--and this because itfurnishes what appears to be another important distinction between theProtozoa and the Metazoa. In nearly all cases where a Protozoön multiplies itself by fission, theprocess begins by a simple division of the nucleus. But when a Metazoönis developed from a germ-cell, although the process likewise begins by adivision of the nucleus, this division is not a simple or direct one; onthe contrary, it is inaugurated by a series of processes going on withinthe nucleus, which are so enormously complex, and withal so beautifullyordered, that to my mind they constitute the most wonderful--if not alsothe most suggestive--which have ever been revealed by microscopicalresearch. It is needless to say that I refer to the phenomena ofkaryokinesis. A few pages further on they will be described more fully. For our present purposes it is sufficient to give merely a pictorialillustration of their successive phases; for a glance at such arepresentation serves to reveal the only point to which attention hasnow to be drawn--namely, the immense complexity of the processes inquestion, and therefore the contrast which they furnish to the simple(or "direct") division of the nucleus preparatory to cell-division inthe unicellular organisms. Here, then (Fig. 29), we see the complexprocesses of karyokinesis in the first two stages of egg-cell division. But similar processes continue to repeat themselves in subsequentstages; and this, there is now good reason to believe, throughout _all_the stages of cell-division, whereby the original egg-cell eventuallyconstructs an entire organism. In other words, all the cells composingall the tissues of a multicellular organism, at all stages of itsdevelopment, are probably originated by these complex processes, whichdiffer so much from the simple process of direct division in theunicellular organisms[9]. In this important respect, therefore, it doesat first sight appear that we have a distinction between the Protozoaand the Metazoa of so pronounced a character, as fairly to raise thequestion whether cell-division is fundamentally identical in unicellularand in multicellular organisms. [9] I say "probably, " because analogy points in this direction. As a matter of fact, in many cases of tissue-formation karyokinesis has not hitherto been detected. But even if in such cases it does not occur--i. E. If failure to detect its occurrence be not due merely to still remaining imperfections of our histological methods, --the large number of cases in which it has been seen to occur in the formation of sundry tissues are of themselves sufficient to indicate some important difference between cells derived from ova (metazoal), and cells which have not been so derived (protozoal). Which is the point now under discussion. [Illustration: FIG. 29. --Successive stages in the division of the ovum, or egg-cell, of a worm. (After Strasburger. ) _a_ to _d_ show the changes taking place in the nucleus and surrounding cell-contents, which result in the first segmentation of the ovum at _e_; _f_ and _g_ show a repetition of these changes in each of the two resulting cells, leading to the second segmentation stage at _h_. ] Lastly, the only other distinction of a physiologically significant kindbetween a single cell when it occurs as a Protozoön and when it does soas the unfertilized ovum of a Metazoön is, that in the latter case thenucleus discharges from its own substance two minute protoplasmic masses("polar bodies"), which are then eliminated from the cell altogether. This process, which will be more fully described later on, appears to beof invariable occurrence in the case of all egg-cells, while nothingresembling it has ever been observed in any of the Protozoa. We must now consider these several points of difference _seriatim_. First, with regard to sexual propagation, we have already seen that thisis by no means the only method of propagation among the multicellularorganisms; and it now remains to add that, on the other hand, there is, to say the least, a suggestive foreshadowing of sexual propagation amongthe unicellular organisms. For although simple binary fission is herethe more usual mode of multiplication, very frequently two (rarely threeor more) Protozoa of the same species come together, fuse into a singlemass, and thus become very literally "one flesh. " This process of"conjugation" is usually (though by no means invariably) followed by aperiod of quiescent "encystation"; after which the contents of the cystescape in the form of a number of minute particles, or "spores, " andthese severally develope into the parent type. Obviously this process ofconjugation, when it is thus a preliminary to multiplication, appears tobe in its essence the same as fertilization. And if it be objected thatencystation and spore-formation in the Protozoa are not always precededby conjugation, the answer would be that neither is oviparouspropagation in the Metazoa invariably preceded by fertilization. Nevertheless, that there are great distinctions between true sexualpropagation and this foreshadowing of it in conjugation I do not deny. The question, however, is whether they be so great as to justify anyargument against an historical continuity between them. What, then, arethese remaining distinctions? Briefly, as we have seen, they are theextrusion from egg-cells of polar bodies, and the occurrence, both inegg-cells and their products (tissue-cells), of the process ofkaryokinesis. But, as regards the polar bodies, it is surely notdifficult to suppose that, whatever their significance may be, it isprobably in some way or another connected with the high specializationof the functions which an egg-cell has to discharge. Nor is there anydifficulty in further supposing that, whatever purpose is served bygetting rid of polar bodies, the process whereby they are got rid of wasoriginally one of utilitarian development--i. E. A process which at itscommencement did not betoken any difference of kind, or breach ofcontinuity, between egg-cells and cells of simpler constitution. Lastly, with respect to karyokinesis, although it is true that themicroscope has in comparatively recent years displayed this apparentlyimportant distinction between unicellular and multicellular organisms, two considerations have here to be supplied. The first is, that in someof the Protozoa processes very much resembling those of karyokinesishave already been observed taking place in the nucleus preparatory toits division. And although such processes do not present quite the sameappearances as are to be met with in egg-cells, neither do thekaryokinetic processes in tissue-cells, which in their sundry kindsexhibit great variations in this respect. Moreover, even if such werenot the case, the bare fact that nuclear division is not invariably ofthe simple or direct character in the case of all Protozoa, issufficient to show that the distinction now before us--like the one lastdealt with--is by no means absolute. As in the case of sexualpropagation, so in that of karyokinesis, processes which are common toall the Metazoa are not wholly without their foreshadowings in theProtozoa. And seeing how greatly exalted is the office of egg-cells--andeven of tissue-cells--as compared with that of their supposed ancestryin protozoal cells, it seems to me scarcely to be wondered at if theirspecializations of function should be associated with correspondingpeculiarities of structure--a general fact which would in no waymilitate against the doctrine of evolution. Could we know the wholetruth, we should probably find that in order to endow the most primitiveof egg-cells with its powers of marshalling its products into a livingarmy of cell-battalions, such an egg-cell must have been passed througha course of developmental specialization of so elaborate a kind, thateven the complex processes of karyokinesis are but a very inadequateexpression thereof. Probably I have now said enough to show that, remarkable and altogetherexceptional as the properties of germ-cells of the multicellularorganisms unquestionably show themselves to be, yet when theseproperties are traced back to their simplest beginnings in theunicellular organisms, they may fairly be regarded as fundamentallyidentical with the properties of living cells in general. Thus viewed, no line of real demarcation can be drawn between growth andreproduction, even of the sexual kind. The one process is, so to speak, physiologically continuous with the other; and hence, so far as thepre-embryonic stage of life-history is concerned, the facts cannotfairly be regarded as out of keeping with the theory of evolution. I will now pass on to consider the embryogeny of the Metazoa, beginningat its earliest stage in the fertilization of the ovum. And here it isthat the constructive argument in favour of evolution which is derivedfrom embryology may be said properly to commence. For it is surely initself a most suggestive fact that all the Metazoa begin their life inthe same way, or under the same form and conditions. _Omne vivum exovo. _ This is a formula which has now been found to apply throughout thewhole range of the multicellular organisms. And seeing, as we have justseen, that the ovum is everywhere a single cell, the formula amounts tosaying that, physiologically speaking, every Metazoön begins its life asa Protozoön, and every Metaphyton as a Protophyton[10]. [10] Even when propagated by budding, a multicellular organism has been ultimately derived from a germ-cell. Now, if the theory of evolution is true, what should we expect to happenwhen these germ-cells are fertilized, and so enter upon their severallydistinct processes of development? Assuredly we should expect to findthat the higher organisms pass through the same phases of development asthe lower organisms, up to the time when their higher characters beginto become apparent. If in the life-history of species these highercharacters were gained by gradual improvement upon lower characters, andif the development of the higher individual is now a generalrecapitulation of that of its ancestral species, in studying thisrecapitulation we should expect to find the higher organism successivelyunfolding its higher characters from the lower ones through which itsancestral species had previously passed. And this is just what we dofind. Take, for example, the case of the highest organism, Man. Likethat of all other organisms, unicellular or multicellular, hisdevelopment starts from the nucleus of a single cell. Again, like thatof all the Metazoa and Metaphyta, his development starts from thespecially elaborated nucleus of an egg-cell, or a nucleus which has beenformed by the fusion of a male with a female element[11]. When hisanimality becomes established, he exhibits the fundamental anatomicalqualities which characterize such lowly animals as polyps andjelly-fish. And even when he is marked off as a Vertebrate, it cannot besaid whether he is to be a fish, a reptile, a bird, or a beast. Later onit becomes evident that he is to be a Mammal; but not till later stillcan it be said to which order of mammals he belongs. [11] It has already been stated that both parthenogenesis and gemmation are ultimately derived from sexual reproduction. It may now be added, on the other hand, that the earlier stages of parthenogenesis have been observed to occur sporadically in all sub-kingdoms of the Metaxoa, including the Vertebrata, and even the highest class, Mammalia. These earlier stages consist in _spontaneous_ segmentations of the ovum; so that even if a virgin has ever conceived and borne a son, and even if such a fact in the human species has been unique, still it would not betoken any breach of physiological continuity. Indeed, according to Weismann's not improbable hypothesis touching the physiological meaning of polar bodies, such a fact need betoken nothing more than a slight disturbance of the complex machinery of ovulation, on account of which the ovum failed to eliminate from its substance an almost inconceivably minute portion of its nucleus. Here, however, we must guard against an error which is frequently metwith in popular expositions of this subject. It is not true that theembryonic phases in the development of a higher form always resemble somany adult stages of lower forms. This may or may not be the case; butwhat always is the case is, that the embryonic phases of the higherform resemble the corresponding phases of the lower forms. Thus, forexample, it would be wrong to suppose that at any stage of hisdevelopment a man resembles a jelly-fish. What he does resemble at anearly stage of his development is the essential or groundplan of thejelly-fish, which that animal presents in _its_ embryonic condition, orbefore it begins to assume its more specialized characters fitting itfor its own particular sphere of life. The similarities, therefore, which it is the function of comparative embryology to reveal are thesimilarities of type or morphological plan: not similarities of specificdetail. Specific details may have been added to this, that, and theother species for their own special requirements, after they hadseverally branched off from the common ancestral stem; and so could notbe expected to recur in the life-history of an independent specificbranch. The comparison therefore must be a comparison of embryo withembryo; not of embryos with adult forms. * * * * * In order to give a general idea of the results thus far yielded by astudy of comparative embryology in the present connexion, I will devotethe rest of this chapter to giving an outline sketch of the mostimportant and best established of these results. Histologically the ovum, or egg-cell, is nearly identical in allanimals, whether vertebrate or invertebrate. Considered as a cell it isof large size, but actually it is not more than 1/100, and may be lessthan 1/200 of an inch in diameter. In man, as in most mammals, it isabout 1/120. It is a more or less spherical body, presenting a thintransparent envelope, called the _zona pellucida_, whichcontains--first, the protoplasmic cell-substance or "yolk, " within whichlies, second, the nucleus or germinal vesicle, within which again lies, third, the nucleolus or germinal spot. This description is true of theegg-cells of all animals, if we add that in the case of the lowestanimals--such as sponges, &c. --there is no enveloping membrane: theegg-cell is here a naked cell, and its constituent protoplasm, beingthus unconfined, is free to perform protoplasmic movements, which itdoes after the manner, and with all the activity, of an amoeba. Buteven with respect to this matter of an enveloping membrane, there is noessential difference between an ovum of the lowest and an ovum of thehighest animals. For in their early stages of development within theovary the ova of the highest animals are likewise in the condition ofnaked cells, exhibiting amoebiform movements; the enveloping membraneof an ovum being the product of a later development. Moreover thismembrane, when present, is usually provided with one or more minuteapertures, through which the spermatozoön passes when fertilizing theovum. It is remarkable that the spermatozoa know, so to speak, of theexistence of these gate-ways, --their snake-like movements being directedtowards them, presumably by a stimulus due to some emanationtherefrom[12]. In the mammalian ovum, however, these apertures areexceedingly minute, and distributed all round the circumference of thepellucid envelope, as represented in this illustration (Fig. 32). [12] The spermatozooids of certain plants can be strongly attracted towards a pipette which is filled with malic acid--crowding around and into it with avidity. [Illustration: FIG. 30. --Ovarian ovum of a Mammal, (_a_) magnified and viewed under pressure, (_b_) burst by increased pressure, with yolk and nucleus escaping: (_c_) the nucleus more freed from yolk-substance. (From _Quain's Anatomy_, after Allen Thomson. )] [Illustration: FIG. 31. --Amoeboid movements of young egg-cells, _a_, Amoeboid ovum of _Hydra_ (from Balfour, after Kleitnenberg); _b_, early ovum of _Toxopneustes variegatus_, with pseudopodia-like processes (from Balfour, after Selenka); _c_, ovum of _Toxopneustes lividus_, more nearly ripe (from Balfour, Hertwig). A1 to A4, the primitive egg-cell of a Chalk-Sponge (_Leuculmis echinus_), in four successive conditions of motion. B1 to B8, ditto of a Hermit-Crab (_Chondracanthus cornutus_), in eight successive stages (after E. Von Beneden). C1 to C5, ditto of a Cat, in five successive stages (after Pflüger). D, ditto of Trout; E, of a Hen; F, of Man. The first series is taken from the _Encycl. Brit. _; the second from Häckel's _Evolution of Man_. ] [Illustration: FIG. 32. --Human ovum, mature and greatly magnified. (After Häckel. )] In thus saying that the ova of all animals are, so far as microscopescan reveal, _substantially_ similar, I am of course speaking of theegg-cell proper, and not of what is popularly known as the egg. The eggof a bird, for example, is the egg-cell, _plus_ an enormous aggregationof nutritive material, an egg-shell, and sundry other structures suitedto the subsequent development of the egg-cell when separated from theparent's body. But all these accessories are, from our present point ofview, accidental or adventitious. What we have now to understand by theovum, the egg, or the egg-cell, is the microscopical germ which I havejust described. So far then as this germ is concerned, we find that allmulticellular organisms begin their existence in the same kind ofstructure, and that this structure is anatomically indistinguishablefrom that of the permanent form presented by the lowest, or unicellularorganisms. But although anatomically indistinguishable, physiologicallythey present the sundry peculiarities already mentioned. Now I have endeavoured to show that none of these peculiarities are suchas to exclude--or even so much as to invalidate--the supposition ofdevelopmental continuity between the lowest egg-cells and the highestprotozoal cells. It remains to show in this place, and on the otherhand, that there is no breach of continuity between the lowest and thehighest egg-cells; but, on the contrary, that the remarkable uniformityof the complex processes whereby their peculiar characters are exhibitedto the histologist, is such as of itself to sustain the doctrine ofcontinuity in a singularly forcible manner. On this account, therefore, and also because the facts will again have to be considered in anotherconnexion when we come to deal with Weismann's theory of heredity, Iwill here briefly describe the processes in question. We have already seen that the young egg-cell multiplies itself by simplebinary division, after the manner of unicellular organisms ingeneral--thereby indicating, as also by its amoebiform movements, itsfundamental identity with such organisms in kind. But, as we havelikewise seen, when the ovum ceases to resemble these organisms, bytaking on its higher degree of functional capacity, it is no longer ableto multiply itself in this manner. On the contrary, its cell-divisionsare now of an endogenous character, and result in the formation of manydifferent kinds of cells, in the order required for constructing themulticellular organism to which the whole series of processes eventuallygive rise. We have now to consider these processes _seriatim_. [Illustration: FIG. 33. --Stages in the formation of the polar bodies in the ovum of a star-fish. (After Hertwig. ) _g. V. _, germinal vesicle transformed into a spindle-shaped system of fibres; _p. '_, the first polar body becoming extruded; _p. _, _p. _, both polar bodies fully extruded; _f. Pn. _, female pronucleus, or residue of the germinal vesicle. ] First of all the nucleus discharges its polar bodies, as previouslymentioned, and in the manner here depicted on the previous page. (Fig. 33. ) It will be observed that the nucleus of the ovum, or the germinalvesicle as it is called, gets rid first of one and afterwards of theother polar body by an "indirect, " or karyokinetic, process of division. (Fig. 33. ) Extrusion of these bodies from the ovum (or it may be onlyfrom the nucleus) having been accomplished, what remains of the nucleusretires from the circumference of the ovum, and is called the femalepronucleus. (Fig. 33. _f. Pn. _) The ovum is now ready for fertilization. A similar emission of nuclear substance is said by some good observersto take place also from the male germ-cell, or spermatozoön, at or aboutthe close of _its_ development. The theories to which these facts havegiven rise will be considered in future chapters on Heredity. Turning now to the mechanism of fertilization, the diagrams (Figs. 34, 35) represent what happens in the case of star-fish. [Illustration: FIG. 34. --Fertilization of the ovum of an echinoderm. (From _Quain's Anatomy_, after Selenka. ) S, spermatozoön; _m. Pr. _, male pronucleus; _f. Pr. _, female pronucleus. 1 to 4 correspond to D to G in the next figure. ] [Illustration: FIG. 35. --Fertilization of the ovum of a star-fish. (From the _Encycl. Brit. _ after Fol. ) A, spermatozoa in the mucilaginous coat of the ovum; a prominence is rising from the surface of the ovum towards a spermatozoön; B, they have almost met; C, they have met; D, the spermatozoön enters the ovum through a distinct opening; H, the entire ovum, showing extruded polar bodies on its upper surface, and the moving together of the male and female pronuclei; E, F, G, meeting and coalescence of the pronuclei. ] The sperm-cell, or spermatozoön, is seen in the act of penetrating theovum. In the first figure it has already pierced the mucilaginous coatof the ovum, the limit of which is represented by a line through whichthe tail of the spermatozoön is passing: the head of the spermatozoön isjust entering the ovum proper. It may be noted that, in the case of manyanimals, the general protoplasm of the ovum becomes aware, so to speak, of the approach of a spermatozoön, and sends up a process to meet it. (Fig. 35, A, B, C. ) Several--or even many--spermatozoa may thus enterthe coat of the ovum; but normally only one proceeds further, or rightinto the substance of the ovum, for the purpose of effectingfertilization. This spermatozoön, as soon as it enters the periphery ofthe yolk, or cell-substance proper, sets up a series of remarkablephenomena. First, its own head rapidly increases in size, and takes onthe appearance of a cell-nucleus: this is called the male pronucleus. Atthe same time its tail begins to disappear, and the enlarged headproceeds to make its way directly towards the nucleus of the ovum which, as before stated, is now called the female pronucleus. The latter in itsturn moves towards the former, and when the two meet they fuse into onemass, forming a new nucleus. Before the two actually meet, thespermatozoön has lost its tail altogether; and it is noteworthy thatduring its passage through the protoplasmic cell-contents of the ovum, it appears to exercise upon this protoplasm an attractive influence; forthe granules of the latter in its vicinity dispose themselves around itin radiating lines. All these various phenomena are depicted in theabove wood-cuts. (Figs. 34, 35. ) Fertilization having been thus effected by fusion of the male and femalepronuclei into a single (or new) nucleus, this latter body proceeds toexhibit complicated processes of karyokinesis, which, as before shown, are preliminary to nuclear division in the case of egg-cells. Indeed thekaryokinetic process may begin in both the pronuclei before theirjunction is effected; and, even when their junction is effected, it doesnot appear that complete fusion of the so-called chromatin elements ofthe two pronuclei takes place. For the purpose of explaining what thismeans, and still more for the purpose of giving a general idea of thekaryokinetic processes as a whole, I will quote the followingdescription of them, because, for terseness combined with lucidity, itis unsurpassable. [Illustration: FIG. 36. --Karyokinesis of a typical tissue-cell (epithelium of Salamander). (After Flemming and Klein. ) The series from A to I represents the successive stages in the movement of the chromatin fibres during division, excepting G, which represents the "nucleus-spindle" of an egg-cell. A, resting nucleus; D, wreath-form; E, single star, the loops of the wreath being broken; F, separation of the star into two groups of U-shaped fibres; H, diaster or double star; I, completion of the cell-division and formation of two resting nuclei. In G the chromatin fibres are marked _a_, and correspond to the "equatorial plate"; _b_, achromatin fibres forming the nucleus-spindle; _c_, granules of the cell-protoplasm forming a "polar star. " Such a polar star is seen at each end of the nucleus-spindle, and is not to be confused with the diaster H, the two ends of which are composed of _chromatin_. ] Researches, chiefly due to Flemming, have shown that the nucleus in very many tissues of higher plants and animals consists of a capsule containing a plasma of "achromatin, " not deeply stained by re-agents, ramifying in which is a reticulum of "chromatin" consisting of fibres which readily take a deep stain. (Fig. 36, A). Further it is demonstrated that, when the cell is about to divide into two, definite and very remarkable movements take place in the nucleus, resulting in the disappearance of the capsule and in the arrangement of its fibres first in the form of a wreath (D), and subsequently (by the breaking of the loops formed by the fibres) in the form of a star (E). A further movement within the nucleus leads to an arrangement of the broken loops in two groups (F), the position of the open ends of the broken loops being reversed as compared with what previously obtained. Now the two groups diverge, and in many cases a striated appearance of the achromatin substance between the two groups of chromatin loops is observable (H). In some cases (especially egg-cells) this striated arrangement of the achromatin is then termed a "nucleus-spindle, " and the group of chromatin loops (G, _a_) is known as "the equatorial plate. " At each end of the nucleus-spindle in these cases there is often seen a star consisting of granules belonging to the general protoplasm of the cell (G, _c_). These are known as "polar stars. " After the separation of the two sets of loops (H) the protoplasm of the general substance of the cell becomes constricted, and division occurs, so as to include a group of chromatin loops in each of the two fission products. Each of these then rearranges itself together with the associated chromatin into a nucleus such as was present in the mother cell to commence with (I)[13]. [13] Ray Lankester, _Encyclop. Brit. _, 9th ed. , Vol. XIX, pp. 832-3. Since the above was published, however, further progress has been made. In particular it has been found that the chromatin fibres pass fromphase D to phase F by a process of longitudinal splitting (Fig. 37 _g_, _h_; Fig. 38, VI, VII)--which is a point of great importance forWeismann's theory of heredity, --and that the protoplasm outside thenucleus seems to take as important a part in the karyokinetic process asdoes the nuclear substance. For the so-called "attraction-spheres" (Fig. 38 II _a_, III, III _a_, VIII to XII), which were at first supposed tobe of subordinate importance in the process as a whole, are now known totake an exceedingly active part in it (see especially IX to XI). Lastly, it may be added that there is a growing consensus of authoritativeopinion, that the chromatin fibres are the seats of the material ofheredity, or, in other words, that they contain those essential elementsof the cell which endow the daughter-cells with their distinctivecharacters. Therefore, where the parent-cell is an ovum, it follows fromthis view that all hereditary qualities of the future organism arepotentially present in the ultra-microscopical structure of thechromatin fibres. [Illustration: FIG. 37. --Study of successive changes taking place in the nucleus of an epithelium cell, preparatory to division of the cell. (From _Quain's Anatomy_, after Flemming. ) _a_, resting cell, showing the nuclear network; _b_, first stage of division, the chromatoplasm transformed into a skein of closely contorted filaments; _c_ to _f_, further stages in the growth and looping arrangement of the filaments; _g_, stellate phase, or aster; _h_, completion of the splitting of the filaments, already begun in _f_ and _g_; _i_, _j_, _k_, successive stages in separation of the filaments into two groups; _l_, the final result of this (diaster); _m_ to _q_, stages in the division of the whole cell into two, showing increasing contortion of the filaments, until they reach the resting stage at _q_]. [Illustration: FIG. 38. --Formation and conjugation of the pronuclei in _Ascaris megalocephala_. (From _Quain's Anatomy_, after E. Von Beneden. ) _f_, female pronucleus; _m_, male pronucleus; _p_, one of the polar bodies. I. The second polar body has just been extruded; both male and female pronuclei contain two chromatin particles; those of the male pronucleus are becoming transformed into a skein. II. The chromatin in both pronuclei now forms into a skein. II _a_. The skeins are more distinct. Two attraction (or protoplasmic) spheres, each with a central particle united with a small spindle of achromatic fibres, have made their appearance in the general substance of the egg close to the mutually approaching pronuclei. The male pronucleus has the remains of the body of the spermatozoön adhering to it. III. Only the female pronucleus is shown in this figure. The skein is contracted and thickened. The attraction-spheres are near one side of the ovum, and are connected with its periphery by a cone of fibres forming a polar circle, _p. C. _; _e. C. _, equatorial circle. III _a_. The pronuclei have come into contact, and the spindle-system is now arranged across their common axis. IV. Contraction of the skein, and formation of two U-or V-shaped chromatin fibres in each pronucleus. V. The V-shaped chromatin filaments are now quite distinct: the male and female pronuclei are in close contact. ] [Illustration: (38 continued) VI. , VII. The V-shaped filaments are splitting longitudinally; their structure of fine granules of chromatin is apparent in VII. , which is more highly magnified. The conjugation of the pronuclei is apparently complete in VII. The attraction-spheres and achromatic spindle, although present, are not depicted in IV. , V. , VI. , and VII. VIII. Equatorial arrangement of the four chromatin loops in the middle of the now segmenting ovum: the achromatic substance forming a spindle-shaped system of granules with fibres radiating from the poles of the spindle (attraction-spheres); the chromatin forms an equatorial plate. (Compare Fig. 36 G. ) IX. Shows diagrammatically the commencing separation of the chromatin fibres of the conjugated nuclei, and the system of fibres radiating from the attraction-spheres. (Compare again Fig. 36 G. ) _p. C. _, polar circle; _e. C. _, equatorial circle; _c. C. _, central particle. X. Further separation of the chromatin filaments. Each of the central particles of the attraction-spheres has divided into two. XI. The chromatin fibres are becoming developed into the skeins of the two daughter-nuclei. These are still united by fibres of achromatin. The general protoplasm of the ovum is becoming divided. XII. The two daughter-nuclei exhibit a chromatin network. Each of the attraction-spheres has divided into two, which are joined by fibres of achromatin, and connected with the periphery of the cell in the same way as in the original or parent sphere, III. ] As I shall have more to say about these processes in the next volume, when we shall see the important part which they bear in Weismann'stheory of heredity, it is with a double purpose that I here introducethese yet further illustrations of them upon a somewhat larger scale. The present purpose is merely that of showing, more clearly thanhitherto, the great complexity of these processes on the one hand, and, on the other, the general similarity which they display in egg-cells andin tissue-cells. But as in relation to this purpose the illustrationsspeak for themselves, I may now pass on at once to the history ofembryonic development, which follows fertilization of the ovum. * * * * * We have seen that when the new nucleus of the fertilized ovum (which isformed by a coalescence of the male pronucleus with the female) hascompleted its karyokinetic processes, it is divided into two equalparts; that these are disposed at opposite poles of the ovum; and thatthe whole contents of the ovum are thereupon likewise divided into twoequal parts, with the result that there are now two nucleated cellswithin the spherical wall of the ovum where before there had only beenone. Moreover, we have also seen that a precisely similar series ofevents repeat themselves in each of these two cells, thus giving rise tofour cells (see Fig. 29). It must now be added that such duplication iscontinued time after time, as shown in the accompanying illustrations(Figs. 39, 40). [Illustration: FIG. 39. --Segmentation of ovum. (After Häckel. ) Successive stages are marked by the letters A, B, C. D represents several stages in advance of C. ] [Illustration: FIG. 40. --The contents of an ovum in an advanced stage of segmentation, drawn in perspective. (After Häckel. )] All this, it will be noticed, is a case of cell-multiplication, whichdiffers from that which takes place in the unicellular organisms only inits being _invariably_ preceded (as far as we know) by karyokinesis, andin the resulting cells being all confined within a common envelope, andso in not being free to separate. Nevertheless, from what has alreadybeen said, it will also be noticed that this feature makes all thedifference between a Metazoön and a Protozoön; so that already the ovumpresents the distinguishing character of a Metazoön. I have dealt thus at considerable length upon the processes whereby theoriginally unicellular ovum and spermatozoön become converted into themulticellular germ, because I do not know of any other exposition of theargument from Embryology where this, the first stage of the argument, has been adequately treated. Yet it is evident that the fact of all theprocesses above described being so similar in the case of sexual (ormetazoal) reproduction among the innumerable organisms where it occurs, constitutes in itself a strong argument in favour of evolution. For themechanism of fertilization, and all the processes which even thus far wehave seen to follow therefrom, are hereby shown to be not only highlycomplex, but likewise highly specialized. Therefore, the remarkablesimilarity which they present throughout the whole animal kingdom--notto speak of the vegetable--is expressive of organic continuity, ratherthan of absolute discontinuity in every case, as the theory of specialcreation must necessarily suppose. And it is evident that this argumentis strong in proportion to the uniformity, the specialization, and thecomplexity of the processes in question. Having occupied so much space with supplying what appear to me thedeficiencies in previous expositions of the argument from Embryology, Ican now afford to take only a very general view of the more importantfeatures of this argument as they are successively furnished by all thelater stages of individual development. But this is of littleconsequence, seeing that from the point at which we have now arrivedprevious expositions of the argument are both good and numerous. Thefollowing then is to be regarded as a mere sketch of the evidences ofphyletic (or ancestral) evolution, which are so abundantly furnished byall the subsequent phases of ontogenetic (or individual) evolution. The multicellular body which is formed by the series of segmentationsabove described is at first a sphere of cells (Fig. 40). Soon, however, a watery fluid gathers in the centre, and progressively pushes the cellstowards the circumference, until they there constitute a single layer. The ovum, therefore, is now in the form of a hollow sphere containingfluid, confined within a continuous wall of cells (Fig. 41 A). The nextthing that happens is a pitting in of one portion of the sphere (B). Thepit becomes deeper and deeper, until there is a complete invagination ofthis part of the sphere--the cells which constitute it beingprogressively pushed inwards until they come into contact with those atthe opposite pole of the ovum. Consequently, instead of a hollow sphereof cells, the ovum now becomes an open sac, the walls of which arecomposed of a double layer of cells (C). The ovum is now what has beencalled a gastrula; and it is of importance to observe that probably allthe Metazoa pass through this stage. At any rate it has been found tooccur in all the main divisions of the animal kingdom, as a glance atthe accompanying figures will serve to show (Fig. 42)[14]. Moreover manyof the lower kinds of Metazoa never pass beyond it; but are all theirlives nothing else than gastrulæ, wherein the orifice becomes the mouthof the animal, the internal or invaginated layer of cells the stomach, and the outer layer the skin. So that if we take a child's india-rubberball, of the hollow kind with a hole in it, and push in one side withour fingers till internal contact is established all round, by thenholding the indented side downwards we should get a very fair anatomicalmodel of a gastræa form, such as is presented by the adult condition ofmany of the most primitive Metazoa--especially the lower_Coelenterata_. The preceding figures represent two other such formsin nature, the first locomotive and transitory, the second fixed andpermanent (Figs. 43, 44). [14] In most vertebrated animals this process of gastrulation has been more or less superseded by another, which is called delamination; but it scarcely seems necessary for our present purposes to describe the latter. For not only does it eventually lead to the same result as gastrulation--i. E. The converting of the ovum into a double-walled sac, --but there is good evidence among the lower Vertebrata of its being preceded by gastrulation; so that, even as to the higher Vertebrata, embryologists are pretty well agreed that delamination has been but a later development of, or possibly improvement upon, gastrulation. [Illustration: FIG. 41. --Formation of the gastrula of _Amphioxus_. (After Kowalevsky. ) A, wall of the ovum, composed of a single layer of cells; B, a stage in the process of gastrulation; C, completion of the process; S, original or segmentation cavity of ovum; _al_, alimentary cavity of gastrula; _ect_, outer layer of cells; _ent_, inner layer of cells; _b_, orifice, constituting the mouth in permanent forms. ] [Illustration: FIG. 42. --Gastrulation. A, Gastrula of a Zoophyte (_Gastrophysema_). (After Häckel. ) B, Gastrula of a Worm (_Sagitta_). (After Kowalevsky. ) C, Gastrula of an Echinoderm (_Uraster_). (After A. Agassiz. ) D, Gastrula of an Arthropod (_Nauplius_). (After Häckel. ) E, Gastrula of a Mollusk (_Limnæus_). (After Rabl. ) F, Gastrula of a Vertebrate (_Amphioxus_). (After Kowalevsky. ) In all, _d_, indicates the intestinal cavity; _o_, the primitive mouth; _s_, the cleavage-cavity; _i_, the endoderm, or intestinal layer; _e_, the ectoderm or skin-layer. ] [Illustration: FIG. 43. --Gastrula of a Chalk Sponge. (After Häckel. ) A, External view. B, Longitudinal section. _g_, digestive cavities; _o_, mouth; _i_, endoderm; _e_, ectoderm. ] [Illustration: FIG. 44. --_Prophysema primordiale_, an extant gastræa-form. (After Häckel. ) (A). External view of the whole animal, attached by its foot to seaweed. (B). Longitudinal section of the same. The digestive cavity (_d_) opens at its upper end in the mouth (_m_). Among the cells of the endoderm (_g_) lie amoeboid egg-cells of large size (_e_). The ectoderm (_h_) is encrusted with grains of sand, above the sponge spicules. ] Here, then, we leave the lower forms of Metazoa in their condition ofpermanent gastrulæ. They differ from the transitory stage of otherMetazoa only in being enormously larger (owing to greatly further_growth_, without any further _development_ as to matters of fundamentalimportance), and in having sundry tentacles and other organs added lateron to meet their special requirements. The point to remember is, that inall cases a gastrula is an open sac composed of two layers of cells--theouter layer being called the ectoderm, and the inner the endoderm. Theyhave also been called the animal layer and the vegetative layer, becauseit is the outer layer (ectoderm) that gives rise to all the organs ofsensation and movement--viz. The skin, the nervous system, and themuscular system; while it is the inner layer (endoderm) that gives riseto all the organs of nutrition and reproduction. It is desirable onlyfurther to explain that gastrulation does not take place in all theMetazoa after exactly the same plan. In different lines of descentvarious and often considerable modifications of the original and mostsimple plan have been introduced; but I will not burden the presentexposition by describing these modifications[15]. It is enough for usthat they always end in the formation of the two primary layers ofectoderm and endoderm. [15] The most extreme of them is that which is mentioned in the last foot-note. The next stage of differentiation is common to all the Metazoa, exceptthose lowest forms which, as we have just seen, remain permanently aslarge gastrulæ, with sundry specialized additions in the way oftentacles, &c. This stage of differentiation consists in the formationof either a pouch or an additional layer between the ectoderm and theendoderm, which is called the mesoderm. It is probably in most casesderived from the endoderm, but the exact mode of its derivation is stillsomewhat obscure. Sometimes it has the appearance of itself constitutingtwo layers; but it is needless to go into these details; for in any casethe ultimate result is the same--viz. That of converting the Metazoöninto the form of a tube, the walls of which are composed of concentriclayers of cells. The outermost layer afterwards gives rise to theepidermis with its various appendages, and also to the central nervoussystem with its organs of special sense. The median layer gives rise tothe voluntary muscles, bones, cartilages, &c. , the nutritive systems ofthe blood, the chyle, the lymph, and the muscular tube of the intestine. Lastly, the innermost layer developes into the epithelium lining of theintestine, with its various appendages of liver, lungs, intestinalglands, &c. I have just said that this three or four layered stage is shared by allthe Metazoa, except those very lowest forms--such as sponges andjelly-fish--which do not pass on to it. But from this point thedevelopmental histories of all the main branches of the Metazoadiverge--the Vermes, the Echinodermata, the Mollusca, the Articulata, and the Vertebrata, each taking a different road in their subsequentevolution. I will therefore confine attention to only one of theseseveral roads or methods, namely, that which is followed by theVertebrata--observing merely that, if space permitted, the sameprinciples of progressive though diverging histories of evolution wouldequally well admit of being traced in all the other sub-kingdoms whichhave just been named. In order to trace these principles in the case of the Vertebrata, it isdesirable first of all to obtain an idea of the anatomical featureswhich most essentially distinguish the sub-kingdom as a whole. Thefollowing, then, is what may be termed the ideal plan of vertebrateorganization, as given by Prof. Häckel. First, occupying the major axisof body we perceive the primitive vertebral column. The parts lyingabove this axis are those which have been developed from the ectodermand mesoderm--viz. Voluntary muscles, central nervous system, and organsof special sense. The parts lying below this axis are for the most partthose which have been developed from the endoderm--namely, thedigestive tract with its glandular appendages, the circulating systemand the respiratory system. In transverse section, therefore, the idealvertebrate consists of a solid axis, with a small tube occupied by thenervous system above, and a large tube, or body-cavity, below. Thisbody-cavity contains the viscera, breathing organs, and heart, with itsprolongations into the main blood-vessels of the organism. Lastly, oneither side of the central axis are to be found large masses ofmuscle--two on the dorsal and two on the ventral. As yet, however, thereare no limbs, nor even any bony skeleton, for the primitive vertebralcolumn is hitherto unossified cartilage. This ideal animal, therefore, is to all appearance as much like a worm as a fish, and swims by meansof a lateral undulation of its whole body, assisted, perhaps, by adorsal fin formed out of skin. [Illustration: FIG. 45. --Ideal primitive vertebrate, seen from the left side. (After Häckel. ) _na_, nose; _au_, eye; _g_, ear; _md_, mouth; _ks_, gill-openings; _x_, notochord; _mr_, spinal tube; _kg_, gill-vessels; _k_, gill-intestine; _hz_, heart; _ms_, muscles; _ma_, stomach; _v_, intestinal vein; _c_, body-cavity; _a_, aorta; _l_, liver; _d_, small intestine; _e_, ovary; _h_, testes; _n_, kidney canal; _af_, anus; _lh_, true or leather-skin; _oh_, outer-skin (epidermis); _f_, skin-fold, acting as a fin. ] [Illustration: FIG. 46. --The same in transverse section through the ovaries; lettering as in the preceding Fig. ] [Illustration: FIG. 47. --_Amphioxus lanceolatus_. (After Häckel. ) _a_, anus; _au_, eye; _b_, ventral muscles; _c_, body-cavity; _ch_, notochord; _d_, intestine; _do_ and _du_, dorsal and ventral walls of intestine; _f_, fin-seam; _h_, skin; _k_, gills; _ka_, gill-artery; _lb_, liver; _lv_, liver-vein; _m 1_, brain-bladder; _m 2_, spinal marrow; _mg_, stomach; _o_, mouth; _p_, ventral pore; _r_, dorsal muscle; _s_, tail-fin; _t_, aorta; _v_, intestinal vein; _x_, boundary between gill-intestine and stomach-intestine; _y_, hypobranchial groove. ] Now I should not have presented this ideal representation of a primitivevertebrate--for I have very little faith in the "scientific use of theimagination" where it aspires to discharge the functions of a Creator inthe manufacture of archetypal forms--I say I should not have presentedthis ideal representative of a primitive vertebrate, were it not thatthe ideal is actually realized in a still existing animal. For therestill survives what must be an immensely archaic form of vertebrate, whose anatomy is almost identical with that of the imaginary type whichhas just been described. I allude, of course, to _Amphioxus_, which isby far the most primitive or generalized type of vertebrated animalhitherto discovered. Indeed, we may say that this remarkable creature isalmost as nearly allied to a worm as it is to a fish. For it has nospecialized head, and therefore no skull, brain, or jaws: it isdestitute alike of limbs, of a centralized heart, of developed liver, kidneys, and, in short, of most of the organs which belong to the otherVertebrata. It presents, however, a rudimentary backbone, in the form ofwhat is called a notochord. Now a primitive dorsal axis of this kindoccurs at a very early period of embryonic life in all vertebratedanimals; but, with the exception of _Amphioxus_, in all other existingVertebrata this structure is not itself destined to become the permanentor bony vertebral column. On the contrary, it gives way to, or isreplaced by, this permanent bony structure at a later stage ofdevelopment. Consequently, it is very suggestive that so distinctivelyembryonic a structure as this temporary cartilaginous axis of all theother known Vertebrata should be found actually persisting to thepresent day as the permanent axis of _Amphioxus_. In many otherrespects, likewise, the early embryonic history of other Vertebratarefers us to the permanent condition of _Amphioxus_. In particular, wemust notice that the wall of the neck is always perforated by what in_Amphioxus_ are the gill-openings, and that the blood-vessels as theyproceed from the heart are always distributed in the form of what arecalled gill-arches, adapted to convey the blood round or through thegills for the purpose of aeration. In all existing fish and othergill-breathing Vertebrata, this arrangement is permanent. It islikewise met with in a peculiar kind of worm, called _Balanoglossus_--acreature so peculiar, indeed, that it has been constituted by Gegenbaura class all by itself. We can see by the wood-cuts that it presents aseries of gill-slits, like the homologous parts of the fishes with whichit is compared--i. E. Fishes of a comparatively low type oforganization, which dates from a time before the development of externalgills. (Figs. 48, 49, 50. ) Now, as I have already said, thesegill-_slits_ are supported internally by the gill-_arches_, or theblood-vessels which convey the blood to be oxygenized in the branchialapparatus (see below, Figs. 51, 52, 53); and the whole arrangement isdeveloped from the anterior part of the intestine--as is likewise therespiratory mechanism of all the gill-breathing Vertebrata. That soclose a parallel to this peculiar mechanism should be met with in aworm, is a strong additional piece of evidence pointing to thederivation of the Vertebrata from the Vermes. [Illustration: FIG. 48. --_Balanoglossus_. (After A. Agassiz. ) _r_, proboscis; _h_, collar; _k_, gill-slits; _d_, digestive posterior intestine; _v_, intestinal vessel; _a_, anus. ] [Illustration: FIG. 49. --A large Sea-lamprey (_Petromyzon marinus_), much reduced in size. (After Cuvier and Häckel. ) A series of seven gill-slits are visible. ] [Illustration: FIG. 50--Adult Shark (_Carcharias melanopterus_). (After Cuvier and Häckel. )] [Illustration: FIG. 51. --Diagram of heart and gill-arches of a fish. (After Owen. )] [Illustration: FIG. 52. --One gill-arch, with branchial fringe attached. (After Owen. ) H, Heart. ] [Illustration: FIG. 53. --Diagram of heart and gill-arches in a lizard. (After Owen. ) The gill-arches, _a a' a''_, and _b b' b''_, are called aortic arches in air-breathing vertebrata. ] Well, I have just said that in all the gill-breathing Vertebrata, thismechanism of gill-slits and vascular gill-arches in the front part ofthe intestinal tract is permanent. But in the air-breathing Vertebratasuch an arrangement would obviously be of no use. Consequently, thegill-slits in the sides of the neck (see Figs. 16 and 57, 58), and thegill-arches of the large blood-vessels (Figs. 54, 55, 56), are hereexhibited only as transitory phases of development. But as such theyoccur in all air-breathing Vertebrata. And, as if to make the homologiesas striking as possible, at the time when the gill-slits and thegill-arches are developed in the embryonic young of air-breathingVertebrata, the heart is constructed upon the fish-like type. That is tosay, it is placed far forwards, and, from having been a simple tube asin Worms, is now divided into two chambers, as in Fish. Later on itbecomes progressively pushed further back between the developing lungs, while it progressively acquires the three cavities distinctive ofAmphibia, and finally the four cavities belonging only to the completedouble circulation of Birds and Mammals. Moreover, it has now beensatisfactorily shown that the lungs of air-breathing Vertebrata, whichare thus destined to supersede the function of gills, are themselves themodified swim-bladder or float, which belongs to Fish. Consequently, allthese progressive modifications in the important organs of circulationand respiration in the air-breathing Vertebrata, together make up ascomplete a history of their aquatic pedigree as it would be possible forthe most exacting critic to require. [Illustration: FIG. 54. --Ideal diagram, of primitive gill-or aortic-arches. (After Rathke. ) H, outline of heart. The arrows show the course of the blood. ] [Illustration: FIG. 55. --The same, modified for a bird. (After Le Conte. ) The dark lines show the aortic arches which persist. A, aorta; _p_, pulmonary arches; SC, S'C', sub-clavian; C, C', carotids. ] [Illustration: FIG. 56. --The same, modified for a mammal. (After Le Conte. )] [Illustration: FIG. 57. --A series of embryos at three comparable and progressive stages of development (marked I, II, III), representing each of the classes of vertebrated animals below the Mammalia (After Häckel. )] [Illustration: FIG. 58. --Another series of embryos, also at three comparable and progressive stages of development (marked I, II, III), representing four different divisions of the class Mammalia. (After Häckel. )] If space permitted, it would be easy to present abundance of additionalevidence to the same effect from the development of the skeleton, theskull, the brain, the sense-organs, and, in short, of every constituentpart of the vertebrate organization. Even without any anatomicaldissection, the similarity of all vertebrated embryos at comparablestages of development admits of being strikingly shown, if we merelyplace the embryos one beside the other. Here, for instance, are theembryos of a fish, a salamander, a tortoise, a bird, and four differentmammals. In each case three comparable stages of development arerepresented. Now, if we read the series horizontally, we can see thatthere is very little difference between the eight animals at theearliest of the three stages represented--all having fish-like tails, gill-slits, and so on. In the next stage further differentiation hastaken place, but it will be observed that the limbs are still sorudimentary that even in the case of Man they are considerably shorterthan the tail. But in the third stage the distinctive characters arewell marked. * * * * * So much then for an outline sketch of the main features in the embryonichistory of the Vertebrata. But it must be remembered that the science ofcomparative embryology extends to each of the other three great branchesof the tree of life, where these take their origin, through the worms, from the still lower, or gastræa, forms. And in each of these threegreat branches--namely, the Echinodermata, the Mollusca, and theArthropoda--we have a repetition of just the same kind of evidence infavour of continuous descent, with adaptive modification in sundrylines, as that which I have thus briefly sketched in the case of theVertebrata. The roads are different, but the method of travelling is thesame. Moreover, when the embryology of the Worms is closely studied, theorigin of these different roads admits of being clearly traced. So thatwhen all this mass of evidence is taken together, we cannot wonder thatevolutionists should now regard the science of comparative embryology asthe principal witness to their theory. CHAPTER V. PALÆONTOLOGY. The present Chapter will be devoted to a consideration of the evidenceof organic evolution which has been furnished by the researches ofgeologists. On account of its direct or historical nature, this branchof evidence is popularly regarded as the most important--so much so, indeed, that in the opinion of most educated persons the whole doctrineof organic evolution must stand or fall according to the so-called"testimony of the rocks. " Now, without at all denying the peculiarimportance of this line of evidence, I must begin by remarking that itdoes not present the denominating importance which popular judgmentassigns to it. For although popular judgment is right in regarding thetestimony of the rocks as of the nature of a history, this judgment, asa rule, is very inadequately acquainted with the great imperfections ofthat history. Knowing in a general way what magnificent advances thescience of geology has made during the present century, the public mindis more or less imbued with the notion, that because we now possess atolerably complete record of the chronological succession of geologicalformations, we must therefore possess a correspondingly complete recordof the chronological succession of the forms of life which from time totime have peopled the globe. Now in one sense this notion is partlytrue, but in another sense it is profoundly false. It is partly true ifwe have regard only to those larger divisions of the vegetable or animalkingdoms which naturalists designate by the terms classes and orders. But the notion becomes progressively more untrue when it is applied tofamilies and genera, while it is most of all untrue when applied tospecies. That this must be so may be rendered apparent by twoconsiderations. In the first place, it does not follow that because we have a tolerablycomplete record of the succession of geological formations, we havetherefore any correspondingly complete record of their fossiliferouscontents. The work of determining the relative ages of the rocks doesnot require that every cubic mile of the earth's surface should beseparately examined, in order to find all the different fossils which itmay contain. Were this the case, we should hitherto have made but verysmall progress in our reading of the testimony of the rocks. Therelative ages of the rocks are determined by broad comparative surveysover extensive areas; and although the identification of widelyseparated deposits is often greatly assisted by a study of theirfossiliferous contents, the mere pricking of a continent here and thereis all that is required for this purpose. Hence, the accuracy of ourinformation touching the relative ages of geological strata does notdepend upon--and, therefore, does not betoken--any equivalent accuracyof knowledge touching the fossiliferous material which these strata mayat the present time actually contain. And, as we well know, theopportunities which the geologist has of discovering fossils areextremely limited, if we consider these opportunities in relation to thearea of geological formations. The larger portion of the earth's surfaceis buried beneath the sea; and much the larger portion of thefossiliferous deposits on shore are no less hopelessly buried beneaththe land. Therefore it is only upon the fractional portion of theearth's surface which at the present time happens to be actually exposedto his view that the geologist is able to prosecute his search forfossils. But even here how miserably inadequate this search has hithertobeen! With the exception of a scratch or two in the continents of Asiaand America, together with a somewhat larger number of similar scratchesover the continent of Europe, even that comparatively small portion ofthe earth's surface which is available for the purpose has been hithertoquite unexplored by the palæontologist. How enormously rich a store ofmaterial remains to be unearthed by the future scratchings of thissurface, we may dimly surmise from the astonishing world of bygone lifewhich is now being revealed in the newly discovered fossiliferousdeposits on the continent of America. But, besides all this, we must remember, in the second place, that allthe fossiliferous deposits in the world, even if they could bethoroughly explored, would still prove highly imperfect, considered as ahistory of extinct forms of life. In order that many of these formsshould have been preserved as fossils, it is necessary that they shouldhave died upon a surface neither too hard nor too soft to admit of theirleaving an impression; that this surface should afterwards havehardened sufficiently to retain the impression; that it should then havebeen protected from the erosion of water, as well as from thedisintegrating influence of the air; and yet that it should not havesunk far enough beneath the surface to have come within the no lessdisintegrating influence of subterranean heat. Remembering thus, as ageneral rule, how many conditions require to have met before a fossilcan have been both formed and preserved, we must conclude that thegeological record is probably as imperfect in itself as are ouropportunities of reading even the little that has been recorded. If wespeak of it as a history of the succession of life upon the planet, wemust allow, on the one hand, that it is a history which merits the nameof a "chapter of accidents"; and, on the other hand, that during thewhole course of its compilation pages were being destroyed as fast asothers were being formed, while even of those that remain it is only aword, a line, or at most a short paragraph here and there, that we arepermitted to see. With so fragmentary a record as this to study, I donot think it is too much to say that no conclusions can be fairly basedupon it, merely from the absence of testimony. Only if the testimonywere positively opposed to the theory of descent, could any argument befairly raised against that theory on the grounds of this testimony. Inother words, if any of the fossils hitherto discovered prove the orderof succession to have been incompatible with the theory of geneticdescent, then the record may fairly be adduced in argument, because weshould then be in possession of definite information of a positivekind, instead of a mere absence of information of any kind. But if theadverse argument reaches only to the extent of maintaining that thegeological record does not furnish us with so complete a series of"connecting links" as we might have expected, then, I think, theargument is futile. Even in the case of human histories, written withthe intentional purpose of conveying information, it is an unsafe thingto infer the non-occurrence of an event from a mere silence of thehistorian--and this especially in matters of comparatively small detail, such as would correspond (in the present analogy) to the occurrence of_species_ and _genera_ as connecting links. And, of course, if thehistory had only come down to us in fragments, no one would attach anyimportance at all to what might have been only the _apparent_ silence ofthe historian. In view, then, of the unfortunate imperfection of the geological record_per se_, as well as of the no less unfortunate limitation of our meansof reading even so much of the record as has come down to us, I concludethat this record can only be fairly used in two ways. It may fairly beexamined for positive testimony against the theory of descent, or forproof of the presence of organic remains of a high order of developmentin a low level of strata. And it may be fairly examined for negativetestimony, or for the absence of connecting links, if the search beconfined to the larger taxonomic divisions of the fauna and flora of theworld. The more minute these divisions, the more restricted must havebeen the areas of their origin, and hence the less likelihood of theirhaving been preserved in the fossil state, or of our finding them evenif they have been. Therefore, if the theory of evolution is true, weought not to expect from the geological record a full history of_specific_ changes in any but at most a comparatively small number ofinstances, where local circumstances happen to have been favourable forthe writing and preservation of such a history. But we might reasonablyexpect to find a general concurrence of geological testimony to thelarger fact--namely, of there having been throughout all geological timea uniform progression as regards the larger taxonomic divisions. And, asI will next proceed to show, this is, in a general way, what we do find, although not altogether without some important exceptions, with which Ishall deal in an Appendix. There is no _positive_ proof _against_ the theory of descent to be drawnfrom a study of palæontology, or proof of the presence of any kind offossils in strata where the fact of their presence is incompatible withthe theory of evolution. On the other hand, there is an enormous body ofuniform evidence to prove two general facts of the highest importance inthe present connexion. The first of these general facts is, that anincrease in the diversity of types both of plants and animals has beenconstant and progressive from the earliest to the latest times, as weshould anticipate that it must have been on the theory of descent inever-ramifying lines of pedigree. And the second general fact is, thatthrough all these branching lines of ever-multiplying types, from thefirst appearance of each of them to their latest known conditions, thereis overwhelming evidence of one great law of organic nature--the law ofgradual advance from the general to the special, from the low to thehigh, from the simple to the complex. Now, the importance of these large and general facts in the presentconnexion must be at once apparent; but it may perhaps be rendered moreso if we try to imagine how the case would have stood supposinggeological investigation to have yielded in this matter an oppositeresult, or even so much as an equivocal result. If it had yielded anopposite result, if the lower geological formations were found tocontain as many, as diverse, and as highly organized types as the latergeological formations, clearly there would have been no room at all forany theory of progressive evolution. And, by parity of reasoning, inwhatever degree such a state of matters were found to prevail, in thatdegree would the theory in question have been discredited. But seeingthat these opposite principles do not prevail in any (relativelyspeaking) considerable degree[16], we have so far positive testimony ofthe largest and most massive character in favour of this theory. Forwhile all these large and general facts are very much what they ought tobe according to this theory, they cannot be held to lend any support atall to the rival theory. In other words, it is clearly no essential partof the theory of special creation that species should everywhere exhibitthis gradual multiplication as to number, coupled with a gradualdiversification and general elevation of types, in all the growingbranches of the tree of life. No one could adopt seriously the jocularlines of Burns, to the effect that the Creator required to practise hisprentice hand on lower types before advancing to the formation ofhigher. Yet, without some such assumption, it would be impossible toexplain, on the theory of independent creations, why there should havebeen this gradual advance from the few to the many, from the general tothe special, from the low to the high. [16] For objections which may be brought against this and similar statements, see the Appendix. +---+--------------------------+--------------------------------------- | |_Epochs and Formations. _ |_Faunal Characters. _ |C | | |a |--------------------------+--------------------------------------- |i |POST-PLIOCENE. |Man. Mammalia principally of living |n | Glacial Period. | species. Mollusca exclusively recent. |o | +--------------------------------------- |z |PLIOCENE, 3, 000 feet. |Mammalia principally of recent genera |o | | --living species rare. Mollusca very |i | | modern. |c | +--------------------------------------- | |MIOCENE, 4, 000 ft. |Mammalia principally of living |o | | families; extinct genera numerous; |r | | species all extinct. Mollusca largely | |OLIGOCENE, 8, 000 ft. | of recent species. |T | +--------------------------------------- |e |EOCENE, 10, 000 ft. |Mammalia with numerous extinct families |r | | and orders; all the species and |i | | most of the genera extinct. Modern |a | | type Shell-Fish. |r | | |y | | +---|--------------------------+--------------------------------------- | |LARAMIE, 4, 000 ft. |Passage beds. | |--------------------------+--------------------------------------- |M |CRETACEOUS, 12, 000 ft. |Dinosaurian (bird-like) Reptiles; |e | Chalk. | Pterodactyls (flying Reptiles); |s | | toothed Birds; earliest Snake; bony |o | | Fishes; Crocodiles; Turtles; |z | | Ammonites. |o | +--------------------------------------- |i |JURASSIC, 6, 000 ft. |Earliest Birds; giant Reptiles |c | Oolite. | (Ichthyosaurs, Dinosaurs, | | Lias. | Pterodactyls); Ammonites; Clam- and |o | | Snail-Shells very abundant; decline |r | | of Brachiopods; Butterfly. | | +--------------------------------------- |S |TRIAS, 5, 000 ft. |First Mammalian (Marsupial); 2-gilled |e | New Red Sandstone. | Cephalopods (Cuttle-Fishes, |c | | Belemnites); reptilian Foot-Prints. |o | | |n | | |d | | |a | | |r | | |y | | |---|--------------------------+--------------------------------------- |P |PERMIAN, 5, 000 ft. |Earliest true Reptiles. |a | +--------------------------------------- |l |CARBONIFEROUS, 26, 000 ft. |Earliest Amphibian (Labyrinthodont); |e | | extinction of Trilobites; first |o | Coal. | Cray-fish; Beetles; Cockroaches; |z | | Centipedes; Spiders. |o | +--------------------------------------- |i |DEVONIAN, 18, 000 ft. |Cartilaginous and Ganoid Fishes; |c | Old Red Sandstone. | earliest and (snail) and freshwater | | | Shells; Shell-Fish abundant; decline |o | | of Trilobites; May-flies; Crab. |r | +--------------------------------------- | |SILURIAN, 33, 000 ft. |Earliest Fish; the first Air-Breathers |P | | (Insect, Scorpion); Brachiopods and |r | | 4-gilled Cephalopods very abundant; |i | | Trilobites; Corals; Graptolites. |m | +--------------------------------------- |a |CAMBRIAN, 24, 000 ft. |Trilobites; Brachiopod Mollusks. |r | | |y | | |---|--------------------------+--------------------------------------- |A |ARCHÆEAN, 30, 000 ft. | |z | Huronian. |Eozoön, (probably not a fossil). |o | Laurentian. | |i |--------------------------+--------------------------------------- |c |PRIMEVAL. |Non-sedimentary. +------------------------------+--------------------------------------+ I submit, then, that so far as the largest and most general principlesin the matter of palæontology are concerned, we have about as strong andmassive a body of evidence as we could reasonably expect this branch ofscience to yield; for it is at once enormous in amount and positive incharacter. Therefore, if I do not further enlarge upon the evidencewhich we here have, as it were _en masse_, it is only because I do notfeel that any words could add to its obvious significance. It may bestbe allowed to speak for itself in the millions of facts which arecondensed in this tabular statement of the order of succession of allthe known forms of animal life, as presented by the eminentpalæontologist, Professor Cope[17]. [17] For difficulties and objections, see Appendix. Or, taking a still more general survey, this tabular statement may bestill further condensed, and presented in a diagrammatic form, as it hasbeen by another eminent American palæontologist, Prof. Le Conte, in hisexcellent little treatise on _Evolution and its Relations to ReligiousThought_. The following is his diagrammatic representation, with hisremarks thereon. When each ruling class declined in importance, it did not perish, but continued in a subordinate position. Thus, the whole organic kingdom became not only higher and higher in its highest forms, but also more and more complex in its structure and in the interaction of its correlated parts. The whole process and its result is roughly represented in the accompanying diagram, in which A B represents the course of geological time, and the curve, the rise, culmination, and decline of successive dominate classes. [Illustration: FIG. 59. --Diagram of Geological Succession of the Classes of the Animal Kingdom. (After Le Conte. )] I will here leave the evidence which is thus yielded by the most generalprinciples that have been established by the science of palæontology;and I will devote the rest of this chapter to a detailed considerationof a few highly special lines of evidence. By thus suddenly passing fromone extreme to the other, I hope to convey the best idea that can beconveyed within a brief compass of the minuteness, as well as theextent, of the testimony which is furnished by the rocks. * * * * * When Darwin first published his _Origin of Species_, adverse criticsfastened upon the "missing-link" argument as the strongest that theycould bring against the theory of descent. Although Darwin had himselfstrongly insisted on the imperfection of the geological record, and theconsequent precariousness of any negative conclusions raised upon it, these critics maintained that he was making too great a demand upon theargument from ignorance--that, even allowing for the imperfection of therecord, they would certainly have expected at least a few cases oftestimony to _specific_ transmutation. For, they urged in effect, looking to the enormous profusion of the extinct species on the onehand, and to the immense number of known fossils on the other, it wasincredible that no satisfactory instances of specific transmutationshould ever have been brought to light, if such transmutation had everoccurred in the universal manner which the theory was bound to suppose. But since Darwin first published his great work palæontologists havebeen very active in discovering and exploring fossiliferous beds insundry parts of the world; and the result of their labours has been tosupply so many of the previously missing links that the voice ofcompetent criticism in this matter has now been well-nigh silenced. Indeed, the material thus furnished to an advocate of evolution at thepresent time is so abundant that his principal difficulty is to selecthis samples. I think, however, that the most satisfactory result will begained if I restrict my exposition to a minute account of some fewseries of connecting links, rather than if I were to take a more generalsurvey of a larger number. I will, therefore, confine the survey to theanimal kingdom, and there mention only some of the cases which haveyielded well-detailed proof of continuous differentiation. It is obvious that the parts of animals most likely to have beenpreserved in such a continuous series of fossils as the present line ofevidence requires, would have been the hard parts. These are horns, bones, teeth, and shells. Therefore I will consider each of these fourclasses of structures separately. * * * * * Horns wherever they occur, are found to be of high importance forpurposes of classification. They are restricted to the Ruminants, andappear under three different forms or types--namely solid, as inantelopes; hollow, as in sheep; and deciduous, as in deer. Now, in eachof these divisions we have a tolerably complete palæontological historyof the evolution of horns. The early ruminants were altogether hornless(Fig. 60). Then, in the middle Miocene, the first antelopes appearedwith tiny horns, which progressively increased in size among theever-multiplying species of antelopes until the present day. But it isin the deer tribe that we meet with even better evidence touching theprogressive evolution of horns; because here not only size, but shape, is concerned. For deer's horns, or antlers, are arborescent; and hencein their case we have an opportunity of reading the history, not only ofa progressive growth in size, but also of an increasing development ofform. Among the older members of the tribe, in the lower Miocene, thereare no horns at all. In the mid-Miocene we meet with two-pronged horns(_Cervus dicrocerus_, Figs. 61, 62, 1/5 nat. Size). Next, in the upperMiocene (_C. Matheronis_, Fig. 63, 1/8 nat. Size), and extending intothe Pliocene (_C. Pardinensis_, Fig. 64, 1/18 nat. Size), we meet withthree-pronged horns. Then, in the Pliocene we find also four-prongedhorns (_C. Issiodorensis_, Fig. 65, 1/16 nat. Size), leading us tofive-pronged (_C. Tetraceros_). Lastly, in the Forest-bed of Norfolk wemeet with arborescent horns (_C. Sedgwickii_, Fig. 66, 1/35 nat. Size). The life-history of existing stags furnishes a parallel development(Fig. 67), beginning with a single horn (which has not yet been foundpalæontologically), going on to two prongs, three prongs, four prongs, and afterwards branching. [Illustration: FIG. 60. --Skull of _Oreodon Culbertsoni_. (After Leidy. )] [Illustration: FIGS. 61-66. The series is reduced from Gaudry's illustrations, after Farge, Croizet, Jobert and Boyd Dawkins. ] [Illustration: FIG. 67. --Successive stages in the development of an existing Deer's Antlers. (After Gaudry, but a better illustration has already been given on p. 100. )] * * * * * Coming now to bones, we have a singularly complete record of transitionfrom one type or pattern of structure to another in the phylogenetichistory of tails. This has been so clearly and so tersely conveyed byProf. Le Conte, that I cannot do better than quote his statement. It has long been noticed that there are among fishes two styles of tail-fins. These are the even-lobed, or homocercal (Fig. 68), and the uneven-lobed, or heterocercal (Fig. 69). The one is characteristic of ordinary fishes (teleosts), the other of sharks and some other orders. In structure the difference is even more fundamental than in form. In the former style the backbone stops abruptly in a series of short, enlarged joints, and thence sends off rays to form the tail-fin (Fig. 68); in the latter the backbone runs through the fin to its very point, growing slenderer by degrees, and giving off rays above and below from each joint, but the rays on the lower side are much longer (Fig. 69). This type of fin is, therefore, _vertebrated_, the other _non-vertebrated_. Figs. 68 and 69 show these two types in form and structure. But there is still another type found only in the lowest and most generalized forms of fishes. In these the tail-fin is vertebrated and yet symmetrical. This type is shown in Fig. 70. [Illustration: FIG. 68. --Homocercal Tail, showing (A) external form and (B) internal structure. ] [Illustration: FIG. 69. --Heterocercal Tail, showing (A) external form and (B) internal structure. ] [Illustration: FIG. 70. --Vertebrated but symmetrical fin (diphycercal), showing (A) external form and (B) internal structure. ] Now, in the development of a teleost fish (Fig. 68), as has been shown by Alexander Agassiz, the tail-fin is first like Fig. 70; then becomes heterocercal, like Fig. 69; and, finally, becomes homocercal like Fig. 68. Why so? Not because there is any special advantage in this succession of forms; for the changes take place either in the egg or else in very early embryonic states. The answer is found in the fact that _this is the order of change in the phylogenetic series_. The earliest fish-tails were either like Fig. 69 or Fig. 70; never like Fig. 68. The earliest of all were almost certainly like Fig. 70; then they became like Fig. 69; and, finally, only much later in geological history (Jurassic or Cretaceous), they became like Fig. 68. This order of change is still retained in the embryonic development of the last introduced and most specialized order of existing fishes. The family history is repeated in the individual history. Similar changes have taken place in the form and structure of birds' tails. The earliest bird known--the Jurassic _Archæopteryx_--had a long reptilian tail of twenty-one joints, each joint bearing a feather on each side, right and left (Fig. 71): [see also Fig. 73]. In the typical modern bird, on the contrary, the tail-joints are diminished in number, shortened up, and enlarged, and give out long feathers, fan-like, to form the so-called tail (Fig. 72). The _Archæopteryx'_ tail is _vertebrated_, the typical bird's _non-vertebrated_. This shortening up of the tail did not take place at once, but gradually. The Cretaceous birds, intermediate in time, had tails intermediate in structure. The _Hesperornis_ of Marsh had twelve joints. At first--in Jurassic strata--the tail is fully a half of the whole vertebral column. It then gradually shortens up until it becomes the aborted organ of typical modern birds. Now, in embryonic development, the tail of the modern typical bird _passes through all these stages_. At first the tail is nearly one half the whole vertebral column; then, as development goes on, while the rest of the body grows, the growth of the tail stops, and thus finally becomes the aborted organ we now find. The ontogeny still passes through the stages of the phylogeny. The same is true of all tailless animals. [Illustration: FIG. 71. --Tail of _Archæopteryx_. A indicates origin of simply-jointed tail. ] [Illustration: FIG. 72. --Tail of modern Bird. The numerals indicate the foreshortened, enlarged, and consolidated joints; _f_, terminal segment of the vertebral column; D, shafts of feathers. ] [Illustration: FIG. 73. --_Archæopteryx macura_, restored, 1/2 nat. Size. (After Flower. ) The section of the tail is copied from Owen, nat. Size. ] The extinct _Archæopteryx_ above alluded to presents throughout itswhole organization a most interesting assemblage of "generalizedcharacters. " For example, its teeth, and its still unreduced digits ofthe wings (which, like those of the feet, are covered with scales), refer us, with almost as much force as does the vertebrated tail, to theSauropsidian type--or the trunk from which birds and reptiles havediverged. We will next consider the palæontological evidence which we nowpossess of the evolution of mammalian limbs, with special reference tothe hoofed animals, where this line of evidence happens to be mostcomplete. I may best begin by describing the bones as these occur in the sundrybranches of the mammalian type now living. As we shall presently see, the modifications which the limbs have undergone in these sundrybranches chiefly consist in the suppression of some parts and theexaggerated development of others. But, by comparing all mammalian limbstogether, it is easy to obtain a generalized type of mammalian limb, which in actual life is perhaps most nearly conformed to in the case ofbears. I will therefore choose the bear for the purpose of brieflyexpounding the bones of mammalian limbs in general--merely asking it tobe understood, that although in the case of many other mammalia some ofthese bones may be dwindled or altogether absent, while others may begreatly exaggerated as to relative size, in no case do any _additional_bones appear. On looking, then, at the skeleton of a bear (Fig. 74), the first thingto observe is that there is a perfect serial homology between the bonesof the hind legs and of the fore legs. The thigh-bone, or femur, corresponds to the shoulder-bone, or humerus; the two shank bones (tibiaand fibula) correspond to the two arm-bones (radius and ulna); the manylittle ankle-bones (tarsals) correspond to the many little wrist-bones(carpals); the foot-bones (meta-tarsals) correspond to the hand-bones(meta-carpals); and, lastly, the bones of each of the toes correspond tothose of each of the fingers. [Illustration: FIG. 74. --Skeleton of Polar Bear, drawn from nature (_Brit. Mus. _). ] The next thing to observe is, that the disposition of bones in the caseof the bear is such that the animal walks in the way that has beencalled plantigrade. That is to say, all the bones of the fingers, aswell as those of the toes, feet, and ankles, rest upon the ground, orhelp to constitute the "soles. " Our own feet are constructed on aclosely similar pattern. But in the majority of living mammalian formsthis is not the case. For the majority of mammals are what has beencalled digitigrade. That is to say, the bones of the limb are sodisposed that both the foot and hand bones, and therefore also the ankleand wrist, are removed from the ground altogether, so that the animalwalks exclusively upon its toes and fingers--as in the case of thisskeleton (Fig. 75), which is the skeleton of a lion. The next figuresdisplay a series of limbs, showing the progressive passage of acompletely plantigrade into a highly digitigrade type--the curved linesof connexion serving to indicate the homologous bones (Figs. 76, 77). [Illustration: _Fig_. 75. --Skeleton of Lion. (After Huxley. )] [Illustration: FIG. 76. --Anterior limb of Man, Dog, Hog, Sheep, and Horse. (After Le Conte. ) _Sc_, shoulder-blade; _c_, coracoid; _a_, _b_, bones of fore-arm; 5, bones of the wrist; 6, bones of the hand; 7, bones of the fingers. ] [Illustration: FIG. 77. --Posterior limb of Man, Monkey, Dog, Sheep and Horse. (After Le Conte. ) 1, Hip-joint; 2, thigh-bone; 3, knee-joint; 4, bones of leg; 5, ankle-joint; 6, bones of foot; 7, bones of toes. ] I will now proceed to detail the history of mammalian limbs, as this hasbeen recorded for us in fossil remains. The most generalized or primitive types of limb hitherto discovered inany vertebrated animal above the class of fishes, are those which aremet with in some of the extinct aquatic reptiles. Here, for instance, isa diagram of the left hind limb of _Baptanodon discus_ (Fig. 78). It hassix rows of little symmetrical bones springing from a leg-like origin. But the whole structure resembles the fin of a fish about as nearly asit does the leg of a mammal. For not only are there six rows of bones, instead of five, suggestive of the numerous rays which characterise thefin of a fish; but the structure as a whole, having been covered overwith blubber and skin, was throughout flexible and unjointed--thus infunction, even more than in structure, resembling a fin. In thisrespect, also, it must have resembled the paddle of a whale (see Fig. 79); but of course the great difference will be noted, that the paddleof a whale reveals the dwindled though still clearly typical bones of atrue mammalian limb; so that although in outward form and function thesetwo paddles are alike, their inward structure clearly shows that whilethe one testifies to the absence of evolution, the other testifies tothe presence of degeneration. If the paddle of _Baptanodon_ had occurredin a whale, or the paddle of a whale had occurred in _Baptanodon_, either fact would in itself have been well-nigh destructive of the wholetheory of evolution. [Illustration: FIG. 78. --A, posterior limb of _Baptanodon discus_. (After Marsh. ) F, thigh-bone; I to VI, undifferentiated bones of the leg and foot. B, anterior limb of _Chelydra serpentina_. (After Gegenbaur. ) U and R, bones of the fore-arm; I to V, fully differentiated bones of the hand, following those of the wrist. ] [Illustration: FIG. 79. --Paddle of a Whale. ] Such, then, is the most generalized as it is the most ancient type ofvertebrate limb above the class of fishes. Obviously it is a typesuited only to aquatic life. Consequently, when aquatic Vertebrata beganto become terrestrial, the type would have needed modification in orderto serve for terrestrial locomotion. In particular, it would have neededto gain in consolidation and in firmness, which means that it would haveneeded also to become jointed. Accordingly, we find that this archaictype gave place in land-reptiles to the exigencies of theserequirements. Here for example is a diagram, copied from Gegenbaur, ofthe right fore-foot of _Chelydra serpentina_ (Fig. 78). As compared withthe homologous limb of its purely aquatic predecessor, there is to benoticed the disappearance of one of the six rows of small bones, aconfluence of some of the remainder in the other five rows, aduplication of the arm-bone into a radius and ulna, in order to admit ofjointed rotation of the hand, and a general disposition of the smallbones below these arm-bones, which clearly foreshadows the joint of thewrist. Indeed, in this fore-foot of _Chelydra_, a child could trace allthe principal homologies of the mammalian counterpart, growing, like thenext stage in a dissolving view, out of the primitive paddle of_Baptanodon_--namely, first the radius and ulna, next the carpals, thenthe meta-carpals, and, lastly, the three phalanges in each of the fivedigits. Such a type of foot no doubt admirably meets the requirements of slowreptilian locomotion over swampy ground. But for anything like rapidlocomotion over hard and uneven ground, greater modifications would beneeded. Such modifications, however, need not be other in kind: it isenough that they should continue in the same line of advance, so as toreach a higher degree of firmness, combined with better joints. Accordingly we find that this took place, not indeed among reptiles, whose habits of cold-blooded life have not changed, but among theirwarm-blooded descendants, the mammals. Moreover, when we examine thewhole mammalian series, we find that the required modifications musthave taken place in slightly different ways in three lines of descentsimultaneously. We have first the plantigrade and digitigrademodifications already mentioned (pp. 178, 179) Of these the plantigradewalking entailed least change, because most resembling the ancestral orlizard-like mode of progression. All that was here needed was a generalimprovement as to relative lengths of bones, with greater consolidationand greater flexibility of joints. Therefore I need not say anythingmore about the plantigrade division. But the digitigrade modificationnecessitated a change of structural plan, to the extent of raising thewrist and ankle joints off the ground, so as to make the quadruped walkon its fingers and toes. We meet with an interesting case of thistransition in the existing hare, which while at rest supports itself onthe whole hind foot after the manner of a plantigrade animal, but whenrunning does so upon the ends of its toes, after the manner of adigitigrade animal. It is of importance for us to note that this transition from theoriginal plantigrade to the more recent digitigrade type, has beencarried out on two slightly different plans in two different lines ofmammalian descent. The hoofed mammals--which are all digitigrade--aresub-classified as artiodactyls and perissodactyls, i. E. Even-toed andodd-toed. Now, whether an animal has an even or an odd number of toesmay seem a curiously artificial distinction on which to found soimportant a classification of the mammalian group. But if we look at thematter from a less empirical and more intelligent point of view, weshall see that the alternative of having an even or an odd number oftoes carries with it alternative consequences of a practically importantkind to any animal of the digitigrade type. For suppose an aboriginalfive-toed animal, walking on the ends of its five toes, to be calledupon to resign some of his toes. If he is left with an even number, itmust be two or four; and in either case the animal would gain thefirmest support by so disposing his toes as to admit of the axis of hisfoot passing between an equal number of them--whether it be one or twotoes on each side. On the other hand, if our early mammal were calledupon to retain an odd number of toes, he would gain best support byadjusting matters so that the axis of his foot should be coincident withhis middle toe, whether this were his only toe, or whether he had one oneither side of it. This consideration shows that the classification intoeven-toed and odd-toed is not so artificial as it no doubt at firstsight appears. Let us, then, consider the stages in the evolution ofboth these types of feet. Going back to the reptile _Chelydra_, it will be observed that the axisof the foot passes down the middle toe, which is therefore supported bytwo toes on either side (Fig. 78). It may also be noticed that the wristor ankle bones do not interlock, either with one another or with thebones of the hand or foot below them. This, of course, would give aweak foot, suited to slow progression over marshy ground--which, as wehave seen, was no doubt the origin of the mammalian plantigrade foot. Here, for instance, to all intents and purposes, is a similar type offoot, which belonged to a very early mammal, antecedent to the elephantseries, the horse series, the rhinoceros, the hog, and, in short, allthe known hoofed mammalia (Fig. 80). It was presumably an inhabitant ofswampy ground, slow in its movements, and low in its intelligence. [Illustration: FIG. 80. --Fossil skeleton of _Phenacodus primavus_. (After Cope. )] But now, as we have seen, for more rapid progression on hard unevenground, a stronger and better jointed foot would be needed. Therefore wefind the bones of the wrist and ankle beginning to interlock, both amongthemselves and also with those of the foot and hand immediately belowthem. Such a stage of evolution is still apparent in the now existingelephant. (See Fig. 81. ) [Illustration: FIG. 81. --Bones of the foot of four different forms of the perissodactyl type, showing gradual reduction in the number of digits, coupled with a greater consolidation of the bones above the digits. The series reads from right to left. Drawn from nature (_Brit. Mus. _). ] Next, however, a still stronger foot was made by the still furtherinterlocking of the wrist and ankle bones, so that both the first andsecond rows of them were thus fitted into each other, as well as intothe bones of the hand and foot beneath. This further modification isclearly traceable in some of the earlier perissodactyls, and occurs inthe majority at the present time. Compare, for example, the greaterinterlocking and consolidation of these small bones in the Rhinoceros ascontrasted with the Elephant (Fig. 81). Moreover, simultaneously withthese consolidating improvements in the mechanism of the wrist and anklejoints, or possibly at a somewhat later period, a reduction in thenumber of digits began to take place. This was a continuation of thepolicy of consolidating the foot, analogous to the dropping out of thesixth row of small bones in the paddle of _Baptanodon_. (Fig. 78. ) Inthe pentadactyl plantigrade foot of the early mammals, the first digit, being the shortest, was the first to leave the ground, to dwindle, and finally to disappear. More work being thus thrown on the remainingfour, they were strengthened by interlocking with the wrist (or ankle)bones above them, as just mentioned; and also by being brought closertogether. [Illustration: FIG. 82. --Bones of the foot of four different forms of the artiodactyl type, showing gradual reduction of the number of digits, coupled with a greater consolidation of the bones above the digits. The series reads from right to left. Drawn from nature (_Brit. Mus. _). ] The changes which followed I will render in the words of ProfessorMarsh. Two kinds of reduction began. One leading to the existing perissodactyl foot, and the other, apparently later, resulting in the artiodactyl type. In the former the axis of the foot remained in the middle of the third digit, as in the pentadactyl foot. [See Fig. 81. ] In the latter, it shifted to the outer side of this digit, or between the third and fourth toe. [See Fig. 82. ] In the further reduction of the perissodactyl foot, the fifth digit, being shorter than the remaining three, next left the ground, and gradually disappeared. [Fig. 81 B. ] Of the three remaining toes, the middle or axial one was the longest, and retaining its supremacy as greater strength and speed were required, finally assumed the chief support of the foot [Fig. 81 C], while the outer digits left the ground, ceased to be of use, and were lost, except as splint-bones [Fig. 81 D]. The feet of the existing horse shows the best example of this reduction in the Perissodactyls, as it is the most specialized known in the Ungulates [Fig. 81 D]. In the artiodactyl foot, the reduction resulted in the gradual diminution of the two outer of the four remaining toes, the third and fourth doing all the work, and thus increasing in size and power. The fifth digit, for the same reasons as in the perissodactyl foot, first left the ground and became smaller. Next, the second soon followed, and these two gradually ceased to be functional, [and eventually disappeared altogether, as shown in the accompanying drawing of the feet of still existing animals, Fig. 82 B, C, D]. The limb of the modern race-horse is a nearly perfect piece of machinery, especially adapted to great speed on dry, level ground. The limb of an antelope, or deer, is likewise well fitted for rapid motion on a plain, but the foot itself is adapted to rough mountain work as well, and it is to this advantage, in part, that the Artiodactyls owe their present supremacy. The plantigrade pentadactyl foot of the primitive Ungulate--and even the perissodactyl foot that succeeded it--both belong to the past humid period of the world's history. As the surface of the earth slowly dried up, in the gradual desiccation still in progress, new types of feet became a necessity, and the horse, antelope, and camel were gradually developed, to meet the altered conditions. The best instance of such progressive modifications in the case ofperissodactyl feet is furnished by the fossil pedigree of the existinghorse, because here, within the limits of the same continuous familyline, we have presented the entire series of modifications. There are now known over thirty species of horse-like creatures, beginning from the size of a fox, then progressively increasing in bulk, and all standing in linear series in structure as in time. Confiningattention to the teeth and feet, it will be seen from the wood-cut onpage 189 that the former grow progressively longer in their sockets, andalso more complex in the patterns of their crowns. On the other hand, the latter exhibit a gradual diminution of their lateral toes, togetherwith a gradual strengthening of the middle one. (See Fig. 83. ) So thatin the particular case of the horse-ancestry we have a practicallycomplete chain of what only a few years ago were "missing links. " Andthis now practically completed chain shows us the entire history of whathappens to be the most peculiar, or highly specialized, limb in thewhole mammalian class--namely, that of the existing horse. Of the othertwo wood-cuts, the former (Fig. 84) shows the skeleton of a very earlyand highly generalized ancestor, while the other is a partialrestoration of a much more recent and specialized one. (Fig. 85. ) [Illustration: FIG. 83. --Feet and teeth in fossil pedigree of the Horse. (After Marsh. ) _a_, bones of the fore-foot; _b_, bones of the hind-foot; _c_, radius and ulna; _d_, tibia and fibula; _e_, roots of a tooth; _f_ and _g_, crowns of upper and lower molar teeth. ] [Illustration: FIG. 84. --_Palæotherium_. (Lower Tertiary of Paris Basin. )] [Illustration: FIG. 85. --_Hipparion_. (New World Pliocene. )] On the other hand, progressive modifications of the artiodactyl feet maybe traced geologically up to the different stages presented by livingruminants, in some of which it has proceeded further than in others. Forinstance, if we compare the pig, the deer, and the camel (Fig. 82), weimmediately perceive that the dwindling of the two rudimentary digitshas proceeded much further in the case of the deer than in that of thepig, and yet not so far as in that of the camel, seeing that here theyhave wholly disappeared. Moreover, complementary differences are to beobserved in the degree of consolidation presented by the two usefuldigits. For while in the pig the two foot-bones are still clearlydistinguishable throughout their entire length, in the deer, and stillmore in the camel, their union is more complete, so that they go toconstitute a single bone, whose double or compound character isindicated externally only by a slight bifurcation at the base. Nevertheless, if we examine the state of matters in the unborn young ofthese animals, we find that the two bones in question are stillseparated throughout their length, and thus precisely resemble what usedto be their permanent condition in some of the now fossil species ofhoofed mammalia. Turning next from bones of the limb to other parts of the mammalianskeleton, let us briefly consider the evidence of evolution that is herelikewise presented by the vertebral column, the skull, and the teeth. As regards the vertebral column, if we examine this structure in any ofthe existing hoofed animals, we find that the bony processes calledzygapophyses, which belong to each of the constituent vertebræ, are soarranged that the anterior pair belonging to each vertebra interlockswith the posterior pair belonging to the next vertebra. In this way thewhole series of vertebræ are connected together in the form of a chain, which, while admitting of considerable movement laterally, is everywhereguarded against dislocation. But if we examine the skeletons of anyungulates from the lower Eocene deposits, we find that in no case isthere any such arrangement to secure interlocking. In all the hoofedmammals of this period the zygapophyses are flat. Now, from this flatcondition to the present condition of full interlocking we obtain acomplete series of connecting links. In the middle Miocene period wefind a group of hoofed animals in which the articulation begins by aslight rounding of the previously flat surfaces: later on this roundingprogressively increases, until eventually we get the completeinterlocking of the present time. As regards teeth, and still confining attention to the hoofed mammals, we find that low down in the geological series the teeth present ontheir grinding surfaces only three simple tubercles. Later on a fourthtubercle is added, and later still there is developed that complicatedsystem of ridges and furrows which is characteristic of these teeth atthe present time, and which was produced by manifold and variousinvolutions of the three or four simple tubercles of Eocene and lowerMiocene times. In other words, the principle of gradual improvement inthe construction of teeth, which has already been depicted as regardsthe particular case of the Horse-family (Fig. 83), is no less apparentin the pedigree of all the other mammalia, wherever the palæontologicalhistory is sufficiently intact to serve as a record at all. Lastly, as regards the skull, casts of the interior show that all theearlier mammals had small brains with comparatively smooth orunconvoluted surfaces; and that as time went on the mammalian braingradually advanced in size and complexity. Indeed so small were thecerebral hemispheres of the primitive mammals that they did not overlapthe cerebellum, while their smoothness must have been such as in thisrespect to have resembled the brain of a bird or reptile. This, ofcourse, is just as it ought to be, if the brain, which the skull has toaccommodate, has been gradually evolved into larger and largerproportions in respect of its cerebral hemispheres, or the upper massesof it which constitute the seat of intelligence. Thus, if we look at theabove series of wood-cuts, which represents the comparative structure ofthe brain in the existing classes of the Vertebrata, we can immediatelyunderstand why the fossil skulls of Mammalia should present a gradualincrease in size and furrowing, so as to accommodate the generalincrease of the brain in both these respects between the level marked"maml" and that marked "man, " in the last of the diagrams. (Fig. 87. ) [Illustration: FIG. 86. --Comparative series of Brains. (After Le Conte. ) The series reads from above downwards, and represents diagrammatically the brain of a Fish, a Reptile, a Bird, a Mammal, and a Man. In each case the letter A marks a side view, and the letter B a top view. The small italics throughout signify the following homologous parts: _m_, medulla; _cb_, cerebellum; _op_, optic lobes; _cr_, cerebrum and thalamus; _ol_, olfactory lobes. The series shows a progressive consolidation and enlargement of the brain in general, and of the cerebrum and cerebellum in particular, which likewise exhibit continually advancing structure in respect of convolution. In the case of Man, these two parts of the brain have grown to so great a size that they conceal all the other parts from the superficial points of view represented in the diagram. ] [Illustration: FIG. 87. --Ideal section through all the above stages. (After Le Conte. ]) The tabular statement on the following diagram, which I borrow fromProf. Cope, will serve at a glance to reveal the combined significanceof so many lines of evidence, united within the limits of the same groupof animals. To give only one special illustration of the principle of evolution asregards the skull, here is one of the most recent instances that hasoccurred of the discovery of a missing link, or connecting form (seeFig. 88). The fossil (B), which was found in New Jersey, stands in anintermediate position between the stag and the elk. In the stag (A) theskull is high, showing but little of that anterior attenuation which issuch a distinctive feature of the skull of the elk (C). The nasal bones(N) of the former, again, are remarkably long when compared with thesimilar bones of the latter, and the premaxillaries (PMX), instead ofbeing projected forward along the horizontal plane of the base of theskull, are deflected sharply downward. In all these points, it will beseen, the newly discovered form (_Cervalces_) holds an intermediateposition (B). "The skull exhibits a partial attenuation anteriorly, the premaxillaries are directed about equally downward and forward, andthe nasal bones are measurably contracted in size. The horns likewisefurnish characters which further serve to establish this dualrelationship[18]. " [18] Heilprin, _Geological Evidences of Evolution_, pp. 73-4 (1888). [Illustration: FIG. 88. --Skulls of--A, Canadian Stag; B, _Cervalces Americanus_; and C, Elk. (After Heilprin. )] Formation. |No. Of toes | |Feet | | |Astragalus. | | | |Carpus and tarsus. | | | | |Ulno-radius. | | | | | |Superior molars. | | | | | | |Zygapophyses. | | | | | | | |Brain. | | | | | | | | Pliocene. |1-1, 2-2 | |Digitigrade. (Plantigrade. ) | | |Grooved. (Flat. ) | | | |Interlocking. (Opposite. ) | | | | |Faceted. | | | | | |4-tubercles, crested and cemented. | | | | | | |Doubly involute. Singly involute. | | | | | | | |Hemispheres larger, convoluted. | | | | | | | | Upper Miocene. (Loup Fork. ) |3-3, 4-4, (5-5) | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Middle. (John Day. ) |2-2, 3-3, 4-4 | |Digitigrade. | | |Grooved. | | | |Interlocking. | | | | |Faceted. Smooth | | | | | |4-tubercles, and crested | | | | | | |Singly involute. Double involute. | | | | | | | |Hemispheres larger, convoluted. | | | | | | | | Lower (White River. ) |3-3, 4-3 | |Digitigrade. Plantigrade. | | |Grooved. | | | |Interlocking. | | | | |Smooth. Faceted. | | | | | |4-tubercles, and crested | | | | | | |? Singly involute. | | | | | | | |Hemispheres small, and largeer. | | | | | | | | Eocene. Upper (Bridger. ) |3-3, 4-3, 4-5 | |(Digitigrade. ) Plantigrade. | | |Grooved. (Flat. ) | | | |Opposite. Interlocking. | | | | |Smooth. | | | | | |4-tubercles. 3-tubercles, and crested. | | | | | | |Singly involute. Plane | | | | | | | |Hemispheres small. | | | | | | | | Middle. (Wasatch. ) |4-3, 4-5, 5-5 | |Plantigrade. (Digitigrade. ) | | |Flat. (Grooved. ) | | | |Opposite. Interlocking. | | | | |Smooth. | | | | | |4-tubercles. 3-tubercles, a few crested. | | | | | | |Plane. Singly involute. | | | | | | | |Hemispheres small; | | | | | | | |mesencephalon sometimes exposed | | | | | | | | Lower (Puerco. ) |5-5 | |Plantigrade. | | |Flat. | | | |Opposite. | | | | |Smooth. | | | | | |3-tubercles. (4-tubercles), none crested. | | | | | | |Plane. | | | | | | | |Mesencephalon exposed; | | | | | | | |hemisphere small and smoother. | | | | | | | | The evidence, then, which is furnished by all parts of the vertebralskeleton--whether we have regard to Fishes, Reptiles, Birds, orMammals--is cumulative and consistent. Nowhere do we meet with anydeviation or ambiguity, while everywhere we encounter similar proofs ofcontinuous transformation--proofs which vary only with the varyingamount of material which happens to be at our disposal, being mostnumerous and detailed in those cases where the greatest number of fossilforms has been preserved by the geological record. Here, therefore, wemay leave the vertebral skeleton; and, having presented a sample of theevidence as yielded by horns and bones, I will conclude by glancing withsimilar brevity at the case of shells--which, as before remarked, constitute the only other sufficiently hard or permanent material toyield unbroken evidence touching the fossil ancestry of animals. Of course it will be understood that I am everywhere giving merelysamples of the now superabundant evidence which is yielded bypalæontology; and, as this chapter is already a long one, I must contentmyself with citing only the case of mollusk-shells, although shells ofother classes might be made to yield highly important additions to thetestimony. Moreover, even as regards the one division of mollusk-shells, I can afford to quote only a very few cases. These, however, are in myopinion the strongest single pieces of evidence in favour oftransmutation which have thus far been brought to light. Near the village of Steinheim, in Würtemberg, there is an ancientlake-basin, dating from Tertiary times. The lake has long ago dried up;but its aqueous deposits are extraordinarily rich in fossil shells, especially of different species of the genus _Planorbis_. The followingis an authoritative _résumé_ of the facts. As the deposits seem to have been continuous for ages, and the fossil shells very abundant, this seemed to be an excellent opportunity to test the theory of derivation. With this end in view, they have been made the subject of exhaustive study by Hilgendorf in 1866, and by Hyatt in 1880. In passing from the lowest to the highest strata the species change greatly and many times, the extreme forms being so different that, were it not for the intermediate forms, they would be called not only different species, but different genera. And yet the gradations are so insensible that the whole series is nothing less than a demonstration, in this case at least, of origin of species by derivation with modifications. The accompanying plate of successive forms (Fig. 89), which we take from Prof. Hyatt's admirable memoir, will show this better than any mere verbal explanation. It will be observed that, commencing with four slight varieties--probably sexually isolated varieties--of one species, each series shows a gradual transformation as we go upward in the strata--i. E. Onward in time. Series I branches into three sub-series, in two of which the change of form is extreme. Series IV is remarkable for great increase in size as well as change in form. In the plate we give only selected stages, but in the fuller plates of the memoir, and still more in the shells themselves, the subtilest gradations are found[19]. [19] Le Conte, _loc. Cit. _, pp. 236-7. [Illustration: FIG. 89. --Transmutations of _Planorbis_. (After Hyatt. )] Here is another and more recently observed case of transmutation in thecase of mollusks. The recent species, _Strombus accipitrinus_, still inhabits the coastsof Florida. Its extinct prototype, _S. Leidy_, was discovered a fewyears ago by Prof. Heilprin in the Pliocene formations of the interiorof Florida. The peculiar shape of the wing, and tuberculation of thewhorl, are thus proved to have grown but of a previously more conicalform of shell. [Illustration: FIG. 90. --Transformation of _Strombus_. (After Heilprin. ) 1, 1_a_, _Strombus Leidy_ (1, typical), Pliocene; 2, 2_a_, _Strombus accipitrinus_ (2_a_ typical) Recent. ] Lastly, attention may here again be directed to the very instructiveseries of shells which has already been shown in a previous chapter, andwhich serves to illustrate the successive geological forms of _Paludina_from the Tertiary beds of Slavonia, as depicted by Prof. Neumayr ofVienna. (Fig. 1, p. 19. ) CHAPTER VI. GEOGRAPHICAL DISTRIBUTION. The argument from geology is the argument from the distribution ofspecies in time. I will next take the argument from the distribution ofspecies in space--that is, the present geographical distribution ofplants and animals. Seeing that the theory of descent with adaptive modification impliesslow and gradual change of one species into another, and progressivelystill more slow and gradual changes of one genus, family, or order intoanother genus, family, or order, we should expect on this theory thatthe organic types living on any given geographical area would be foundto resemble or to differ from organic types living elsewhere, accordingas the area is connected with or disconnected from other geographicalareas. For instance, the large continental islands of Australia and NewZealand are widely disconnected from all other lands of the world, anddeep sea soundings show that they have probably been thus disconnected, either since the time of their origin, or, at the least, through immensegeological epochs. The theory of evolution, therefore, would expect tofind two general facts with regard to the inhabitants of these islands. First, that the inhabitants should form, as it were, little worlds oftheir own, more or less unlike the inhabitants of any other parts of theglobe. And next, that some of these inhabitants should present us withindependent information touching archaic forms of life. For it ismanifestly most improbable that the course of evolutionary historyshould have run exactly parallel in the case of these isolated oceaniccontinents and in continents elsewhere. Australia and New Zealand, therefore, ought to present a very large number, not only of peculiarspecies and genera, but even of families, and possibly of orders. Nowthis is just what Australia and New Zealand do present. The case of thedog being doubtful, there is an absence of all mammalian life, exceptthat of one of the oldest and least highly developed orders, theMarsupials. There even occurs a unique order, still lower in the scaleof organization--so low, in fact, that it deserves to be regarded as butnascent mammalian: I mean, of course, the Monotremata. As regards Birds, we have the peculiar wingless forms alluded to in a previous chapter(viz. That on Morphology); and, without waiting to go into details, itis notorious that the faunas of Australia and New Zealand are not onlyhighly peculiar, but also suggestively archaic. Therefore, in both therespects above mentioned, the anticipations of our theory are fullyborne out. But as it would take too long to consider, even cursorily, the faunas and floras of these immense islands, I here allude to themonly for the sake of illustration. In order to present the argument fromgeographical distribution within reasonable limits, I think it is bestto restrict our examination to smaller areas; for these will betteradmit of brief and yet adequate consideration. But of course it will beunderstood that the less isolated the region, and the shorter the timethat it has been isolated, the smaller amount of peculiarity should weexpect to meet with on the part of its present inhabitants. Or, conversely stated, the longer and the greater the isolation, the morepeculiarity of species would our theory expect to find. The object ofthe present chapter will be to show that these, and other cognateexpectations, are fully realized by facts; but, before proceeding to dothis, I must say a few words on the antecedent standing of the argument. Where the question is, as at present, between the rival theories ofspecial creation and gradual transmutation, it may at first sight wellappear that no test can be at once so crucial and so easily applied asthis of comparing the species of one geographical area with those ofanother, in order to see whether there is any constant correlationbetween differences of type and degrees of separation. But a littlefurther thought is enough to show that the test is not quite so simpleor so absolute--that it is a test to be applied in a large and generalway over the surface of the whole earth, rather than one to be reliedupon as exclusively rigid in every special case. In the first place, there is the obvious consideration that lands orseas which are discontinuous now may not always have been so, or not forlong enough to admit of the effects of separation having been exerted toany considerable extent upon their inhabitants. Next, there is thescarcely less important consideration, that although land areas may longhave been separated from one another by extensive tracts of ocean, birds and insects may more or less easily have been able to fly fromone to the other; while even non-flying animals and plants may oftenhave been transported by floating ice or timber, wind or water currents, and sundry other means of dispersal. Again, there is the importantinfluence of climate to be taken into account. We know from geologicalevidence that in the course of geological time the self-same continentshave been submitted to enormous changes of temperature--varying in factfrom polar cold to almost tropical heat; and as it is manifestlyimpossible that forms of life suited to one of these climates could havesurvived during the other, we can here perceive a further and mostpotent cause interfering with the test of geographical distribution asindiscriminately applied in all cases. When the elephant andhippopotamus were flourishing in England amid the luxuriant vegetationwhich these large animals require, it is evident that scarcely any onespecies of either the fauna or the flora of this country can have beenthe same as it was when its African climate gave place to that ofGreenland. Therefore, as Mr. Wallace observes, "If glacial epochs intemperate lands and mild climates near the poles have, as now believedby men of eminence, occurred several times over in the past history ofthe earth, the effects of such great and repeated changes both onmigration, modification, and extinction of species, must have been ofoverwhelming importance--of more importance perhaps than even thegeological changes of sea and land. " But although for these, and certain other less important reasons which Ineed not wait to detail, we must conclude that the evidence fromgeographical distribution is not to be regarded as a crucial testbetween the rival theories of creation and evolution in all casesindiscriminately, I must next remark that it is undoubtedly one of thestrongest lines of evidence which we possess. When we once rememberthat, according to the general theory of evolution itself, the presentgeographical distribution of plants and animals is "the visible outcomeor residual product of the whole past history of the earth, " and, therefore, that of the conditions determining the characters of lifeinhabiting this and that particular area continuity or discontinuitywith other areas is but one, --when we remember this, we find that nofurther reservation has to be made: all the facts of geographicaldistribution speak with one consent in favour of the naturalistictheory. * * * * * The first of these facts which I shall adduce is, that although thegeographical range of any given species is, as a rule, continuous, suchis far from being always the case. Very many species have more or lessdiscontinuous ranges--the mountain-hare, for instance, extending fromthe Arctic regions over the greater portion of Europe to the UralMountains and the Caucasus, and yet over all this enormous tractappearing only in isolated or discontinuous patches, where there happento be either mountain ranges or climates cold enough to suit its nature. Now, in all such cases of discontinuity in the range of a species thetheory of evolution has a simple explanation to offer--namely, eitherthat some representatives of the species have at some former period beenable to migrate from one region to the other, or else that at one timethe species occupied the whole of the range in question, but afterwardsbecame broken up as geographical, climatic, or other changes renderedparts of the area unfit for the species to inhabit. Thus, for instance, it is easy to understand that during the last cold epoch themountain-hare would have had a continuous range; but that as the Arcticclimate gradually receded to polar regions, the species would be able tosurvive in southern latitudes only on mountain ranges, and thus wouldbecome broken up into many discontinuous patches, corresponding withthese ranges. In the same way we can explain the occurrence of Arcticvegetation on the Alps and Pyrenees--namely, as left behind by theretreat of the Arctic climate at the close of the glacial period. But now, on the other hand, the theory of special creation cannot sowell afford to render this obvious explanation of discontinuity. In thecase of the Arctic flora of the Alps, for instance, although it is truethat much of this vegetation is of an Arctic type, it is not true thatthe species are all identical with those which occur in the Arcticregions. Therefore the theory of special creation would here have toassume that, although the now common species were left behind on theAlps by the retreat of glaciation northwards, the peculiar Alpinespecies were afterwards created separately upon the Alps, and yetcreated with such close affinities to the pre-existing species as to beincluded with them under the same genera. Looking to the absurdity ofthis supposition, as well as of others which I need not wait to mention, certain advocates of special creation have sought to take refuge inanother hypothesis--namely, that species which present a markedlydiscontinuous range may have had a corresponding number of differentcentres of creation, the same specific type having been turned down, soto speak, on widely separated areas. But to me it seems that thisexplanation presents even greater difficulty than the other. If it isdifficult to say why the Divinity should have chosen to create newspecies of plants on the Alps on so precisely the same pattern as theold, much more would it be difficult to say why, in addition to thesenew species, he should also have created again the old species which hehad already placed in the Arctic regions. * * * * * So much, then, for discontinuity of distribution. The next general factto be adduced is, that there is no constant correlation between habitatsand animals or plants suited to live upon them. Of course all theanimals and plants living upon any given area are well suited to liveupon that area; for otherwise they could not be there. But the point nowis, that besides the area on which they do live, there are usually manyother areas in different parts of the globe where they might have livedequally well--as is proved by the fact that when transported by man theythrive as well, or even better, than in their native country. Therefore, upon the supposition that all species were separately created in thecountries where they are respectively found, we must conclude that theywere created in only some of the places where they might equally wellhave lived. Probably there is at most but a small percentage either ofplants or animals which would not thrive in some place, or places, onthe earth's surface other than that in which they occur; and hence wemust say that one of the objects of special creation--if this be thetrue theory--was that of depositing species in only some among theseveral parts of the earth's surface equally well suited to supportthem. Now, I do not contend that this fact in itself raises any difficultyagainst the theory of special creation. But I do think that a veryserious difficulty is raised when to this fact we add another--namely, that on every biological region we encounter species related to otherspecies in genera, and usually also genera related to other genera infamilies. For if each of all the constituent species of a genus, andeven of a family, were separately created, we must hence conclude thatin depositing them there was an unaccountable design manifested to makeareas of distribution correspond to the natural affinities of theirinhabitants. For example, the humming-birds are geographicallyrestricted to America, and number 120 genera, comprising over 400species. Hence, if this betokens 400 separate acts of creation, itcannot possibly have been due to chance that they were all performed onthe same continent: it must have been design which led to every speciesof this large family of birds having been deposited in one geographicalarea. Or, to take a case where only the species of a single genus areconcerned. The rats and mice proper constitute a genus which comprisesaltogether more than 100 species, and they are all exclusivelyrestricted to the Old World. In the New World they are represented byanother genus comprising about 70 species, which resemble their OldWorld cousins in form and habits; but differ from them in dentition andother such minor points. Now, the question is, --Why should all the 100species have been separately created on one side of the Atlantic withone pattern of dentition, and all the 70 species on the other side withanother pattern? What has the Atlantic Ocean got to do with any"archetypal plan" of rats' teeth? Or again, to recur to Australia, why should all the mammalian forms oflife be restricted to the one group of Marsupials, when we know that notonly the Rodents, such as the rabbit, but all other orders of mammals, would thrive there equally well. And similarly, of course, in countlessother instances. Everywhere we meet with this same correlation betweenareas of distribution and affinities of classification. Now, it is at once manifest how completely this general fact harmonizeswith the theory of evolution. If the 400 species of humming-birds, forinstance, are all modified descendants of common ancestors, and if noneof their constituent individuals have ever been large enough to maketheir way across the oceans which practically isolate their territoryfrom all other tropical and sub-tropical regions of the globe, then wecan understand why it is that all the 400 species occupy the samecontinent. But on the special-creation theory we can see no reason whythe 400 species should all have been deposited in America. And, asalready observed, we must remember that this correlation between ageographically restricted habitat and the zoological or botanicalaffinities of its inhabitants, is repeated over and over and over againin the faunas and floras of the world, so that merely to enumerate theinstances would require a separate chapter. Furthermore, the general argument thus presented in favour of descentwith continuous modification admits of being enormously strengthened bythree different classes of additional facts. The first is, that the correlation in question--namely, that between ageographically restricted habitat and the zoological or botanicalaffinities of its inhabitants--is not limited to the now existingspecies, but extends also to the extinct. That is to say, the deadspecies are allied to the living species, as we should expect that theymust be, if the latter are modified descendants of the former. On thealternative theory, however, we have to suppose that the policy ofmaintaining a correlation between geographical restriction and naturalaffinity extends very much further back than even the existing speciesof plants and animals; indeed we must suppose that a practicallyinfinite number of additional acts of separate creation were governed bythe same policy, in the case of long lines of species long sinceextinct. Thus far, then, the only answer which an advocate of special creationcan adduce is, that for some reason unknown to us such a policy may havebeen more wise than it appears: it may have served some inscrutablepurpose that allied products of distinct acts of creation should all bekept together on the same areas. Well, in answer to this unjustifiableappeal to the argument from ignorance, I will adduce the second of thethree considerations. This is, that in cases where the geographicalareas are not restricted the policy in question fails. In other words, where the inhabitants of an area are free to migrate to other areas, thepolicy of correlating affinity with distribution is most significantlyforgotten. In this case species wander away from their native homes, andthe course of their wanderings is marked by the origination of newspecies springing up en route. Now, is it reasonable to suppose that themere circumstance of some members of a species being able to leave theirnative home should furnish any occasion for creating new and alliedspecies upon the tracts over which they travel, or the territories towhich they go? When the 400 existing species of humming-birds have allbeen created on the same continent for some reason supposed to beunknown, why should this reason give way before the accident of anymeans of migration being furnished to humming-birds, so that they shouldbe able to visit, say the continents of Africa and Asia, there gain afooting beside the sun-birds, and henceforth determine a new centre forthe separate creation of additional species of humming-birds peculiar tothe Old World--as has happened in the case of the majority of specieswhich, unlike the humming-birds, have been at any time free to migratefrom their original homes? Lastly, my third consideration is, that the supposed policy in questiondoes not extend to affinities which are wider than those between speciesand genera--more rarely to families, scarcely ever to orders, and neverto classes. In other words, nature shows a double correlation in hergeographical distribution of organic types:--first, that which we havealready considered between geographical restriction and naturalaffinity among inhabitants of the same areas; second, another of a moredetailed character between _degrees_ of geographical restriction and_degrees_ of natural affinity. The more distant the affinity, the moregeneral is the extension. This, of course, is what we should expect onthe theory of descent with modification, because the more distant theaffinity, and therefore, _ex hypothesi_, the larger and the older theoriginal group of organisms, the greater must be the chance ofdispersal. The 400 species of humming-birds may well be unable tomigrate from their native continent; but it would indeed have been anunaccountable fact if no other species of all the class of birds hadever been able to have crossed the Atlantic Ocean. Thus, on the theoryof evolution, we can well understand the second correlation now beforeus--namely, between remoteness of affinity and generality ofdispersal, --so that there is no considerable portion of the habitableglobe without representatives of all the classes of animals, fewportions without representatives of all the orders, but many portionswithout many of the families, innumerable portions without innumerablegenera, and, of course, all portions without the great majority ofspecies. Now, while this general correlation thus obviously supports thetheory of natural descent with progressive modification, it makesdirectly against the opposite theory of special creation. For we haverecently seen that when we restrict our view to the case of species andgenera, the theory of special creation is obliged to suppose that forsome inscrutable reason the Deity had regard to systematic affinitywhile determining on what large areas to create his species[20]. Butnow we see that he must be held to have neglected this inscrutablereason (whatever it was) when he passed beyond the range of genera--andthis always in proportion to the remoteness of systematic affinity onthe part of the species concerned. [20] I say "_large_ areas" for the sake of argument; but the same correlation between distribution and affinity extends likewise to _small_ areas where only _small_ differences of affinity are concerned. Thus, for instance, speaking of smaller areas, Moritz Wagner says:--"The broader and more rapid the river, the higher and more regular the mountain-chain, the calmer and more extensive the sea, the more considerable, as a general rule, will be the taxonomic separation between the populations"; and he shows that, in correlation with such differences in the _degrees_ of separation, are the _degrees_ of diversification--i. E. , the _numbers_ of species, and even of varieties, which these topographical barriers determine. I cannot well conceive a _reductio ad absurdum_ more complete than this. But, having now presented these most general facts of geographicaldistribution in their relation to the issue before us, we may nextproceed to consider a few illustrations of them in detail, for in thisway I think that their overwhelming weight may become yet moreabundantly apparent. * * * * * It will assist us in dealing with these detailed illustrations if webegin by considering the means of dispersal of organisms from one placeto another. Of course the most ordinary means is that of continuouswandering, or emigration; but where geographical barriers of any kindhave to be surmounted, organisms may only be able to pass them by moreexceptional and accidental means. The principal barriers of ageographical kind are oceans, rivers, mountain-chains, anddesert-tracts, in the case of terrestrial organisms; and, in the caseof aquatic organisms, the presence of land. But it is to be observedthat, as regards marine organisms, any considerable difference in thetemperature of the water may constitute a barrier as effectual as thepresence of land; and also that, in the case of all shallow-waterfaunas, a tract of deep ocean constitutes almost as complete a barrieras it does to terrestrial faunas. Now, the means whereby barriers admit of being accidentally oroccasionally surmounted are, of course, various; and they differ in thecase of different organisms. Birds, bats, and insects, on account oftheir powers of flight, are particularly apt to be blown out greatdistances to sea, and hence of all animals are most likely to become theinvoluntary colonists of distant shores. Floating timber serves toconvey seeds and eggs of small animals over great distances; and Darwinhas shown that many kinds of seeds are able of themselves to float formore than a month in sea-water without losing their powers ofgermination. For instance, out of 87 kinds, 64 germinated after animmersion of 28 days, and a few survived an immersion of 137 days. As aresult of all his experiments he concludes, that the seeds of at leastten per cent. Of the species of plants of any country might be floatedby sea-currents during 28 days, without losing their powers ofgermination; and this, at the average rate of flow of several Atlanticcurrents, would serve to transport the seeds to a distance of at least900 miles. Again, he proved that even seeds which are quickly destroyedby contact with sea-water admit of being successfully transported during30 days, if they be contained within the crop of a dead bird. He alsoproved that living birds are most active agents in the work ofdissemination, and this not only by taking seeds into their crops(where, so long as they remain, the seeds are uninjured), but likewiseby carrying seeds (and even young mollusks) attached to their feet andfeathers. In the course of these experiments he found that a smallcup-full of mud, which he gathered from the edges of three ponds inFebruary, was so charged with seeds that when sown in the ground thesefew ounces of mud yielded no less than 537 plants, belonging to manydifferent species. It is therefore evident what opportunities are thusafforded for the transportation of seeds on the feet and bills ofwading-birds. Lastly, floating ice is well known to act as a carrier ofany kind of life which may prove able to survive this mode of transit. Such being the nature of geographical barriers, and the means thatorganisms of various kinds may occasionally have of overcoming them, Iwill now give a few detailed illustrations of the argument fromgeographical distribution, as previously presented in its general form. To begin with aquatic animals. As Darwin remarks, "the marineinhabitants of the Eastern and Western shores of South America are verydistinct; with extremely few shells, crustacea, or echinodermata incommon. " Again, westward of the shores of America, a wide space of openocean extends, which, as we have seen, furnishes as effectual a barrieras does the land to any emigration of shallow-water animals. Now, assoon as this reach of deep water is passed, we meet in the easternislands of the Pacific with another and totally distinct fauna. "So thatthree marine faunas range northward and southward in parallel lines notfar from each other, under corresponding climates": they are, however, "separated from each other by impassable barriers, either of land oropen sea": and it is in exact coincidence with the course of thesebarriers that we find so remarkable a differentiation of the faunas[21]. Obviously, therefore, it is impossible to suggest that this correlationis accidental. Altogether many thousands of species are involved, andwithin this comparatively limited area they are sharply marked off intothree groups as to their natural affinities, and into three groups as totheir several basins. Hence, if all these species were separatelycreated, there is no escape from the conclusion that for some reason oranother the act of creation was governed by the presence of thesebarriers, so that species deposited on the Eastern shores of SouthAmerica were formed with one set of natural affinities, while speciesdeposited on the Western shore were formed with another set; andsimilarly with regard to the third set of species in the third basin, which, extending over a whole hemisphere to the coast of Africa withoutany further barrier, nowhere presents, over this vast area, any othercase of a distinct marine fauna. But what conceivable reason can therehave been thus to consult these geographical barriers in the originalcreation of specific types? Even if such a case stood alone, it wouldbe strongly suggestive of error on the part of the special creationtheory. But let us take another case, this time from fresh-water faunas. [21] The only exception is in the case of the fish on each side of the isthmus of panama, where about 30 per cent, of the species are identical. But it is possible enough that at some previous time this narrow isthmus may have been even narrower than at present, if not actually open. At all events, the fact that this partial exception occurs just where the land-barrier is so narrow, is more suggestive of migration than of independent creation. Although the geographical distribution of fresh-water fish andfresh-water shells is often surprisingly extensive and apparentlycapricious, this may be explained by the means of dispersal being hereso varied--not only aquatic birds, floods, and whirlwinds, but alsogeographical changes of water-shed having all assisted in the process. Moreover, in some cases it is possible that the habits of more widelydistributed fresh-water fish may have originally been wholly or partlymarine--which, of course, would explain the existing discontinuity oftheir existing fresh-water distribution. But, be this as it may (and itis not a question that affects the issue between special creation andgradual evolution, since it is only a question as to how a given specieshas been dispersed from its original home, whether or not in that homeit was specially created), the point I desire to bring forward is, thatwhere we find a barrier to the emigration of fresh-water forms which ismore formidable than a thousand miles of ocean--a barrier over whichneither water-fowl nor whirlwinds are likely to pass, and which is abovethe reach of any geological changes of water-shed, --where we find such abarrier, we always find a marked difference in the fresh-water faunas oneither side of it. The kind of barrier to which I allude is a highmountain-chain. It may be only a few miles wide; yet it exercises agreater influence on the diversification of specific types, wherefresh-water faunas are concerned, than almost any other. But why shouldthis be the case on any intelligible theory of special creation? Why, inthe depositing of species of newly created fresh-water fish, should thepresence of an impassable mountain-chain have determined so uniformly adifference of specific affinity on either side of it? The question, sofar as I can see, does not admit of an answer from any reasonableopponent. * * * * * Turning now from aquatic organisms to terrestrial, the body of factsfrom which to draw is so large, that I think the space at my disposalmay be best utilized by confining attention to a single division ofthem--that, namely, which is furnished by the zoological study ofoceanic islands. In the comparatively limited--but in itself extensive--class of factsthus presented, we have a particularly fair and cogent test as betweenthe alternative theories of evolution and creation. For where we meetwith a volcanic island, hundreds of miles from any other land, andrising abruptly from an ocean of enormous depth, we may be quite surethat such an island can never have formed part of a now submergedcontinent. In other words, we may be quite sure that it always has beenwhat it now is--an oceanic peak, separated from all other land byhundreds of miles of sea, and therefore an area supplied by nature forthe purpose, as it were, of testing the rival theories of creation andevolution. For, let us ask, upon these tiny insular specks of land whatkind of life should we expect to find? To this question the theories ofspecial creation and of gradual evolution would agree in giving thesame answer up to a certain point. For both theories would agree insupposing that these islands would, at all events in large part, derivetheir inhabitants from accidental or occasional arrivals of wind-blownor water-floated organisms from other countries--especially, of course, from the countries least remote. But, after agreeing upon this point, the two theories must part company in their anticipations. Thespecial-creation theory can have no reason to suppose that a smallvolcanic island in the midst of a great ocean should be chosen as thetheatre of any extraordinary creative activity, or for any particularlyrich manufacture of peculiar species to be found nowhere else in theworld. On the other hand, the evolution theory would expect to find thatsuch habitats are stocked with more or less peculiar species. For itwould expect that when any organisms chanced to reach a wholly isolatedrefuge of this kind, their descendants should forthwith have startedupon an independent course of evolutionary history. Protected fromintercrossing with any members of their parent species elsewhere, andexposed to considerable changes in their conditions of life, it wouldindeed be fatal to the general theory of evolution if these descendants, during the course of many generations, were not to undergo appreciablechange. It has happened on two or three occasions that European ratshave been accidentally imported by ships upon some of these islands, andeven already it is observed that their descendants have undergone aslight change of appearance, so as to constitute them what naturalistscall local varieties. The change, of course, is but slight, because thetime allowed for it has been so short. But the longer the time that acolony of a species is thus completely isolated under changed conditionsof life the greater, according to the evolution theory, should we expectthe change to become. Therefore, in all cases where we happen to know, from independent evidence of a geological kind, that an oceanic islandis of very ancient formation, the evolution theory would expect toencounter a great wealth of peculiar species. On the other hand, as Ihave just observed, the special-creation theory can have no reason tosuppose that there should be any correlation between the age of anoceanic island and the number of peculiar species which it may be foundto contain. Therefore, having considered the principles of geographical distributionfrom the widest or most general point of view, we shall pass to theopposite extreme, and consider exhaustively, or in the utmost possibledetail, the facts of such distribution where the conditions are bestsuited to this purpose--that is, as I have already said, upon oceanicislands, which may be metaphorically regarded as having been formed bynature for the particular purpose of supplying naturalists with acrucial test between the theories of creation and evolution. Thematerial upon which my analysis is to be based will be derived from themost recent works upon geographical distribution--especially from themagnificent contributions to this department of science which we owe tothe labours of Mr. Wallace. Indeed, all that follows may be regarded asa condensed filtrate of the facts which he has collected. Even as thusrestricted, however, our subject-matter would be too extensive to bedealt with on the present occasion, were we to attempt an exhaustiveanalysis of the floras and faunas of all oceanic islands upon the faceof the globe. Therefore, what I propose to do is to select for suchexhaustive analysis a few of what may be termed the most oceanic ofoceanic islands--that is to say, those oceanic islands which are mostwidely separated from mainlands, and which, therefore, furnish the mostunquestionable of test cases as between the theories of special creationand genetic descent. * * * * * _Azores. _--A group of volcanic islands, nine in number, about 900 milesfrom the coast of Portugal, and surrounded by ocean depths of 1, 800 to2, 500 fathoms. There is geological evidence that the origin of the groupdates back at least as far as Miocene times. There is a total absence ofall terrestrial Vertebrata, other than those which are known to havebeen introduced by man. Flying animals, on the other hand, are abundant;namely, 53 species of birds, one species of bat, a few species ofbutterflies, moths, and hymenoptera, with 74 species of indigenousbeetles. All these animals are unmodified European species, with theexception of one bird and many of the beetles. Of the 74 indigenousspecies of the latter, 36 are not found in Europe; but 19 are natives ofMadeira or the Canaries, and 3 are American, doubtless transplanted bydrift-wood. The remaining 14 species occur nowhere else in the world, though for the most part they are allied to other European species. There are 69 known species of land-shells, of which 37 are European, and32 peculiar, though all allied to European forms. Lastly, there are 480known species of plants, of which 40 are peculiar, though allied toEuropean species. _Bermudas. _--A small volcanic group of islands, 700 miles from NorthCarolina. Although there are about 100 islands in the group, their totalarea does not exceed 50 square miles. The group is surrounded by watervarying in depth from 2, 500 to 3, 800 fathoms. The only terrestrialVertebrate (unless the rats and mice are indigenous) is a lizard alliedto an American form, but specifically distinct from it, and therefore asolitary species which does not occur anywhere else in the world. Noneof the birds or bats are peculiar, any more than in the case of theAzores; but, as in that case, a large percentage of the land-shells areso--namely, at least one quarter of the whole. Neither the botany northe entomology of this group has been worked out; but I have said enoughto show how remarkably parallel are the cases of these two volcanicgroups of islands situated in different hemispheres, but at about thesame distance from large continents. In both there is an extraordinarypaucity of terrestrial vertebrata, and of any peculiar species of birdor beast. On the other hand, there is in both a marvellous wealth ofpeculiar species of insects and land-shells. Now these correlations areall abundantly intelligible. It is a difficult matter for anyterrestrial animal to cross 900, or even 700, miles of ocean: thereforeonly one lizard has succeeded in doing so in one of the two parallelcases; and, living cut off from intercrossing with its parent form, thedescendants of that lizard have become modified so as to constitute apeculiar species. But it is more easy for large flying animals to crossthose distances of ocean: consequently, there is only one instance of apeculiar species of bird or bat--namely, a bull-finch in the Azores, which, being a small land-bird, is not likely ever to have had any othervisitors from its original parent species coming over from Europe tokeep up the original breed. Lastly, it is very much more easy forinsects and land-mollusca to be conveyed to such islands by wind andfloating timber than it is for terrestrial mammals, or even than it isfor small birds and bats; but yet such means of transit are notsufficiently sure to admit of much recruiting from the mainland for thepurpose of keeping up the specific types. Consequently, the insects andthe land-shells present a much greater proportion of peculiarspecies--namely, one half and one fourth of the land-shells in the onecase, and one eighth of the beetles in the other. All thesecorrelations, I say, are abundantly intelligible on the theory ofevolution; but who shall explain, on the opposite theory, why orders ofbeetles and land-mollusca should have been chosen from among all otheranimals for such superabundant creation on oceanic islands, so that inthe Azores alone we find no less than 32 of the one and 14 of the other?And, in this connexion, I may again allude to the peculiar species ofbeetles in the island of Madeira. Here there are an enormous number ofpeculiar species, though they are nearly all related to, or includedunder the same genera as, beetles on the neighbouring continent. Now, aswe have previously seen, no less than 200 of these species have lost theuse of their wings. Evolutionists explain this remarkable fact by theirgeneral laws of degeneration under disuse, and the operation of naturalselection, as will be shown later on; but it is not so easy for specialcreationists to explain why this enormous number of peculiar species ofbeetles should have been deposited on Madeira, all allied to beetles onthe nearest continent, and nearly all deprived of the use of theirwings. And similarly, of course, with all the peculiar species of theBermudas and the Azores. For who will explain, on the theory ofindependent creation, why all the peculiar species, both of animals andplants, which occur on the Bermudas should so unmistakably presentAmerican affinities, while those which occur on the Azores no lessunmistakably present European affinities? But to proceed to other, andstill more remarkable, cases. _The Galapagos Islands. _--This archipelago is of volcanic origin, situated under the equator between 500 and 600 miles from the West Coastof South America. The depth of the ocean around them varies from 2, 000to 3, 000 fathoms or more. This group is of particular interest, from thefact that it was the study of its fauna which first suggested toDarwin's mind the theory of evolution. I will, therefore, begin byquoting a short passage from his writings upon the zoological relationsof this particular fauna. Here almost every product of the land and of the water bears the unmistakeable stamp of the American continent. There are twenty-six land birds; of these, twenty-one, or perhaps twenty-three, are ranked as distinct species, and would commonly be assumed to have been here created; yet the close affinity of most of these birds to American species is manifest in every character, in their habits, gestures, and tones of voice. So it is with the other animals, and with a large proportion of the plants, as shown by Dr. Hooker in his admirable Flora of this archipelago. The naturalist, looking at the inhabitants of these volcanic islands in the Pacific, distant several hundred miles from the continent, feels that he is standing on American land. Why should this be so? Why should the species which are supposed to have been created in the Galapagos Archipelago, and nowhere else, bear so plainly the stamp of affinity to those created in America? There is nothing in the conditions of life, in the geological nature of the islands, in their height or climate, or in the proportions in which the several classes are associated together, which closely resembles the conditions of the South American coast; in fact, there is a considerable dissimilarity in all these respects. On the other hand, there is a considerable degree of resemblance in the volcanic nature of the soil, in the climate, height, and size of the islands, between the Galapagos and Cape de Verde Archipelagoes; but what an entire and absolute difference in their inhabitants! The inhabitants of the Cape de Verde Islands are related to those of Africa, like those of the Galapagos to America. Facts such as these admit of no sort of explanation on the ordinary view of independent creation; whereas on the view here maintained, it is obvious that the Galapagos Islands would be likely to receive colonists from America, and the Cape de Verde Islands from Africa; such colonists would be liable to modification--the principle of inheritance still betraying their original birthplace[22]. [22] _origin of species_, pp. 353-4. The following is a synopsis of the fauna and flora of this archipelago, so far as at present known. The only terrestrial vertebrates are twopeculiar species of land-tortoise, and one extinct species; five speciesof lizards, all peculiar--two of them so much so as to constitute apeculiar genus;--and two species of snakes, both closely allied to SouthAmerican forms. Of birds there are 57 species, of which no less than 38are peculiar; and all the non-peculiar species, except one, belong toaquatic tribes. The true land birds are represented by 31 species, ofwhich all, except one, are peculiar; while more than half of them go toconstitute peculiar genera. Moreover, while they are all unquestionablyallied to South American forms, they present a beautiful series ofgradations, "from perfect identity with the continental species, togenera so distinct that it is difficult to determine with what formsthey are most nearly allied; and it is interesting to note that thisdiversity bears a distinct relation to the probabilities of, andfacilities for, migration to the islands. The excessively abundantrice-bird, which breeds in Canada, and swarms over the whole UnitedStates, migrating to the West Indies and South America, visiting thedistant Bermudas almost every year, and extending its range as far asParaquay, is the only species of land-bird which remains completelyunchanged in the Galapagos; and we may therefore conclude that somestragglers of the migrating host reach the islands sufficiently often tokeep up the purity of the breed[23]. " Again, of the thirty peculiarland-birds, it is observable that the more they differ from any otherspecies or genera on the South American continent, the more certainlyare they found to have their nearest relations among those SouthAmerican forms which have the more restricted range, and are thereforethe least likely to have found their way to the islands with anyfrequency. [23] Wallace, _Island Life_, pp. 271-2. The insect fauna of the Galapagos islands is scanty, and chieflycomposed of beetles. These number 35 species, which are nearly allpeculiar, and in some cases go to constitute peculiar genera. The sameremarks apply to the twenty species of land-shells. Lastly, of the totalnumber of flowering plants (332 species) more than one half (174species) are peculiar. It is observable in the case of these peculiarspecies of plants--as also of the peculiar species of birds--that manyof them are restricted to single islands. It is also observable that, with regard both to the fauna and flora, the Galapagos Islands as awhole are very much richer in peculiar species than either the Azores orBermudas, notwithstanding that both the latter are considerably moreremote from their nearest continents. This difference, which at firstsight appears to make against the evolutionary interpretation, reallytends to confirm it. For the Galapagos Islands are situated in a calmregion of the globe, unvisited by those periodic storms and hurricaneswhich sweep over the North Atlantic, and which every year convey somestraggling birds, insects, seeds, &c. , to the Azores and Bermudas. Notwithstanding their somewhat greater isolation geographically, therefore, the Azores and Bermudas are really less isolated biologicallythan are the Galapagos Islands; and hence the less degree of peculiarityon the part of their endemic species. But, on the theory of specialcreation, it is impossible to understand why there should be any suchcorrelation between the prevalence of gales and a comparative inertnessof creative activity. And, as we have seen, it is equally impossible onthis theory to understand why there should be a further correlationbetween the _degree_ of peculiarity on the part of the isolatedspecies, and the degree in which their nearest allies on the mainlandare there confined to narrow ranges, and therefore less likely to keepup any biological communication with the islands. _St. Helena. _--A small volcanic island, ten miles long by eight wide, situated in mid-ocean, 1100 miles from Africa, and 1800 from SouthAmerica. It is very mountainous and rugged, bounded for the most part byprecipices, rising from ocean depths of 17, 000 feet, to a height abovethe sea-level of nearly 3, 000. When first discovered it was richlyclothed with forests; but these were all destroyed by human agencyduring the 16th, 17th and 18th centuries. The records of civilizationpresent no more lamentable instance of this kind of destruction. From amerely pecuniary point of view the abolition of these primeval forestshas proved an irreparable loss; but from a scientific point of view theloss is incalculable. These forests served to harbour countless forms oflife, which extended at least from the Miocene age, and which, havingfound there an ocean refuge, survived as the last remnants of a remotegeological epoch. In those days, as Mr. Wallace observes, St. Helenamust have formed a kind of natural museum or vivarium of archaic speciesof all classes, the interest of which we can now only surmise from thefew remnants of those remnants, which are still left among the moreinaccessible portions of the mountain peaks and crater edges. Theseremnants of remnants are as follows. There is a total absence of all indigenous mammals, reptiles, fresh-water fish, and true land-birds. There is, however, a species ofplover, allied to one in South Africa; but it is specifically distinct, and therefore peculiar to the island. The insect life, on the otherhand, is abundant. Of beetles no less than 129 species are believed tobe aboriginal, and, with one single exception, the whole number arepeculiar to the island. "But in addition to this large amount ofspecific peculiarity (perhaps unequalled anywhere else in the world), the beetles of this island are remarkable for their generic isolation, and for the altogether exceptional proportion in which the greatdivisions of the order are represented. The species belong to 39 genera, of which no less than 25 are peculiar to the island; and many of theseare such isolated forms that it is impossible to find their allies inany particular country[24]. " More than two-thirds of all the speciesbelong to the group of weevils--a circumstance which serves to explainthe great wealth of beetle-population, the weevils being beetles whichlive in wood, and St. Helena having been originally a densely woodedisland. This circumstance is also in accordance with the view that thepeculiar insect fauna has been in large part evolved from ancestorswhich reached the island by means of floating timber; for, of course, noexplanation can be suggested why special creation of this highlypeculiar insect fauna should have run so disproportionately into theproduction of weevils. About two-thirds of the whole number of beetles, or over 80 species, show no close affinity with any existing insects, while the remaining third have some relations, though often very remote, with European and African forms. That this high degree of peculiarityis due to high antiquity is further indicated, according to our theory, by the large number of species which some of the types comprise. Thus, the 54 species of _Cossonidæ_ may be referred to three types; the 11species of _Bembidium_ form a group by themselves; and the _Heteromera_form two groups. "Now, each of these types may well be descended from asingle species, which originally reached the island from some otherland; and the great variety of generic and specific forms into whichsome of them have diverged is an indication, and to some extent ameasure, of the remoteness of their origin[25]. " But, on thecounter-supposition that all these 128 peculiar species were separatelycreated to occupy this particular island, it is surely unaccountablethat they should thus present such an arborescence of natural affinitiesamongst themselves. [24] Wallace, _Island Life_, p. 287. [25] Wallace, _Island Life_, p. 287. Passing over the rest of the insect fauna, which has not yet beensufficiently worked out, we next find that there are only 20 species ofindigenous land-shells--which is not surprising when we remember by whatenormous reaches of ocean the island is surrounded. Of these 20 speciesno less than 13 have become extinct, three are allied to Europeanspecies, while the rest are so highly peculiar as to have no near alliesin any other part of the globe. So that the land-shells tell exactly thesame story as the insects. Lastly, the plants likewise tell the same story. The truly indigenousflowering plants are about 50 in number, besides 26 ferns. Forty of theformer and ten of the latter are peculiar to the island, and, as SirJoseph Hooker tells us, "cannot be regarded as very close specificallies of any other plants at all" Seventeen of them belong to peculiargenera, and the others all differ so markedly as species from theircongeners, that not one comes under the category of being an insularform of a continental species. So that with respect to its plants noless than with respect to its animals, we find that the island of St. Helena constitutes a little world of unique species, allied amongthemselves, but diverging so much from all other known forms that inmany cases they constitute unique genera. _Sandwich Islands. _--These are an extensive group of islands, largerthan any we have hitherto considered--the largest of the group beingabout the size of Devonshire. The entire archipelago is volcanic, withmountains rising to a height of nearly 14, 000 feet. The group issituated in the middle of the North Pacific, at a distance ofconsiderably over 2, 000 miles from any other land, and surrounded byenormous ocean depths. The only terrestrial vertebrata are two lizards, one of which constitutes a peculiar genus. There are 24 aquatic birds, five of which are peculiar; four birds of prey, two of which arepeculiar; and 16 land-birds, all of which are peculiar. Moreover, these16 land-birds constitute no less than 10 peculiar genera, and even onepeculiar family of five genera. This is an amount of peculiarity farexceeding that of any other islands, and, of course, corresponds withthe great isolation of this archipelago. The only other animals whichhave here been carefully studied are the land-shells, and these tell thesame story as the birds. For there are no less than 400 species whichare all, without any exception, peculiar; while about three-quarters ofthem go to constitute peculiar genera. Again, of the plants, 620 speciesare believed to be endemic; and of these 377 are peculiar, yielding noless than 39 peculiar genera. * * * * * Prejudice apart, I think we must all now agree that it is needless tocontinue further this line of proof. I have chosen the smallest and mostisolated islands for the purposes of our present argument, first becausethese furnish the most crucial kind of test, and next because they bestadmit of being dealt with in a short space. But, if necessary, a vastamount of additional material could be furnished, not only from othersmall oceanic islands, but still more from the largest islands of theworld, such as Australia and New Zealand. However, after the detailedinventories which have now been given in the case of some of the smallerislands most remote from mainlands, we may well be prepared to accept itas a general law, that _wherever_ there is evidence of land-areas havingbeen for a long time separated from other land-areas, there we meet witha more or less extraordinary profusion of unique species, often runningup into unique genera. And, in point of fact, so far as naturalists havehitherto been able to ascertain, _there is no exception to this generallaw in any region of the globe_. Moreover, there is everywhere aconstant correlation between the _degree_ of this peculiarity on thepart of the fauna and flora, and the _time_ during which they have beenisolated. Thus, for instance, among the islands which I have calledinto evidence, those that are at once the most isolated and giveindependent proofs of the highest antiquity, are the Galapagos Islands, the Sandwich Islands, and St. Helena. Now, if we apply the method oftabular analysis to these three cases, we obtain the following mostastonishing results. For the sake of simplicity I will omit theenumeration of peculiar genera, and confine attention to peculiarspecies. Moreover, I will consider only terrestrial animals; for, as wehave already seen, aquatic animals are so much more likely to reachoceanic islands that they do not furnish nearly so fair a test of theevolutionary hypothesis. PECULIAR SPECIES. +------------+---------+----------+-----------+----------+----------+ | | Shells. | Insects. | Reptiles. | Birds. | Mammals. | | +---------+----------+-----------+----------+----------+ | Sandwich. | 400 | ? | 2 | 16 | 0 | | Galapagos | 15 | 35 | 10 | 30 | 0 | | St. Helena | 20 | 128 | 0 | 1 | 0 | +------------+---------+----------+-----------+----------+----------+ | Totals. | 435 | 163 | 12 | 47 | 0 | +------------+---------+----------+-----------+----------+----------+ NON-PECULIAR SPECIES. +------------+---------+----------+-----------+----------+----------+ | | Shells. | Insects. | Reptiles. | Birds. | Mammals. | | +---------+----------+-----------+----------+----------+ | Sandwich. | 0 | ? | 0 | 0 | 0 | | Galapagos | ? | ? | 0 | 1 | 0 | | St. Helena | 0 | ? | 0 | 0 | 0 | +------------+---------+----------+-----------+----------+----------+ | Totals. | 0 | ? | 0 | 1 | 0 | +------------+---------+----------+-----------+----------+----------+ From this synopsis we perceive that out of a total of 658 species ofterrestrial animals known to inhabit these three oceanic territories, all are peculiar, with the exception of a single land-bird which isfound in the Galapagos Islands. This is the rice-bird, so very abundanton the American continent that its representatives must not unfrequentlybecome the involuntary colonists of the Archipelago. There are, however, a few species of non-peculiar insects inhabiting the Sandwich andGalapagos Islands, the exact number of which is doubtful, and on thisaccount are not here quoted. But at most they would be represented byunits, and therefore do not affect the general result. Lastly, theremarkable fact will be noted, that there is no single representative ofthe mammalian class in any of these islands. If we turn next to consider the case of plants, we obtain the followingresult:-- +-----------+-----------+--------------+ | |_Peculiar |_Non-peculiar | | | Species. _ | Species. _ | +-----------|-----------|--------------+ |Sandwich | 377 | 243 | |Galapagos | 174 | 158 | |St. Helena | 50 | 26 | | | ---- | ---- | | Totals | 601 | 427 | +-----------+-----------+--------------+ So that by adding together peculiar species both of land-animals andplants, we find that on these three limited areas alone there are 1258forms of life which occur nowhere else upon the globe--not to speak ofthe peculiar aquatic species, nor of the presumably large number ofpeculiar species of all kinds not hitherto discovered in theseimperfectly explored regions. Now let us compare these facts with those which are presented by thefaunas and floras of islands less remote from continents, and knownfrom independent geological evidence to be of comparatively recentorigin--that is, to have been separated from their adjacent mainlands incomparatively recent times, and therefore as islands to be comparativelyyoung. The British Isles furnish as good an instance as could be chosen, for they together comprise over 1000 islands of various sizes, which arenowhere separated from one another by deep seas, and in the opinion ofgeologists were all continuous with the European continent since theglacial period. BRITISH ISLES. NON-PECULIAR SPECIES. +---------+---------+----------+-----------+---------+---------+ | | Land | | Reptiles | Land | Land | | Plants. | Shells. | Insects. | and | Birds. | Mammals. | | | | | Amphibia. | | | +---------+---------+----------+-----------+---------+---------+ | 1462 | 83 | 12, 551 | 13 | 130 | 40 | +---------+---------+----------+-----------+---------+---------+ PECULIAR SPECIES. +---------+---------+----------+-----------+---------+---------+ | | Land | | Reptiles | Land | Land | | Plants. | Shells. | Insects. | and | Birds. | Mammals. | | | | | Amphibia. | | | +---------+---------+----------+-----------+---------+---------+ | 46 | 4 | 149 | 0 | 1 | 0 | +---------+---------+----------+-----------+---------+---------+ Total Peculiar Plants 46 Total Peculiar Animals 154 ---- Grand Total 200 I have drawn up this table in the most liberal manner possible, including as peculiar species forms which many naturalists regard asmerely local varieties. But, even as thus interpreted, how wonderful isthe contrast between the 1000 islands of Great Britain and the singlevolcanic rock of St. Helena, where almost all the animals and about halfthe plants are peculiar, instead of about 1/80 of the animals, and 1/30of the plants. Of course, if no peculiar species of any kind hadoccurred in the British Isles, advocates of special creation might haveargued that it was, so to speak, needless for the Divinity to have addedany new species to those European forms which fully populated theislands at the time when they were separated from the continent. But, asthe matter stands, advocates of special creation must face the fact thata certain small number of new and peculiar species have been formed onthe British Isles; and, therefore, that creative activity has not beenwholly suspended in their case. Why, then, has it been so meagre in thiscase of a thousand islands, when it has proved so profuse in the case ofall single islands more remote from mainlands, and presenting a higherantiquity? Or why should the Divinity have thus appeared so uniformly toconsult these merely accidental circumstances of space and time in thedepositing of his unique specific types? Do not such facts rather speakwith irresistible force in favour of the view, that while all ancientand solitary islands have had time enough, and separation enough, toadmit of distinct histories of evolution having been written in theirliving inhabitants, no one of the thousand islands of Great Britain hashad either time enough, or separation enough, to have admitted of morethan some of the first pages of such a history having been commenced? But this allusion to Great Britain introduces us to another point. Itwill have been observed that, unlike oceanic islands remote frommainlands, Great Britain is well furnished both with reptiles (includingamphibia) and mammals. For there is no instance of any oceanic islandsituated at more than 300 miles from a continent where any singlespecies of the whole class of mammals is to be found, excepting speciesof the only order which is able to fly--namely, the bats. And the samehas to be said of frogs, toads, and newts, whose spawn is quickly killedby contact with sea-water, and therefore could never have reached remoteislands in a living state. Hence, on evolutionary principles; it isquite intelligible why oceanic islands should not present any species ofmammals or batrachians--peculiar or otherwise, --save such species ofmammals as are able to fly. But on the theory of special creation we canassign no reason why, notwithstanding the extraordinary profusion ofunique types of other kinds which we have seen to occur on oceanicislands, the Deity should have made this curious exception to thedetriment of all frogs, toads, newts, and mammals, save only such as areable to fly. Or, if any one should go so far to save a desperatehypothesis as to maintain that there must have been some hidden reasonwhy batrachians and quadrupeds were not specially created on oceanicislands, I may mention another small--but in this relation a mostsignificant--fact. This is that on some of these islands there occurcertain peculiar species of plants, the seeds of which are provided withnumerous tiny hooks, obviously and beautifully adapted--like those onthe seeds of allied plants elsewhere--to catch the wool or hair ofmoving quadrupeds, and so to further their own dissemination. But, as wehave just seen, there are no quadrupeds in the islands to meet thesebeautiful adaptations on the part of the plants; so that specialcreationists must resort to the almost impious supposition that in thesecases the Deity has only carried out half his plan, in that while hemade an elaborate provision for these uniquely created species ofplants, which depended for its efficiency on the presence of quadrupeds, he nevertheless neglected to place any quadrupeds on the islands wherehe had placed the plants. Such one-sided attempts at adaptation surelyresolve the thesis of special creation to a _reductio ad absurdum_; andhence the only reasonable interpretation of them is, that while theseeds of allied or ancestral plants were able to float to the islands, no quadrupeds were ever able over so great a distance to swim. * * * * * Although much more evidence might still be given under the head ofgeographical distribution, I must now close with a brief summary of themain points that have been adduced. After certain preliminary considerations, I began by noticing that thetheory of evolution has a much more intelligible account to give thanhas its rival of the facts of discontinuous distribution--the Alpineflora, for instance, being allied to the Arctic, not because the samespecies were separately created in both places, but because during theglacial period these species extended all over Europe, and were leftbehind on the Alps as the Arctic flora receded northwards--which wassufficiently long ago to explain why some of the Alpine species areunique, though closely allied to Arctic forms. Next we saw that, although living things are always adapted to theclimates under which they live (since otherwise they could not livethere at all), it is equally true that, as a rule, besides the area onwhich they do live, there are many other areas in different parts of theglobe where they might have lived equally well. Consequently we mustconclude that, if all species were separately created, many species wereseverally created on only one among a number of areas where they mightequally well have thrived. Now, although this conclusion in itself maynot seem opposed to the theory of special creation, a most seriousdifficulty is raised when it is taken in connexion with another fact ofan equally general kind. This is, that on every biological region weencounter chains of allied species constituting allied genera, families, and so on; while we scarcely ever meet with allied species in differentbiological regions, notwithstanding that their climates may be similar, and, consequently, just as well suited to maintain some of the alliedspecies. Hence we must further conclude, if all species were separatelycreated, that in the work of creation some unaccountable regard was paidto making areas of distribution correspond to degrees of structuralaffinity. A great many species of the rat genus were created in the OldWorld, and a great many species of another, though allied, genus werecreated in the New World: yet no reason can be assigned why no onespecies of the Old World series should not just as well have beendeposited in the New World, and _vice versa_. On the other hand, thetheory of evolution may claim as direct evidence in its support all theinnumerable cases such as these--cases, indeed, so innumerable that, asMr. Wallace remarks, it may be taken as a law of nature that "everyspecies has come into existence coincident both in space and time with apre-existing and closely allied species. " A general law which, while initself most strongly suggestive of evolution, is surely impossible toreconcile with any reasonable theory of special creation. Furthermore, this law extends backwards through all geological time, with the resultthat the extinct species which now occur only as fossils on any givengeological area, resemble the species still living upon that area, as weshould expect that they must, if the former were the natural progenitorsof the latter. On the other hand, if they were not the naturalprogenitors, but all the species, both living and extinct, were thesupernatural and therefore independent creations which the rival theorywould suppose, then no reason can be given why the extinct speciesshould thus resemble the living--any more than why the living speciesshould resemble one another. For, as we have seen, there are almostalways many other habitats on other parts of the globe, where anymembers of any given group of species might equally well have beendeposited; and this, of course, applies to geological no less than tohistorical time. Yet throughout all time we meet with this mostsuggestive correlation between continuity of a geographical area andstructural affinity between the forms of life which have lived, or arestill living, upon that area. Similarly, we find the further, and no less suggestive, correlationbetween the birth of new species and the immediate pre-existence ofclosely allied species on the same area--or, at most, on closelycontiguous areas. Where a continuous area has long been circumscribed by barriers of anykind, which prevent the animals from wandering beyond it, then we findthat all the species, both extinct and living, constitute more or less aworld of their own; while, on the other hand, where the animals are freeto migrate from one area to another, the course of their migrations ismarked by the origination of new species springing up _en route_, andserving to connect the older, or metropolitan, forms with the younger, or colonising, forms in the way of a graduated series. This principle, however, admits of being traced only in certain cases of speciesbelonging to the same genus, of genera belonging to the same family, or, at most, of families belonging to the same order. In other words, themore general the structural affinity, the more general is thegeographical extension--as we should expect to be the case on the theoryof descent with branching modifications, seeing that the larger, theolder, and the more diverse the group of organisms compared, the greatermust be their chances of dispersal. These general considerations led us to contemplate more in detail thecorrelation between structural affinity and barriers to free migration. Such barriers, of course, differ in the cases of different organisms. Marine organisms are stopped by land, unsuitable temperature, orunsuitable depths; fresh-water organisms by sea and by mountain-chains;terrestrial organisms chiefly by water. Now it is a matter of factwhich admits of no dispute, that in each of these cases we meet with adirect correlation between the kind of barrier and the kind of organismswhose structural affinities are affected thereby. Where we have to dowith marine organisms, barriers such as the Isthmus of Panama and thevarying depth of the Western Pacific determine three very distinctfaunas, ranging north and south in closely parallel lines, and undercorresponding climates. Where we have to do with fresh-water organisms, we find that a mountain-chain only a few miles wide has more influencein determining differences of organic type on either side of it than isexercised by even thousands of miles of a continuous land-area, if thisbe uninterrupted by any mountains high enough to prevent water-fowl, whirlwinds, &c. , from dispersing the ova. Again, where we have to dowith terrestrial organisms, the most effectual barriers are wide reachesof ocean; and, accordingly, we find that these exercise an enormousinfluence on the modification of terrestrial types. Moreover, we findthat the _more_ terrestrial an organism, or the _greater_ the difficultyit has in traversing a wide reach of ocean, the _greater_ is themodifying influence of such a barrier upon that type. In oceanicislands, for example, many of the plants and aquatic birds usuallybelong to the same species as those which occur on the nearestmainlands, and where there are any specific differences, these butrarely run up to generic differences. But the land-birds, insects, andreptiles which are found on such islands are nearly always specifically, and very often generically, distinct from those on the nearestmainland--although invariably allied with sufficient closeness to leaveno manner of doubt as to their affinities with the fauna of thatmainland. Lastly, no amphibians and no mammals (except bats) are everfound on any oceanic islands. Yet, as we have seen, on the theory ofspecial creation, these islands must all be taken to have been thetheatres of the most extraordinary creative activity, so that on onlythree of them we found no less than 1258 unique species, whereof 657were unique species of land animals, to be set against one singlespecies known to occur elsewhere. Nevertheless, notwithstanding thisprodigious expenditure of creative energy in the case of land-birds, land-shells, insects, and reptiles, no single new amphibian, or nosingle new mammal, has been created on any single oceanic island, if weexcept the only kind of mammal that is able to fly, and the ancestors ofwhich, like those of the land-birds and insects, might therefore havereached the islands ages ago. Moreover, with regard to mammals, even incases where allied forms occur on either side of a sea-channel, it isfound to be a general rule that if the channel is shallow, the specieson either side of it are much more closely related than if it bedeep--and this irrespective of its width. Therefore we can onlyconclude, in the words of Darwin--"As the amount of modification whichanimals of all kinds undergo partly depends on lapse of time, and as theislands which are separated from each other or from the mainland byshallow channels are more likely to have been continuously united withina recent period than islands separated by deeper channels, we canunderstand how it is that a relation exists between the depth of the seaseparating two mammalian faunas, and the degree of their affinity--arelation which is quite inexplicable on the theory of independent actsof creation. " * * * * * Looking to all these general principles of geographical distribution, and remembering the sundry points of smaller detail relating to oceanicislands which I will not wait to recapitulate, to my mind it seems thatthere is no escape from the following conclusion, with which I willbring my brief epitome of the evidence to a close. The conclusion towhich, I submit, all the evidence leads is, that if the doctrine ofspecial creation is taken to be true, then it must be further taken thatthe one and only principle which has been consistently followed in thegeographical deposition of species, is that of so depositing them as tomake it everywhere appear that they were not thus deposited at all, butcame into existence where they now occur by way of genetic descent withperpetual migration and correlative modification. On no other principle, so far as I can see, would it be possible to account for the fact that"every species has come into existence coincident both in space and timewith a pre-existing and closely allied species, " together with thecarefully graduated regard to physical barriers which the Creator musthave displayed while depositing his newly formed species on either sidesof them--everywhere making _degrees_ of structural affinity correspondto _degrees_ of geographical continuity, and _degrees_ of structuraldifference correspond to _degrees_ of geographical separation, whetherby mountain-chains in the case of fresh-water faunas, by land and bydeep sea in the case of marine faunas, or by reaches of ocean in thecase of terrestrial faunas--stocking oceanic islands with an enormousprofusion of peculiar species all allied to those on the nearestmainlands, yet everywhere avoiding the creation upon them of anyamphibian or mammal, except an occasional bat. We are familiar with thedoctrine that God is a God who hideth himself; here, however, it seemsto me, we should have but a thinly-veiled insinuation, not merely thatin his works he is hidden, but that in these works he is untrue. Thanwhich I cannot conceive a stronger condemnation of the theory which ithas been my object fairly to represent and dispassionately tocriticise. SECTION II _SELECTION_ CHAPTER VII. THE THEORY OF NATURAL SELECTION. Thus far we have been considering the main evidences of organicevolution considered as a fact. We now enter a new field, namely, theevidences which thus far have been brought to light touching the causesof organic evolution considered as a process. As was pointed out in the opening chapter, this is obviously themethodical course to follow: we must have some reasonable assurance thata fact is a fact before we endeavour to explain it. Nevertheless, it isnot necessary that we should actually demonstrate a fact to be a factbefore we endeavour to explain it. Even if we have but a reasonablepresumption as to its probability, we may find it well worth while toconsider its explanation; for by so doing we may obtain additionalevidence of the fact itself. And this because, if it really is a fact, and if we hit upon the right explanation of it, by proving theexplanation probable, we may thereby greatly increase our evidence ofthe fact. In the very case before us, for example, the evidence ofevolution as a fact has from the first been largely derived from testingDarwin's theory concerning its method. It was this theoreticalexplanation of its method which first set him seriously to enquire intothe evidences of evolution as a fact; and ever since he published hisresults, the evidences which he adduced in favour of natural selectionas a method have constituted some of the strongest reasons whichscientific men have felt for accepting evolution as a fact. Of coursethe evidence in favour of this fact has gone on steadily growing, quiteindependently of the assistance which was thus so largely lent to it bythe distinctively Darwinian theory of its method; and, indeed, so muchhas this been the case, that in the present treatise we have been ableto consider such direct evidence of the fact itself, without anyreference at all to the indirect or accessory evidence which is derivedfrom that of natural selection as a method. From which it follows thatin most of what I am about to say in subsequent chapters on theevidences of natural selection as a method, there will be furnished alarge addition to the evidences which have already been detailed ofevolution as a fact. But, as a matter of systematic treatment, I havethought it desirable to keep these two branches of our subject separate. Which means that I have made the evidences of evolution as a fact tostand independently on their own feet--feet which in my opinion areamply strong enough to bear any weight of adverse criticism that can beplaced upon them. Our position, then, is this. On the foundation of the previous chapters, I will henceforth assume that we all accept organic evolution as a fact, without requiring any of the accessory evidence which is gained byindependent proof of natural selection as a method. But in making thisassumption--namely, that we are all now firmly persuaded of the fact ofevolution--I do not imagine that such is really the case. I make theassumption for the purposes of systematic exposition, and in order thatdifferent parts of the subject may be kept distinct. I confess it doesappear to me remarkable that there should still be a doubt in anyeducated mind touching the general fact of evolution; while it becomesto me unaccountable that such should be the case with a few still livingmen of science, who cannot be accused of being ignorant of the evidenceswhich have now been accumulated. But in whatever measure we mayseverally have been convinced--or remained unconvinced--on this matter, for the purposes of exposition I must hereafter assume that we are allagreed to the extent of regarding the process of evolution as, at least, sufficiently probable to justify enquiry touching its causes onsupposition of its truth. Now, the causes of evolution have been set forth in a variety ofdifferent hypotheses, only the chief of which need be mentioned here. Historically speaking the first of these was that which was put forwardby Erasmus Darwin, Lamarck, and Herbert Spencer. It consists in puttingtogether the following facts and inferences. We know that, in the lifetime of the individual, increased use ofstructures leads to an increase of their functional efficiency; while, on the other hand, disuse leads to atrophy. The arms of a blacksmith, and the legs of a mountaineer, are familiar illustrations of the firstprinciple: our hospital wards are full of illustrations of the second. Again, we know that the characters of parents are transmitted to theirprogeny by means of heredity. Now the hypothesis in question consistsin supposing that if any particular organs in a species are habituallyused for performing any particular action, they must undergo astructural improvement which would more and more adapt them to theperformance of that action; for in each generation constant use wouldbetter and better adapt the structures to the discharge of theirfunctions, and they would then be bequeathed to the next generation inthis their improved form by heredity. So that, for instance, if therehad been a thousand generations of blacksmiths, we might expect the sonsof the last of them to inherit unusually strong arms, even if theseyoung men had themselves taken to some other trade not requiring anyspecial use of their arms. Similarly, if there had been a thousandgenerations of men who used their arms but slightly, we should expecttheir descendants to show but a puny development of the upperextremities. Now let us apply all this to the animal kingdom in general. The giraffe, for instance, is a ruminant whose entire frame has beenadapted to support an enormously long neck, which is of use to theanimal in reaching the foliage of trees. The ancestors of the giraffe, having had ordinary necks, were supposed by Lamarck to have graduallyincreased the length of them, through many successive generations, byconstantly stretching to reach high foliage; and he further supposedthat, when the neck became so long as to require for its support specialchanges in the general form of the animal as a whole, these specialchanges would have brought about the dwindling of other parts from whichso much activity was no longer required--the general result being thatthe whole organization of the animal became more and more adapted tobrowsing on high foliage. And so in the cases of other animals, Lamarckbelieved that the adaptation of their forms to their habits could beexplained by this simple hypothesis that the habits created the forms, through the effects of use and disuse, coupled with heredity. Such is what is ordinarily known as Lamarck's theory of evolution. Wemay as well remember, however, that it really constitutes only one partof his theory; for besides this hypothesis of the cumulative inheritanceof functionally-produced modifications--to which we may add theinherited effects of any direct action exercised by surroundingconditions of life, --Lamarck believed in some transcendental principletending to produce gradual improvement in pre-determined lines ofadvance. Therefore it would really be more correct to designate theformer hypothesis by the name either of Erasmus Darwin, or, stillbetter, of Herbert Spencer. Nevertheless, in order to avoid confusion, Iwill follow established custom, and subsequently speak of thishypothesis as the Lamarckian hypothesis--understanding, however, that inemploying this designation I am not referring to any part or factor ofLamarck's general theory of evolution other than the one which has justbeen described--namely, the hypothesis of the cumulative transmission offunctionally-produced, or otherwise "acquired, " modifications. This, then, was the earliest hypothesis touching the causes of organicevolution. But we may at once perceive that it is insufficient toexplain all that stands to be explained. In the first place, it refersin chief part only to the higher animals, which are actuated to effortby intelligence. Its explanatory power in the case of mostinvertebrata--as well as in that of all plants--is extremely limited, inasmuch as these organisms can never be moved to a greater or less useof their several parts by any discriminating volition, such as thatwhich leads to the continued straining of a giraffe's neck for thepurpose of reaching foliage. In the second place, even among the higheranimals there are numberless tissues and organs which unquestionablypresent a high degree of adaptive evolution, but which neverthelesscannot be supposed to have fallen within the influence of Lamarckianprinciples. Of such are the shells of crustacea, tortoises, &c. , whichalthough undoubtedly of great use to the animals presenting them, cannotever have been _used_ in the sense required by Lamarck's hypothesis, i. E. Actively exercised, so as to increase a flow of nutrition to thepart. Lastly, in the third place, the validity of Lamarck's hypothesisin any case whatsoever has of late years become a matter of seriousquestion, as will be fully shown and discussed in the next volume. Meanwhile it is enough to observe that, on account of all these reasons, the theory of Lamarck, even if it be supposed to present any truth atall, is clearly insufficient as a full or complete theory of organicevolution. * * * * * In historical order the next theory that was arrived at was the theoryof natural selection, simultaneously published by Darwin and Wallace onJuly 1st, 1858. If we may estimate the importance of an idea by the change of thoughtwhich it effects, this idea of natural selection is unquestionably themost important idea that has ever been conceived by the mind of man. Yet the wonder is that it should not have been hit upon long before. Orrather, I should say, the wonder is that its immense and immeasurableimportance should not have been previously recognised. For, since thepublication of this idea by Darwin and Wallace, it has been found thatits main features had already occurred to at least two otherminds--namely, Dr. Wells in 1813, and Mr. Patrick Matthew in 1831. Butneither of these writers perceived that in the few scattered sentenceswhich they had written upon the subject they had struck the key-note oforganic nature, and resolved one of the principal chords of theuniverse. Still more remarkable is the fact that Mr. HerbertSpencer--notwithstanding his great powers of abstract thought and hisgreat devotion of those powers to the theory of evolution, when as yetthis theory was scorned by science--still more remarkable, I say, is thefact that Mr. Herbert Spencer should have missed what now appears soobvious an idea. But most remarkable of all is the fact that Dr. Whewell, with all his stores of information on the history of theinductive sciences, and with all his acumen on the matter of scientificmethod, should not only have conceived the idea of natural selection, but expressly stated it as a logically possible explanation of theorigin of species, and yet have so stated it merely for the purpose ofdismissing it with contempt[26]. This, I think, is most remarkable, because it serves to prove how very far men's minds at that time musthave been from entertaining, as in any way antecedently probable, thedoctrine of transmutation. In order to show this I will here quote onepassage from the writings of Whewell, and another from a distinguishedFrench naturalist referred to by him. [26] For quotations, see Note A. In 1846 Whewell wrote:-- Not only is the doctrine of the transmutation of species in itself disproved by the best physiological reasonings, but the additional assumptions which are requisite to enable its advocates to apply it to the explanation of the geological and other phenomena of the earth, are altogether gratuitous and fantastical[27]. [27] whewell, _indications of the creator_, 2nd ed. , 1846. Then he quotes with approval the following opinion:-- Against this hypothesis, which, up to the present time, I regard as purely gratuitous, and likely to turn geologists out of the sound and excellent road in which they now are, I willingly raise my voice, with the most absolute conviction of being in the right[28]. [28] de blainville, _compte rendu_, 1837. And, after displaying the proof rendered by Lyell of uniformitarianismin geology, and cordially subscribing thereto, Whewell adds:-- We are led by our reasonings to this view, that the present order of things was commenced by an act of creative power entirely different to any agency which has been exerted since. None of the influences which have modified the present races of animals and plants since they were placed in their habitations on the earth's surface can have had any efficacy in producing them at first. We are necessarily driven to assume, as the beginning of the present cycle of organic nature, an event not included in the course of nature[29]. [29] Whewell, _ibid. _, p. 162. So much, then, for the state of the most enlightened and representativeopinions on the question of evolution before the publication ofDarwin's work; and so much, likewise, for the only reasonablesuggestions as to the causes of evolution which up to that time had beenput forward, even by those few individuals who entertained any belief inevolution as a fact. It was the theory of natural selection that changedall this, and created a revolution in the thought of our time, themagnitude of which in many of its far-reaching consequences we are noteven yet in a position to appreciate; but the action of which hasalready wrought a transformation in general philosophy, as well as inthe more special science of biology, that is without a parallel in thehistory of mankind. * * * * * Although every one is now more or less well acquainted with the theoryof natural selection, it is necessary, for the sake of completeness, that I should state the theory; and I will do so in full detail. It is a matter of observable fact that all plants and animals areperpetually engaged in what Darwin calls a "struggle for existence. "That is to say, in every generation of every species a great many moreindividuals are born than can possibly survive; so that there is inconsequence a perpetual battle for life going on among all theconstituent individuals of any given generation. Now, in this strugglefor existence, which individuals will be victorious and live? Assuredlythose which are best fitted to live, in whatever respect, or respects, their superiority of fitness may consist. Hence it follows that Nature, so to speak, _selects_ the best individuals out of each generation tolive. And not only so; but as these favoured individuals transmit theirfavourable qualities to their offspring, according to the fixed laws ofheredity, it further follows that the individuals composing eachsuccessive generation have a general tendency to be better suited totheir surroundings than were their forefathers. And this follows, notmerely because in every generation it is only the "flower of the flock"that is allowed to breed, but also because, if in any generation somenew and beneficial qualities happen to arise as slight variations fromthe ancestral type, they will (other things permitting) be seized uponby natural selection, and, being transmitted by heredity to subsequentgenerations, will be added to the previously existing type. Thus thebest idea of the whole process will be gained by comparing it with theclosely analogous process whereby gardeners, fanciers, andcattle-breeders create their wonderful productions; for just as thesemen, by always "_selecting_" their best individuals to breed from, slowly but continuously improve their stock, so Nature, by a similarprocess of "_selection_" slowly but continuously makes the variousspecies of plants and animals better and better suited to the conditionsof their life. Now, if this process of continuously adapting organisms to theirenvironment takes place in nature at all, there is no reason why weshould set any limits on the extent to which it is able to go, up to thepoint at which a complete and perfect adaptation is achieved. Thereforewe might suppose that all species would eventually reach this conditionof perfect harmony with their environment, and then remain fixed. Andso, according to the theory, they would, if the environment were itselfunchanging. But forasmuch as the environment (i. E. The sum total of theexternal conditions of life) of almost every organic type alters moreor less from century to century--whether from astronomical, geological, and geographical changes, or from the immigrations and emigrations ofother species living on contiguous areas, and so on--it follows that theprocess of natural selection need never reach a terminal phase. Andforasmuch as natural selection may thus continue, _ad infinitum_, slowlyto alter a specific type in adaptation to a gradually changingenvironment, if in any case the alteration thus effected is sufficientin amount to lead naturalists to name the result as a distinct species, it follows that natural selection has transmuted one specific type intoanother. Similarly, by a continuation of the process, specific typeswould become transmuted into generic, generic into family types, and soon. Thus the process is supposed to go on throughout all the countlessforms of life continuously and simultaneously--the world of organictypes being thus regarded as in a state of perpetual, though gradual, flux. * * * * * Now, the first thing we have to notice about this theory is, that in allits main elements it is merely a statement of observable facts. It is anobservable fact that in all species of plants and animals a very muchlarger number of individuals are born than can possibly survive. Thus, for example, it has been calculated that if the progeny of a single pairof elephants--which are the slowest breeding of animals--were allallowed to reach maturity and propagate, in 750 years there would beliving 19, 000, 000 descendants. Again, in the case of vegetables, if aspecies of annual plant produces only two seeds a year, if these insuccessive years were all allowed to reproduce their kind, in twentyyears there would be 11, 000, 000 plants from a single ancestor. Yet weknow that nearly all animals and plants produce many more young at atime than in either of these two supposed cases. Indeed, as individualsof many kinds of plants, and not a few kinds of animals, produce everyyear several thousand young, we may make a rough estimate and say, thatover organic nature as a whole probably not one in a thousand young areallowed to survive to the age of reproduction. How tremendous, therefore, must be the struggle for existence! It is thought a terriblething in battle when one half the whole number of combatants perish. Butwhat are we to think of a battle for life where only one in a thousandsurvives? This, then, is the first fact. The second is the fact so long agorecognised, that the battle is to the strong, the race to the swift. Thethousandth individual which does survive in the battle forexistence--which does win the race for life--is, without question, oneof the individuals best fitted to do so; that is to say, best fitted tothe conditions of its existence considered as a whole. Nature is, therefore, always picking out, or selecting, such individuals to liveand to breed. The third fact is, that the individuals so selected transmit theirfavourable qualities to their offspring by heredity. There is no doubtabout this fact, so far as we are concerned with it. For although, as Ihave already hinted, considerable doubt has of late years been cast uponLamarck's doctrine of the hereditary transmission of _acquired_characters, it remains as impossible as ever it was to question thehereditary transmission of what are called _congenital_ characters. Andthis is all that Darwin's theory necessarily requires. The fourth fact is, that although heredity as a whole produces awonderfully exact copy of the parent in the child, there is never aprecise reduplication. Of all the millions of human beings upon the faceof the earth, no one is so like another that we cannot see somedifference; the resemblance is everywhere specific, nowhere individual. Now this same remark applies to all specific types. The only reason whywe notice individual differences in the case of the human type more thanwe do in the case of any other types, is because our attention is heremore incessantly focussed upon these differences. We are compelled tonotice them in the case of our own species, however small they mayappear to a naturalist, because, unless we do so, we should notrecognise the members of our own family, or be able to distinguishbetween a man whom we know is ready to do us an important service, andanother man whom we know is ready to cut our throats. But our commonmother Nature is able thus to distinguish between all her children. Hereyes are much more ready to detect small individual peculiarities thanare the eyes of any naturalist. No slight variations in the cast offeature or disposition of parts, no minute difference in the arrangementof microscopical cells, can escape her ever vigilant attention. And, consequently, when among all the innumerable multitudes of individualvariations any one arises which--no matter in how slight a degree--givesto that individual a better chance of success in the struggle for life, Nature chooses that individual to survive, and so to perpetuate theimprovement in his or her progeny. Now I say that all these several component parts of Darwinian doctrineare not matters of theory, but matters of fact. The only element oftheory in his doctrine of evolution by natural selection has referenceto the degree in which these observable facts, when thus broughttogether, are adequate to account for the process of evolution. * * * * * So much, then, as a statement of the theory of natural selection. Butfrom this statement--i. E. From the theory of natural selectionitself--there follow certain matters of general principle which it isimportant to bear in mind. These, therefore, I shall here proceed tomention. First of all, it is evident that the theory is applicable as anexplanation of organic changes in specific types only in so far as thesechanges are of _use_, or so far as such changes endow the species withbetter chances of success in the general struggle for existence. This isthe only sense in which I shall always employ the terms use, utility, service, benefit, and so forth--that is to say, in the sense oflife-preserving. * * * * * Next, it must be clearly understood that the life which it is theobject, so to speak, of natural selection to preserve, is primarily thelife of the _species_; not that of the _individual_. Natural selectionpreserves the life of the individual only in so far as this is conduciveto that of the species. Wherever the life-interests of the individualclash with those of the species, that individual is sacrificed in favourof others who happen better to subserve the interests of the species. For example, in all organisms a greater or less amount of vigour iswasted, so far as individual interests are concerned, in the formationand the nourishment of progeny. In the great majority of plants andanimals an enormous amount of physiological energy is thus expended. Look at the roe or the milt of a herring, for instance, and see what ahuge drain has been made upon the individual for the sake of itsspecies. Again, all unselfish instincts have been developed for the sakeof the species, and usually against the interests of the individual. Anant which will allow her head to be slowly drawn from her body ratherthan relinquish her hold upon a pupa, is clearly acting in response toan instinct which has been developed for the benefit of the hive, thoughfatal to the individual. And, in a lesser degree, the parentalinstincts, wherever they occur, are more or less detrimental to theinterests of the individual, though correspondingly essential to thoseof the race. These illustrations will serve to show that natural selection alwaysworks primarily for the life-interests of the species--and, indeed, onlyworks for those of the individual at all in so far as the latter happento coincide with the former. Or, otherwise stated, the object of naturalselection is always that of producing and maintaining specific types inthe highest degree of efficiency, no matter what may become of theconstituent individuals. Which is a striking republication by Science ofa general truth previously stated by Poetry:-- So careful of the type she seems, So careless of the single life. Tennyson thus noted the fact, and a few years later Darwin supplied theexplanation. But of course in many, if not in the majority of cases, anything thatadds to the life-sustaining power of the single life thereby ministersalso to the life-sustaining power of the type; and thus we canunderstand why all mechanisms and instincts which minister to the singlelife have been developed--namely, because the life of the species ismade up of the lives of all its constituent individuals. It is onlywhere the interests of the one clash with those of the other thatnatural selection works against the individual. So long as the interestsare coincident, it works in favour of both. Natural selection, then, is a theory which seeks to explain by naturalcauses the occurrence of every kind of adaptation which is to be metwith in organic nature, on the assumption that adaptations of every kindhave primary reference to the preservation of species, and thereforealso, as a general rule, to the preservation of their constituentindividuals. And from this it follows that where it is for the benefitof a species to change its type, natural selection will effect thatchange, thus leading to a specific transmutation, or the evolution of anew species. In such cases the old species may or may not becomeextinct. If the transmutation affects the species as a whole, orthroughout its entire range, of course _that_ particular type becomesextinct, although it does so by becoming changed into a still moresuitable type in the course of successive generations. If, on the otherhand, the transmutation affects only a part of the original species, ornot throughout its entire range, then the other parts of that speciesmay survive for any number of ages as they originally were. In the onecase there is a ladder-like transmutation of species in time; in theother case a possibly tree-like multiplication of species in space. Butwhether the evolution of species be thus serial in time or divergent inspace, the object of natural selection, so to speak, is in either casethe same--namely, that of preserving all types which prove best suitedto the conditions of their existence. * * * * * Once more, the term "struggle for existence" must be understood tocomprehend, not only a competition for life among contemporaryindividuals of the same species, but likewise a struggle by all suchindividuals taken collectively for the continuance of their own specifictype. Thus, on the one hand, while there is a perpetual civil war beingwaged between members of the same species, on the other hand there is aforeign war being waged by the species as a whole against its world as awhole. Hence it follows that natural selection does not secure survivalof the fittest as regards individuals only, but also survival of thefittest as regards types. This is a most important point to remember, because, as a general rule, these two different causes produce exactlyopposite effects. Success in the civil war, where each is fightingagainst all, is determined by _individual_ fitness and _self-reliance_. But success in the foreign war is determined by what may be termed_tribal_ fitness and _mutual dependence_. For example, among socialinsects the struggle for existence is quite as great between differenttribes or communities, as it is between different individuals of thesame community; and thus we can understand the extraordinary degree inwhich not only co-operative instincts, but also largely intelligentsocial habits, have here been developed[30]. Similarly, in the case ofmankind, we can understand the still more extraordinary development ofthese things--culminating in the moral sense. I have heard a sermon, preached at one of the meetings of the British Association, entirelydevoted to arguing that the moral sense could not have been evolved bynatural selection, seeing that the altruism which this sense involves isthe very opposite of selfishness, which alone ought to have been theproduct of survival of the fittest in a struggle for life. And, ofcourse, this argument would have been perfectly sound had Darwin limitedthe struggle for existence to individuals, without extending it tocommunities. But if the preacher had ever read Darwin's works he wouldhave found that, when thus extended, the principle of natural selectionis bound to work in favour of the co-operative instincts in the case ofso highly social an animal as man; and that of these instinctsconscience is the highest imaginable exhibition. [30] For cases, see _Animal Intelligence_, in the chapters on Ants and Bees; and, for discussion of principles, _Mental Evolution in Animals_, in the chapters on Instinct. What I have called tribal fitness--in contradistinction to individualfitness--begins with the family, developes in the community (herd, hive, clan, &c. ), and usually ends with the limits of the species. On the onehand, however, it is but seldom that it extends so far as to embrace theentire species; while, on the other hand, it may in some cases, and asit were sporadically, extend beyond the species. In these latter casesmembers of different species mutually assist one another, whether in theway of what is called symbiosis, or in a variety of other ways which Ineed not wait to mention. For the only point which I now desire to makeclear is, that all cases of mutual aid or co-operation, whether withinor beyond the limits of species, are cases which fall under theexplanatory sweep of the Darwinian theory[31]. [31] Prince Kropotkin in the _Nineteenth Century_ (Feb. 1888, Apr. 1891) has adduced a large and interesting body of facts, showing the great prevalence of the principle of co-operation in organic nature. * * * * * Another important point to notice is, that it constitutes no part of thetheory of natural selection to suppose that survival of the fittest mustinvariably lead to _improvement_ of type, in the sense of superiororganization. On the contrary, if from change of habits or conditions oflife an organic type ceases to have any use for previously usefulorgans, natural selection will not only allow these organs in successivegenerations to deteriorate--by no longer placing any selective premiumupon their maintenance--but may even proceed to assist the agenciesengaged in their destruction. For, being now useless, they may becomeeven deleterious, by absorbing nutriment, causing weight, occupyingspace, &c. , without conferring any compensating benefit. Thus we canunderstand why it is that parasites, for example, present the phenomenaof what is called _degeneration_, i. E. Showing by their whole structurethat they have descended from a possibly very much higher type oforganization than that which they now exhibit. Having for innumerablegenerations ceased to require their legs, their eyes, and so forth, allsuch organs of high elaboration have either disappeared or becomevestigial, leaving the parasite as a more or less effete representativeof its ancestry. These facts of degeneration, as we have previously seen, are of verygeneral occurrence, and it is evident that their importance in the fieldof organic evolution as a whole has been very great. Moreover, it oughtto be particularly observed that, as just indicated, the facts may bedue either to a passive _cessation_ of selection, or to an active_reversal_ of it. Or, more correctly, these facts are probably _always_due to the cessation of selection, although in most cases where speciesin a state of nature are concerned, the process of degeneration has beenboth hastened and intensified by the super-added influence of thereversal of selection. In the next volume I shall have occasion to recurto this distinction, when it will be seen that it is one of no smallimportance to the general theory of descent. * * * * * We may now proceed to consider certain misconceptions of the Darwiniantheory which are largely, not to say generally, prevalent amongsupporters of the theory. These misconceptions, therefore, differ fromthose which fall to be considered in the next chapter, i. E. Misconceptions which constitute grounds of objection to the theory. * * * * * Of all the errors connected with the theory of natural selection, perhaps the one most frequently met with--especially among supporters ofthe theory--is that of employing the theory to explain all cases ofphyletic modification (or inherited change of type) indiscriminately, without waiting to consider whether in particular cases its applicationis so much as logically possible. The term "natural selection" thusbecomes a magic word, or Sesame, at the utterance of which every closeddoor is supposed to be immediately opened. Be it observed, I am not herealluding to that merely blind faith in natural selection, which of lateyears has begun dogmatically to force this principle as the sole causeof organic evolution in every case where it is _logically possible_ thatthe principle can have come into play. Such a blind faith, indeed, Ihold to be highly inimical, not only to the progress of biologicalscience, but even to the true interests of the natural selection theoryitself. As to this I shall have a good deal to say in the next volume. Here, however, the point is, that the theory in question is ofteninvoked in cases where it is not even logically possible that it canapply, and therefore in cases where its application betokens, not merelyan error of judgment or extravagance of dogmatism, but a fallacy ofreasoning in the nature of a logical contradiction. Almost any number ofexamples might be given; but one will suffice to illustrate what ismeant. And I choose it from the writings of one of the authors of theselection theory itself, in order to show how easy it is to be cheatedby this mere juggling with a phrase--for of course I do not doubt that amoment's thought would have shown the writer the untenability of hisstatement. In his most recent work Mr. Wallace advances an interesting hypothesisto the effect that differences of colour between allied species, whichare apparently too slight to serve any other purpose, may act as"recognition marks, " whereby the opposite sexes are enabled at once todistinguish between members of their own and of closely resemblingspecies. Of course this hypothesis can only apply to the higher animals;but the point here is that, supposing it to hold for them, Mr. Wallaceproceeds to argue thus:--Recognition marks "have in all probability beenacquired in the process of differentiation for the purpose of checkingthe intercrossing of allied forms, " because "one of the first needs of anew species would be to keep separate from its nearest allies, and thiscould be more readily done by some easily seen external mark[32]. "Now, it is clearly not so much as logically possible that theserecognition-marks (supposing them to be such) can have been acquiredby natural selection, "for the purpose of checking intercrossing ofallied forms. " For the theory of natural selection, from its ownessential nature as a theory, is logically exclusive of the suppositionthat survival of the fittest ever provides changes in anticipation offuture uses. Or, otherwise stated, it involves a contradiction of thetheory itself to say that the colour-changes in question were originatedby natural selection, in order to meet "one of the _first_ needs of a_new_ species, " or for the purpose of _subsequently_ preventingintercrossing with allied forms. If it had been said that thesecolour-differentiations were originated by some cause other than naturalselection (or, if by natural selection, still with regard to some_previous_, instead of _prophetic_, "purpose"), and, when so "acquired, "_then_ began to serve the "purpose" assigned, the argument would nothave involved the fallacy which we are now considering. But, as itstands, the argument reverts to the teleology of pre-Darwinian days--orthe hypothesis of a "purpose" in the literal sense which sees the endfrom the beginning, instead of a "purpose" in the metaphorical sense ofan adaptation that is evolved by the very modifications which subserveit[33]. [32] _Darwinism_, pp. 218 and 227. [33] Since the above was written Prof. Lloyd Morgan has published a closely similar notice of the passage in question. "This language, " he says, "seems to savour of teleology (that pitfall of the evolutionist). The cart is put before the horse. The recognition-marks were, I believe, not produced to prevent intercrossing, but intercrossing has been prevented because of preferential mating between individuals possessing special recognition-marks. To miss this point is to miss an important segregation-factor. "--(_Animal Life and Intelligence_, p. 103. ) Again, on pp. 184-9, he furnishes an excellent discussion on the whole subject of the fallacy alluded to in the text, and gives illustrative quotations from other prominent Darwinians. I should like to add that Darwin himself has nowhere fallen into this, or any of the other fallacies, which are mentioned in the text. * * * * * Another very prevalent, and more deliberate, fallacy connected with thetheory of natural selection is, _that it follows deductively from thetheory itself_ that the principle of natural selection must be the solemeans of modification in all cases where modification is of an_adaptive_ kind, --with the consequence that no other principle can everhave been concerned in the production of structures or instincts whichare of any use to their possessors. Whether or not natural selectionactually has been the sole means of adaptive modification in the race, as distinguished from the individual, is a question of biologicalfact[34]; but it involves a grave error of reasoning to suppose thatthis question can be answered deductively from the theory of naturalselection itself, as I shall show at some length in the next volume. [34] Of course adaptive modifications produced in the individual lifetime, and not _inherited_, do not concern the question at all. In this and the following paragraphs, therefore, "adaptations, " "adaptive modifications, " &c. , refer exclusively to such as are hereditary, i. E. Phyletic. * * * * * A still more extravagant, and a still more unaccountable fallacy is theone which represents it as following deductively from the theory ofnatural selection itself, that all _hereditary_ characters are"necessarily" due to natural selection. In other words, not only alladaptive, but likewise all non-adaptive hereditary characters, it issaid, _must_ be due to natural selection. For non-adaptive charactersare taken to be due to "correlation of growth, " in connexion with someof the adaptive ones--natural selection being thus the _indirect_ meansof producing the former _wherever_ they may occur, on account of itsbeing the _direct_ and the _only_ means of producing the latter. Thus itis deduced from the theory of natural selection itself, --1st, that theprinciple of natural selection is the only possible cause of adaptivemodification: 2nd, that non-adaptive modifications can only occur in therace as correlated appendages to the adaptive: 3rd, that, consequently, natural selection is the only possible cause of modification, whetheradaptive or non-adaptive. Here again, therefore, we must observe thatnone of these sweeping generalizations can possibly be justified bydeductive reasoning from the theory of natural selection itself. Anyattempt at such deductive reasoning must necessarily end in circularreasoning, as I shall likewise show in the second volume, where thiswhole "question of utility" will be thoroughly dealt with. * * * * * Once more, there is an important oversight very generally committed bythe followers of Darwin. For even those who avoid the fallacies abovementioned often fail to perceive, that natural selection can only beginto operate if the _degree_ of adaptation is already given assufficiently high to count for something in the struggle for_existence_. Any adaptations which fall below this level of importancecannot possibly have been produced by survival of the fittest. Yet thefollowers of Darwin habitually speak of adaptative characters, which _intheir own opinion_ are subservient merely to comfort or convenience, ashaving been produced by such means. Clearly this is illogical; for itbelongs to the essence of Darwin's theory to suppose, that naturalselection can have no jurisdiction beyond the line where structures orinstincts already present a sufficient degree of adaptational value toincrease, in some measure, the expectation of life on the part of theirpossessors. We cannot speak of adaptations as due to natural selection, without thereby affirming that they present what I have elsewhere termeda "selection value. " * * * * * Lastly, as a mere matter of logical definition, it is well-nighself-evident that the theory of natural selection is a theory of theorigin, and cumulative development, of _adaptations_, whether these bedistinctive of species, or of genera, orders, families, classes, andsub-kingdoms. It is only when the adaptations happen to be distinctiveof the first (or lowest) of these taxonomic divisions, that the theorywhich accounts for _these_ adaptations accounts also for the forms whichpresent them, --i. E. Becomes _also_ a theory of the origin of species. This, however, is clearly but an accident of particular cases; and, therefore, even in them the theory is _primarily_ a theory ofadaptations, while it is but secondarily a theory of the species whichpresent them. Or, otherwise stated, the theory is no more a theory ofthe origin of species than it is of the origin of genera, families, andthe rest; while, on the other hand, it is _everywhere_ a theory of theadaptive modifications whereby each of these taxonomic divisions hasbeen differentiated as such. Yet, sufficiently obvious as the accuracyof this definition must appear to any one who dispassionately considersit, several naturalists of high standing have denounced it in violentterms. I shall therefore have to recur to the subject at somewhatgreater length hereafter. At present it is enough merely to mention thematter, as furnishing another and a curious illustration of the notinfrequent weakness of logical perception on the part of minds wellgifted with the faculty of observation. It may be added, however, thatthe definition in question is in no way hostile to the one which isvirtually given by Darwin in the title of his great work. _The Origin ofSpecies by means of Natural Selection_ is beyond doubt the best titlethat could have been given, because at the time when the work waspublished the _fact_, no less than the _method_, of organic evolutionhad to be established; and hence the most important thing to be done atthat time was to prove the transmutation of species. But now that thishas been done to the satisfaction of naturalists in general, it is as Ihave said, curious to find some of them denouncing a wider definition ofthe principle of natural selection, merely because the narrower (orincluded) definition is invested with the charm of verbalassociations[35]. [35] The question as to whether natural selection has been the only principle concerned in the origination of species, is quite distinct from that as to the accuracy of the above definition. * * * * * So much for fallacies and misconceptions touching Darwin's theory, whichare but too frequently met with in the writings of its supporters. Wemust now pass on to mention some of the still greater fallacies andmisconceptions which are prevalent in the writings of its opponents. And, in order to do this thoroughly, I shall begin by devoting theremainder of the present chapter to a consideration of the antecedentstanding of the two theories of natural selection and supernaturaldesign. This having been done, in the succeeding chapters I shall dealwith the evidences for, and the objections against, the former theory. * * * * * Beginning, then, with the antecedent standing of these alternativetheories, the first thing to be noticed is, that they are both concernedwith the same subject-matter, which it is their common object toexplain. Moreover, this subject-matter is clearly and sharply divisibleinto two great classes of facts in organic nature--namely, those ofAdaptation and those of Beauty. Darwin's theory of descent explains theformer by his doctrine of natural selection, and the latter by hisdoctrine of sexual selection. In the first instance, therefore, I shallhave to deal only with the facts of adaptation, leaving for subsequentconsideration the facts of beauty. Innumerable cases of the adaptation of organisms to their surroundingsbeing the facts which now stand before us to be explained either bynatural selection or by supernatural intention, we may first consider astatement which is frequently met with--namely, that even if all suchcases of adaptation were proved to be fully explicable by the theory ofdescent, this would constitute no disproof of the theory of design: allthe cases of adaptation, it is argued, might still be due to design, even though they admit of being hypothetically accounted for by thetheory of descent. I have heard an eminent Professor tell his class thatthe many instances of mechanical adaptation discovered and described byDarwin as occurring in orchids, seemed to him to furnish better proof ofsupernatural contrivance than of natural causes; and another eminentProfessor has informed me that, although he had read the _Origin ofSpecies_ with care, he could see in it no evidence of natural selectionwhich might not equally well have been adduced in favour of intelligentdesign. But here we meet with a radical misconception of the wholelogical attitude of science. For, be it observed, this exception _inlimine_ to the evidence which we are about to consider does not questionthat natural selection _may_ be able to do all that Darwin ascribes toit. The objection is urged against his interpretation of the factsmerely on the ground that these facts might _equally well_ be ascribedto intelligent design. And so undoubtedly they might, if we were allsimple enough to adopt a supernatural explanation whenever a natural oneis found sufficient to account for the facts. Once admit the irrationalprinciple that we may assume the operation of higher causes where theoperation of lower ones is sufficient to explain the observed phenomena, and all our science and all our philosophy are scattered to the winds. For the law of logic which Sir William Hamilton called the law ofparsimony--or the law which forbids us to assume the operation of highercauses when lower ones are found sufficient to explain the observedeffects--this law constitutes the only barrier between science andsuperstition. It is always possible to give a hypothetical explanationof any phenomenon whatsoever, by referring it immediately to theintelligence of some supernatural agent; so that the only differencebetween the logic of science and the logic of superstition consists inscience recognising a validity in the law of parsimony whichsuperstition disregards. Therefore one can have no hesitation in sayingthat this way of looking at the evidence in favour of natural selectionis not a scientific or a reasonable way of looking at it, but a purelysuperstitious way. Let us take, as an illustration, a perfectly parallelcase. When Kepler was unable to explain by any known causes the pathsdescribed by the planets, he resorted to a supernatural explanation, andsupposed that every planet was guided in its movements by some presidingangel. But when Newton supplied a beautifully simple physicalexplanation, all persons with a scientific habit of mind at onceabandoned the metaphysical one. Now, to be consistent, theabove-mentioned Professors, and all who think with them, ought still toadhere to Kepler's hypothesis in preference to Newton's explanation;for, excepting the law of parsimony, there is certainly no otherlogical objection to the statement, that the movements of the planetsafford as good evidence of the influence of guiding angels as they do ofthe influence of gravitation. So much, then, for the illogical position that, granting the evidence infavour of natural descent and supernatural design to be equal andparallel, we should hesitate in our choice between the two theories. But, of course, if the evidence is supposed _not_ to be equal andparallel--i. E. If it is supposed that the theory of natural selectionis not so good a theory whereby to explain the facts of adaptation as isthat of supernatural design, --then the objection is no longer the onewhich we are considering. It is quite another objection, and one whichis not _prima facie_ absurd. Therefore let us state clearly the distinctquestion which thus arises. Innumerable cases of adaptation of organisms to their environments arethe observed facts for which an explanation is required. To supply thisexplanation, two, and only two, hypotheses are in the field. Of thesetwo hypotheses one is intelligent design manifested directly in specialcreation; the other is natural causation operating through countlessages of the past. Now, the adaptations in question involve aninnumerable multitude of special mechanisms, in most cases even withinthe limits of any one given species; but when we consider the sum of allthese mechanisms presented by organic nature as a whole, the mind mustindeed be dull which does not feel astounded. For, be it furtherobserved, these mechanical contrivances[36] are, for the most part, nomerely simple arrangements, which might reasonably be supposed due, like the phenomena of crystallization, to comparatively simple physicalcauses. On the contrary, they everywhere and habitually exhibit sodeep-laid, so intricate, and often so remote an adaptation of means toends, that no machinery of human contrivance can properly be said toequal their perfection from a mechanical point of view. Therefore, without question, the hypothesis which first of all they suggest--orsuggest most readily--is the hypothesis of design. And this hypothesisbecomes virtually the only hypothesis possible, if it be assumed--as itgenerally was assumed by natural theologians of the past, --that allspecies of plants and animals were introduced into the world _suddenly_. For it is quite inconceivable that any known cause, other thanintelligent design, could be competent to turn out instantaneously anyone of these intricate pieces of machinery, already adapted to theperformance of its special function. But, on the other hand, if there isany evidence to show that one species becomes slowly transformed intoanother--or that one set of adaptations becomes slowly changed intoanother set as changing circumstances require, --then it becomes quitepossible to imagine that a strictly natural causation may have hadsomething to do with the matter. And this suggestion becomes greatlymore probable when we discover, from geological evidence andembryological research, that in the history both of races and ofindividuals the various mechanisms in question have themselves had ahistory--beginning in the forms of most uniformity and simplicity, gradually advancing to forms more varied and complex, nowhere exhibitingany interruptions in their upward progress, until the world of organicmachinery as we now have it is seen to have been but the last phase of along and gradual growth, the ultimate roots of which are to be found inthe soil of undifferentiated protoplasm. [36] It is often objected to Darwin's terminology, that it embraces such words as "contrivance, " "purpose, " &c. , which are strictly applicable only to the processes or the products of thought. But when it is understood that they are used in a neutral or metaphorical sense, I cannot see that any harm arises from their use. Lastly, when there is supplied to us the suggestion of natural selectionas a cause presumably adequate to account for this continuous growth inthe number, the intricacy, and the perfection of such mechanisms, it isonly the most unphilosophical mind that can refuse to pause as betweenthe older hypothesis of design and the newer hypothesis of descent. Thus it is clear that the _a priori_ standing of the rival hypotheses ofnaturalism and supernaturalism in the case of all these pieces oforganic machinery, is profoundly affected by the question whether theycame into existence suddenly, or whether they did so gradually. For, ifthey all came into existence suddenly, the fact would constitutewell-nigh positive proof in favour of supernaturalism, or creation bydesign; whereas, if they all came into existence gradually, this factwould in itself constitute presumptive evidence in favour of naturalism, or of development by natural causes. And, as shown in the previouschapters, the proof that all species of plants and animals came intoexistence gradually--or the proof of evolution as a fact--is simplyoverwhelming. From a still more general point of view I may state the case in anotherway, by borrowing and somewhat expanding an illustration which, Ibelieve, was first used by Professor Huxley. If, when the tide is out, we see lying upon the shore a long line of detached sea-weed, markingthe level which is reached by full tide, we should be free to concludethat the separation of the sea-weed from the sand and the stones was dueto the intelligent work of some one who intended to collect the sea-weedfor manure, or for any other purpose. But, on the other hand, we mightexplain the fact by a purely physical cause--namely, the separation bythe sea-waves of the sea-weed from the sand and stones, in virtue of itslower specific gravity. Now, thus far the fact would be explainedequally well by either hypothesis; and this fact would be the fact of_selection_. But whether we yielded our assent to the one explanation orto the other would depend upon a due consideration of all collateralcircumstances. The sea-weed might not be of a kind that is of any use toman; there might be too great a quantity of it to admit of our supposingthat it had been collected by man; the fact that it was all deposited onthe high-water-mark would in itself be highly suggestive of the agencyof the sea; and so forth. Thus, in such a case any reasonable observerwould decide in favour of the physical explanation, or against theteleological one. Now the question whether organic evolution has been caused by physicalagencies or by intelligent design is in precisely the same predicament. There can be no logical doubt that, theoretically at all events, thephysical agencies which the present chapter is concerned with, and whichare conveniently summed up in the term natural selection, are ascompetent to produce these so-called mechanical contrivances, and theother cases of adaptation which are to be met with in organic nature, as intelligent design could be. Hence, our choice as between these twohypotheses must be governed by a study of all collateral circumstances;that is to say, by a study of the evidences in favour of the physicalexplanation. To this study, therefore, we shall now address ourselves, in the course of the following chapters. CHAPTER VIII. EVIDENCES OF THE THEORY OF NATURAL SELECTION. I will now proceed to state the main arguments in favour of the theoryof natural selection, and then, in the following chapter, the mainobjections which have been urged against it. In my opinion, the main arguments in favour of the theory are three innumber. First, it is a matter of observation that the struggle for existence innature does lead to the extermination of forms less fitted for thestruggle, and thus makes room for forms more fitted. This general factmay be best observed in cases where an exotic species proves itselfbetter fitted to inhabit a new country than is some endemic specieswhich it exterminates. In Great Britain, for example, the so-calledcommon rat is a comparatively recent importation from Norway, and it hasso completely supplanted the original British rat, that it is nowextremely difficult to procure a single specimen of the latter: thenative black rat has been all but exterminated by the foreign brown rat. The same thing is constantly found in the case of imported species ofplants. I have seen the river at Cambridge so choked with the inordinatepropagation of a species of water-weed which had been introduced fromAmerica, that considerable expense had to be incurred in order to clearthe river for traffic. In New Zealand the same thing has happened withthe European water-cress, and in Australia with the common rabbit. So itis doubtless true, as one of the natives is said to have philosophicallyremarked, "the white man's rat has driven away our rat, the European flydrives away our fly, his clover kills our grass, and so will the Maorisdisappear before the white man himself. " Innumerable other cases to thesame effect might be quoted; and they all go to establish the fact thatforms less fitted to survive succumb in their competition with formsbetter fitted. * * * * * Secondly, there is a general consideration of the largest possiblesignificance in the present connexion--namely, that among all themillions of structures and instincts which are so invariably, and forthe most part so wonderfully, adapted to the needs of the speciespresenting them, we cannot find a single instance, either in thevegetable or animal kingdom, of a structure or an instinct which isdeveloped for the exclusive benefit of another species. Now this greatand general fact is to my mind a fact of the most enormous, not to sayoverwhelming, significance. The theory of natural selection has now beenbefore the world for more than thirty years, and during that time it hadstood a fire of criticism such as was never encountered by anyscientific theory before. From the first Darwin invited this criticismto adduce any single instance, either in the vegetable or animalkingdom, of a structure or an instinct which should unquestionably beproved to be of exclusive use to any species other than the onepresenting it. He even went so far as to say that if any one suchinstance could be shown he would surrender his whole theory on thestrength of it--so assured had he become, by his own prolongedresearches, that natural selection was the true agent in the productionof adaptive structures, and, as such, could never have permitted such astructure to occur in one species for the benefit of another. Now, asthis invitation has been before the world for so many years, and has notyet been answered by any naturalist, we may by this time be prettyconfident that it never will be answered. How tremendous, then, is thesignificance of this fact in its testimony to Darwin's theory! Thenumber of animal and vegetable species, both living and extinct, is tobe reckoned by millions, and every one of these species presents on anaverage hundreds of adaptive structures, --at least one of which in many, possibly in most, if not actually in all cases, is peculiar to thespecies that presents it. In other words, there are millions of adaptivestructures (not to speak of instincts) which are peculiar to the speciespresenting them, and also many more which are the common property ofallied species: yet, notwithstanding this inconceivable profusion ofadaptive structures in organic nature, there is no single instance thathas been pointed out of the occurrence of such a structure save for thebenefit of the species that presents it. Therefore, I say that thisimmensely large and general fact speaks with literally immeasurableforce in favour of natural selection, as at all events one of the maincauses of organic evolution. For the fact is precisely what we shouldexpect if this theory is true, while upon no other theory can itsuniversality and invariability be rendered intelligible. On thebeneficent design theory, for instance, it is inexplicable that nospecies should ever be found to present a structure or an instincthaving primary reference to the welfare of another species, when, _exhypothesi_, such an endless amount of thought has been displayed in thecreation of structures and instincts having primary reference to thespecies which present them. For how magnificent a display of divinebeneficence would organic nature have afforded, if all--or evensome--species had been so inter-related as to have ministered to eachothers wants. Organic species might then have been likened to acountless multitude of voices, all singing in one great harmoniouspsalm. But, as it is, we see absolutely no vestige of suchco-ordination: every species is for itself, and for itself alone--anoutcome of the always and everywhere fiercely raging struggle for life. In order that the force of this argument may not be misapprehended, itis necessary to bear in mind that it is in no way affected by caseswhere a structure or an instinct is of primary benefit to its possessor, and then becomes of secondary benefit to some other species on accountof the latter being able in some way or another to utilise its action. Of course organic nature is full of cases of this kind; but they only goto show the readiness which all species display to utilise forthemselves everything that can be turned to good account in their ownenvironments, and so, among other things, the structures and instinctsof other animals. For instance, it would be no answer to Darwin'schallenge if any one were to point to a hermit-crab inhabiting thecast-off shell of a mollusk; because the shell was primarily of use tothe mollusk itself, and, so far as the mollusk is concerned, the fact ofits shell being afterwards of a secondary use to the crab is quiteimmaterial. What Darwin's challenge requires is, that some structure orinstinct should be shown which is not merely of such secondary oraccidental benefit to another species, but clearly adapted to the needsof that other species in the first instance--such, for example, as wouldbe the case if the tail of a rattle-snake were of no use to itspossessor, while serving to warn other animals of the proximity of adangerous creature; or, in the case of instincts, if it were true that apilot-fish accompanies a shark for the purpose of helping the shark todiscover food. Both these instances have been alleged; but both havebeen shown untenable. And so it has proved of all the other cases whichthus far have been put forward. Perhaps the most remarkable of all the allegations which ever have beenput forward in this connexion are those that were current with regard toinstincts before the publication of Darwin's work. These allegations arethe most remarkable, because they serve to show, in a degree which I donot believe could be shown anywhere else, the warping power ofpreconceived ideas. A short time ago I happened to come across the 8thedition of the _Encyclopædia Britannica_, and turned up the article on"Instinct" there, in order to see what amount of change had been wroughtwith regard to our views on this subject by the work of Darwin--the 8thedition of the _Encyclopædia Britannica_ having been published shortlybefore _The Origin of Species by means of Natural Selection_. I cannotwait to give any lengthy quotations from this representative exponent ofscientific opinion upon the subject at that time; but its general driftmay be appreciated if I transcribe merely the short concludingparagraph, wherein he sums up his general results. Here he says:-- It thus only remains for us to regard instinct as a mental faculty, _sui generis_, the gift of God to the lower animals, that man in his own person, and by them, might be relieved from the meanest drudgery of nature. Now, here we have the most extraordinary illustration that is imaginableof the obscuring influence of a preconceived idea. Because he startedwith the belief that instincts _must_ have been implanted in animals forthe benefit of man, this writer, even when writing a purely scientificessay, was completely blinded to the largest, the most obvious, and themost important of the facts which the phenomena of instinct display. For, as a matter of fact, among all the many thousands of instinctswhich are known to occur in animals, there is no single one that can bepointed to as having any special reference to man; while, on the otherhand, it is equally impossible to point to one which does not refer tothe welfare of the animal presenting it. Indeed, when the point issuggested, it seems to me surprising how few in number are the instinctsof animals which have proved to be so much as of secondary or accidentalbenefit to man, in the same way as skins, furs, and a whole host ofother animal products are thus of secondary use to him. Therefore, thiswriter not only failed to perceive the most obvious truth that everyinstinct, without any single exception, has reference to the animalwhich presents it; but he also conceived a purely fictitious inversionof this truth, and wrote an essay to prove a statement which all theinstincts in the animal kingdom unite in contradicting. This example will serve to show, in a striking manner, not only thedistance that we have travelled in our interpretation of organic naturebetween two successive editions of the _Encyclopædia Britannica_, butalso the amount of verification which this fact furnishes to the theoryof natural selection. For, inasmuch as it belongs to the very essence ofthis theory that all adaptive characters (whether instinctive orstructural) must have reference to their own possessors, we findoverpowering verification furnished to the theory by the fact now beforeus--namely, that immediately prior to the enunciation of this theory, the truth that all adaptive characters have reference only to thespecies which present them was not perceived. In other words, it was thetesting of this theory by the facts of nature that _revealed_ tonaturalists the general law which the theory, as it were, predicted--thegeneral law that all adaptive characters have primary reference to thespecies which present them. And when we remember that this is a kind ofverification which is furnished by millions of separate cases, the wholemass of it taken together is, as I have before said, overwhelming. It is somewhat remarkable that the enormous importance of this argumentin favour of natural selection as a prime factor of organic evolutionhas not received the attention which it deserves. Even Darwin himself, with his characteristic reserve, has not presented its incalculablesignificance; nor do I know any of his followers who have made anyapproach to an adequate use of it in their advocacy of his views. Inpreparing the present chapter, therefore, I have been particularlycareful not to pitch too high my own estimate of its evidential value. That is to say, I have considered, both in the domain of structures andof instincts, what instances admit of being possibly adduced _percontra_, or as standing outside the general law that adaptive structuresand instincts are of primary use only to their possessors. In the resultI can only think of two such instances. These, therefore, I will nowdispose of. The first was pointed out, and has been fully discussed, by Darwinhimself. Certain species of ants are fond of a sweet fluid that issecreted by aphides, and they even keep the aphides as we keep cows forthe purpose of profiting by their "milk. " Now the point is, that the useof this sweet secretion to the aphis itself has not yet been made out. Of course, if it is of no use to the aphis, it would furnish a casewhich completely meets Darwin's own challenge. But, even if thissupposition did not stand out of analogy with all the other facts oforganic nature, most of us would probably deem it prudent to hold thatthe secretion must primarily be of some use to the aphis itself, although the matter has not been sufficiently investigated to inform usof what this use is. For, in any case, the secretion is not of any vitalimportance to the ants which feed upon it: and I think but few impartialminds would go so far to save an hypothesis as to maintain, that theDivinity had imposed this drain upon the internal resources of onespecies of insect for the sole purpose of supplying a luxury toanother. On the whole, it seems most probable that the fluid is of thenature of an excretion, serving to carry off waste products. Such, atall events, was the opinion at which Darwin himself arrived, as a resultof observing the facts anew, and in relation to his theory. * * * * * The other instance to which I have alluded as seeming at first sightlikely to answer Darwin's challenge is the formation of vegetable galls. The great number and variety of galls agree in presenting a more or lesselaborate structure, which is not only foreign to any of the uses ofplant-life, but singularly and specially adapted to those of theinsect-life which they shelter. Yet they are produced by a growth of theplant itself, when suitably stimulated by the insects' inoculation--or, according to recent observations, by emanations from the bodies of thelarvæ which develop from the eggs deposited in the plant by the insect. Now, without question, this is a most remarkable fact; and if there weremany more of the like kind to be met with in organic nature, we mightseriously consider whether the formation of galls should not be held tomake against the ubiquitous agency of natural selection. But inasmuch asthe formation of galls stands out as an exception to the otherwiseuniversal rule of every species for itself, and for itself alone, we arejustified in regarding this one apparent exception with extremesuspicion. Indeed, I think we are justified in regarding the peculiarpathological effect produced in the plant by the secretions of theinsect as having been in the first instance accidentally beneficial tothe insects. Thus, if any other effect than that of a growing tumour hadbeen produced in the first instance, or if the needs of the insectprogeny had not been such as to have derived profit from being enclosedin such a tumour, then, of course, the inoculating instinct of theseanimals could not have been developed by natural selection. But, giventhese two conditions, and it appears to me there is nothing very muchmore remarkable about an accidental correlation between the effects of aparasitic larva on a plant and the needs of that parasite, than there isbetween the similarly accidental correlation between a hydated parasiteand the nutrition furnished to it by the tissues of a warm-bloodedanimal. Doubtless the case of galls is somewhat more remarkable, inasmuch as the morbid growth of the plant has more concern in thecorrelation--being, in many instances, a more specialized structure onthe part of a host than occurs anywhere else, either in the animal orvegetable world. But here I may suggest that although natural selectioncannot have acted upon the plant directly, so as to have produced gallsever better and better adapted to the needs of the insect, it may haveso acted upon the plants indirectly _though the insects_. For it mayvery well have been that natural selection would ever tend to preservethose individual insects, the quality of whose emanations tended toproduce the form of galls best suited to nourish the insect progeny; andthus the character of these pathological growths may have become everbetter and better adapted to the needs of the insects. Lastly, lookingto the enormous number of relations and inter-relations between allorganic species, it is scarcely to be wondered at that even soextraordinary an instance of correlation as this should have arisen thusby accident, and then have been perfected by such an _indirect_ agencyof natural selection as is here suggested[37]. [37] Note B. * * * * * The third general class of facts which tell so immensely in favour ofnatural selection as an important cause of organic evolution, are thoseof domestication. The art of the horticulturist, the fancier, thecattle-breeder, &c. , consists in producing greater and greaterdeviations from a given wild type of plant or animal, in any particulardirection that may be desired for purposes either of use or of beauty. Cultivated cereals, fruits, and flowers are known to have been allderived from wild species; and, of course, the same applies to all ourdomesticated varieties of animals. Yet if we compare a cabbage rose witha wild rose, a golden pippin apple with a crab, a toy terrier with anyspecies of wild dog, not to mention any number of other instances, therecan be no question that, if such differences had appeared in nature, theorganisms presenting them would have been entitled to rank as distinctspecies--or even, in many cases, as distinct genera. Yet we know, as amatter of fact, that all these differences have been produced by aprocess of artificial selection, or pairing, which has been continuouslypractised by horticulturists and breeders through a number ofgenerations. It is the business of these men to note the individualorganisms which show most variation in the directions required, and thento propagate from these individuals, in order that the progeny shallinherit the qualities desired. The results thus become cumulative fromgeneration to generation, until we now have an astonishing manifestationof useful qualities on the one hand, and of beautiful qualities on theother, according as the organisms have been thus bred for purposes ofuse or for those of beauty. Now it is immediately obvious that in these cases the process ofartificial selection is precisely analogous to that of natural selection(and of sexual selection which will be considered later on), in allrespects save one: the utility or the beauty which it is the aim ofartificial selection continually to enhance, is utility or beauty inrelation to the requirements or to the tastes of man; whereas theutility or the beauty which is produced by natural selection and sexualselection has reference only to the requirements or the tastes of theorganisms themselves. But, with the exception of this one point ofdifference, the processes and the products are identical in kind. Persevering selection by man is thus proved to be capable of creatingwhat are virtually new specific types, and this in any requireddirection. Hence, when we remember how severe is the struggle forexistence in nature, it becomes impossible to doubt that selection bynature is able to do at least as much as artificial selection in the wayof thus creating new types out of old ones. Artificial selection, indeed, notwithstanding the many and marvellous results which it hasaccomplished, can only be regarded as but a feeble imitation of naturalselection, which must act with so much greater vigilance and throughsuch immensely greater periods of time. In a word, the provedcapabilities of artificial selection furnish, in its best conceivableform, what is called an argument _a fortiori_ in favour of naturalselection. Or, to put it in another way, it may be said that for thousands of yearsmankind has been engaged in making a gigantic experiment to test, as itwere by anticipation, the theory of natural selection. For, althoughthis prolonged experiment has been carried on without any such intentionon the part of the experimenters, it is none the less an experiment inthe sense that its results now furnish an overwhelming verification ofMr. Darwin's theory. That is to say, they furnish overwhelming proof ofthe efficacy of the selective principle in the modification of organictypes, when once this principle is brought steadily and continuously tobear upon a sufficiently long series of generations. In order to furnish ocular evidence of the value of this line ofverification, I have had the following series of drawings prepared. Another and equally striking series might be made of the products ofartificial selection in the case of plants; but it seems to me that thecase of animals is more than sufficient for the purpose just stated. Perhaps it is desirable to add that considerable care has been bestowedupon the execution of these portraits; and that in every case the latterhave been taken from the most typical specimens of the artificialvariety depicted. Those of them which have not been drawn directly fromlife are taken from the most authoritative sources; and, before beingsubmitted to the engraver, they were all examined by the best judges ineach department. In none of the groups, however, have I aimed at anexhaustive representation of all the varieties: I have merely introducedrepresentatives of as many as the page would in each case accommodate. [Illustration: FIG. 91. --Pigeons. Drawn from life (prize specimens). ] [Illustration: FIG. 92. --Pigeons, continued. Drawn from life (prize specimens). ] [Illustration: FIG. 93. --Fowls. Drawn from life (prize specimens). ] [Illustration: FIG. 94. --Fowls, continued. Drawn from life (prize specimens). ] [Illustration: FIG. 95. --Pair of Japanese Fowls, long-tailed breed. Drawn from stuffed specimens in the British Museum. ] [Illustration: FIG. 96. --Canaries. Drawn from life (prize specimens). ] [Illustration: FIG. 97. --Sebastopol, or Frizzled Goose. Drawn from a photograph. ] [Illustration: FIG. 98. --The Dingo, or wild dog of Australia, 1/10 nat. Size. Drawn from life (_Zoological Gardens_). ] [Illustration: FIG. 99. --Dogs. Drawn from life (prize specimens). ] [Illustration: FIG. 100. --Dogs, continued. Drawn from life (prize specimens). ] [Illustration: FIG. 101. --The Hairless Dog of Japan, 1/10 nat. Size. Drawn from a photograph kindly lent for the purpose by the proprietor. ] [Illustration: FIG. 102. --The skull of a Bull-dog compared with that of a Deerhound. Drawn from nature. ] [Illustration: FIG. 103. Rabbits. Drawn from life (prize specimens). ] [Illustration: FIG. 104. --Horses. Drawn from life (prize specimens). ] [Illustration: FIG. 105. --Sheep. The illustrations are confined to British breeds. Drawn from life (prize specimens). ] [Illustration: FIG. 106. --Cattle. The illustrations are confined to British breeds. Drawn from life (prize specimens). ] [Illustration: FIG. 107. --Wild Boar contrasted with a modern Domesticated Pig. Drawn from life (_Zoological Gardens_, and prize specimen). ] The exigencies of space have prevented, in some of the groups, strictadherence to a uniform scale--with the result that contrasts betweendifferent breeds in respect of size are not adequately rendered. Thisremark applies especially to the dogs; for although the artist hasendeavoured to draw them in perspective, unless the distance betweenthose in the foreground and those in the background is understood to bemore considerable than it appears, an inadequate idea is given of therelative differences of size. The most instructive of the groups, Ithink, is that of the Canaries; because the many and great changes indifferent directions must in this case have been produced by artificialselection in so comparatively short a time--the first mention of thisbird that I can find being by Gesner, in the sixteenth century. * * * * * Now, it is surely unquestionable that in these typical proofs of theefficacy of artificial selection in the modification of specific types, we have the strongest conceivable testimony to the power of naturalselection in the same direction. For it thus appears that wherevermankind has had occasion to operate by selection for a sufficiently longtime--that is to say, on whatever species of plant or animal he choosesthus to operate for the purpose of modifying the type in any requireddirection, --the results are always more or less the same: he finds thatall specific types lend themselves to continuous deflection in anyparticulars of structure, colour, &c. , that he may desire to modify. Nevertheless, to this parallel between the known effects of artificialselection, and the inferred effects of natural selection, two objectionshave been urged. The first is, that in the case of artificial selectionthe selecting agent is a voluntary intelligence, while in the case ofnatural selection the selecting agent is Nature herself; and whether ornot there is any counterpart of man's voluntary intelligence in natureis a question with which Darwinism has nothing to do. Therefore, it isalleged, the analogy between natural selection and artificial selectionfails _ab initio_, or at the fountain-head of the causes which are takenby the analogy to be respectively involved. The second objection to the analogy is, that the products of artificialselection, closely as they may resemble natural species in all otherrespects, nevertheless present one conspicuous and highly importantpoint of difference: they rarely, if ever, present the physiologicalcharacter of mutual infertility, which is a character of extremelygeneral occurrence in the case of natural species, even when these aremost nearly allied. I will deal with these two objections in the next chapter, where I shallbe concerned with the meeting of all the objections which have ever beenurged against the theory of natural selection. Meanwhile I am engagedonly in presenting the general arguments which support the theory, andtherefore mention these objections to one of them merely _en passant_. And I do so in order to pledge myself effectually to dispose of themlater on, so that for the purposes of my present argument both theseobjections may be provisionally regarded as non-existent; which means, in other words, that we may provisionally regard the analogy betweenartificial selection and natural selection as everywhere logicallyintact. * * * * * To sum up, then, the results of the foregoing exposition thus far, whatI hold to be the three principal, or most general, arguments in favourof the theory of natural selection, are as follows. First, there is the _a priori_ consideration that, if on independentgrounds we believe in the theory of evolution at all, it becomes obviousthat natural selection _must_ have had _some_ part in the process. Forno one can deny the potent facts of heredity, variability, the strugglefor existence, and survival of the fittest. But to admit these facts isto admit natural selection as a principle which must be, at any rate, one of the factors of organic evolution, supposing such evolution tohave taken place. Next, when we turn from these _a priori_considerations, which thus show that natural selection _must_ have beenconcerned to some extent in the process of evolution, we find in organicnature evidence _a posteriori_ of the extent to which this principle_has_ been thus concerned. For we find that among all the countlessmillions of adaptive structures which are to be met with in organicnature, it is an invariable rule that they exist in relation to theneeds of the particular species which present them: they never have anyprimary reference to the needs of other species. And as thisextraordinarily large and general fact is exactly what the theory ofnatural selection would expect, the theory is verified by the fact in anextraordinarily cogent manner. In other words, the fact goes to provethat in _all_ cases where adaptive structures or instincts areconcerned, natural selection must have been either the sole cause atwork, or, at the least, an influence controlling the operation of allother causes. Lastly, an actually experimental verification of the theory has beenfurnished on a gigantic scale by the operations of breeders, fanciers, and horticulturists. For these men, by their process of selectiveaccumulation, have empirically proved what immense changes of type maythus be brought about; and so have verified by anticipation, and in amost striking manner, the theory of natural selection--which, as now sofully explained, is nothing more than a theory of cumulativemodifications by means of selective breeding. So much, then, by way of generalities. But perhaps the proof of naturalselection as an agency of the first importance in the transmutation ofspecies may be best brought home to us by considering a few of itsapplications in detail. I will therefore devote the rest of the presentchapter to considering a few cases of this kind. There are so many large fields from which such special illustrations maybe supplied, that it is difficult to decide which of them to draw upon. For instance, the innumerable, always interesting, and often astonishingadaptations on the part of flowers to the fertilising agency of insects, has alone given rise to an extensive literature since the time whenDarwin himself was led to investigate the subject by the guidance of hisown theory. The same may be said of the structures and movements ofclimbing plants, and in short, of all the other departments of naturalhistory where the theory of natural selection has led to the study ofthe phenomena of adaptation. For in all these cases the theory ofnatural selection, which first led to their discovery, still remains theonly scientific theory by which they can be explained. But among all thepossible fields from which evidences of this kind may be drawn, I thinkthe best is that which may be generically termed defensive colouring. Tothis field, therefore, I will restrict myself. But, even so, the casesto be mentioned are but mere samples taken from different divisions ofthis field; and therefore it must be understood at the outset that theycould easily be multiplied a hundred-fold. _Protective Colouring. _ A vast number of animals are rendered more or less inconspicuous byresembling the colours of the surfaces on which they habitually rest. Such, for example, are grouse, partridges, rabbits, &c. Moreover, thereare many cases in which, if the needs of the creature be such that itmust habitually frequent surfaces of different colours, it has acquiredthe power of changing its colour accordingly--e. G. Cuttle-fish, flat-fish, frogs, chameleons, &c. The physiological mechanism wherebythese adaptive changes of colour are produced differs in differentanimals; but it is needless for our purposes to go into this part of thesubject. Again, there are yet other cases where protective colouringwhich is admirably suited to conceal an animal through one part of theyear, would become highly conspicuous during another part of it--namely, when the ground is covered with snow. Accordingly, in these cases theanimals change their colour in the winter months to a snowy white:witness stoats, mountain hares, ptarmigan, &c. (Fig. 108. ) [Illustration: FIG. 108. --Seasonal changes of colour in Ptarmigan (_Lagopus mutus_). Drawn from stuffed specimens in the British Museum, 1/6 nat. Size, with appropriate surroundings supplied. ] Now, it is sufficiently obvious that in all these classes of cases theconcealment from enemies or prey which is thus secured is of advantageto the animals concerned; and, therefore, that in the theory of naturalselection we have a satisfactory theory whereby to explain it. And thiscannot be said of any other theory of adaptive mechanisms in nature thathas ever been propounded. The so-called Lamarckian theory, for instance, cannot be brought to bear upon the facts at all; and on the theory ofspecial creation it is unintelligible why the phenomena of protectivecolouring should be of such general occurrence. For, in as far asprotective colouring is of advantage to the species which present it, itis of corresponding disadvantage to those other species against thepredatory nature of which it acts as a defence. And, of course, thesame applies to yet other species, if they serve as prey. Moreover, the more minutely this subject is investigated in all its details, the more exactly is it found to harmonise with the naturalisticinterpretation[38]. [38] Were it not that some of Darwin's critics have overlooked the very point wherein the great value of protective colouring as evidence of natural selection consists, it would be needless to observe that it does so in the _minuteness_ of the protective resemblance which in so many cases is presented. Of course where the resemblance is only very general, the phenomena might be ascribed to mere coincidence, of which the instincts of the animal have taken advantage. But in the measure that the resemblance becomes minutely detailed, the supposition of mere coincidence is excluded, and the agency of some specially adaptive cause demonstrated. Again, it is almost needless to say, no real difficulty is presented (as has been alleged) by the cases above quoted of seasonal imitations, on the ground that natural selection could not act alternately on the same individual. Natural selection is not supposed to act alternately on the same individual. It is supposed to act always in the same manner, and if, as in the case of a regularly recurring change in the colours of the environment, correspondingly recurrent changes are required to appear in the colours of the animals, natural selection sets its premium upon those individuals the constitutions of which best lend themselves to seasonal changes of the needful kind--probably under the influence of stimuli supplied by the changes of external conditions (temperature, moisture, &c. ). In the first place, we always find a complete correspondence betweenimitative colouring and instinctive endowment. If a caterpillar exactlyresembles the colour of a twig, it also presents the instinct ofhabitually reposing in the attitude which makes it most resemble atwig--standing out from the branch on which it rests at the same angleas is presented by the real twigs of the tree on which it lives. Here, again, is a bird protectively coloured so as to resemble stonesupon the rough ground where it habitually lives; and the drawing showsthe attitude in which the bird instinctively reposes, so as stillfurther to increase its resemblance to a stone. (Fig. 109. ) [Illustration: FIG. 109. --_Oedicnemus crepitans_, showing the instinctive attitude of concealment. Drawn from a stuffed specimen in the British Museum, 1/6 nat. Size, with appropriate surroundings supplied. To take only one other instance, hares and rabbits, like grouse andpartridges--or like the plover just alluded to, --instinctively crouchupon those surfaces the colours of which they resemble; and I have oftenremarked that if, on account of any individual peculiarity ofcoloration, the animal is not able thus to secure concealment, itnevertheless exhibits the instinct of crouching which is of benefit toall its kind, although, from the accident of its own abnormal colouring, this instinct is then actually detrimental to the animal itself. Forexample, every sportsman must have noticed that the somewhat raremelanic variety of the common rabbit will crouch as steadily as thenormal brownish-gray type, notwithstanding that, owing to its abnormalcolour, a "nigger-rabbit" thus renders itself the most conspicuousobject in the landscape. In all such cases, of course, there has been adeviation from the normal type in respect of colour, with the resultthat the inherited instinct is no longer in tune with the otherendowments of the animal. Such a variation of colour, therefore, willtend to be suppressed by natural selection; while any variations whichmay bring the animal still more closely to resemble its habitualsurroundings will be preserved. Thus we can understand the trulywonderful extent to which this principle of protective colouring hasbeen carried in many cases where the need of it has been most urgent. Not only colour, but structure, may be profoundly modified for thepurposes of protective concealment. Thus, caterpillars which resembletwigs do so not only in respect of colour, but also of shape; and thiseven down to the most minute details in cases where the adaptation ismost complete: certain butterflies and leaf-insects so preciselyresemble the leaves upon which, or among which, they live, that it isalmost impossible to detect them in the foliage--not only the colour, the shape, and the venation being all exactly imitated, but in somecases even the defects to which the leaves are liable, in the way offungoid growths, &c. There are other insects which with similarexactness resemble moss, lichens, and so forth. A species of fishsecures a complete resemblance to bunches of sea-weed by a frond-likemodification of all its appendages, and so on through many otherinstances. Now, in all such cases where there is so precise animitation, both in colour and structure, it seems impossible to suggestany other explanation of the facts than the one which is supplied byMr. Darwin's theory--namely, that the more perfect the resemblance iscaused to become through the continuous influence of natural selectionalways picking out the best imitations, the more highly discriminativebecomes the perception of those enemies against the depredations ofwhich this peculiar kind of protection is developed; so that, in virtueof this action and re-action, eventually we have a degree of imitationwhich renders it almost impossible for a naturalist to detect the animalwhen living in its natural environment. [Illustration: FIG. 110. --Imitative forms and colours in insects. Drawn from nature (_R. Coll. Surg. Mus. _). ] _Warning Colours. _ In strange and glaring contrast to all these cases of protectivecolouring, stand other cases of conspicuous colouring. Thus, forexample, although there are numberless species of caterpillars whichpresent in an astonishing degree the phenomena of protective colouring, there are numberless other species which not only fail to present thesephenomena in any degree, but actually go to the opposite extreme ofpresenting colours which appear to have been developed for the sake oftheir conspicuousness. At all events, these caterpillars are usually themost conspicuous objects in their surroundings, and therefore in theearly days of Darwinism they were regarded by Darwin himself aspresenting a formidable difficulty in the way of his theory. To Mr. Wallace belongs the merit of having cleared up this difficulty in anextraordinarily successful manner. He virtually reasoned thus. If the_raison d'être_ of protective colouring be that of concealing agreeablyflavoured caterpillars from the eye-sight of birds, may not the _raisond'être_ of conspicuous colouring be that of protecting disagreeablyflavoured caterpillars from any possibility of being mistaken by birds?Should this be the case, of course the more conspicuous the colouringthe better would it be for the caterpillars presenting it. Now as soonas this suggestion was acted upon experimentally, it was found to beborne out by facts. Birds could not be induced to eat caterpillars ofthe kinds in question; and there is now no longer any doubt that theirconspicuous colouring is correlated with their distastefulness to birds, in the same way as the inconspicuous or imitative colouring of othercaterpillars is correlated with their tastefulness to birds. Here thenis yet another instance, added to those already given, of theverification yielded to the theory of natural selection by its provedcompetency as a guide to facts in nature; for assuredly this particularclass of facts would never have been suspected but for its suggestiveagency. As in the case of protective imitation, so in this case of warningconspicuousness, not only colour, but structure may be greatly modifiedfor the purpose of securing immunity from attack. Here, of course, theobject is to assume, as far as possible, a touch-me-not appearance; sothat, although destitute of any real means of offence, the creatures inquestion present a fictitiously dangerous aspect. As theDevil's-coach-horse turns up his stingless tail when threatened by anenemy, so in numberless ways do many harmless animals of all classespretend to be formidable. But the point now is that these instincts ofself-defence are often helped out by structural modifications, expressly and exclusively adapted to this end. For example, what aremarkable series of protective adjustments occurs in the life-historyof the Puss Moth--culminating with so comical an instance of theparticular device now under consideration as the following. I quote thefacts from Mr. E. B. Poulton's admirable book on _The Colours ofAnimals_ (pp. 269-271). [Illustration: FIG. 111. --The larva of Puss Moth (_C. Vinula_) when undisturbed; full-fed; natural size. ] The larva of the Puss Moth (Cerura vinula) is very common upon poplar and willow. The circular dome-like eggs are laid, either singly or in little groups of two or three, upon the upper side of the leaf, and being of a reddish colour strongly suggest the appearance of little galls, or the results of some other injury to the leaf. The youngest larvæ are black, and also rest upon the upper surface of the leaf, resembling the dark patches which are commonly seen in this position. As the larva grows, the apparent black patch would cover too large a space, and would lead to detection if it still occupied the whole surface of the body. The latter gains a green ground-colour which harmonises with the leaf, while the dark marking is chiefly confined to the back. As growth proceeds the relative amount of green increases, and the dark mark is thus prevented from attaining a size which would render it too conspicuous. In the last stage of growth the green larva becomes very large, and usually rests on the twigs of its food-plant (Fig. 111). The dark colour is still present on the back but is softened to a purplish tint, which tends to be replaced by a combination of white and green in many of the largest larvæ. Such a larva is well concealed by General Protective Resemblance, and one may search a long time before finding it, although assured of its presence from the stripped branches of the food-plant and the fæces on the ground beneath. [Illustration: FIG. 112. --The larva of Puss Moth in its terrifying attitude after being disturbed; full-fed; natural size. ] As soon as a large larva is discovered and disturbed it withdraws its head into the first body-ring, inflating the margin, which is of a bright red colour. There are two intensely black spots on this margin in the appropriate position for eyes, and the whole appearance is that of a large flat face extending to the outer edge of the red margin (see Fig. 112). The effect is an intensely exaggerated caricature of a vertebrate face, which is probably alarming to the vertebrate enemies of the caterpillar. The terrifying effect is therefore mimetic. The movements entirely depend on tactile impressions: when touched ever so lightly a healthy larva immediately assumes the terrifying attitude, and turns so as to present its full face towards the enemy; if touched on the other side or on the back it instantly turns its face in the appropriate direction. The effect is also greatly strengthened by two pink whips which are swiftly protruded from the prongs of the fork in which the body terminates. The prongs represent the last pair of larval legs which have been greatly modified from their ordinary shape and use. The end of the body is at the same time curved forward over the back (generally much further than in Fig. 112), so that the pink filaments are brandished above the head. _Mimicry. _ Lastly, these facts as to imitative and conspicuous colouring lead on tothe yet more remarkable facts of what is called mimicry. By mimicry ismeant the imitation in form and colour of one species by another, inorder that the imitating species may be mistaken for the imitated, andthus participate in some advantage which the latter enjoys. Forinstance, if, as in the case of the conspicuously-coloured caterpillars, it is of advantage to an ill-savoured species that it should hold out awarning to enemies, clearly it may be of no less advantage to awell-savoured species that it should borrow this flag, and thus bemistaken for its ill-savoured neighbour. Now, the extent to which thisdevice of mimicry is carried is highly remarkable, not only in respectof the number of its cases, but also in respect of the astonishingaccuracy which in most of these cases is exhibited by the imitation. There need be little or virtually no zoological affinity between theimitating and the imitated forms; that is to say, in some cases thezoological affinity is not closer than ordinal, and therefore cannotpossibly be ascribed to kinship. Like all the other branches of thegeneral subject of protective resemblance in form or colouring, thisbranch has already been so largely illustrated by previous writers, that, as in the previous cases, I need only give one or two examples. Those which I choose are chosen on account of the colours concerned notbeing highly varied or brilliant, and therefore lending themselves toless ineffectual treatment by wood-engraving than is the case whereattempts are made to render by this means even more remarkableinstances. (Figs. 113, 114, 115. ) [Illustration: FIG. 113. --Three cases of mimicry. Drawn from nature: first two pairs nat. Size, last pair 2/3 (_R. Coll. Surg. Mus. _). ] [Illustration: FIG. 114. --Two further cases of mimicry; flies resembling a wasp in the one and a bee in the other. Drawn from nature: nat. Size (_R. Coll. Surg. Mus. _). ] [Illustration: FIG. 115. --A case of mimicry where a non-venomous species of snake resembles a venomous one. Drawn from nature: 1/3 nat. Size (_R. Coll. Surg. Mus. _). ] It is surely apparent, without further comment, that it is impossible toimagine stronger evidence in favour of natural selection as a true causein nature, than is furnished by this culminating fact in the matter ofprotective resemblance, whereby it is shown that a species of onegenus, family, or even order, will accurately mimic the appearance of aspecies belonging to another genus, family, or order, so as to deceiveits natural enemies into mistaking it for a creature of so totallydifferent a kind. And it must be added that while this fact of mimicryis of extraordinarily frequent occurrence, there can be no possibilityof our mistaking its purpose. For the fact is never observable except inthe case of species which occupy the same area or district. Such being what appears to me the only reasonable view of the matter, Iwill now conclude this chapter on the evidences of natural selection asat all events the main factor of organic evolution, by simply addingillustrations of two further cases of mimicry, which are perhaps evenmore remarkable than any of the foregoing examples. The first of the two(Fig. 115) speaks for itself. The second will be rendered intelligibleby the following few words of explanation. There are certain ants of the Amazons which present the curious instinctof cutting off leaves from trees, and carrying them like banners overtheir heads to the hive, as represented in Fig. 116, B, where one ant isshown without a leaf, and the others each with a leaf. Their object inthus collecting leaves is probably that of growing a fungus upon the"soil" which is furnished by the leaves when decomposing. But, be thisas it may[39], the only point we are now concerned with is theappearance which these ants present when engaged in their habitualoperation of carrying leaves. For it has been recently observed by Mr. W. L. Sclater, that in the localities where these hymenopterous insectsoccur, there occurs also a _homopterous_ insect which mimics the ant, leaf and all, in a wonderfully deceptive manner. The leaf is imitated bythe thin flattened body of the insect, "which in its dorsal aspect is socompressed laterally that it is no thicker than a leaf, and terminatesin a sharp jagged edge. " The colour is exactly the same as that of aleaf, and the brown legs show themselves beneath the green body in justthe same way as those of the ant show themselves beneath the leaf. Sothat both the form and the colouring of the homopterous insect has beenbrought to resemble, with singular exactness, those belonging to adifferent order of insect, when the latter is engaged in its peculiaravocation. A glance at the figure is enough to show the means employedand the result attained. In A, an ant and its mimic are represented asabout 2-1/2 times their natural size, and both proceeding in the samedirection. It ought to be mentioned, however, that in reality the marginof the leaf is seldom allowed to retain its natural serrations as heredepicted: the ants usually gnaw the edge of the real leaf, so that themargin of the false one bears an even closer resemblance to it than theillustration represents. B is a drawing from life of a group of fiveants carrying leaves, and their mimic walking beside them[40]. [39] For a full account of this instinct and its probable purpose, see _Animal Intelligence_, pp. 93-6. [40] Both drawings are reproduced from Mr. Poulton's paper upon the subject (_Proc. Zool. Soc. _, June 16, 1891). [Illustration: FIG. 116 PROTECTIVE MIMICRY] CHAPTER IX. CRITICISMS OF THE THEORY OF NATURAL SELECTION. I will now proceed to consider the various objections and difficultieswhich have hitherto been advanced against the theory of naturalselection. Very early in the day Owen hurled the weight of his authority againstthe new theory, and this with a strength of onslaught which was onlyequalled by its want of judgment. Indeed, it is painfully apparent thathe failed to apprehend the fundamental principles of the Darwiniantheory. For he says:-- Natural Selection is an explanation of the process [of transmutation] of the same kind and value as that which has been proffered of the mystery of "secretion. " For example, a particular mass of matter in a living animal takes certain elements out of the blood, and rejects them as "bile. " Attributes were given to the liver which can only be predicated of the whole animal; the "appetency" of the liver, it was said, was for the elements of bile, and "biliosity, " or the "hepatic sensation, " guided the gland to their secretion. Such figurative language, I need not say, explains absolutely nothing of the nature of bilification[41]. [41] _Anatomy of Vertebrates_, vol. Iii. P. 794. Assuredly, it was needless for Owen to say that figurative language ofthis kind explains nothing; but it was little less than puerile in himto see no more in the theory of natural selection than such a merefigure of speech. To say that the liver selects the elements of bile, orthat nature selects specific types, may both be equally unmeaningre-statements of facts; but when it is explained that the term naturalselection, unlike that of "hepatic sensation, " is used as a shorthandexpression for a whole group of well-known natural causes--struggle, variation, survival, heredity, --then it becomes evidence of an almostchildish want of thought to affirm that the expression is figurative andnothing more. The doctrine of natural selection may be a huge mistake;but, if so, this is not because it consists of any unmeaning metaphor:it can only be because the combination of natural causes which itsuggests is not of the same adequacy in fact as it is taken to be intheory. Owen further objected that the struggle for existence could only act asa cause of the extinction of species, not of their origination--a viewof the case which again shows on his part a complete failure to graspthe conception of Darwinism. Acting alone, the struggle for existencecould only cause extermination; but acting together with variation, survival, and heredity, it may very well--for anything that Owen, orothers who followed in this line of criticism, show to thecontrary--have produced every species of plant and animal that has everappeared upon the face of the earth. Another and closely allied objection is, that the theory of naturalselection "personifies an abstraction. " Or, as the Duke of Argyll statesit, the theory is "essentially the image of mechanical necessityconcealed under the clothes, and parading in the mask, of mentalpurpose. The word 'natural' suggests Matter, and the physical forces. The word 'selection' suggests Mind, and the powers of choice. " This, however, is a mere quarrelling about words. Darwin called the principlewhich he had discovered by the name natural selection in order to markthe analogy between it and artificial selection. No doubt in thisanalogy there is not necessarily supposed to be in nature anycounterpart to the mind of the breeder, nor, therefore, to his powers ofintelligent choice. But there is no need to limit the term _selection_(_se_ and _lego_, Gr. [Greek: legô]) to powers of intelligent choice. Aspreviously remarked, a bank of sea-weed on the sea-shore may be said tohave been selected by the waves from all the surrounding sand andstones. Similarly, we may say that grain is selected from chaff by thewind in the process of winnowing corn. Or, if it be thought that thereis any ambiguity involved in such a use of the term in the case of"Natural Selection, " there is no objection to employing the phrase whichhas been coined by Mr. Spencer as its equivalent--namely, "Survival ofthe Fittest. " The point of the theory is, that those organisms which arebest suited to their surroundings are allowed to live and to propagate, while those which are less suited are eliminated; and whether we callthis process a process of selection, or call it by any other name, isclearly immaterial. A material question is raised only when it is asked whether the processis one that can be ascribed to causation strictly natural. It is oftendenied that such is the case, on the ground that natural selection doesnot originate the variations which it favours, but depends upon thevariations being supplied by some other means. For, it is said, all thatnatural selection does is to preserve the suitable variations _afterthey have arisen_. Natural selection does not _cause_ these suitablevariations; and therefore, it is argued, Darwin and his followers areprofoundly mistaken in representing the principle as one which_produces_ adaptations. Now, although this objection has been putforward by some of the most intelligent minds in our generation, itappears to me to betoken some extraordinary failure to appreciate thevery essence of Darwinian doctrine. No doubt it is perfectly true thatnatural selection does not produce variations of any kind, whetherbeneficial or otherwise. But if it be granted that variations of manykinds are occurring in every generation, and that natural selection iscompetent to preserve the more favourable among them, then it appears tome unquestionable that this principle of selection deserves to beregarded as, in the full sense of the word, a natural cause. Thevariations being expressly regarded by the theory as more or lesspromiscuous[42], survival of the fittest becomes the winnowing fan, whose function it is to eliminate all the less fit in each generation, in order to preserve the good grain, out of which to constitute the nextgeneration. And as this process is supposed to be continuous throughsuccessive generations, its action is supposed to be cumulative, tillfrom the eye of a worm there is gradually developed the eye of an eagle. Therefore it follows from these suppositions (which are not disputed bythe present objection), that if it had not been for the process ofselection, such development would never have been begun; and that in theexact measure of its efficiency will the development proceed. But anyagency without the operation of which a result cannot take place mayproperly be designated the cause of that result: it is the agency which, in co-operation with all the other agencies in the cosmos, produces thatresult. [42] The degree in which variability is indefinite, or, on the contrary, determinate, is a question which is not yet ripe for decision--nor even, in my opinion, for discussion. But I may here state the following general principles with regard to it. (1) It is evident that up to some point or another variations _must_ be pre-determined in definite lines. Men do not gather grapes from thorns, figs from thistles, nor even moss-roses from sweet-briars. In other words, "the nature of the organism" in all cases necessitates the limiting of variations within certain bounds. (2) But when the question is as to what these bounds may be, we can only answer in a general way that, according to the general theory of evolution, they must be such as are imposed by heredity, coupled with the degree to which external conditions of life (and possibly also use-inheritance) are capable, in given cases, of modifying congenital characters. These are the only causes which the theory of descent can consistently recognise as producing variations in determinate directions. (3) Inasmuch as variation presupposes the existence of parts that vary, and inasmuch as the variation of parts can only be in the alternative directions of increase or decrease around an average, it follows that, in the first instance at all events, every variation, if determinate, must be so only in one or other of these two opposite directions. (4) In as far as variations are summated in successive generations, so as eventually to give rise to new structures, organs, mechanisms, &c. , natural selection is theoretically competent to explain the facts, without our having to postulate the operation of unknown causes producing variations in determinate lines, --or not further than is stated in paragraphs 1 and 2. (5) Nevertheless, it does not follow that there are not such other unknown causes; and, if there are, of course the importance of natural selection as a cause of adaptive modification would be limited in proportion to their number and the extent of their operation. But it is for those who, like the late Professors Asa Gray and Nägeli, maintain the existence of such causes, to substantiate their belief by indicating them. Take any analogous case. The selective agency of specific gravity whichis utilised in gold-washing does not create the original differencesbetween gold-dust and dust of all other kinds. But these differencesbeing presented by as many different bodies in nature, the gold-washertakes advantage of the selective agency in question, and, by using it asa cause of segregation, is enabled to separate the gold from all theearths with which it may happen to be mixed. So far as the objects ofthe gold-washer are concerned, it is immaterial with what other earthsthe gold-dust may happen to be mixed. For although gold-dust may occurin intimate association with earths of various kinds in variousproportions, and although in each case the particular admixture whichoccurs must have been due to definite causes, these things, in relationto the selective process of the washer, are what is called accidental:that is to say, they have nothing to do with the causative action of theselective process. Now, in precisely the same sense Darwin calls themultitudinous variations of plants and animals accidental. By so callingthem he expressly says he does not suppose them to be accidental in thesense of not all being due to definite causes. But they are accidentalin relation to the sifting process of natural selection: all that theyhave to do is to furnish the promiscuous material on which this siftingprocess acts. Or let us take an even closer analogy. The power of selective breedingby man is so wonderful, that in the course of successive generations allkinds of peculiarities as to size, shape, colour, special appendages orabortions, &c. , can be produced at pleasure, as we saw in the lastchapter. Now all the promiscuous variations which are supplied to thebreeder, and out of which, by selecting only those that are suited tohis purpose, he is able to produce the required result--all thosepromiscuous variations, in relation to that purpose, are accidental. Therefore the selective agency of the breeder deserves to be regarded asthe cause of that which it produces, or of that which could not havebeen produced but for the operation of such agency. But where is thedifference between artificial and natural selection in this respect?And, if there is no difference, is not natural selection as muchentitled to be regarded as a true cause of the origin of naturalspecies, as artificial selection is to be regarded as a true cause ofour domesticated races? Here, as in the case of the previousillustration, if there be any ambiguity in speaking of variations asaccidental, it arises from the incorrect or undefined manner in whichthe term "accidental" is used by Darwin's critics. In its original andphilosophically-correct usage, the term "accident" signifies a propertyor quality not essential to our conception of a substance: hence, it hascome to mean anything that happens as a result of unforeseen causes--or, lastly, that which is causeless. But, as we know that nothing can happenwithout causes of some kind, the term "accident" is divested of realmeaning when it is used in the last of these senses. Yet this is thesense that is sought to be placed upon it by the objection which we areconsidering. If the objectors will but understand the term in itscorrect philosophical sense--or in the only sense in which it presentsany meaning at all, --they will see that Darwinians are both logicallyand historically justified in employing the word "accidental" as theword which serves most properly to convey the meaning that theyintend--namely, variations due to causes accidental to the struggle forexistence. Similarly, when it is said that variations are "spontaneous, "or even "fortuitous, " nothing further is meant than that we do not knowthe causes which lead to them, and that, so far as the principle ofselection is concerned, it is immaterial what these causes may be. Or, to revert to our former illustration, the various weights of differentkinds of earths are no doubt all due to definite causes; but, inrelation to the selective action of the gold-washer, all the differentweights of whatever kinds of earth he may happen to include in hiswashing-apparatus are, _strictly speaking_, accidental. And as atdifferent washings he meets with different proportions of heavy earthswith light ones, and as these "variations" are immaterial to him, he maycolloquially speak of them as "fortuitous, " or due to "chance, " eventhough he knows that at each washing they must have been determined bydefinite causes. More adequately to deal with this merely formal objection, however, would involve more logic-chopping than is desirable on the presentoccasion. But I have already dealt with it fully elsewhere, --viz. In_The Contemporary Review_ for June, 1888, to which therefore I may referany one who is interested in dialectics of this kind[43]. [43] Within the last few months this objection has been presented anew by Mr. D. Syme, whose book _On the Modification of Organisms_ exhibits a curious combination of shrewd criticisms with almost ludicrous misunderstandings. One of the latter it is necessary to state, because it pervades the quotation which I am about to supply. He everywhere compares "natural selection" with "the struggle for existence, " uses them as convertible terms, and while absurdly stating that "Darwin defines natural selection as the struggle for existence, " complains of "the liability of error, both on his own part and on the part of his readers, " which arises from his not having everywhere adhered to this definition! (p. 8). "Darwin has put forth two distinct and contradictory theories of the functions of natural selection. According to the one theory natural selection is selective or preservative, and nothing more. According to the other theory natural selection creates the variations(!) ... It certainly seems absurd to speak of natural selection, or the struggle for existence, as selective or preservative, for the struggle for existence does not preserve at all, not even the fit variations, as both the fit and the unfit struggle for existence, the unfit naturally more than the fit, and the fit are preserved, not in consequence of the struggle, but in consequence of their fitness. Suppose two varieties of the same species are driven, by an increase of their numbers, to seek for subsistence in a colder region than they have been accustomed to, and that one of these varieties had a hardier constitution than the other; and let us suppose that the former withstood the severe climate better than the latter, and consequently survived, while the other perished. In this case the hardier survived, not because of the struggle, but because it had a constitution better adapted to the climate. I wish to ascertain if a certain metal in my possession is gold or some baser metal, and I apply the usual test; but the mere fact of my testing this metal would not make it gold or any other kind of metal. " I have thought it worth while to quote this passage for the sake of showing the extraordinary confusion of mind which still prevails on the part of Darwin's critics, even with reference to the very fundamental parts of his theory. For, as I have said, the writer of this passage shows himself a shrewd critic in some other parts of his essay, where he is not engaged especially on the theory of natural selection. I will now pass on to consider another misconception of the Darwiniantheory, which is very prevalent in the public mind. It is virtuallyasked, If some species are supposed to have been improved by naturalselection, why have not all species been similarly improved? Why shouldnot all invertebrated animals have risen into vertebrated? Or whyshould not all monkeys have become men? The answers are manifold. In the first place, it by no means followsthat because an advance in organization has proved itself of benefit inthe case of one form of life, therefore any or every other form wouldhave been similarly benefited by a similar advance. The business ofnatural selection is to bring this and that form of life into theclosest harmony with its environment that all the conditions of the casepermit. Sometimes it will happen that the harmony will admit of beingimproved by an improvement of organization. But just as often it willhappen that it will be best secured by leaving matters as they are. If, therefore, an organism has already been brought into a tolerably fulldegree of harmony with its environment, natural selection will not tryto change it so long as the environment remains unchanged; and this, nodoubt, is the reason why some species have survived through enormousperiods of geological time without having undergone any change. Again, as we saw in a previous chapter, there are yet other cases where, onaccount of some change in the environment or even in the habits of theorganisms themselves, adaption will be best secured by an active_reversal_ of natural selection, with the result of causing_degeneration_. But, it is sometimes further urged, there are cases where we cannotdoubt that improvement of organization would have been of benefit tospecies; and yet such improvement has not taken place--as, for instance, in the case all monkeys not turning into men. Here, however, we mustremember that the operation of natural selection in any case dependsupon a variety of highly complex conditions; and, therefore, that thefact of all those conditions having been satisfied in one instance is noreason for concluding that they must also have been satisfied in otherinstances. Take, for example, the case of monkeys passing into men. Thewonder to me appears to be that this improvement should have taken placein even one line of descent; not that, having taken place in one line, it should not also have taken place in other lines. For how enormouslycomplex must have been the conditions--physical, anatomical, physiological, psychological, sociological--which by their happyconjunction first began to raise the inarticulate cries of an ape intothe rational speech of a man. Therefore, the more that we appreciate thesuperiority of a man to an ape, the less ought we to countenance thissupposed objection to Darwin's theory--namely, that natural selectionhas not effected the change in more than one line of descent. Even in the case of two races of mankind where one has risen higher inthe scale of civilization than another, it is now generally impossibleto assign the particular causes of the difference; much more, then, mustthis be impossible in the case of still more remote conditions whichhave led to the divergence of species. The requisite variations may nothave arisen in the one line of descent which did arise in the other; orif they did arise in both, some counterbalancing disadvantages may haveattended their initial development in the one case which did not obtainin the other. In short, where so exceedingly complex a play ofconditions are concerned, the only wonder would be if two differentlines of descent _had_ happened to present two independent and yetperfectly parallel lines of history. These general considerations would apply equally to the great majorityof other cases where some types have made great advances upon others, notwithstanding that we can see no reason why the latter should not inthis respect have imitated the former. But there is yet a furtherconsideration which must be taken into account. The struggle forexistence is always most keen between closely allied species, because, from the similarity of their forms, habits, needs, &c. , they are inclosest competition. Therefore it often happens that the mere fact ofone species having made an advance upon others of itself precludes theothers from making any similar advance: the field, so to speak, hasalready been occupied as regards that particular improvement, and wherethe struggle for existence is concerned possession is emphatically ninepoints of the law. For example, to return to the case of apes becomingmen, the fact of one rational species having been already evolved (evenif the rational faculty were at first but dimly nascent) must make anenormous change in the conditions as regards the possibility of anyother such species being subsequently evolved--unless, of course, it beby way of descent from the rational one. Or, as Sir Charles Lyell haswell put it, two rational species can never _coexist_ on the globe, although the descendants of one rational species may in time become_transformed_ into another single rational species[44]. [44] _Principles of Geology_, vol. Ii. P. 487 (11th ed. ). In view of such considerations, another and exactly opposite objectionhas sometimes been urged--viz. That we ought never to find inferiorforms of organization in company with superior, because in the strugglefor existence the latter ought to have exterminated the former. Or, toquote the most recent expression of this view, "in every locality therewould only be one species, and that the most highly organized; and thusa few superior races would partition the earth amongst them to theentire exclusion of the innumerable varieties, species, genera, andorders which now inhabit it[45]. " Of course to this statement it wouldbe sufficient to enquire, On what would these few supremely organizedspecies subsist? Unless manna fell from heaven for their especialbenefit, it would appear that such forms could under no circumstances bethe most improved forms; in exterminating others on such a scale asthis, they would themselves be quickly, and very literally, improved offthe face of the earth. But even when the statement is not made in soextravagant a form as this, it must necessarily be futile as anobjection unless it has first been shown that we know exactly all theconditions of the complex struggle for existence between the higher andlower forms in question. And this it is impossible that we ever canknow. The mere fact that one form has been changed in virtue of thisstruggle must in many cases of itself determine a change in theconditions of the struggle. Again, the other and closely allied forms(and these furnish the best grounds for the objection) may also haveundergone defensive changes, although these may be less conspicuous toour observation, or perhaps less suggestive of "improvement" to ourimperfect means of judging. Lastly, not to continue citing an endlessnumber of such considerations, there is the broad fact that it is onlyto those cases where, for some reason or another, the lower forms havenot been exposed to a struggle of fatal intensity, that the objectionapplies. But we know that in millions of other cases the lower (i. E. Less fitted) forms _have_ succumbed, and therefore I do not see that theobjection has any ground to stand upon. That there is a general tendencyfor lower forms to yield their places to higher is shown by the gradualadvance of organization throughout geological time; for if _all_ theinferior forms had survived, the earth could not have contained them, unless she had been continually growing into something like the size ofJupiter. And if it be asked why any of the inferior forms have survived, the answer has already been given, as above. [45] Syme, on the _Modification of Organisms_, p. 46. There is only one other remark to be made in this connexion. Mr. Symechooses two cases as illustrations of the supposed difficulty. These aresufficiently diverse--viz. Foraminifera and Man. Touching the former, there is nothing that need be added to the general answer just given. But with regard to the latter it must be observed that the dominion ofnatural selection as between different races of mankind is greatlyrestricted by the presence of rationality. Competition in the humanspecies is more concerned with wits and ideas than with nails and teeth;and therefore the "struggle" between man and man is not so much foractual _being_, as for _well-being_. Consequently, in regard to thepresent objection, the human species furnishes the worst example thatcould have been chosen. * * * * * Hitherto I have been considering objections which arise frommisapprehensions of Darwin's theory. I will now go on to consider alogically sound objection, which nevertheless is equally futile, because, although it does not depend on any misapprehension of thetheory, it is not itself supported by fact. The objection is the same as that which we have already considered inrelation to the general theory of descent--namely, that similar organsor structures are to be met with in widely different branches of thetree of life. Now this would be an objection fatal to the theory ofnatural selection, supposing these organs or structures in the casescompared are not merely analogous, but also homologous. For it would beincredible that in two totally different lines of descent one and thesame structure should have been built up independently by two parallelseries of variations, and that in these two lines of descent it shouldalways and independently have ministered to the same function. On theother hand, there would be nothing against the theory of naturalselection in the fact that two structures, _not_ homologous, should comeby independent variation in two different lines of descent to be adaptedto perform the same function. For it belongs to the very essence of thetheory of natural selection that a useful function should be secured byfavourable variations of whatever structural material may happen to bepresented by different organic types. Flying, for instance, is a veryuseful function, and it has been developed independently in at leastfour different lines of descent--namely, the insects, reptiles, birds, and mammals. Now if in all, or indeed in any, of these four cases thewings had been developed on the same anatomical pattern, so as not onlyto present the analogical resemblance which it is necessary that theyshould present in order to discharge their common function of flying, but likewise an homologous or structural resemblance, showing that theyhad been formed on the same anatomical "plan, "--if such has been thecase, I say, the theory of natural selection would certainly bedestroyed. Now it has been alleged by competent naturalists that there are severalsuch cases in organic nature. We have already noticed in a previouschapter (pp. 58, 59), that Mr. Mivart has instanced the eye of thecuttle-fish as not only analogous to, but also homologous with, the eyeof a true fish--that is to say, the eye of a mollusk with the eye of avertebrate. And he has also instanced the remarkable resemblance of ashrew to a mouse--that is, of an insectivorous mammal to a rodent--notto mention other cases. In the chapter alluded to these instances ofhomology, alleged to occur in different branches of the tree of life, were considered with reference to the process of organic evolution as afact: they are now being considered with reference to the agency ofnatural selection as a method. And just as in the former case it wasshown, that if any such alleged instances could be proved, the proofwould be fatal to the general theory of organic evolution by physicalcauses, so in the present case, if this could be proved, it would beequally fatal to the more special theory of natural selection. But, aswe have before seen, no single case of this kind has ever been madeout; and, therefore, not only does this supposed objection fall to theground, but in so doing it furnishes an additional argument in favour ofnatural selection. For in the earlier chapter just alluded to I showedthat this great and general fact of our nowhere being able to find twohomologous structures in different branches of the tree of life, was thestrongest possible testimony in favour of the theory of evolution. And, by parity of reasoning, I now adduce it as equally strong evidence ofnatural selection having been the cause of _adaptive_ structures, independently developed in all the different lines of descent. For thealternative is between adaptations having been caused by naturalselection or by supernatural design. Now, if adaptations were caused bynatural selection, we can very well understand why they should never behomologous in different lines of descent, even in cases where they havebeen brought to be so closely analogous as to have deceived so good anaturalist as Mr. Mivart. Indeed, as I have already observed, so wellcan we understand this, that any single instance to the contrary wouldbe sufficient to destroy the theory of natural selection _in toto_, unless the structure be one of a very simple type. But on the otherhand, it is impossible to suggest any rational explanation why, if alladaptations are due to supernatural design, such scrupulous care shouldhave been taken never to allow homologous adaptations to occur indifferent divisions of the animal or vegetable kingdoms. Why, forinstance, should the eye of a cuttle-fish _not_ have been constructed onthe same ideal pattern as that of vertebrate? Or why, among thethousands of vertebrated species, should no one of their eyes beconstructed on the ideal pattern that was devised for the cuttle-fish?Of course it may be answered that perhaps there was some hidden reasonwhy the design should never have allowed an adaptation which it haddevised for one division of organic nature to appear in another--even incases where the new design necessitated the closest possible resemblancein everything else, save in the matter of anatomical homology. Undoubtedly such may have been the case--or rather such _must_ have beenthe case--if the theory of special design is true. But where thequestion is as to the truth of this theory, I think there can be nodoubt that its rival gains an enormous advantage by being able to_explain_ why the facts are such as they are instead of being obliged totake refuge in hypothetical possibilities of a confessedlyunsubstantiated and apparently unsubstantial kind. Therefore, as far as this objection to the theory of natural selectionis concerned--or the allegation that homologous structures occur indifferent divisions of organic nature--not only does it fall to theground, but positively becomes itself converted into one of thestrongest arguments in favour of the theory. As soon as the allegationis found to be baseless, the very fact that it cannot be brought to bearupon any one of all the millions of adaptive structures in organicnature becomes a fact of vast significance on the opposite side. * * * * * The next difficulty to which I shall allude is that of explaining by thetheory of natural selection the preservation of the first beginnings ofstructures which are then useless, though afterwards, when more fullydeveloped, they become useful. For it belongs to the very essence of thetheory of natural selection, that a structure must be supposed alreadyuseful before it can come under the influence of natural selection:therefore the theory seems incapable of explaining the origin andconservation of _incipient_ organs, or organs which are not yetsufficiently developed to be of any service to the organisms presentingthem. This objection is one that has been advanced by all the critics ofDarwinism; but has been presented with most ability and force by theDuke of Argyll. I will therefore state it in his words. If the doctrine of evolution be true--that is to say, if all organic creatures have been developed by ordinary generation from parents--then it follows of necessity that the primæval germs must have contained potentially the whole succeeding series. Moreover, if that series has been developed gradually and very slowly, it follows, also as a matter of necessity, that every modification of structure must have been functionless at first, when it began to appear.... Things cannot be selected until they have first been produced. Nor can any structure be selected by utility in the struggle for existence until it has not only been produced, but has been so far perfected as to actually be used. The Duke proceeds to argue that all adaptive structures must thereforeoriginally have been due to special design: in the earlier stages oftheir development they must all have been what he calls "propheticgerms. " Not yet themselves of any use, and therefore not yet capable ofbeing improved by natural selection, both in their origin and in thefirst stages (at all events) of their development, they must beregarded as intentionally preparatory to the various uses which theysubsequently acquire. Now this argument, forcible as it appears at first sight, is really atfault both in its premiss and in its conclusion. By which I mean that, in the first place the premiss is not true, and, in the next place, thateven if it were, the conclusion would not necessarily follow. Thepremiss is, "that every modification of structure must have beenfunctionless at first, when it began to appear;" and the conclusion is, that, _quâ_ functionless, such a modification cannot have been caused bynatural selection. I will consider these two points separately. First as to the premiss, it is not true that every modification ofstructure must necessarily be functionless when it first begins toappear. There are two very good reasons why such should not be the casein all instances, even if it should be the case in some. For, as amatter of observable fact, a very large proportional number of incipientorgans are useful from the very moment of their inception. Take, forexample, what is perhaps the most wonderful instance of refinedmechanism in nature--the eye of a vertebrated animal. Comparativeanatomy and embryology combine to testify that this organ had its originin modifications of the endings of the ordinary nerves of the skin. Nowit is evident that from the very first any modification of a cutaneousnerve whereby it was rendered able, in however small a degree, to bedifferently affected by light and by darkness would be of benefit to thecreature presenting it; for the creature would thus be able to seek theone and shun the other according to the requirements of its life. Andbeing thus useful from the very moment of its inception, it wouldafterwards be gradually improved as variations of more and more utilitypresented themselves, until not only would finer and finer degrees ofdifference between light and shade become perceptible, but even theoutlines of solid bodies would begin to be appreciated. And so on, stageby stage, till from an ordinary nerve-ending in the skin is evolved theeye of an eagle. Moreover, in this particular instance there is very good reason tosuppose that the modification of the cutaneous nerves in question beganby a progressive increase in their sensitiveness to temperature. Wherever dark pigment happened to be deposited in the skin--and we knowthat in all animals it is apt to be deposited in points and patches, asit were by accident, or without any "prophecy" as to future uses, --thecutaneous nerves in its vicinity would be better able to appreciate thedifference between sun and shade in respect of temperature, even thoughas yet there were no change at all in these cutaneous nerves tending tomake them responsive to light. Now it is easy to see how, from such apurely accidental beginning, natural selection would have had from thefirst sufficient material to act upon. It being of advantage to a lowlycreature that it should distinguish with more and more delicacy, or withmore and more rapidity, between light and darkness by means of itsthermal sensations, the pigment spots in the skin would be renderedpermanent by natural selection, while the nerves in that region would bythe same agency be rendered more and more specialized as organs adaptedto perceive changes of temperature, until from the stage of respondingto the thermal rays of the non-luminous spectrum alone, they becomecapable of responding also to luminous. So much, then, for the first consideration which serves to invalidatethe Duke's premiss. The second consideration is, that very often anorgan which began by being useful for the performance of one function, after having been fully developed for the performance of that function, finds itself, so to speak, accidentally fitted to the performance ofsome other and even more important function, which it thereupon beginsto discharge, and so to undergo a new course of adaptive development. Insuch cases, and so far as the new function is concerned, the difficultytouching the first inception of an organ does not apply; for here theorgan has already been built up by natural selection for one purpose, before it begins to discharge the other. As an example of such a case wemay take the lung of an air-breathing animal. Originally the lung was aswim-bladder, or float, and as such it was of use to the aquaticancestors of terrestrial animals. But as these ancestors graduallybecame more and more amphibious in their habits, the swim-bladder beganmore and more to discharge the function of a lung, and so to take awholly new point of departure as regards its developmental history. Butclearly there is here no difficulty with regard to the inception of itsnew function, because the organ was already well developed for onepurpose before it began to serve another. Or, to take only oneadditional example, there are few structures in the animal kingdom soremarkable in respect of adaptation as is the wing of a bird or a bat;and at first sight it might well appear that a wing could be of noconceivable use until it had already acquired enormous proportionaldimensions, as well as an immense amount of special elaboration as toits general form, size of muscle, amount of blood-supply, and so on. For, obviously, not until it had attained all these things could it evenbegin to raise the animal in the air. But observe how fallacious is thisargument. Although it is perfectly true that a wing could be of no use_as a wing_ until sufficiently developed to serve the purpose of flight, this is merely to say that until it has become a wing it is no use as awing. It does not, however, follow that on this account it was of noprior use for any other purpose. The first modifications of thefore-limb which ended in its becoming an organ of flight may very wellhave been due to adapting it as an organ for increased rapidity oflocomotion of other kinds--whether on land as in the case of its nowdegenerated form in the ostrich, or in water as in the case of theexpanded fins of fish. Indeed, we may see the actual process oftransition from the one function to the other in the case of"flying-fish. " Here the progressive expansion of the pectoral fins mustcertainly have been always of use for continuously promoting rapidity oflocomotion through water; and thus natural selection may havecontinuously increased their development until they now begin to servealso as wings for carrying the animal a short distance through air. Again, in the case of the so-called flying squirrels we find the limbsunited to the body by means of large extensions of the skin, so-thatwhen jumping from one tree to another the animal is able to sustainitself through a long distance in the air by merely spreading out itslimbs, and thus allowing the skin-extensions to act after the manner ofa parachute. Here, of course, we have not yet got a wing, any more thanwe have in the case of the flying-fish; but we have the foundations laidfor the possible development of a future wing, upon a somewhat similarplan as that which has been so wonderfully perfected in the case ofbats. And through all the stages of progressive expansion which the skinof the squirrel has undergone, the expansion has been of use, eventhough it has not yet so much as begun to acquire the distinctivefunctions of a wing. Here, then, there is obviously nothing "prophetic"in the matter, any more than there was in the case of the swim-bladderand the lung, or in that of the nerve-ending and the eye. In short, itis the business of natural selection to secure the highest availabledegree of adaptation for the time being; and, in doing this, it notunfrequently happens that an extreme development of a structure in onedirection (produced by natural selection for the sake of better andbetter adapting the structure to perform some particular function) endsby beginning to adapt it to the performance of some other function. And, whenever this happens to be the case, natural selection forthwith beginsto act upon the structure, so to speak, from a new point of departure. So much, then, for the Duke's premiss--namely, that "every modificationof structure _must_ have been functionless _at first_, when it began toappear. " This premiss is clearly opposed to observable fact. But now, the second position is that, even if this were not so, the Duke'sconclusion would not follow. This conclusion, it will be remembered, is, that if incipient structures are useless, it necessarily follows thatnatural selection can have had no part whatever in their inception. Now, this is a conclusion which does not "necessarily" follow. Even if it begranted that there are structures which in their first beginnings arenot of any use at all for any purpose, it is still possible that theymay owe their origin to natural selection--not indeed directly, butindirectly. This possibility arises from the occurrence in nature of aprinciple which has been called the Correlation of Growth. Mr. Darwin, who has paid more attention to this matter than any otherwriter, has shown, in considerable detail, that all the parts of anygiven organism are so intimately bound together, or so mutuallydependent upon each other, that when one part is caused to change bymeans of natural selection, some other parts are very likely to undergomodification as a consequence. For example, there are several kinds ofdomesticated pigeons and fowls, which grow peculiar wing-like featherson the feet. These are quite unlike all the other feathers in theanimal, except those of the wing, to which they bear a very remarkableresemblance. Mr. Darwin records the case of a bantam where thesewing-like feathers were nine inches in length, and I have myself seen apigeon where they reproduced upon the feet a close imitation of thedifferent kinds of feathers which occupy homologous positions in thewing--primaries, secondaries, and tertiaries all being distinctlyrepeated in their proper anatomical relations. Furthermore, in thiscase, as in most cases where such wing-feathers occur upon the feet, thethird and fourth toes were partly united by skin; and, as is well known, in the wing of a bird the third and fourth digits are completely unitedby skin; "so that in feather-footed pigeons, not only does the exteriorsurface support a row of long feathers, like wing-feathers [which, asjust stated, may in some cases be obviously differentiated intoprimaries, secondaries and tertiaries], but the very same digits whichin the wing are completely united by skin become partially united byskin in the feet; and thus by the law of correlated variation ofhomologous parts, we can understand the curious connexion of featheredlegs and membrane between the two outer toes[46]. " The illustration isdrawn from the specimen to which I have referred. [46] _Variation of Plants and Animals_, vol. Ii. P. 315. [Illustration: FIG. 117. --Feather-footed pigeon. Drawn from nature. ] Many similar instances of the same law are to be met with throughoutorganic nature; and it is evident that in this principle we find aconceivable explanation of the origin of such adaptive structures ascould not have been originated by natural selection acting directly uponthemselves: they may have been originated by natural selectiondeveloping other adaptive structures elsewhere in the organism, thegradual evolution of which has entailed the production of these bycorrelation of growth. And, if so, when once started in this way, thesestructures, because thus accidentally useful, will now themselves comeunder the _direct_ action of natural selection, and so have theirfurther evolution determined with or without the correlated associationwhich first led to their inception. Of course it must be understood that in thus applying the principle ofcorrelated growth, to explain the origin of adaptive structures where itis impossible to explain such origin by natural selection having fromthe first acted directly upon these structures themselves, Darwinistsdo not suppose that in all--or even in most--cases of correlated growththe correlated structures are of use. On the contrary, it is well knownthat structures due to correlated growth are, as a rule, useless. Beingonly the by-products of adaptive changes going on elsewhere, in anygiven case the chances are against these correlated effects beingthemselves of any utilitarian significance; and, therefore, as a matterof fact, correlated growths appear to be usually meaningless from thepoint of view of adaptation. Still, on the doctrine of chances, it isto be expected that sometimes a change of structure which has thus beenindirectly produced by correlation of growth might happen to proveuseful for some purpose or another; and in as many cases as suchindirectly produced structures do prove useful, they will straightwaybegin to be improved by the direct action of natural selection. In allsuch cases, therefore, we should have an explanation of the _origin_ ofsuch a structure, which is the only point that we are now considering. I think, then, that all this effectually disposes of the doctrine of"prophetic germs. " But, before leaving the subject, I should like tomake one further statement of greater generality than any which I havehitherto advanced. This statement is, that we must remember how large astock of meaningless structures are always being produced in the courseof specific transmutations, not only by correlation of growth, which wehave just been considering, but also by the direct action of externalconditions, together with the constant play of all the many and complexforces internal to organisms themselves. In other words, important asthe principle of correlation undoubtedly is, we must remember that eventhis is very far from being the only principle which is concerned in theorigination of structures that may or may not chance to be useful. Therefore, it is not only natural selection when operating indirectlythrough the correlation of growth that is competent to produce newstructures without reference to utility. In all the complex action andreaction of internal and external forces, new variations are perpetuallyarising without any reference to utility, either present or future. Among all this multitude of promiscuous variations, the chances must bethat some percentage will prove of some service, either from the firstmoment of their appearance, or else after they have undergone someamount of development. Such development prior to utility may be due, either to correlation of growth, to the structure having previouslyperformed some other function, as already explained, or else to acontinued operation of the causes which were concerned in the firstappearance of originally useless characters. In a series of chapterswhich will be devoted to the whole question of utility in the nextvolume, I shall hope to give very good reasons for concluding thatuseless characters are not only of highly frequent occurrence, but aredue to a variety of other causes besides correlation of growth. And, ifso, the possibility of originally useless characters happening in somecases to become, by increased development, useful characters, iscorrespondingly increased. Among a hundred varietal or specificcharacters which are directly produced in as many different species by achange of climate, for example, some five or six may be _potentially_useful: that is to say, characters thus adventitiously produced in anincipient form may only require to be further developed by a continuanceof the same causes as first originated them, in order that somepercentage of the whole number shall become of some degree of use. Thoseprofessed followers of Darwin, therefore, who without any reason--or, asit appears to me, against all reason--deny the possibility of uselessspecific characters in any case or in any degree (unless correlated withuseful characters), are playing into the hands of Darwin's critics byindirectly countenancing the difficulty which we are now considering. For, if correlation of growth is unreasonably supposed to be the onlypossible cause of the origin of incipient structures which are notuseful from the first moment of their inception, clearly the field isgreatly narrowed as regards the occurrence of incipient characterssufficient in amount--and, still more, in constancy of appearance andpersistency of transmission--to admit of furnishing material for theworking of natural selection. But in the measure that incipientcharacters--whether varietal or specific--are recognised as not alwaysor "necessarily" useful from the moment of their inception, and yetcapable of being developed to a certain extent by the causes which firstled to their occurrence, in that measure is this line of criticismclosed. For of all the variations which thus occur, it is only thosewhich afterwards prove of any use that are laid hold upon and wrought upby natural selection into adaptive structures, or working organs. And, therefore, what we see in organic nature is the net outcome of thedevelopment of all the happy chances. So it comes that the appearancepresented by organic nature as a whole is that of a continual fulfilmentof structural prophecies, when, in point of fact, if we had a similarrecord of all the other variations it would be seen that possibly notone such prophecy in a thousand is ever destined to be fulfilled. * * * * * Here, then, I feel justified in finally taking leave of the difficultyfrom the uselessness of incipient organs, as this difficulty has beenpresented, in varying degrees of emphasis, by the Duke of Argyll, Mr. Mivart, Professors Nägeli, Bronn, Broca, Eimer, and, indeed, by allother writers who have hitherto advanced it. For, as thus presented, Ithink I have shown that it admits of being adequately met. But now, Imust confess, to me individually it does appear that behind thiserroneous presentation of the difficulty there lies another question, which is deserving of much more serious attention. For although itadmits of being easily shown--as I have just shown--that the difficultyas ordinarily presented fails on account of its extravagance, thequestion remains whether, if stated with more moderation, a realdifficulty might not be found to remain. My quarrel with the conclusion, like my quarrel with the premiss, is dueto its universality. By saying in the premiss that _all_ incipientorgans are _necessarily_ useless at the time of their inception, thesewriters admit of being controverted by fact; and by saying in theconclusion that, _if_ all incipient organs are useless, it necessarilyfollows that in _no_ case can natural selection have been the cause ofbuilding up an organ until it becomes useful, they admit of beingcontroverted by logic. For, even if the premiss were true infact--namely, that all incipient organs are useless at the time of theirinception, --it would not necessarily follow that in no case couldnatural selection build up a useless structure into a useful one;because, although it is true that in no case can natural selection dothis by acting on a useless structure _directly_, it may do so by actingon the useless structure _indirectly_, through its direct action on someother part of the organism with which the useless structure happens tobe correlated. Moreover, as I believe, and will subsequently endeavourto prove, there is abundant evidence to show that incipient charactersare often developed to a large extent by causes other than naturalselection (or apart from any reference to utility), with the result thatsome of them thus happen to become of use, when, of course, the supposeddifficulty is at an end. But although it is thus easy to dispose of both the propositions inquestion, on account of their universality, stated more carefully theywould require, as I have said, more careful consideration. Thus, if ithad been said that some incipient organs are _presumably_ useless at thetime of their inception, and that in _some of these cases_ it isdifficult, or impossible, to conceive how the principle of correlation, or any other principle hitherto suggested, can apply--then the questionwould have been raised from the sphere of logical discussion to that ofbiological fact. And the new question thus raised would have to bedebated, no longer on the ground of general or abstract principles, buton that of special or concrete cases. Now until within the last year ortwo it has not been easy to find such a special or concrete case--thatis to say, a case which can be pointed to as apparently excluding thepossibility of natural selection having had anything to do with thegenesis of an unquestionably adaptive structure. But eventually such acase has arisen, and the Duke of Argyll has not been slow in perceivingits importance. This case is the electric organ in the tail of theskate. No sooner had Professor Cossar Ewart published an abstract of hisfirst paper on this subject, than the Duke seized upon it as a case forwhich, as he said, he had long been waiting--namely, the case of an_adaptive_ organ the genesis of which _could not possibly_ be attributedto natural selection, and must therefore be attributed to supernaturaldesign. Now, I do not deny that he is here in possession of an admirablecase--a case, indeed, so admirable that it almost seems to have beenspecially designed for the discomfiture of Darwinians. Therefore, inorder to do it full justice, I will show that it is even more formidablethan the Duke of Argyll has represented. Electric organs are known to occur in several widely different kinds offish--such as the _Gymnotus_ and _Torpedo_. Wherever these organs dooccur, they perform the function of electric batteries in storing anddischarging electricity in the form of more or less powerful shocks. Here, then, we have a function which is of obvious use to the fish forpurposes both of offence and defence. These organs are everywherecomposed of a transformation of muscular, together with an enormousdevelopment of nervous tissue; but inasmuch as they occupy differentpositions, and are also in other respects dissimilar in the differentzoological groups of fishes where they occur, no difficulty can bealleged as to these analogous organs being likewise homologous indifferent divisions of the aquatic vertebrata. Now, in the particular case of the skate, the organ is situated in thetail, where it is of a spindle-like form, measuring, in a large fish, about two feet in length by about an inch in diameter at the middle ofthe spindle. Although its structure is throughout as complex and perfectas that of the electric organ in _Gymnotus_ or _Torpedo_, its smallersize does not admit of its generating a sufficient amount ofelectricity to yield a discharge that can be felt by the hand. Nevertheless, that it does discharge under suitable stimulation has beenproved by Professor Burdon Sanderson by means of a telephone; for hefound that every time he stimulated the animal its electrical dischargewas rendered audible by the telephone. Here, then, the difficultyarises. For of what conceivable use is such an organ to its possessor?We can scarcely suppose that any aquatic animal is more sensitive toelectric shocks than is the human hand; and even if such were the case, a discharge of so feeble a kind taking place in water would beshort-circuited in the immediate vicinity of the skate itself. So therecan be no doubt that such weak discharges as the skate is able todeliver must be wholly imperceptible alike to prey and to enemies. Yetfor the delivery of such discharges there is provided an organ of suchhigh peculiarity and huge complexity, that, regarded as a piece ofliving mechanism, it deserves to rank as at once the most extremelyspecialized and the most highly elaborated structure in the whole animalkingdom. Thousands of separately formed elements are ranged in row afterrow, all electrically insulated one from another, and packed away intothe smallest possible space, with the obvious end, or purpose, ofconspiring together for the simultaneous delivery of an electric shock. Nevertheless, the shock when delivered is, as we have just seen, tooslight to be of any conceivable use to the skate. Therefore it appearsimpossible to suggest how this astonishing structure--much moreastonishing, in my opinion, than the human eye or the human hand--canever have been begun, or afterwards developed, by means of naturalselection. For if it be not even yet of any conceivable use to itspossessor, clearly thus far survival of the fittest can have had nothingto do with its formation. On the other hand, seeing that electric organswhen of larger size, as in the _Gymnotus_ and _Torpedo_, are of obvioususe to their possessors, the facts of the case, so far as the skate isconcerned, assuredly do appear to sanction the doctrine of "propheticgerms. " The organ in the skate seems to be on its way towards becomingsuch an organ as we meet with in these other animals; and, therefore, unless we can show that it is now, and in all previous stages of itsevolution has throughout been, of use to the skate, the facts do presenta serious difficulty to the theory of natural selection, while theyreadily lend themselves to the interpretation of a disposing orfore-ordaining mind, which knows how to construct an electric battery bythus transforming muscular tissue into electric tissue, and is nowactually in process of constructing such an apparatus for theprospective benefit of future creatures. Should it be suggested that possibly the electric organ of the skate maybe in process of degeneration, and therefore that it is now thepractically functionless remnant of an organ which in the ancestors ofthe skate was of larger size and functional use--against so obvious asuggestion there lie the whole results of Professor Ewart'sinvestigations, which go to indicate that the organ is here not in astage of degeneration, but of evolution. For instance, in _Raiaradiata_, it does not begin to be formed out of the muscular tissueuntil some time after the animal has left the egg-capsule, and assumedall the normal proportions (though not yet the size) of the adultcreature. The organ, therefore, is one of the very latest to appear inthe ontogeny of _R. Radiata_; and, moreover, it does not attain its full_development_ (i. E. Not merely _growth_, but transforming of muscularfibres into electrical elements) till the fish attains maturity. Read inthe light of embryology, these facts prove, (1) that the electric organof _R. Radiata_ must be one of the very latest products of theanimal's phylogeny; and, (2) that as yet, at all events, it has notbegun to degenerate. But, if not, it must either be at a stand-still, orit must be in course of further evolution; and, whichever of thesealternatives we adopt, the difficulty of accounting for its presentcondition remains. In this connexion also it is worth while to remarkthat the electric organ, even after it has attained its full_development_, continues its _growth_ with the growth of the fish, andthis in a much higher ratio, either than the tail alone, or the wholeanimal. Lastly, Prof. Burdon Sanderson finds that _section for section_the organ in the skate is as efficient as it is in _Torpedo_. It isevident that these facts also point to the skate's organ being in courseof phylogenetic evolution. [Illustration: Fig. 118. --_Raia radiata_, representing the life size of the youngest individual in which muscle fibres have been found developing into electric cells]. [Illustration: Fig. 119. --Electric organ of the skate. The left-hand drawing (i) represents the entire organ (natural size) of a full-grown _r. Radiata_. This is a small skate, which rarely exceeds 50 centms. In length; but in the large _r. Batis_, the organ may exceed two feet in length. The other drawings represent single muscle-fibres in successive stages of transition. In the first of the series (ii) the motor plate, and the nerves connected with it, have already been considerably enlarged. In the other three specimens, the fibre becomes more and more club-like, and eventually cup-like. These changes of shape are expressive of great changes of structure, as may be seen in the last of the series (v), where the shallow cup is seen in partial section. The electric plate lines the concavity of the cup, and is richly supplied with nerves (only a few of which are represented in the last drawing); the thick walls of the cup are composed of muscular fibres, the striation of which is distinctly visible. ] [Illustration: Fig. 120. --Electric cells of _raia radiata_. The drawing on the left represents one of the clubs magnified, as in the preceding wood-cut. The drawing on the right represents a number of these clubs, less highly magnified, _in situ_. ] Again, it cannot be answered that the principle of correlation may bedrawn upon in mitigation of the difficulty. The structure of theelectric organ is far too elaborate, far too specialized, and far tooobviously directed to a particular end, to admit of our conceivablysupposing it due to any accidental correlation with structural changesgoing on elsewhere. Even as regards the initial changes ofmuscle-elements into electrical-elements, I do not think the principleof correlation can be reasonably adduced by way of explanation; for, asshown in the illustrations, even this initial change is mostextraordinarily peculiar, elaborate, and specialized. But, be this as itmay, I am perfectly certain that the principle of correlation cannotpossibly be adduced to explain the subsequent _association of theseelectrical elements into an electric battery_, actuated by a specialnervous mechanism of enormous size and elaboration--unless of course, the progress of such a structure were assumed to have been throughout ofsome utility. Under this supposition, however, the principle ofcorrelation would be forsaken in favour of that of natural selection;and we should again be in the presence of the same difficulty as thatwith which we started. But now, and further, if we do thus abandon correlation in favour ofnatural selection, and therefore if for the sake of saving an hypothesiswe assume that the organ as it now stands _must_ be of some use to theexisting skate, we should still have to face the question--Of whatconceivable use can those initial stages of its formation have been, when first the muscle-elements began to be changed into the verydifferent electrical-elements, and when therefore they became useless asmuscles while not yet capable of performing even so much of theelectrical function as they now perform? Lastly, we must remember that not only have we here the most highlyspecialized, the most complex, and altogether the most elaborativelyadaptive organ in the animal kingdom; but also that in the formation ofthis structure there has been needed an altogether unparalleledexpenditure of the most physiologically expensive of allmaterials--namely, nervous tissue. Whether estimated by volume or byweight, the quantity of nervous tissue which is consumed in the electricorgan of the skate is in excess of all the rest of the nervous systemput together. It is needless to say that nowhere else in the animalkingdom--except, of course, in other electric fishes--is there anyapproach to so enormous a development of nervous tissue for thedischarge of a special function. Therefore, as nervous tissue is, physiologically speaking, the most valuable of all materials, we areforced to conclude that natural selection ought strongly to have_opposed_ the evolution of such organs, unless from the first moment oftheir inception, and throughout the whole course of their development, they were of some such paramount importance as biologically to justifyso unexampled an expenditure. Yet this paramount importance does notadmit of being so much as surmised, even where the organ has alreadyattained the size and degree of elaboration which it presents in theskate. In view of all these considerations taken together, I freely confessthat the difficulty presented by this case appears to me of a magnitudeand importance altogether unequalled by that of any other singlecase--or any series of cases--which has hitherto been encountered by thetheory of natural selection. So that, if there were many other cases ofthe like kind to be met with in nature, I should myself at once allowthat the theory of natural selection would have to be discarded. Butinasmuch as this particular case stands so far entirely by itself, andtherefore out of analogy with thousands, or even millions, of othercases throughout the whole range of organic nature, I am constrained tofeel it more probable that the electric organ of the skate will some dayadmit of being marshalled under the general law of natural selection--injust the same way as proved to be the case with the conspicuouscolouring of those caterpillars, which, as explained in the lastchapter, at one time seemed to constitute a serious difficulty to thetheory, and yet, through a better knowledge of all the relationsinvolved, has now come to constitute one of the strongest witnesses inits favour. * * * * * I have now stated all the objections of any importance which havehitherto been brought against the theory of natural selection, exceptingthree, which I left to be dealt with together because they form alogically connected group. With a brief consideration of these, therefore, I will bring this chapter to a close. The three objections to which I allude are, (1) that a largeproportional number of specific, as well as of higher taxonomiccharacters, are seemingly useless characters, and therefore do not lendthemselves to explanation by the Darwinian theory; (2) that the mostgeneral of all specific characters--viz. Cross-infertility betweenallied species--cannot possibly be due to natural selection, as isdemonstrated by Darwin himself; (3) that the swamping effects of freeintercrossing must always render impossible by natural selection aloneany evolution of species in divergent (as distinguished from serial)lines of change. These three objections have been urged from time to time by not a few ofthe most eminent botanists and zoologists of our century; and from onepoint of view I cannot myself have the smallest doubt that theobjections thus advanced are not only valid in themselves, but also byfar the most formidable objections which the theory of natural selectionhas encountered. From another point of view, however, I am equallyconvinced that they all admit of absolute annihilation. This strongantithesis arises, as I have said, from differences of standpoint, orfrom differences in the view which we take of the theory of naturalselection itself. If we understand this theory to set forth naturalselection as the sole cause of organic evolution, then all the aboveobjections to the theory are not merely, as already stated, valid andformidable, but as I will now add, logically insurmountable. On theother hand, if we take theory to consist merely in setting forth naturalselection as a factor of organic evolution, even although we believe itto have been the chief factor or principal cause, all the threeobjections in question necessarily vanish. For in this case, even if itbe satisfactorily proved that the theory of natural selection is unableto explain the three classes of facts above mentioned, the theory is notthereby affected: facts of each and all of these classes may beconsistently left by the theory to be explained by causes other thannatural selection--whether these be so far capable or incapable ofhypothetical formulation. Thus it is evident that whether the threeobjections above named are to be regarded as logically insurmountable bythe theory, or as logically non-existent in respect to it, dependssimply upon the manner in which the theory itself is stated. In the next volume a great deal more will have to be said upon thesematters--especially with regard to the causes other than naturalselection which in my opinion are capable of explaining these so-called"difficulties. " In the present connexion, however, all I have attemptedto show is, that, whatever may be thought touching the supplementarytheories whereby I shall endeavour to explain the facts of inutility, cross-sterility, and non-occurrence of free intercrossing, no one ofthese facts is entitled to rank as an objection against the theory ofnatural selection, unless we understand this theory to claim anexclusive prerogative in the field of organic evolution. This, as wehave previously seen, is what Mr. Wallace does claim for it; while onthe other hand, Mr. Darwin expressly--and even vehemently--repudiatesthe claim: from which it follows that all the three main objectionsagainst the theory of natural selection are objections which vitallyaffect the theory only as it has been stated and upheld by Wallace. Asthe theory has been stated and upheld by Darwin, all these objectionsare irrelevant. This is a fact which I had not myself perceived at thetime when I mentioned these objections in a paper entitled_Physiological Selection_, which was published in 1886. The discussionsto which that paper gave rise, however, led me to consider these mattersmore closely; and further study of Darwin's writings, with these mattersspecially in view, has led me to see that none of the objections inquestion are relevant to his theory, as distinguished from that of Mr. Wallace. This, I acknowledge, I ought to have perceived before Ipublished the paper just alluded to; but in those days I had had nooccasion to follow out the differences between Darwin and Wallace to alltheir consequences, and therefore adopted the prevalent view that theirtheories of evolution were virtually identical. Now, however, I haveendeavoured to make it clear that the points wherein they differ involvethe important consequences above set forth. All these the mostformidable objections against the theory of natural selection arisesimply and solely from what I conceive to be the erroneous manner inwhich the theory has been presented by Darwin's distinguished colleague. * * * * * I have now considered, as impartially as I can, all the main criticismsand objections which have been brought against the theory of naturalselection; and the result is to show that, neither singly norcollectively, are they entitled to much weight. On the other hand, aswe have seen in the preceding chapter, there is a vast accumulation ofevidence in favour of the theory. Hence, it is no wonder that the theoryhas now been accepted by all naturalists, with scarcely any one notableexception, as at any rate the best working hypothesis which has everbeen propounded whereby to explain the facts of organic evolution. Moreover, in the opinion of those most competent to judge, the theory isentitled to be regarded as something very much more than a workinghypothesis: it is held to be virtually a completed induction, or, inother words, the proved exhibition of a general law, whereby thecausation of organic evolution admits of being in large part--if notaltogether--explained. Now, whether or not we subscribe to this latter conclusion ought, Ithink, to depend upon what we mean by an explanation in the case whichis before us. If we mean only that, given the large class of known factsand unknown causes which are conveniently summarized under the termsHeredity and Variability, then the further facts of Struggle andSurvival serve, in some considerable degree or another, to account forthe phenomena of adaptive evolution, I cannot see any room to questionthat the evidence is sufficient to prove the statement. But it is clearthat by taking for granted these great facts of Heredity andVariability, we have assumed the larger part of the problem as a whole. Or, more correctly, by thus generalizing, in a merely verbal form, allthe unknown causes which are concerned in these two great factors of theprocess in question, we are not so much as attempting to explain theprecedent causation which serves as a condition to the process. Muchmore than half the battle would already have been won, had Darwin'spredecessors been able to explain the causes of Heredity and Variation;hence it is but a very partial victory which we have hitherto gained inour recent discovery of the effects of Struggle and Survival. Yet partial though it be in relation to the whole battle, in itself, orconsidered absolutely, there can be no reasonable doubt that itconstitutes the greatest single victory which has ever been gained bythe science of Biology. For this very reason, however, it behoves us toconsider all the more carefully the extent to which it goes. But mydiscussion of this matter must be relegated to the next volume, where Ihope to give abundant proof of the soundness of Darwin's judgment asconveyed in the words:--"I am convinced that natural selection has beenthe main, but not the exclusive, means of modification. " CHAPTER X. THE THEORY OF SEXUAL SELECTION, AND CONCLUDING REMARKS. Although the explanatory value of the Darwinian theory of naturalselection is, as we have now seen, incalculably great, it neverthelessdoes not meet those phenomena of organic nature which perhaps more thanany other attract the general attention, as well as the generaladmiration, of mankind: I mean all that class of phenomena which go toconstitute the Beautiful. Whatever value beauty as such may have, itclearly has not a life-preserving value. The gorgeous plumage of apeacock, for instance, is of no advantage to the peacock in his strugglefor life, and therefore cannot be attributed to the agency of naturalselection. Now this fact of beauty in organic structures is a fact ofwide generality--almost as wide, indeed, as is the fact of theirutility. Mr. Darwin, therefore, suggested another hypothesis whereby torender a scientific explanation of this fact. Just as by his theory ofnatural selection he sought to explain the major fact of utility, so didhe endeavour to explain the minor fact of beauty by a theory of what hetermed Sexual Selection. It is a matter of observation that the higher animals do not pairindiscriminately; but that the members of either sex prefer thoseindividuals of the opposite sex which are to them most attractive. It isimportant to understand _in limine_ that nobody has ever attempted tochallenge this statement. In other words, it is an unquestionable factthat among many of the higher animals there literally and habituallyoccurs a _sexual selection_; and this fact is not a matter of inference, but, as I have said, a matter of observation. The inference only beginswhere, from this observable fact, it is argued, --1st, that the sexualselection has reference to an æsthetic taste on the part of the animalsthemselves; and 2nd, that, supposing the selection to be determined bysuch a taste, the cause thus given is adequate to explain the phenomenaof beauty which are presented by these animals. I will consider thesetwo points separately. From the evidence which Darwin has collected, it appears to meimpossible to doubt that an æsthetic sense is displayed by many birds, and not a few mammals. This of course does not necessarily imply thatthe _standards_ of such a sense are the same as our own; nor does itnecessarily imply that there is any constant relation between such asense and high levels of intelligence in other respects. In point offact, such is certainly not the case, because the best evidence that wehave of an æsthetic sense in animals is derived from birds, and not frommammals. The most cogent cases to quote in this connexion are those ofthe numerous species of birds which habitually adorn their nests withgaily coloured feathers, wool, cotton, or any other gaudy materialswhich they may find lying about the woods and fields. In many cases amarked preference is shown for particular objects--as, for instance, inthe case of the Syrian nut-hatch, which chooses the iridescent wings ofinsects, or that of the great crested fly-catcher, which similarlychooses the cast-off skins of snakes. But no doubt the most remarkableof these cases is that of the baya-bird of Asia, which after havingcompleted its bottle-shaped and chambered nest[47], studs it over withsmall lumps of clay, both inside and out, upon which the cock-birdsticks fire-flies, apparently for the sole purpose of securing abrilliantly decorative effect. Other birds, such as the hammer-head ofAfrica, adorn the surroundings of their nests (which are built upon theground) with shells, bones, pieces of broken glass and earthenware, orany objects of a bright and conspicuous character which they may happento find. The most consummate artists in this respect are, however, thebower-birds; for the species of this family construct elaborateplay-houses in the form of arched tunnels, built of twigs upon theground. Through and around such a tunnel they chase one another; and itis always observable that not only is the floor paved with a greatcollection of shells, bones, coloured stones, and any other brilliantobjects which they are able to carry in their beaks, but also that thewalls are decorated with the most gaudy articles which the birds canfind. There is one genus, in Papua, which even goes so far as to providethe theatre with a surrounding garden. A level piece of ground isselected as a site for the building. The latter is about two feet high, and constructed round the growing stalk of a shrub, which thereforeserves as a central pillar to which the frame-work of the roof isattached. Twigs are woven into this frame-work until the whole isrendered rain-proof. The tent thus erected is about nine feet incircumference at its base, and presents a large arch as an entrance. Thecentral pillar is banked up with moss at its base, and a gallery isbuilt round the interior of the edifice. This gallery is decorated withflowers, fruits, fungi, &c. These are also spread over the garden, whichcovers about the same area as the play-house. The flowers are said tobe removed when they fade, while fresh ones are gathered to supply theirplaces. Thus the garden is always kept bright with flowers, as well aswith the brilliant green of mosses, which are collected and distributedin patches, resembling tiny lawns. [47] The chambers are three in number. The two upper ones are occupied respectively by the male and the sitting female. The lower one serves as a general living room when the young are hatched. [Illustration: FIG. 121. --The Garden Bower-bird (_Amblyornis inornata_). Reduced from _Gould's Birds of New Guinea_ to 1/4 nat. Size. ] Now these sundry cases alone seem to prove a high degree of the æstheticsense as occurring among birds; for, it is needless to say, none of thefacts just mentioned can be due to natural selection, seeing that theyhave no reference to utility, or the preservation of life. But if anæsthetic sense occurs in birds, we should expect, on _a priori_ grounds, that it would probably be exercised with reference to the personalappearance of the sexes. And this expectation is fully realized. For itis an observable fact that in most species of birds where the males areremarkable for the brilliancy of their plumage, not only is thisbrilliancy most remarkable during the pairing season, but at this seasonalso the male birds take elaborate pains to display their charms beforethe females. Then it is that the peacock erects his tail to strut roundand round the hens, taking care always to present to them a front view, where the coloration is most gorgeous. And the same is true of all othergaily coloured male birds. During the pairing season they activelycompete with one another in exhibiting their attractiveness to thefemales; and in many cases there are added all sorts of extraordinaryantics in the way of dancings and crowings. Again, in the case of allsong-birds, the object of the singing is to please the females; and forthis purpose the males rival one another to the best of their musicalability. Thus there can be no question that the courtship of birds is a highlyelaborate business, in which the males do their best to surpass oneanother in charming the females. Obviously the inference is that themales do not take all this trouble for nothing; but that the femalesgive their consent to pair with the males whose personal appearance, orwhose voice, proves to be the most attractive. But, if so, the young ofthe male bird who is thus _selected_ will inherit his superior beauty;and thus, in successive generations, a continuous advance will be madein the beauty of plumage or of song, as the case may be, --both theorigin and development of beauty in the animal world being thus supposeddue to the æsthetic taste of animals themselves. Such is the theory of sexual selection in its main outlines; and withregard to it we must begin by noting two things which are of mostimportance. In the first place, it is a theory wholly and completelydistinct from the theory of natural selection; so that any truth orerror in the one does not in the least affect the other. The secondpoint is, that there is not so great a wealth of evidence in favour ofsexual selection as there is in favour of natural selection; and, therefore, that while all naturalists nowadays accept natural selectionas _a_ (whether or not _the_) cause of adaptive, useful, orlife-preserving structures, there is no such universal--but only a verygeneral--agreement with reference to sexual selection as a cause ofdecorative, beautiful, or life-embellishing structures. Nevertheless, the evidence in favour of sexual selection is both large in amount andmassive in weight. Our consideration of this evidence will bring us to the second divisionof our subject, as previously marked out for discussion--namely, granting that an æsthetic sense occurs in certain large divisions of theanimal kingdom, what is the proof that such a sense is a cause of thebeauty which is presented by the animals in question? Before proceeding to state this proof, however, it is desirable toobserve that under the theory of sexual selection Darwin has includedtwo essentially different classes of facts. For besides the large classof facts to which I have thus far been alluding, --i. E. The cases wheretwo sexes of the same species differ from one another in respect ofornamentation, --there is another class of facts equally important, namely, the cases where the two sexes of the same species differ fromone another in respect of size, strength, and the possession of naturalweapons, such as spurs, horns, &c. In most of these cases it is themales which are thus superiorly endowed; and it is a matter ofobservation that in all cases where they are so endowed they use theirsuperior strength and natural weapons for fighting together, in order tosecure possession of the females. Hence results what Mr. Darwin hascalled the Law of Battle between males of the same species; and this lawof battle he includes under his theory of sexual selection. But it isevident that the principle which is operative in the law of battlediffers from the principle which is concerned in the form of sexualselection that has to do with embellishment, and consequent charm. Thelaw of battle, in fact, more nearly approaches the law of naturalselection; seeing that it expresses the natural advantages of bruteforce in the struggling of rival animals, and so frequently results in_death of the less fitted_, as distinguished from a mere failure topropagate. Now against this doctrine of the law of battle, and theconsequences to which it leads in the superior fighting powers of maleanimals, no objection has been raised in any quarter. It is only withregard to the other aspect of the theory of sexual selection--or thatwhich is concerned with the superior embellishment of male animals--thatany difference of opinion obtains. I will now proceed to give the mainarguments on both sides of this question, beginning with a _résumé_ ofthe evidences in favour of sexual selection. In the first place, the fact that secondary sexual characters of theembellishing kind are so generally restricted to the male sex in itselfseems to constitute very cogent proof that, in some way or another, suchcharacters are connected with the part which is played by the male inthe act of propagation. Moreover, secondary sexual characters of thiskind are of quite as general occurrence as are those of the other kindwhich have to do with rivalry in battle; and the former are usually ofthe more elaborate description. Therefore, as there is no doubt thatsecondary sexual characters of the one order have an immediate purposeto serve in the act of propagation, we are by this close analogyconfirmed in our surmise that secondary sexual characters of the other, and still more elaborate, order are likewise so concerned. Moreover, this view of their meaning becomes still further strengthened when wetake into consideration the following facts. Namely, (_a_) secondarysexual characters of the embellishing kind are, as a rule, developedonly at maturity; and most frequently during only a part of the year, which is invariably the breeding season: (_b_) they are always more orless seriously affected by emasculation: (_c_) they are always, andonly, displayed in perfection during the act of courtship: (_d_) then, however, they are displayed with the most elaborate pains; yet always, and only, before the females: (_e_) they appear, at all events in manycases, to have the effect of charming the females into a performance ofthe sexual act; while it is certain that in many cases, both amongquadrupeds and birds, individuals of the one sex are capable of feelinga strong antipathy against, or a strong preference for, certainindividuals of the opposite sex. Such are the main lines of evidence in favour of the theory of sexualselection. And although it is enough that some of them should be merelystated as above in order that their immense significance should becomeapparent, in the case of others a bare statement is not sufficient forthis purpose. More especially is this the case as regards the enormousprofusion, variety, and elaboration of sexually-embellishing characterswhich occur in birds and mammals--not to mention several divisions ofArthropoda; together with the extraordinary amount of trouble which, ina no less extraordinary number of different ways, is taken by the maleanimals to display their embellishments before the females. And even inmany cases where to our eyes there is no particular embellishment todisplay, the process of courtship consists in such an elaborateperformance of dancings, struttings, and attitudinizings that it isscarcely possible to doubt their object is to incite the opposite sex. Here, for instance, is a series of drawings illustrating the courtshipof spiders. I choose this case as an example, partly because it is theone which has been published most recently, and partly because it is ofparticular interest as occurring so low down in the zoological scale. Iam indebted to the kindness of Mr. And Mrs. Peckham for permission toreproduce these few selected drawings from their very admirable work, which is published by the Natural History Society of Wisconsin, U. S. Itis evident at a glance that all these elaborate, and to our eyesludicrous, performances are more suggestive of incitation than of anyother imaginable purpose. And this view of the matter is stronglycorroborated by the fact that it is the most brightly coloured parts ofthe male spiders which are most obtruded upon the notice of the femaleby these peculiar attitudes--in just the same way as is invariably thecase in the analogous phenomena of courtship among birds, insects, &c. [Illustration: FIG. 122. --Courtship of Spiders. A few examples of some of the attitudes adopted by different species of males when approaching their females. (After Peckham. )] [Illustration: FIG. 123. --Courtship of Spiders. Continued from Fig. 122, similarly showing some of the attitudes of approach adopted by males of yet other different species. (After Peckham. )] But so great is the mass of material which Darwin has collected in proofof all the points mentioned in the foregoing paragraph, that to attemptanything in the way of an epitome would really be to damage itsevidential force. Therefore I deem it best simply to refer to it as itstands in his _Descent of Man_, concluding, as he concludes, --"Thissurprising uniformity in the laws regulating the differences between thesexes in so many and such widely separated classes is intelligible if weadmit the action throughout all the higher divisions of the animalkingdom of one common cause, namely, sexual selection"; while, as hemight well have added, it is difficult to imagine that all the largeclasses of facts which an admission of this common cause serves toexplain, can ever admit of being rendered intelligible by any othertheory. We may next proceed to consider the objections which have been broughtagainst the theory of sexual selection. And this is virtually the samething as saying that we may now consider Mr. Wallace's views upon thesubject. Reserving for subsequent consideration the most general of theseobjections--namely, that at best the theory can only apply to the moreintelligent animals, and so must necessarily fail to explain thephenomena of beauty in the less intelligent, or in the non-intelligent, as well as in all species of plants--we may take _seriatim_ the otherobjections which, in the opinion of Mr. Wallace, are sufficient todispose of the theory even as regards the higher animals. In the first place, he argues that the principal cause of the greaterbrilliancy of male animals in general, and of male birds in particular, is that they do not so much stand in need of protection arising fromconcealment as is the case with their respective females. Consequentlynatural selection is not so active in repressing brilliancy of colour inthe males, or, which amounts to the same thing, is more active in"repressing in the female those bright colours which are normallyproduced in both sexes by general laws. " Next, he argues that not only does natural selection thus exercise anegative influence in passively permitting more heightened colour toappear in the males, but even exercises a positive influence in activelypromoting its development in the males, while, at the same time, actively repressing its appearance in the females. For heightenedcolour, he says, is correlated with health and vigour; and as there canbe no doubt that healthy and vigorous birds best provide for theiryoung, natural selection, by always placing its premium on health andvigour in the males, thus also incidentally promotes, through correlatedgrowth, their superior coloration. Again, with regard to the display which is practised by male birds, andwhich constitutes the strongest of all Mr. Darwin's arguments in favourof sexual selection, Mr. Wallace points out that there is no evidence ofthe females being in any way affected thereby. On the other hand, heargues that this display may be due merely to general excitement; and helays stress upon the more special fact that moveable feathers arehabitually erected under the influence of anger and rivalry, in order tomake the bird look more formidable in the eyes of antagonists. Furthermore, he adduces the consideration that, even if the females arein any way affected by colour and its display on the part of the males, and if, therefore, sexual selection be conceded a true principle intheory, still we must remember that, as a matter of fact, it can onlyoperate in so far as it is allowed to operate by natural selection. Now, according to Mr. Wallace, natural selection must wholly neutralize anysuch supposed influence of sexual selection. For, unless the survivorsin the general struggle for existence happen to be those which are alsothe most highly ornamented, natural selection must neutralize anddestroy any influence that may be exerted by female selection. Butobviously the chances against the otherwise best fitted males happeningto be likewise the most highly ornamented must be many to one, unless, as Wallace supposes, there is some correlation between embellishment andgeneral perfection, in which case, as he points out, the theory ofsexual selection lapses altogether, and becomes but a special case ofnatural selection. Once more, Mr. Wallace argues that the evidence collected by Mr. Darwinhimself proves that each bird finds a mate under any circumstances--ageneral fact which in itself must quite neutralize any effect of sexualselection of colour or ornament, since the less highly coloured birdswould be at no disadvantage as regards the leaving of healthy progeny. Lastly, he urges the high improbability that through thousands ofgenerations all the females of any particular species--possibly spreadover an enormous area--should uniformly and always have displayedexactly the same taste with respect to every detail of colour to bepresented by the males. Now, without any question, we have here a most powerful array ofobjections against the theory of sexual selection. Each of them is ablydeveloped by Mr. Wallace himself in his work on _Tropical Nature_; andalthough I have here space only to state them in the most abbreviated ofpossible forms, I think it will be apparent how formidable theseobjections appear. Unfortunately the work in which they are mainlypresented was published several years after the second edition of the_Descent of Man_, so that Mr. Darwin never had a suitable opportunity ofreplying. But, if he had had such an opportunity, as far as I can judgeit seems that his reply would have been more or less as follows. In the first place, Mr. Wallace fails to distinguish between brilliancyand ornamentation--or between colour as merely "heightened, " and asdistinctively decorative. Yet there is obviously the greatest possibledifference between these two things. We may readily enough admit that amere heightening of already existing coloration is likely enough--at allevents in many cases--to accompany a general increase of vigour, andtherefore that natural selection, by promoting the latter, may alsoincidentally promote the former, in cases where brilliancy is not asource of danger. But clearly this is a widely different thing fromshowing that not only a _general brilliancy of colour_, but also _theparticular disposition of colours_, in the form of ornamental patterns, can thus be accounted for by natural selection. Indeed, it is expresslyin order to account for the occurrence of such ornamental patterns thatMr. Darwin constructed his theory of sexual selection; and therefore, bythus virtually ignoring the only facts which that theory endeavours toexplain, Mr. Wallace is not really criticizing the theory at all. Byrepresenting that the theory has to do only with brilliancy of colour, as distinguished from disposition of colours, he is going off upon afalse issue which has never really been raised[48]. Look, for example, at a peacock's tail. No doubt it is sufficiently brilliant; but far moreremarkable than its brilliancy is its elaborate pattern on the one hand, and its enormous size on the other. There is no conceivable reason whymere _brilliancy of colour_, as an accidental concomitant of generalvigour, should have run into so extraordinary, so elaborate, and sobeautiful a _design of colours_. Moreover, this design is only unfoldedwhen the tail is erected, and the tail is not erected in battle (as Mr. Wallace's theory of the erectile function in feathers would require), but in courtship; obviously, therefore, the purpose of the pattern, soto speak, is correlated with the act of courtship--it being only then, in fact, that the general purpose of the whole structure, as well as themore special purpose of the pattern, becomes revealed. Lastly, the factof this whole structure being so large, entailing not only a greatamount of physiological material in its production, but also ofphysiological energy in carrying about such a weight, as well as ofincreased danger from impeding locomotion and inviting capture--all thisis obviously incompatible with the supposition of the peacock's tailhaving been produced by natural selection. And such a case does notstand alone. There are multitudes of other instances of ornamentalstructures imposing a drain upon the vital energies of their possessors, without conferring any compensating benefit from a utilitarian point ofview. Now, in all these cases, without any exception, such structuresare ornamental structures which present a plain and obvious reference tothe relationship of the sexes. Therefore it becomes almost impossible todoubt--first, that they exist for the sake of ornament; and next, thatthe ornament exists on account of that relationship. If such structureswere due merely to a superabundance of energy, as Mr. Wallace supposes, not only ought they to have been kept down by the economizing influenceof natural selection; but we can see no reason, either why they shouldbe so highly ornamental on the one hand, or so exclusively related tothe sexual relationship on the other. [48] Note C. Finally, we must take notice of the fact that where peculiar_structures_ are concerned for purposes of display in courtship, the_elaboration_ of these structures is often no less remarkable than thatof patterns where colours are thus concerned. Take, for example, thecase of the Bell-bird, which I select from an innumerable number ofinstances that might be mentioned because, while giving a verbaldescription of this animal, Darwin does not supply a pictorialrepresentation thereof. The bird, which lives in South America, has avery loud and peculiar call, that can be heard at a distance of two orthree miles. The female is dusky-green; but the adult male is abeautiful white, excepting the extraordinary structure with which we areat present concerned. This is a tube about three inches long, whichrises from the base of the beak. It is jet black, and dotted over withsmall downy feathers. The tube is closed at the top, but its cavitycommunicates with the palate, and thus the whole admits of beinginflated from within, when, of course, it stands erect as represented inone of the two drawings. When not thus inflated, it hangs down, asshown in the second figure, which represents the plumage of a youngmale. (Fig. 124. ) [Illustration: FIG. 124. --The Bell-bird (_Chasmorhynchus niveus_, 1/4 natural size). Drawn from nature (_R. Coll. Surg. Mus. _). In the drawing of the adult male the ornamental appendage is represented in its inflated condition, during courtship; in the drawing of the young male it is shown in its flaccid condition. ] In another species of the genus there are three of these appendages--thetwo additional ones being mounted on the corners of the mouth. (Fig. 125. ) In all species of the genus (four in number) the tubes areinflated during courtship, and therefore perform the function of sexualembellishments. Now the point to which I wish to draw attention is, thatso specialized and morphologically elaborate a structure cannot beregarded as merely adventitious. It must have been developed by somedefinite cause, acting through a long series of generations. And as noother function can be assigned to it than that of charming the femalewhen it is erected in courtship, the peculiarity of form and mechanismwhich it presents--like the elaboration of patterns in cases wherecolour only is concerned--virtually compels us to recognise in sexualselection the only conceivable cause of its production. [Illustration: FIG. 125. --_C. Tricarunculatus_, 1/4 natural size. Copied from the _Ibis_. The ornamental appendages of the male are represented in a partly inflated condition. ] For these reasons I think that Mr. Wallace's main objection falls to theground. Passing on to his subsidiary objections, I do not see muchweight in his merely negative difficulty as to there being an absence ofevidence upon hen birds being charmed by the plumage, or the voice, oftheir consorts. For, on the one hand, it is not very safe to infer whatsentiments may be in the mind of a hen; and, on the other hand, it isimpossible to conceive what motive can be in the mind of a cock, otherthan that of making himself attractive, when he performs his variousantics, displays his ornamental plumes, or sings his melodious songs. Considerations somewhat analogous apply to the difficulty of supposingso much similarity and constancy of taste on the part of female animalsas Mr. Darwin's theory undoubtedly requires. Although we know verylittle about the psychology of the lower animals, we do observe in manycases that small details of mental organization are often wonderfullyconstant and uniform throughout all members of a species, even where itis impossible to suggest any utility as a cause. Again, as regards the objection that each bird finds a mate under anycircumstances, we have here an obvious begging of the whole question. That every feathered Jack should find a feathered Jill is perhaps whatwe might have antecedently expected; but when we meet with innumerableinstances of ornamental plumes, melodious songs, and the rest, as somany witnesses to a process of sexual selection having always been inoperation, it becomes irrational to exclude such evidence on account ofour antecedent prepossessions. There remains the objection that the principles of natural selectionmust necessarily swallow up those of sexual selection. And thisconsideration, I doubt not, lies at the root of all Mr. Wallace'sopposition to the supplementary theory of sexual selection. He isself-consistent in refusing to entertain the evidence of sexualselection, on the ground of his antecedent persuasion that in the greatdrama of evolution there is no possible standing-ground for any otheractor than that which appears in the person of natural selection. Buthere, again, we must refuse to allow any merely antecedent presumptionto blind our eyes to the actual evidence of other agencies havingco-operated with natural selection in producing the observed results. And, as regards the particular case now before us, I think I have shown, as far as space will permit, that in the phenomena of decorativecolouring (as distinguished from merely brilliant colouring), ofmelodious song (as distinguished from merely tuneless cries), ofenormous arborescent antlers (as distinguished from merely offensiveweapons), and so forth--I say that in all these phenomena we havephenomena which cannot possibly be explained by the theory of naturalselection; and, further, that if they are to be explained at all, thiscan only be done, so far as we can at present see, by Mr. Darwin'ssupplementary theory of sexual selection. I have now briefly answered all Mr. Wallace's objections to thissupplementary theory, and, as previously remarked, I feel prettyconfident that, at all events in the main, the answer is such as Mr. Darwin would himself have supplied, had there been a third edition ofhis work upon the subject. At all events, be this as it may, we arehappily in possession of unquestionable evidence that he believed allMr. Wallace's objections to admit of fully satisfactory answers. For hisvery last words to science--read only a few hours before his death at ameeting of the Zoological Society--were: I may perhaps be here permitted to say that, after having carefully weighed, to the best of my ability, the various arguments which have been advanced against the principle of sexual selection, I remain firmly convinced of its truth[49]. [49] Since the above exposition of the theory of sexual selection was written, Mr. Poulton has published his work on the _Colours of Animals_. He there reproduces some of the illustrations which occur in Mr. And Mrs. Peckham's work on _Sexual Selection in Spiders_, and furnishes appropriate descriptions. Therefore, while retaining the illustrations, I have withdrawn my own descriptions. Mr. Poulton has also in his book supplied a _résumé_ of the arguments for and against the theory of sexual selection in general. Of course in nearly all respects this corresponds with the _résumé_ which is given in the foregoing pages; but I have left the latter as it was originally written, because all the critical part is reproduced _verbatim_ from a review of Mr. Wallace's _Darwinism_, of a date still earlier than that of Mr. Poulton's book--viz. _Contemporary Review_, August, 1889. _Concluding Remarks. _ I will now conclude this chapter, and with it the present volume, byoffering a few general remarks on what may be termed the philosophicalrelations of Darwinian doctrine to the facts of adaptation on the onehand, and to those of beauty on the other. Of course we are all awarethat before the days of this doctrine the facts of adaptation in organicnature were taken to constitute the clearest possible evidence ofspecial design, on account of the wonderful mechanisms which theyeverywhere displayed; while the facts of beauty were taken asconstituting no less conclusive evidence of the quality of such specialdesign as beneficent, not to say artistic. But now that the Darwiniandoctrine appears to have explained scientifically the former class offacts by its theory of natural selection, and the latter class of factsby its theory of sexual selection, we may fitly conclude this briefexposition of the doctrine as a whole by considering what influence suchnaturalistic explanations may fairly be taken to exercise upon theolder, or super-naturalistic, interpretations. To begin with the facts of adaptation, we must first of all observe thatthe Darwinian doctrine is immediately concerned with these facts only inso far as they occur in organic nature. With the adaptations--if theycan properly be so called--which occur in all the rest of nature, andwhich go to constitute the Cosmos as a whole so wondrous a spectacle ofuniversal law and perfect order, this doctrine is but indirectlyconcerned. Nevertheless, it is of course fundamentally concerned withthem to the extent that it seeks to bring the phenomena of organicnature into line with those of inorganic; and therefore to show thatwhatever view we may severally take as to the kind of causation which isenergizing in the latter we must now extend to the former. This isusually expressed by saying that the theory of evolution by naturalselection is a mechanical theory. It endeavours to comprise all thefacts of adaptation in organic nature under the same category ofexplanation as those which occur in inorganic nature--that is to say, under the category of physical, or ascertainable, causation. Indeed, unless the theory has succeeded in doing this, it has not succeeded indoing anything--beyond making a great noise in the world. If Mr. Darwinhas not discovered a new mechanical cause in the selection principle, his labour has been worse than in vain. Now, without unduly repeating what has already been said in ChapterVIII, I may remark that, whatever we may each think of the measure ofsuccess which has thus far attended the theory of natural selection inexplaining the facts of adaptation, we ought all to agree that, considered as a matter of general reasoning, the theory does certainlyrefer to a _vera causa_ of a strictly physical kind; and, therefore, that no exception can be taken to the theory in this respect on groundsof _logic_. If the theory in this respect is to be attacked at all, itcan only be on grounds of _fact_--namely, by arguing that the cause doesnot occur in nature, or that, if it does, its importance has beenexaggerated by the theory. Even, however, if the latter propositionshould ever be proved, we may now be virtually certain that the onlyresult would be the relegation of all the residual phenomena ofadaptation to other causes of the physical order--whether known orunknown. Hence, as far as the matter of _principle_ is concerned, we maydefinitely conclude that the great naturalistic movement of our centuryhas already brought all the phenomena of adaptation in organic natureunder precisely the same category of mechanical causation, as similarmovements in previous centuries have brought all the known phenomena ofinorganic nature: the only question that remains for solution is thestrictly _scientific_ question touching the particular causes of themechanical order which have been at work. So much, then, for the phenomena of adaptation. Turning next to those ofbeauty, we have already seen that the theory of sexual selection standsto these in precisely the same relation as the theory of naturalselection does to those of adaptation. In other words, it supplies aphysical explanation of them; because, as far as our present purposesare concerned, it may be taken for granted, or for the sake of argument, that inasmuch as psychological elements enter into the question thecerebral basis which they demand involves a physical side. There is, moreover, this further point of resemblance between the twotheories: neither of them has any reference to inorganic nature. Therefore, with the charm or the loveliness of landscapes, of earth andsea and sky, of pebbles, crystals, and so forth, we have at presentnothing to do. How it is that so many inanimate objects are investedwith beauty--why it is that beauty attaches to architecture, music, poetry, and many other things--these are questions which do notspecially concern the biologist. If they are ever to receive anysatisfactory explanation in terms of natural causation, this must befurnished at the hands of the psychologist. It may be possible for himto show, more satisfactorily than hitherto, that all beauty, wheneverand wherever it occurs, is literally "in the eyes of the beholder"; orthat objectively considered, there is no such thing as beauty. It maybe--and in my opinion it probably is--purely an affair of the percipientmind itself, depending on the association of ideas with pleasure-givingobjects. This association may well lead to a liking for such objects, and so to the formation of what is known as æsthetic feeling with regardto them. Moreover, beauty of inanimate nature must be an affair of thepercipient mind itself, unless there be a creating intelligence withorgans of sense and ideals of beauty similar to our own. And, apart fromany deeper considerations, this latter possibility is scarcely entitledto be regarded as a probability, looking to the immense diversities inthose ideals among different races of mankind. But, be this as it may, the scientific problem which is presented by the fact of æstheticfeeling, even if it is ever to be satisfactorily solved, is a problemwhich, as already remarked, must be dealt with by psychologists. Asbiologists we have simply to accept this feeling as a fact, and toconsider how, out of such a feeling as a cause, the beauty of organicnature may have followed as an effect. Now we have already seen how the theory of sexual selection supposesthis to have happened. But against this theory a formidable objectionarises, and one which I have thought it best to reserve for treatment inthis place, because it serves to show the principal difference betweenMr. Darwin's two great generalizations, considered as generalizations inthe way of mechanical theory. For while the theory of natural selectionextends equally throughout the whole range of organic nature, the theoryof sexual selection has but a comparatively restricted scope, which, moreover, is but vaguely defined. For it is obvious that the theory canonly apply to living organisms which are sufficiently intelligent toadmit of our reasonably accrediting them with æsthetic taste--namely, ineffect, the higher animals. And just as this consideration greatlyrestricts the possible scope of the theory, as compared with that ofnatural selection, so does it render undefined the zoological limitswithin which it can be reasonably employed. Lastly, this necessarilyundefined, and yet most important limitation exposes the theory to theobjection just alluded to, and which I shall now mention. The theory, as we have just seen, is necessarily restricted in itsapplication to the higher animals. Yet the facts which it is designed toexplain are not thus restricted. For beauty is by no means restricted tothe higher animals. The whole of the vegetable world, and the whole ofthe animal world at least as high up in the scale as the insects, mustbe taken as incapable of æsthetic feeling. Therefore, the extremebeauty of flowers, sea-anemones, corals, and so forth, cannot possiblybe ascribed to sexual selection. Now, with regard to this difficulty, we must begin by excluding the caseof the vegetable kingdom as irrelevant. For it has been rendered highlyprobable--if not actually proved--by Darwin and others, that the beautyof flowers and of fruits is in large part due to natural selection. Itis to the advantage of flowering plants that their organs offructification should be rendered conspicuous--and in many cases alsoodoriferous, --in order to attract the insects on which the process offertilization depends. Similarly, it is to the advantage of all plantswhich have brightly coloured fruits that these should be conspicuous forthe purpose of attracting birds, which eat the fruits and so disseminatethe seed. Hence all the gay colours and varied forms, both of flowersand fruits, have been thus adequately explained as due to naturalcauses, working for the welfare, as distinguished from the beauty, ofthe plants. For even the distribution of colours on flowers, or thebeautiful patterns which so many of them present, are found to be usefulin guiding insects to the organs of fructification. Again, the green colouring of leaves, which lends so much beauty to thevegetable world, has likewise been shown to be of vital importance tothe physiology of plant-life; and, therefore, may also be ascribed tonatural selection. Thus, there remains only the forms of plants otherthan the flowers. But the forms of leaves have also in many cases beenshown to be governed by principles of utility; and the same is to besaid of the branching structure which is so characteristic of trees andshrubs, since this is the form most effectual for spreading out theleaves to the light and air. Here, then, we likewise find that the causedetermining plant beauty is natural selection; and so we may concludethat the only reason why the forms of trees which are thus determined byutility appeal to us as beautiful, is because we are accustomed to thesethe most ordinary forms. Our ideas having been always, as it were, moulded upon these forms, æsthetic feeling becomes attached to them bythe principle of association. At any rate, it is certain that when wecontemplate almost any forms of plant-structure which, for specialreasons of utility, differ widely from these (to us) more habitualforms, the result is not suggestive of beauty. Many of the tropical andun-tree-like plants--such as the cactus tribe--strike us as odd andquaint, not as beautiful. Be this however as it may, I trust I have saidenough to prove that in the vegetable world, at all events, theattainment of beauty cannot be held to have been an object aimed at, soto speak, for its own sake. Even if, for the purposes of argument, wewere to suppose that all the forms and colours in the vegetable worldare due to special design, there could be no doubt that the purpose ofthis design has been in chief part a utilitarian purpose; it has notaimed at beauty exclusively for its own sake. For most of such beauty aswe here perceive is plainly due to the means adopted for the attainmentof life-preserving ends, which, of course, is a metaphorical way ofsaying that it is probably due to natural selection[50]. [50] The beauty of autumnal tints in fading leaves may possibly be adduced _per contra_. But here we have to remember that it is only some kinds of leaves which thus become beautiful when fading, while, even as regards those that do, it is not remarkable that their chlorophyll should, as it were, accidentally assume brilliant tints while breaking down into lower grades of chemical constitution. The case, in fact, is exactly parallel to those in the animal kingdom which are considered in the ensuing paragraphs. Turning, then, to the animal kingdom below the level of insects, here weare bound to confess that the beauty which so often meets us cannotreasonably be ascribed either to natural or to sexual selection. Not tosexual selection for the reasons already given; the animals in questionare neither sufficiently intelligent to possess any æsthetic taste, nor, as a matter of fact, do we observe that they exercise any choice inpairing. Not to natural selection, because we cannot here, as in thecase of vegetables, point to any benefit as generally arising frombright colours and beautiful forms. On the principles of naturalism, therefore, we are driven to conclude that the beauty here is purelyadventitious, or accidental. Nor need we be afraid to make thisadmission, if only we take a sufficiently wide view of the facts. For, when we do take such a view, we find that beauty here is by no means ofinvariable, or even of general, occurrence. There is no loveliness aboutan oyster or a lob-worm; parasites, as a rule, are positively ugly, andthey constitute a good half of all animal species. The truth seems tobe, when we look attentively at the matter, that in all cases wherebeauty does occur in these lower forms of animal life, its presence isowing to one of two things--either to the radiate form, or to the brighttints. Now, seeing that the radiate form is of such general occurrenceamong these lower animals--appearing over and over again, with theutmost insistence, even among groups widely separated from one anotherby the latest results of scientific classification--seeing this, itbecomes impossible to doubt that the radiate form is due to somemorphological reasons of wide generality. Whether these reasons beconnected with the internal laws of growth, or to the externalconditions of environment, I do not pretend to suggest. But I feel safein saying that it cannot possibly be due to any design to secure beautyfor its own sake. The very generality of the radiate form is in itselfenough to suggest that it must have some physical, as distinguished froman æsthetic, explanation; for, if the attainment of beauty had here beenthe object, surely it might have been even more effectually accomplishedby adopting a greater variety of typical forms--as, for instance, in thecase of flowers. Coming then, lastly, to the case of brilliant tints in the loweranimals, Mr. Darwin has soundly argued that there is nothing forced orimprobable in the supposition that organic compounds, presenting as theydo such highly complex and such varied chemical constitutions, shouldoften present brilliant colouring _incidentally_. Considered merely ascolouring, there is nothing in the world more magnificent than arterialblood; yet here the colouring is of purely utilitarian significance. Itis of the first importance in the chemistry of respiration; but issurely without any meaning from an æsthetic point of view. For thecolour of the cheeks, and of the flesh generally, in the _white_ racesof mankind, could have been produced quite as effectually by the use ofpigment--as in the case of certain monkeys. Now the fact that in thecase of blood, as in that of many other highly coloured fluids andsolids throughout the animal kingdom, the colour is _concealed_, issurely sufficient proof that the colour, if regarded from an æstheticpoint of view, is accidental. Therefore, when, as in other cases, suchcolouring occurs upon the surface, and thus becomes apparent, are we notirresistibly led to conclude that its _exhibition_ in such cases islikewise accidental, so far as any question of æsthetic design isconcerned? I have now briefly glanced at all the main facts of organic nature withreference to beauty; and, as a result, I think it is impossible toresist the general conclusion, that in organic nature beauty does notexist as an end _per se_. All cases where beauty can be pointed to inorganic nature are seemingly due--either to natural selection, actingwithout reference to beauty, but to utility; to sexual selection, actingwith reference to the taste of animals; or else to sheer accident. Andif this general conclusion should be held to need any specialverification, is it not to be found in the numberless cases whereorganic nature not only fails to be beautiful, but reveals itself as thereverse. Not again to refer to the case of parasites, what can be moreunshapely than a hippopotamus, or more generally repulsive than acrocodile? If it be said that these are exceptions, and that the formsof animals as a rule are graceful, the answer--even apart fromparasites--is obvious. In all cases where the habits of life are such asto render rapid locomotion a matter of utilitarian necessity, theoutlines of an animal _must_ be graceful--else, whether the locomotionbe terrestrial, aerial, or aquatic, it must fail to be swift. Hence itis only in such cases as that of the hippopotamus, rhinoceros, elephant, crocodile, and so forth, where natural selection has had noconcern in developing speed, that the accompanying accident ofgracefulness can be allowed to disappear. But if beauty in organicnature had been in itself what may be termed an artistic object on thepart of a divine Creator, it is absurd to suggest that his design inthis matter should only have been allowed to appear where we are able todetect other and very good reasons for its appearance. * * * * * Thus, whether we look to the facts of adaptation or to those of beauty, everywhere throughout organic nature we meet with abundant evidence ofnatural causation, while nowhere do we meet with any independentevidence of supernatural design. But, having led up to this conclusion, and having thus stated it as honestly as I can, I should like to finishby further stating what, in my opinion is its logical bearing upon themore fundamental tenets of religious thought. As I have already observed at the commencement of this brief exposition, prior to the Darwinian theory of organic evolution, the theologian wasprone to point to the realm of organic nature as furnishing a peculiarlyrich and virtually endless store of facts, all combining in theirtestimony to the wisdom and the beneficence of the Deity. Innumerableadaptations of structures to functions appeared to yield convincingevidence in favour of design; the beauty so profusely shed by livingforms appeared to yield evidence, no less convincing, of that design asbeneficent. But both these sources of evidence have now, as it were, been tapped at their fountain-head: the adaptation and the beauty arealike receiving their explanation at the hands of a purely mechanicalphilosophy. Nay, even the personality of man himself is assailed; andthis not only in the features which he shares with the lower animals, but also in his god-like attributes of reason, thought, and conscience. All nature has thus been transformed before the view of the presentgeneration in a manner and to an extent that has never before beenpossible: and inasmuch as the change which has taken place has takenplace in the direction of naturalism, and this to the extent ofrendering the mechanical interpretation of nature universal, it is nowonder if the religious mind has suddenly awakened to a new and aterrible force in the words of its traditional enemy--Where is now thyGod? This is not the place to discuss the bearings of science onreligion[51]; but I think it is a place where one may properly point outthe limits within which no such bearings obtain. Now, from what has justbeen said, it will be apparent that I am not going to minimise thechange which has been wrought. On the contrary, I believe it is onlystupidity or affectation which can deny that the change in question ismore deep and broad than any single previous change in the whole historyof human thought. It is a fundamental, a cosmical, a world-transformingchange. Nevertheless, in my opinion, it is a change of a non-theistic, as distinguished from an a-theistic, kind. It has rendered impossiblethe appearance in literature of any future Paley, Bell, or Chalmers; butit has done nothing in the way of negativing that belief in a SupremeBeing which it was the object of these authors to substantiate. If ithas demonstrated the futility of their proof, it has furnished nothingin the way of disproof. It has shown, indeed, that their line ofargument was misjudged when they thus sought to separate organic naturefrom inorganic as a theatre for the special or peculiar display ofsupernatural design; but further than this it has not shown anything. The change in question therefore, although greater in degree, is thesame in kind as all its predecessors: like all previous advances incosmological theory which have been wrought by the advance of science, this latest and greatest advance has been that of revealing theconstitution of nature, or the method of causation, as everywhere thesame. But it is evident that this change, vast and to all appearancefinal though it be, must end within the limits of natural causationitself. The whole world of life and mind may now have been annexed tothat of matter and energy as together constituting one magnificentdominion, which is everywhere subject to the same rule, or method ofgovernment. But the ulterior and ultimate question touching the natureof this government as mental or non-mental, personal or impersonal, remains exactly where it was. Indeed, this is a question which cannot beaffected by _any_ advance of science, further than science has provedherself able to dispose of erroneous arguments based upon ignorance ofnature. For while the sphere of science is necessarily restricted tothat of natural causation which it is her office to explore, thequestion touching the _nature of this natural causation_ is one which asnecessarily lies without the whole sphere of such causation itself:therefore it lies beyond any possible intrusion by science. And not onlyso. But if the nature of natural causation be that of the highest orderof known existence, then, although we must evidently be incapable ofconceiving what such a Mind is, at least we seem capable of judging whatin many respects it is not. It cannot be more than one; it cannot belimited either in space or time; it cannot be other than at least asself-consistent as its manifestations in nature are invariable. Now, from the latter deduction there arises a point of first-rate importancein the present connexion. For if the so-called First Cause beintelligent, and therefore all secondary causes but the expression of asupreme Will, in as far as such a Will is self-consistent, the operationof all natural causes must be uniform, --with the result that, as seen byus, this operation must needs appear to be what we call mechanical. Themore unvarying the Will, the more unvarying must be this expressionthereof; so that, if the former be absolutely self-consistent, thelatter cannot fail to be as reasonably interpreted by the theory ofmindless necessity, as by that of ubiquitous intention. Such being, asit appears to me, the pure logic of the matter, the proof of organicevolution amounts to nothing more than the proof of a natural process. What mode of being is ultimately concerned in this process--or in whatit is that this process ultimately consists--is a question upon whichscience is as voiceless as speculation is vociferous. [51] The best treatise on this subject is Prof. Le Conte's _Evolution and its Relation to Religious Thought_ (Appleton & Co. 1888). But, it may still be urged, surely the principle of natural selection(with its terrible basis in the struggle for existence) and theprinciple of sexual selection (with its consequence in denying beautyto be an end in itself) demonstrate that, _if_ there be design innature, such design at all events cannot be beneficent. To this, however, I should again reply that, just as touching the major questionof design itself, so as touching this minor question of the quality ofsuch design as beneficent, I do not see how the matter has been muchaffected by a discovery of the principles before us. For we did not needa Darwin to tell us that the whole creation groaneth and travailethtogether in pain. The most that in this connexion Darwin can fairly besaid to have done is to have estimated in a more careful and precisemanner than any of his predecessors, the range and the severity of thistravail. And if it be true that the result of what may be called hisscientific analysis of nature in respect of suffering is to have shownthe law of suffering even more severe, more ubiquitous, and morenecessary than it had ever been shown before, we must remember at thesame time how he has proved, more rigidly than was ever proved before, that suffering is a condition to improvement--struggle for life beingthe _raison d'être_ of higher life, and this not only in the physicalsphere, but also in the mental and moral. Lastly, if it be said that the _choice_ of such a method, wherebyimprovement is only secured at the cost of suffering, indicates a kindof callousness on the part of an intelligent Being supposed to beomnipotent, I confess that such does appear to me a legitimateconclusion--subject, however, to the reservation that higher knowledgemight displace it. For, as far as matters are now actually presented tothe unbiased contemplation of a human mind, this provisional inferenceappears to me unavoidable--namely, that if the world of sentient life bedue to an Omnipotent Designer, the aim or motive of the design must havebeen that of securing a continuous advance of animal improvement, without any regard at all to animal suffering. For I own it does notseem to me compatible with a fair and honest exercise of our reason toset the sum of animal happiness over against the sum of animal misery, and then to allege that, in so far as the former tends to balance--or toover-balance--the latter, thus far is the moral character of the designas a whole vindicated. Even if it could be shown that the sum ofhappiness in the brute creation considerably preponderates over that ofunhappiness--which is the customary argument of theistic apologists, --weshould still remain without evidence as to this state of matters havingformed any essential part of the design. On the other hand, we shouldstill be in possession of seemingly good evidence to the contrary. Forit is clearly a condition to progress by survival of the fittest, thatas soon as organisms become sentient selection must be exercised withreference to sentiency; and this means that, if further progress is totake place, states of sentiency _must_ become so organized withreference to habitual experience of the race, that pleasures and painsshall answer respectively to states of agreement and disagreement withthe sentient creature's environment. Those animals which found pleasurein what was deleterious to life would not survive, while those whichfound pleasure in what was beneficial to life would survive; and soeventually, in every species of animal, states of sentiency as agreeableor disagreeable must approximately correspond with what is good for thespecies or bad for the species. Indeed, we may legitimately surmise thatthe reason why sentiency (and, _a fortiori_, conscious volition) hasever appeared upon the scene at all, has been because itfurnishes--through this continuously selected adjustment of states ofsentiency to states of the sentient organism--so admirable a means ofsecuring rapid, and often refined, adjustments by the organism to thehabitual conditions of its life[52]. But, if so, not only is this stateof matters a _condition_ to progress in the future; it is further, andequally, a _consequence_ of progress in the past. [52] See _Mental Evolution in Animals_, pp. 110-111. However, be this as it may, from all that has gone before does it notbecome apparent that pleasure or happiness on the one hand, and pain ormisery on the other, must be present in sentient nature? And so long asthey are both seen to be equally necessary under the process ofevolution by natural selection, we have clearly no more reason to regardthe pleasure than the pain as an object of the supposed design. Rathermust we see in both one and the same condition to progress under themethod of natural causation which is before us; and therefore I cannotperceive that it makes much difference--so far as the argument forbeneficence is concerned--whether the pleasures of animals outweightheir pains, or _vice versâ_. Upon the whole, then, it seems to me that such evidence as we have isagainst rather than in favour of the inference, that if design beoperative in animate nature it has reference to animal enjoyment orwell-being, as distinguished from animal improvement or evolution. Andif this result should be found distasteful to the religious mind--if itbe felt that there is no desire to save the evidences of design unlessthey serve at the same time to testify to the nature of that design asbeneficent, --I must once more observe that the difficulty thus presentedto theism is not a difficulty of modern creation. On the contrary, ithas always constituted the fundamental difficulty with which naturaltheologians have had to contend. The external world appears, in thisrespect, to be at variance with our moral sense; and when the antagonismis brought home to the religious mind, it must ever be with a shock ofterrified surprise. It has been newly brought home to us by thegeneralizations of Darwin; and therefore, as I said at the beginning, the religious thought of our generation has been more than everstaggered by the question--Where is now thy God? But I have endeavouredto show that the logical standing of the case has not been materiallychanged; and when this cry of Reason pierces the heart of Faith, itremains for Faith to answer now, as she has always answered before--andanswered with that trust which is at once her beauty and herlife--Verily thou art a God that hidest thyself. _APPENDIX AND NOTES_ APPENDIX TO CHAPTER V. ON OBJECTIONS WHICH HAVE BEEN BROUGHT AGAINST THE THEORY OF ORGANICEVOLUTION ON GROUNDS OF PALÆONTOLOGY. While stating in the text, and in a necessarily general way, theevidence which is yielded by palæontology to the theory of organicevolution, I have been desirous of not overstating it. Therefore, in theearlier paragraphs of the chapter, which deal with the most generalheads of such evidence, I introduced certain qualifying phrases; and Iwill now give the reasons which led me to do so. Of all the five biological sciences which have been called intoevidence--viz. Those of Classification, Morphology, Embryology, Palæontology, and Geographical Distribution--it is in the case ofpalæontology alone that any important or professional opinions stillcontinue to be unsatisfied. Therefore, in order that justice may be doneto this line of dissent, I have thought it better to deal with thematter in a separate Appendix, rather than to hurry it over in the text. And, as all the difficulties or objections which have been advancedagainst the theory of evolution on grounds of palæontology must vary, asto their strength, with the estimate which is taken touching the degreeof imperfection of the geological record, I will begin by adding a fewparagraphs to what has already been said in the text upon this subject. First, then, as to the difficulties in the way of fossils being formedat all. We have already noticed in the text that it is only the more orless hard parts of organisms which under any circumstances can befossilized; and even the hardest parts quickly disintegrate if notprotected from the weather on land, or from the water on the sea-bottom. Moreover, as Darwin says, "we probably take a quite erroneous view whenwe assume that sediment is being deposited over nearly the whole bed ofthe sea, at a rate sufficiently quick to embed and preserve fossilremains. Throughout an enormously large proportion of the ocean, thebright blue tint of the water bespeaks its purity. The many cases onrecord of a formation conformably covered, after an immense interval oftime, by another and a later formation, without the underlying bedhaving suffered in the interval any wear and tear, seem explicable onlyon the view of the bottom of the sea not rarely lying for ages in anunaltered condition. " Next, as regards littoral animals, he shows thedifficulty which they must have in becoming fossils, and gives astriking example in several of the existing species of a sub-family ofcirripedes (_Chthamalinæ_), "which coat the rocks all over the world ininfinite numbers, " yet, with the exception of one species which inhabitsdeep water, no vestige of any of them has been found in any tertiaryformation, although it is known that the genus _Chthamalus_ existedthrough the Chalk period. Lastly, "with respect to the terrestrialproductions which lived through the secondary and palæozoic periods, itis superfluous to state our evidence is fragmentary in an extremedegree. For instance, until recently not a land shell was knownbelonging to either of these vast periods, " with one exception; while, "in regard to mammiferous remains, a glance at the historical table inLyell's Manual will bring home the truth, how accidental and rare hasbeen their preservation, far better than pages of detail. Nor is theirrarity surprising, when we remember how large a proportion of the bonesof tertiary mammals have been discovered either in caves or inlacustrine deposits; and that not a cave or true lacustrine bed is knownbelonging to the age of our secondary or palæozoic formations. " But perhaps of even more importance than all these known causes whichprevent the formation of fossils, is the existence of unknown causeswhich make for the same result. For example, the Flysch-formation is aformation of several thousand feet in thickness (as much as 6000 in someplaces), and it extends for at least 300 miles from Vienna toSwitzerland; moreover, it consists of shale and sandstone. Therefore, alike in respect of time, space, and character, it is just such aformation as we should expect to find highly rich in fossils; yet, "although this great mass has been most carefully searched, no fossils, except a few vegetable remains, have been found. " So much then for the difficulty, so to speak, which nature experiencesin the manufacture of fossils. Probably not one per cent. Of the speciesof animals which have inhabited the earth has left a single individualas a fossil, whereby to record its past existence. But of even more importance than this difficulty of making fossils inthe first instance, is the difficulty of preserving them when they aremade. The vast majority of fossils have been formed under water, and alarge proportional number of these--whether the animals were marine, terrestrial, or inhabitants of fresh water--have been formed insedimentary deposits either of sand, gravel, or other porous material. Now, where such deposits have been afterwards raised into the air forany considerable time--and this has been more or less the case with alldeposits which are available for exploration--their fossiliferouscontents will have been, as a general rule, dissolved by the percolationof rain-water charged with carbonic acid. Similarly, sea-water hasrecently been found to be a surprisingly strong solvent of calcareousmaterial: hence, Saturn-like, the ocean devours her own progeny as faras shells and bones of all kinds are concerned--and this to an extent ofwhich we have probably no adequate conception. Of still greater destructive influence, however, than these solventagencies in earth and sea, are the erosive agencies of both. Any one whowatches the pounding of the waves upon the shore; who then observes theeffect of it upon the rocks broken into shingle, and on the shinglereduced to sand; who, looking behind him at the cliffs, sees there theevidence of the gradual advance of this all-pulverising power--anadvance so gradual that no yard of it is accomplished until within thatyard the "white teeth" have eaten well into the "bowels of the earth";who then reflects that this process is going on simultaneously overhundreds of thousands of miles of coast-lines throughout the world; andwho finally extends his mental vision from space to time, by tryingdimly to imagine what this ever-roaring monster must have consumedduring the hundreds of millions of years that slowly rising and slowlysinking continents have exposed their whole areas to her jaws; whoeverthus observes and thus reflects must be a dull man, if he does not beginto feel that in the presence of such a destroyer as this we have noreason to wonder at a frequent silence in the testimony of the rocks. But although the erosive agency of the sea is thus so inconceivablygreat, it is positively small if compared with erosive agencies on land. The constant action of rain, wind, and running water, in wearing downthe surfaces of all lands into "the dust of continents to be"; thedisintegrating effects on all but the very hardest rocks of winterfrosts alternating with summer heats; the grinding power of ice inperiods of glaciation; and last, but not least, the wholesale melting upof sedimentary formations whenever these have sunk for any considerabledistance beneath the earth's surface:--all these agencies taken togetherconstitute so prodigious a sum of energies combined throughimmeasureable ages in their common work of destruction, that when wetry to realise what it must amount to, we can scarcely fail to wonder, not that the geological record is highly imperfect, but that so much ofthe record has survived as we find to have been the case. And, if we addto these erosive and solvent agencies on land the erosive and solventagencies of the sea, we may almost begin to wonder that anythingdeserving the name of a geological record is in existence at all. That such estimates of the destructive powers of nature are not merematters of speculative reasoning may be amply shown by stating onesingle fact, which, like so many others where the present subject isconcerned, we owe to the generalizations of Darwin. Plutonic rocks, being those which have emerged from subterranean heat of meltingintensity, must clearly at some time or another have lain beneath thewhole thickness of sedimentary deposits, which at that time occupied anypart of the earth's surface where we now find the Plutonic rocks exposedto view. Or, in other words, wherever we now find Plutonic rocks at thesurface of the earth, we must conclude that all the sedimentary rocks bywhich they were covered when in a molten state have since been entirelydestroyed; several vertical miles of the only kinds of rocks in whichfossils can possibly occur must in all such cases have been abolished_in toto_. Now, in many parts of the world metamorphic rocks--which havethus gradually risen from Plutonic depths, while miles of various otherrock-formations have been removed from their now exposed surfaces--coverimmense areas, and therefore testify by their present horizontal range, no less than by their previously vertical depth, to the enormous scaleon which a total destruction has taken place of everything that once layabove them. For instance, the granitic region of Parime is at leastnineteen times the size of Switzerland; a similar region south of theAmazon is probably larger than France, Spain, Italy, and Great Britainall put together; and, more remarkable still, over the area of theUnited States and Canada, granitic rocks exceed in the proportion of 19to 12-1/2 the whole of the newer Palæozoic formations. Lastly, aftergiving these examples, Darwin adds the important consideration, that "inmany regions the metamorphic and granitic rocks would be found much morewidely extended than they appear to be, if all the sedimentary beds wereremoved which rest unconformably on them, and which could not haveformed part of the original mantle under which they were crystallized. " The above is a brief condensation of the already condensed statementwhich Darwin has given of the imperfection of the geological record; butI think it is enough to show, in a general way, how precarious must bethe nature of any objections to the theory of evolution which arefounded merely upon the silence of palæontology in cases where, if therecord were anything like complete, we should be entitled to expect fromit some positive information. But, as we have seen in the text, imperfect though the record be, in as far as it furnishes positiveinformation at all, this is well-nigh uniformly in favour of the theory;and therefore, even on grounds of palæontology alone, it appears to methat Darwin is much too liberal where he concludes his discussion bysaying, --"Those who believe that the geological record is in any degreeperfect, will undoubtedly at once reject the theory. " If in any measurereasonable, such persons ought rather to examine their title to such abelief; and even if they disregard the consensus of testimony which isyielded by all the biological sciences to the theory of evolution, theyought at least to hold their judgment in suspense until they shall havenot only set against the apparently negative testimony which is yieldedby geology its unquestionably positive testimony, but also wellconsidered the causes which may--or rather must--have so gravelyimpaired the geological record. However, be this as it may, I will now pass on to consider thedifficulties and objections which have been brought against the theoryon grounds of palæontology. These may be classified under four heads. First, the absence of varietallinks between allied species; second, the sudden appearance of wholegroups of species--not only as genera and families, but even sometimesas orders and classes--without any forms leading up to them; third, theoccurrence of highly organized types at much lower levels of geologicalstrata than an evolutionist would antecedently expect; and, fourth, theabsence of fossils of any kind lower down than the Cambrian strata. Now all these objections depend on estimates of the imperfection of thegeological record much lower than that which is formed by Darwin. Therefore I have arranged the objections in their order of difficulty inthis respect, or in the order that requires successively increasingestimates of the imperfection of the record, if they are to besuccessively answered. I think that the first of them has been already answered in the text, byshowing that even a very moderate estimate of the imperfection of therecord is enough to explain why intermediate _varieties_, connectingallied _species_, are but comparatively seldom met with. Moreover it wasshown that in some cases, where shells are concerned, remarkablywell-connected series of such varieties have been met with. And the sameapplies to species and genera in certain other cases, as in the equinefamily. But no doubt a greater difficulty arises where whole groups of speciesand genera, or even families and orders, appear to arise suddenly, without anything leading up to them. Even this the second difficulty, however, admits of being fully met, when we remember that in very manycases it has been proved, quite apart from the theory of descent, thatsuperjacent formations have been separated from one another by wideintervals of time. And even although it often happens that intermediatedeposits which are absent in one part of the world are present inanother, we have no right to assume that such is always the case. Besides, even if it were, we should have no right further to assume thatthe faunas of widely separated geographical areas were identical duringthe time represented by the intermediate formation. Yet, unless theywere identical, we should not expect the fossils of the intermediateformation, where extant, to yield evidence of what the fossils wouldhave been in this same formation elsewhere, had it not been theredestroyed. Now, as a matter of fact, "geological formations of eachregion are almost invariably intermittent"; and although in many cases amore or less continuous record of past forms of life can be obtained bycomparing the fossils of one region and formation with those of anotherregion and adjacent formations, it is evident (from what we know of thepresent geographical distribution of plants and animals) that not a fewcases there must have been where the interruption of the record in oneregion cannot be made good by thus interpolating the fossils of anotherregion. And we must remember it is by selecting the cases where thiscannot be done that the objection before us is made to appearformidable. In other words, _unless_ whole groups of new species whichare unknown in formation A appear suddenly in formation C of one region(X), where the intermediate formation B is absent; and _unless_ in someother region (Y), where B is present, the fossiliferous contents of Bfail to supply the fossil ancestry of the new species in A (X); _unless_such a state of matters is found to obtain, the objection before us hasnothing to say. But at best this is negative evidence; and, in order toconsider it fairly, we ought to set against it the cases where aninterposition of fossils found in B (Y) _does_ furnish the fossilancestry of what would _otherwise_ have been an abrupt appearance ofwhole groups of new species in A (X). Now such cases are neither fewnor unimportant, and therefore they deprive the objection of the forceit would have had if the selected cases to the contrary were the generalrule. In addition to these considerations, the following, some of which are ofa more special kind, appear to me so important that I will quote themalmost _in extenso_. We continually forget how large the world is, compared with the area over which our geological formations have been carefully examined: we forget that groups of species may elsewhere have long existed, and have slowly multiplied, before they invaded the ancient archipelagoes of Europe and the United States. We do not make due allowance for the intervals of time which have elapsed between our consecutive formations, --longer perhaps in many cases than the time required for the accumulation of each formation. These intervals will have given time for the multiplication of species from some one parent form; and, in the succeeding formation, such groups of species will appear as if suddenly created. I may here recall a remark formerly made, namely, that it might require a long succession of ages, to adapt an organism to some new and peculiar line of life, for instance, to fly through the air; and consequently that the transitional form would often long remain confined to some one region; but that, when this adaptation had once been effected, and a few species had thus acquired a great advantage over other organisms, a comparatively short time would be necessary to produce many divergent forms, which would spread rapidly and widely throughout the world.... In geological treatises, published not many years ago, mammals were always spoken of as having abruptly come in at the commencement of the tertiary series. And now one of the richest known accumulations of fossil mammals belongs to the middle of the secondary series; and true mammals have been discovered in the new red sandstone at nearly the commencement of this great series. Cuvier used to urge that no monkey occurred in any tertiary stratum; but now extinct species have been discovered in India, South America, and in Europe as far back as the miocene stage. Had it not been for the rare accident of the preservation of footsteps in the new red sandstone of the United States, who would have ventured to suppose that, no less than at least thirty kinds of bird-like animals, some of gigantic size, existed during that period? Not a fragment of bone has been discovered in these beds. Not long ago palæontologists maintained that the whole class of birds came suddenly into existence during the eocene period; but now we know, on the authority of Professor Owen, that a bird certainly lived during the deposition of the upper green-sand. And still more recently that strange bird, the Archeopteryx ... Has been discovered in the oolitic slates of Solenhofen. Hardly any recent discovery shows more forcibly than this, how little we as yet know of the former inhabitants of the world. I may give another instance, which, from having passed under my own eyes, has much struck me. In a memoir on Fossil Sessile Cirripedes, I stated that, from the number of existing and extinct tertiary species; from the extraordinary abundance of the individuals of many species all over the world from the Arctic regions to the equator, inhabiting various zones of depths from the upper tidal limits to 50 fathoms; from the perfect manner in which specimens are preserved in the oldest tertiary beds; from the ease with which even a fragment of a valve can be recognized; from all these circumstances, I inferred that had sessile cirripedes existed during the secondary periods, they would certainly have been preserved and discovered; and as not one species had then been discovered in beds of this age, I concluded that this great group had been suddenly developed at the commencement of the tertiary series. This was a sore trouble to me, adding as I thought one more instance of the abrupt appearance of a great group of species. But my work had hardly been published, when a skilful palæontologist, M. Bosquet, sent me a drawing of a perfect specimen of an unmistakeable sessile cirripede, which he had himself extracted from the chalk of Belgium. And, as if to make the case as striking as possible, this sessile cirripede was a Chthamalus, a very common, large, and ubiquitous genus, of which not one specimen has as yet been found even in any tertiary stratum. Still more recently, a Pyrgoma, a member of a distinct sub-family of sessile cirripedes, has been discovered by Mr. Woodward in the upper chalk; so that we now have abundant evidence of the existence of this group of animals during the secondary period. The case most frequently insisted on by palæontologists of the apparently sudden appearance of a whole group of species, is that of the teleostean fishes, low down, according to Agassiz, in the Chalk period. This group includes the large majority of existing species. But certain Jurassic and Triassic forms are now commonly admitted to be teleostean; and even some palæozoic forms have been thus classed by one high authority. If the teleosteans had really appeared suddenly in the northern hemisphere, the fact would have been highly remarkable; but it would not have formed an insuperable difficulty, unless it could likewise have been shown that at the same period the species were suddenly and simultaneously developed in other quarters of the world. It is almost superfluous to remark that hardly any fossil fish are known from south of the equator; and by running through Pictet's Palæontology it will be seen that very few species are known from several formations in Europe. Some few families of fish now have a confined range; the teleostean fish might formerly have had a similarly confined range, and after having been largely developed in some one sea, might have spread widely. Nor have we any right to suppose that the seas of the world have always been so freely open from south to north as they are at present. Even at this day, if the Malay Archipelago were converted into land, the tropical parts of the Indian Ocean would form a large and perfectly enclosed basin, in which any great group of marine animals might be multiplied; and here they would remain confined, until some of the species became adapted to a cooler climate, and were enabled to double the southern capes of Africa or Australia, and thus reach other and distant seas. From these considerations, from our ignorance of the geology of other countries beyond the confines of Europe and the United States; and from the revolution in our palæontological knowledge effected by the discoveries of the last dozen years, it seems to me to be about as rash to dogmatize on the succession of organic forms throughout the world, as it would be for a naturalist to land for five minutes on some one barren point in Australia, and then to discuss the number and range of its productions[53]. [53] _Origin of Species_, 282-5. In view of all the foregoing facts and considerations, it appears to methat the second difficulty on our list is completely answered. Indeed, even on a moderate estimate of the imperfection of the geologicalrecord, the wonder would have been if many cases had _not_ occurredwhere groups of species present the fictitious appearance of having beensuddenly and simultaneously created in the particular formations wheretheir remains now happen to be observable. Turning next to the third objection, there cannot be any question thatevery here and there in the geological series animals occur of a muchhigher grade zoologically than the theory of evolution would haveexpected to find in the strata where they are found. At any rate, speaking for myself, I should not have antecedently expected to meetwith such highly differentiated insects as butterflies and dragonfliesin the middle of the Secondaries: still less should I have expected toencounter beetles, cockroaches, spiders, and May-flies in the upper andmiddle Primaries--not to mention an insect and a scorpion even in thelower. And I think the same remark applies to a whole sub-kingdom in thecase of Vertebrata. For although it is only the lowest class of thesub-kingdom which, so far as we positively know, was represented in theDevonian and Silurian formations, we must remember, on the one hand, that even a cartilaginous or ganoid fish belongs to the highestsub-kingdom of the animal series; and, on the other hand, that suchanimals are thus proved to have abounded in the very lowest strata wherethere is good evidence of there having been any forms of life at all. Lastly, the fact that Marsupials occur in the Trias, coupled with thefact that the still existing Monotremata are what may be termed animatedfossils, referring us by their lowly type of organization to some periodenormously more remote, --these facts render it practically certain thatsome members of this very highest class of the highest sub-kingdom musthave existed far back in the Primaries. These things, I say, I should not have expected to find, and I think allother evolutionists ought to be prepared to make the sameacknowledgment. But as these things have been found, the only possibleway of accounting for them on evolutionary principles is by supposingthat the geological record is even more imperfect than we needed tosuppose in order to meet the previous objections. I cannot see, however, why evolutionists should be afraid to make this acknowledgment. For I donot know any reason which would lead us to suppose that there is anycommon measure between the distances marked on our tables of geologicalformations, and the times which those distances severally represent. Letthe reader turn to the table on page 163, and then let him say why the30, 000 feet of so-called Azoic rocks may not represent a greaterduration of time than does the thickness of all the Primary rocks abovethem put together. For my own part I believe that this is probably thecase, looking to the enormous ages during which these very earlyformations must have been exposed to destructive agencies of all kinds, now at one time and now at another, in different parts of the world. And, of course, we are without any means of surmising what ranges oftime are represented by the so-called Primeval rocks, for the simplereason that they are non-sedimentary, and non-sedimentary rocks cannotbe expected to contain fossils. But, it will be answered, the 30, 000 feet of Azoic rocks, lying abovethe Primeval, _are_ sedimentary to some extent: they are not allcompletely metamorphic: yet they are all destitute of fossils. This isthe fourth and last difficulty which has to be met, and it can only bemet by the considerations which have been advanced by Lyell and Darwin. The former says:-- The total absence of any trace of fossils has inclined many geologists to attribute the origin of the most ancient strata to an azoic period, or one antecedent to the existence of organic beings. Admitting, they say, the obliteration, in some cases, of fossils by plutonic action, we might still expect that traces of them would oftener be found in certain ancient systems of slate, which can scarcely be said to have assumed a crystalline structure. But in urging this argument it seems to be forgotten that there are stratified formations of enormous thickness, and of various ages, some of them even of tertiary date, and which we know were formed after the earth had become the abode of living creatures, which are, nevertheless, in some districts, entirely destitute of all vestiges of organic bodies[54]. [54] _Elements of Geology_, p. 587. He then proceeds to mention sundry causes (in addition to plutonicaction) which are adequate to destroy the fossiliferous contents ofstratified rocks, and to show that these may well have produced enormousdestruction of organic remains in these oldest of known formations. Darwin's view is that, during the vast ages of time now underconsideration, it is probable that the distribution of sea and land overthe earth's surface has not been uniformly the same, even as regardsoceans and continents. Now, if this were the case, "it might well happenthat strata which had subsided some miles nearer to the centre of theearth, and which had been pressed on by an enormous weight ofsuperincumbent water, might have undergone far more metamorphic actionthan strata which have always remained nearer to the surface. Theimmense areas in some parts of the world, for instance in South America, of naked metamorphic rocks, which must have been heated under greatpressure, have always seemed to me to require some special explanation;and we may perhaps believe that we see, in these large areas, the manyformations long anterior to the Cambrian epoch in a completelymetamorphosed and denuded condition[55]. " The probability of this viewhe sustains by certain general considerations, as well as particularfacts touching the geology of oceanic islands, &c. [55] _Origin of Species_, p. 289. On the whole, then, it seems to me but reasonable to conclude, withregard to all four objections in question, as Darwin concludes withregard to them:-- For my part, following out Lyell's metaphor, I look at the geological record as a history of the world imperfectly kept, written in a changing dialect; of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page only here and there a few lines. Each word of the slowly-changing language, more or less different in the successive chapters, may represent the forms of life, which are entombed in our consecutive formations, and which falsely appear to us to have been abruptly introduced. On this view, the difficulties above discussed are greatly diminished, or even disappear[56]. [56] _Ibid. _ As far as I can see, the only reasonable exception that can be taken tothis general view of the whole matter, is one which has been taken fromthe side of astronomical physics. Put briefly, it is alleged by one of the highest authorities in thisbranch of science, that there cannot have been any such enormous reachesof unrecorded time as would be implied by the supposition of therehaving been a lost history of organic evolution before the Cambrianperiod. The grounds of this allegation I am not qualified to examine;but in a general way I agree with Prof. Huxley in feeling that, from thevery nature of the case, they are necessarily precarious, --and this inso high a degree that any conclusions raised on such premises are notentitled to be deemed formidable[57]. [57] See _Lay Sermons_, Lecture on Geological Reform. * * * * * Turning now to plants, the principal and the ablest opponent of thetheory of evolution is here unquestionably Mr. Carruthers[58]. Thedifficulties which he adduces may be classified under three heads, asfollows:-- [58] See especially the following Presidential addresses:--Geol. Assoc. Nov. 1876; Section D. Brit. Assoc. , 1886; Lin. Soc. , 1890. 1. There is no evidence of change in specific forms of existing plants. Not only are the numerous species of plants which have been found inEgyptian mummies indistinguishable from their successors of to-day; but, what is of far more importance, a large number of our own indigenousplants grew in Great Britain during the glacial period (including underthis term the warm periods between those of successive glaciations), andin no one case does it appear that any modification of specific type hasoccurred. This fact is particularly remarkable as regards leaves, because on the one hand they are the organs of plants which are mostprone to vary, while on the other hand they are likewise the organswhich lend themselves most perfectly to the process of fossilization, sothat all details of their structure can be minutely observed in thefossil state. Yet the interval since the glacial period, although not along one geologically speaking, is certainly what may be called anappreciable portion of time in the history of Dicotyledonous plantssince their first appearance in the Cretaceous epoch. Again, if weextend this kind of enquiry so as to include the world as a whole, anumber of other species of plants dating from the glacial epoch arefound to tell the same story--notwithstanding that, in the opinion ofMr. Carruthers, they must all have undergone many changes of environmentwhile advancing before, and retreating after, successive glaciations indifferent parts of the globe. Or, to quote his own words:--"The variousphysical conditions which of necessity affected these {41} species intheir diffusion over such large areas of the earth's surface in thecourse of, say, 250, 000 years, should have led to the production of manyvarieties; but the uniform testimony of the remains of this considerablepre-glacial flora, as far as the materials admit of a comparison, isthat no appreciable change has taken place. " 2. There is no appearance of generalized forms among the earliest plantswith which we are acquainted. For example, in the first dry landflora--the Devonian--we have representatives of the _Filices_, _Equisetaceæ_, and _Lycopodiaceæ_, all as highly specialized as theirliving representatives, and exhibiting the differential characters ofthese closely related groups. Moreover, these plants were even morehighly organized than their existing descendants in regard to theirvegetative structure, and in some cases also in regard to theirreproductive organs. So likewise the Gymnosperms of that time show intheir fossil state the same highly organized woody structure as theirliving representatives. 3. Similarly, and more generally, the Dicotyledonous plants, which firstappear in the Cretaceous rocks, appear there suddenly, without any formsleading up to them--notwithstanding that "we know very well theextensive flora of the underlying Wealden. " Moreover, we have all thethree great divisions of the Dicotyledons appearing together, and sohighly differentiated that all the species are referred to existinggenera, with the exception of a very few imperfectly preserved, andtherefore uncertain fragments. Such being the facts, we may begin by noticing that, even at firstsight, they present different degrees of difficulty. Thus, I cannot seethat there is much difficulty with regard to those in class 2. Only ifwe were to take the popular (and very erroneous) view of organicevolution as a process which is always and everywhere bound to promotethe specialization of organic types--only then ought we to see any realdifficulty in the absence of generalized types preceding these existingtypes. Of course we may wonder why still lower down in the geologicalseries we do not meet with more generalized (or ancestral) types; butthis is the difficulty number 3, which we now proceed to examine. Concerning the other two difficulties, then, the only possible way ofmeeting that as to the absence of any parent forms lower down in thegeological series is by falling back--as in the analogous case ofanimals--upon the imperfection of the geological record. Although it iscertainly remarkable that we should not encounter any forms serving toconnect the Dicotyledonous plants of the Chalk with the lower forms ofthe underlying Wealden, we must again remember that difficulties thusdepending on the absence of any corroborative record, are by no meansequivalent to what would have arisen in the presence of an adverserecord--such, for instance, as would have been exhibited had the florasof the Wealden and the Chalk been inverted. But, as the case actuallystands, the mere fact that Dicotyledonous plants, where they firstoccur, are found to have been already differentiated into their threemain divisions, is in itself sufficient evidence, on the general theoryof evolution, that there must be a break in the record as hitherto knownbetween the Wealden and the Chalk. Nor is it easy to see how theopponents of this theory can prove their negative by furnishing evidenceto the contrary. And although such might justly be deemed an unfair wayof putting the matter, were this the only case where the geologicalrecord is in evidence, it is not so when we remember that there arenumberless other cases where the geological record does testify toconnecting links in a most satisfactory manner. For in view of thisconsideration the burden of proof is thrown upon those who point toparticular cases where there is thus a conspicuous absence oftransitional forms--the burden, namely, of proving that such cases arenot due merely to a break in the record. Besides, the break in therecord as regards this particular case may be apparent rather than real. For I suppose there is no greater authority on the pure geology of thesubject than Sir Charles Lyell, and this is what he says of theparticular case in question. "If the passage seem at present to besomewhat sudden from the flora of the Lower or Neocomian to that of theUpper Cretaceous period, the abruptness of the change will probablydisappear when we are better acquainted with the fossil vegetation ofthe uppermost tracts of the Neocomian and that of the lowest strata ofthe Gault, or true Cretaceous series[59]. " [59] _Elements of Geology_, p. 280. Lastly, the fact of the flora of the glacial epoch not having exhibitedany modifications during the long residence of some of its specifictypes in Great Britain and elsewhere, is a fact of some importance tothe general theory of evolution, since it shows a higher degree ofstability on the part of these specific types than might perhaps havebeen expected, supposing the theory to be true. But I do not see thatthis constitutes a difficulty against the theory, when we have so manyother cases of proved transmutation to set against it. For instance, notto go further afield than this very glacial flora itself, it will beremembered that in an earlier chapter I selected it as furnishingspecially cogent proof of the transmutation of species. What, then, isthe explanation of so extraordinary a difference between Mr. Carruthers'views and my own upon this point? I believe the explanation to be thathe does not take a sufficiently wide survey of the facts. To begin with, it seems to me that he exaggerates the vicissitudes towhich the species of plants that he calls into evidence have beenexposed while advancing before, and retreating after, the ice. Rather doI agree with Darwin that "they would not have been exposed during theirlong migrations to any great diversity of temperature; and as they allmigrated in a body together, their mutual relations will not have beenmuch disturbed; hence, in accordance with the principles indicated inthis volume, these forms will not have been liable to muchmodification[60]. " But, be this matter of opinion as it may, a muchbetter test is afforded by those numerous cases all the world over, where arctic species have been left stranded on alpine areas by theretreat of glaciation; because here there is no room for differences ofopinion as to a "change of environment" having taken place. Not to speakof climatic differences between arctic and alpine stations, considermerely the changes which must have taken place in the relations of thethus isolated species to each other, as well as to those of all theforeign plants, insects, &c. , with which they have long been thrown intoclose association. If in _such_ cases no variation or transmutation hadtaken place since the glacial epoch, then indeed there would have been adifficulty of some magnitude. But, by parity of reasoning, whateverdegree of difficulty would have been thus presented is not merelydischarged, but converted into at least an equal degree ofcorroboration, when it is found that under such circumstances, inwhatever part of the world they have occurred, some considerable amountof variation and transmutation has always taken place, --and this in theanimals as well as in the plants. For instance, again to quote Darwin, "If we compare the present Alpine plants and animals of the severalgreat European mountain-ranges one with another, though many of thespecies remain identically the same, some exist as varieties, some asdoubtful forms or sub-species, and some as distinct yet closely alliedspecies representing each other on the several ranges[61]. " Lastly, ifinstead of considering the case of alpine floras, we take the muchlarger case of the Old and New World as a whole, we meet with muchlarger proofs of the same general facts. For, "during the slowlydecreasing warmth of the Pliocene period, as soon as the species incommon, which inhabited the New and Old Worlds, migrated south of thePolar Circle, they will have been completely cut off from each other. This separation, as far as the more temperate productions are concerned, must have taken place long ages ago. As the plants and animals migratedsouthward, they will have become mingled in one great region with thenative American productions, and would have had to compete with them;and, in the other great region, with those of the Old World. Consequently we have here everything favourable for muchmodification, --for far more modification than with the Alpineproductions left isolated, within a much more recent period, on theseveral mountain ranges and on the arctic lands of Europe and N. America. Hence it has come, that when we compare the now livingproductions of the temperate regions of the New and Old Worlds, we findvery few identical species; but we find in every class many forms, whichsome naturalists rank as geographical races, and others as distinctspecies; and a host of closely allied or representative forms which areranked by all naturalists as specifically distinct[62]. " [60] _Origin of Species_, p. 332. [61] _Origin of Species_, p. 332. [62] _Ibid_. Pp. 333-4. In view then of all the above considerations--and especially thosequoted from Darwin--it appears to me that far from raising anydifficulty against the theory of evolution, the facts adduced by Mr. Carruthers make in favour of it. For when once these facts are taken inconnection with the others above mentioned, they serve to complete thecorrespondence between degrees of modification with degrees of time onthe one hand, and with degrees of evolution, of change of environment, &c. , on the other. Or, in the words of Le Conte, when dealing with thisvery subject, "It is impossible to conceive a more beautifulillustration of the principles we have been trying to enforce[63]. " [63] _Evolution and its Relation to Religious Thought_, p. 194. NOTE A TO PAGE 257. The passages in Dr. Whewell's writings, to which allusion is here made, are somewhat too long to be quoted in the text. But as I think theydeserved to be given, I will here reprint a letter which I wrote to_Nature_ in March, 1888. In his essay on the _Reception of the Origin of Species_, Prof. Huxley writes:-- "It is interesting to observe that the possibility of a fifth alternative, in addition to the four he has stated, has not dawned upon Dr. Whewell's mind" (_Life and Lectures of Charles Darwin_, vol. Ii, p. 195). And again, in the article _Science_, supplied to _The Reign of Queen Victoria_, he says:-- "Whewell had not the slightest suspicion of Darwin's main theorem, even as a logical possibility" (p 365). Now, although it is true that no indication of such a logical possibility is to be met with in the _History of the Inductive Sciences_, there are several passages in the _Bridgewater Treatise_ which show a glimmering idea of such a possibility. Of these the following are, perhaps, worth quoting. Speaking of the adaptation of the period of flowering to the length of a year, he says:-- "Now such an adjustment must surely be accepted as a proof of design, exercised in the formation of the world. Why should the solar year be so long and no longer? or, this being such a length, why should the vegetable cycle be exactly of the same length? Can this be chance?... And, if not by chance, how otherwise could such a coincidence occur than by an intentional adjustment of these two things to one another; by a selection of such an organization in plants as would fit them to the earth on which they were to grow; by an adaptation of construction to conditions; of the scale of construction to the scale of conditions? It cannot be accepted as an explanation of this fact in the economy of plants, that it is necessary to their existence; that no plants could possibly have subsisted, and come down to us, except those which were thus suited to their place on the earth. This is true; but it does not at all remove the necessity of recurring to design as the origin of the construction by which the existence and continuance of plants is made possible. A watch could not go unless there were the most exact adjustment in the forms and positions of its wheels; yet no one would accept it as an explanation of the origin of such forms and positions that the watch would not go if these were other than they were. If the objector were to suppose that plants were originally fitted to years of various lengths, and that such only have survived to the present time as had a cycle of a length equal to our present year, or one which could be accommodated to it, we should reply that the assumption is too gratuitous and extravagant to require much consideration. " Again, with regard to "the diurnal period, " he adds:-- "Any supposition that the astronomical cycle has occasioned the physiological one, that the structure of plants has been brought to be what it is by the action of external causes, or that such plants as could not accommodate themselves to the existing day have perished, would be not only an arbitrary and baseless assumption, but, moreover, useless for the purposes of explanation which it professes, as we have noticed of a similar supposition with respect to the annual cycle. " Of course these passages in no way make against Mr. Huxley's allusions to Dr. Whewell's writings in proof that, until the publication of the _Origin of Species_, the "main theorem" of this work had not dawned on any other mind, save that of Mr. Wallace. But these passages show, even more emphatically than total silence with regard to the principle of survival could have done, the real distance which at that time separated the minds of thinking men from all that was wrapped up in this principle. For they show that Dr. Whewell, even after he had obtained a glimpse of the principle "as a logical possibility, " only saw in it an "arbitrary and baseless assumption. " Moreover, the passages show a remarkable juxtaposition of the very terms in which the theory of natural selection was afterwards formulated. Indeed, if we strike out the one word "intentional" (which conveys the preconceived idea of the writer, and thus prevented him from doing justice to any naturalistic view), all the following parts of the above quotations might be supposed to have been written by a Darwinian. "If not by chance, how otherwise could such a coincidence occur, than by an _adjustment_ of these two things to one another; by a _selection_ of such an organization in plants as would _fit_ them to the earth on which they were to grow; by an adaptation of _construction_ to _conditions_; of the _scale_ of construction to the _scale_ of conditions?" Yet he immediately goes on to say: "If the objector were to suppose that plants were originally _fitted_ to years of various lengths, and that such only have _survived_ to the present time ... _as could be accommodated to it_ (i. E. The actual cycle), we should reply that the assumption is too gratuitous and extravagant to require much consideration. " Was there ever a more curious exhibition of failure to perceive the importance of a "logical possibility"? And this at the very time when another mind was bestowing twenty years of labour on its "consideration. " NOTE B TO PAGE 295. Since these remarks were delivered in my lectures as here printed, Mr. Mivart has alluded to the subject in the following and preciselyopposite sense:-- Many of the more noteworthy instincts lead us from manifestations of purpose directed to the maintenance of the individual, to no less plain manifestations of a purpose directed to the preservation of the race. But a careful study of the interrelations and interdependencies which exist between the various orders of creatures inhabiting this planet shows us yet a more noteworthy teleology--the existence of whole orders of such creatures being directed to the service of other orders in various degrees of subordination and augmentation respectively. This study reveals to us, as a fact, the enchainment of all the various orders of creatures in a hierarchy of activities, in harmony with what we might expect to find in a world the outcome of a First Cause possessed of intelligence and will[64]. [64] _On Truth_, p. 493. Having read this much, a Darwinian is naturally led to expect that Mr. Mivart is about to offer some examples of instincts or structuresexemplifying what in the margin he calls the "Hierarchy ofMinistrations. " Yet the only facts he proceeds to adduce are thesufficiently obvious facts, that the inorganic world existed before theorganic, plants before herbivorous animals, these before carnivorous, and so on: that is to say, everywhere the conditions to the occurrenceof any given stage of evolution preceded such occurrence, as it isobvious that they must, if, as of course it is not denied, thepossibility of such occurrence depended on the precedence of suchconditions. Now, it is surely obvious that such a "hierarchy ofministrations" as this, far from telling against the theory of naturalselection, is the very thing which tells most in its favour. The factthat animals, for instance, only appeared upon the earth after therewere plants for them to feed upon, is clearly a necessity of the case, whether or not there was any design in the matter. Such "ministrations, "therefore, as plant-organisms yield to animal-organisms is just the kindof ministration that the theory of natural selection requires. Thus far, then, both the theories--natural selection and super-naturaldesign--have an equal right to appropriate the facts. But now, if in noone instance can it be shown that the ministration of plant-life toanimal-life is of such a kind as to subserve the interests ofanimal-life without at the same time subserving those of the plant-lifeitself, then the fact makes wholly in favour of the naturalisticexplanation of such ministration as appears. If any plants had presentedany characters pointing prospectively to needs of animals withoutprimarily ministering to their own, then, indeed, there would have beenno room for the theory of natural selection. But as this can nowhere bealleged, the theory of natural selection finds all the facts to beexactly as it requires them to be: such ministration as plants yield toanimals becomes so much evidence of natural selection having slowlyformed the animals to appropriate the nutrition which the plants hadpreviously gathered--and gathered under the previous influence ofnatural selection acting on themselves entirely for their own sakes. Therefore I say it is painfully manifest that "the enchainment of allthe various orders of creatures in a hierarchy of activities, " is _not_"in harmony with what we might expect to find in a world the outcome ofa First Cause possessed of intelligence and [beneficent] will. " So faras any argument from such "enchainment" reaches, it makes entirelyagainst the view which Mr. Mivart is advocating. In point of fact, thereis a total absence of any such "ministration" by one "order ofcreatures" to the needs of any other order, as the beneficent designtheory would necessarily expect; while such ministration as actuallydoes obtain is exactly and universally the kind which the naturalistictheory requires. Again, quite independently, and still more recently, Mr. Mivart alludedin _Nature_ (vol. Xli, p. 41) to the difficulty which the apparentlyexceptional case of gall-formation presents to the theory of naturalselection. Therefore I supplied (vol. Xli, p. 80) the suggestion givenin the text, viz. That although it appears impossible that the sometimesremarkably elaborate and adaptive structures of galls can be due tonatural selection acting directly on the plants themselves--seeing thatthe adaptation has reference to the needs of their parasites--it isquite possible that the phenomena may be due to natural selection actingindirectly on the plants, by always preserving those individual insects(and larvae) the character of whose secretions is such as will bestinduce the particular shapes of galls that are required. Several othercorrespondents took part in the discussion, and most of them acceptedthe above explanation. Mr. T. D. A. Cockerell, however, advanced anotherand very ingenious hypothesis, showing that there is certainly oneconceivable way in which natural selection might have produced all thephenomena of gall-formation by acting directly on the plantsthemselves[65]. Subsequently Mr. Cockerell published another paper uponthe subject, stating his views at greater length. The following is thesubstance of his theory as there presented:-- [65] _Nature_, vol. Xli, p. 344. Doubtless there were internal plant-feeding larvae before there were galls: and, indeed, we have geological evidence that boring insects date very far back indeed. The primitive internal feeders, then, were miners in the roots, stems, twigs, or leaves, such as occur very commonly at the present day. These miners are excessively harmful to plant-life, and form a class of the most destructive insect-pests known to the farmer: they frequently cause the death of the whole or part of the plant attacked. Now, we may suppose that the secretions of certain of these insects caused a swelling to appear where the larvae lived, and on this excrescence the larvae fed. It is easy to see that the greater the excrescence, and the greater the tendency of the larvae to feed upon it, instead of destroying the vital tissues, the smaller is the amount of harm to the plant. Now the continued life and vitality of the plant is beneficial to the larvae, and the larger or more perfect the gall, the greater the amount of available food. Hence natural selection will have preserved and accumulated the gall-forming tendencies, as not only beneficial to the larvae, but as a means whereby the larvae can feed with least harm to the plant. So far from being developed for the exclusive benefit of the larvae, it is easy to see that, allowing a tendency to gall-formation, natural selection would have developed galls exclusively for the benefit of the plants, so that they might suffer a minimum of harm from the unavoidable attacks of insects. But here it may be questioned--have we proof that internal feeders tend to form galls? In answer to this I would point out that gall-formation is a peculiar feature, and cannot be expected to arise in every group of internal feeders. But I think we can afford sufficient proof that wherever it has arisen it has been preserved; and further, that even the highly complex forms of galls are evolved from forms so simple that we hesitate to call them galls at all[66]. [66] _Entomologist_, March, 1890. The paper then proceeds to give a number of individual cases. No doubtthe principal objection to which Mr. Cockerell's hypothesis is open isone that was pointed out by Herr Wetterhan, viz. "the much greaterfacility afforded to the indirect action through insects, by theenormously more rapid succession of generations with the latter thanwith many of their vegetable hosts--oaks above all[67]. " Thisdifficulty, however, Mr. Cockerell believes maybe surmounted by theconsideration that a growing plant need not be regarded as a singleindividual, but rather as an assemblage of such[68]. [67] _Nature_, vol. Xli, p. 394. [68] _Ibid. _ vol. Xli, pp. 559-560. NOTE C TO PAGE 394. The only remarks that Mr. Wallace has to offer on the _pattern ofcolours_, as distinguished from a mere _brilliancy of colour_, are addedas an afterthought suggested to him by the late Mr. Alfred Tylor's bookon _Colouration of Animals and Plants_ (1886). But, in the first place, it appears to me that Mr. Wallace has formed an altogether extravagantestimate of the value of this work. For the object of the work is toshow, "that diversified colouration follows the chief lines ofstructure, and changes at points, such as the joints, where functionchanges. " Now, in publishing this generalization, Mr. Tylor--who was nota naturalist--took only a very limited view of the facts. When appliedto the animal kingdom as a whole, the theory is worthless; and evenwithin the limits of mammals, birds, and insects--which are the classesto which Mr. Tylor mainly applies it--there are vastly more facts tonegative than to support it. This may be at once made apparent by thefollowing brief quotation from Prof. Lloyd Morgan:-- It can hardly be maintained that the theory affords us any adequate explanation of the _specific_ colour-tints of the humming-birds, or the pheasants, or the Papilionidae among butterflies. If, as Mr. Wallace argues, the immense tufts of golden plumage in the bird of paradise owe their origin to the fact that they are attached just above the point where the arteries and nerves for the supply of the pectoral muscles leave the interior of the body--and the physiological rationale is not altogether obvious, --are there no other birds in which similar arteries and nerves are found in a similar position? Why have these no similar tufts? And why, in the birds of paradise themselves, does it require four years ere these nervous and arterial influences take effect upon the plumage? Finally, one would inquire how the colour is determined and held constant in each species. The difficulty of the Tylor-Wallace view, even as a matter of origin, is especially great in those numerous cases in which the colour is determined by delicate lines, thin plates, or thin films of air or fluid. Mr. Poulton, who takes a similar line of argument in his _Colours of Animals_ (p. 326), lays special stress on the production of _white_ (pp. 201-202). As regards the latter point, it may be noticed that not in any part ofhis writings, so far as I can find, does Mr. Wallace allude to thehighly important fact of colours in animals being so largely due tothese purely physical causes. Everywhere he argues as if colours wereuniversally due to pigments; and in my opinion this unaccountableoversight is the gravest defect in Mr. Wallace's treatment both of thefacts and the philosophy of colouration in the animal kingdom. Forinstance, as regards the particular case of sexual colouration, theoversight has prevented him from perceiving that his theory of"brilliancy" as due to "a surplus of vital energy, " is not so much aslogically possible in what must constitute at least one good half of thefacts to which he applies it--unless he shows that there is someconnection between vital energy and the development of striations, imprisonment of air-bubbles, &c. But any such connection--soessentially important for his theory--he does not even attempt to show. Lastly, and quite apart from these remarkable oversights, even if Mr. Tylor's hypothesis were as reasonable and well-sustained as it isfanciful and inadequate, still it could not apply to _sexual_colouration: it could apply only to colouration as affected byphysiological functions common to both sexes. Yet it is in order tofurnish a "preferable substitute" for Mr. Darwin's theory of _sexual_colouration, that Mr. Wallace adduces the hypothesis in question as oneof "great weight"! In this matter, therefore, I entirely agree withPoulton and Lloyd Morgan. INDEX A. Accident, Darwin's use of the word, 334-340; beauty due to, 408, 409. Achromatin, 126-134. Acquired characters, _see_ Characters. _Acræa eurita_, 328. Adaptation, facts of, in relation to theory of natural selection, 401-403, 411. Adaptive characters, _see_ Characters. Æsthetic sense in animals, 380-385; _see_ Beautiful. Agassiz, Prof. A. , on fauna of the Mammoth cave, 70. Alpine plants, 209, 210, 440-442. _Amauris niavius_, 328. _Amblyornis inornata_, 381-383. _Amphioxus_, 137, 138, 145, 146. Analogy, 38, 50-65, 176, 177, 347-350. Anthropoid, _see_ Apes. Antlers, 98-100, 167-169. Ants, co-operative instincts of, 268; leaf-cutting, 332; keeping aphides, 292. Ape, eye of, 75; _appendix vermiformis_ of, 84-86. Apes, ears of, compared with those of man, 88; muscles of, 77, 82, 83; feet of, 77, 78; tail of, compared with that of man, 82-84; hair of, compared with that of man, 89-91; teeth of, compared with those of man, 92-94; flattening of tibiæ of, 95, 96. Aphides, 292. _Appendix vermiformis_ of man compared with that of orang, 84-86. _Apteryx_, 68, 69. _Archæopteryx_, 171-173. Arctic plants, 209, 210, 440-442. Argyll, Duke of, on natural selection, 334-362. Aristotle, his idea of scientific method, 1; on classification, 23, 24. Arm, distribution of hair on, in man and apes, 89-92. _Arthropoda_, embryology of, 155. Artificial selection, analogy of, to natural selection, 295-314; pictorial representations of products of, 298-312. Artiodactyls, 182-191. Association, principle of, in æsthetics, 404-407. Aster, 129-133. Attraction-spheres, 128, 132, 133. Australia, fauna of, 204, 205; thriving of exotic species in, 286; portrait of wild dog of, 304. Azores, 224, 225. B. Bacon, Lord, on scientific method, 2. _Balanoglossus_, 147, 148. _Baptanodon discus_, posterior limb of, 179-181. Barriers, in relation to geographical distribution, 216-224. Bats, 56, 224, 226, 240. Battle, law of, 385, 386. Baya-bird, 381. Bear, skeleton of, 174; feet of, 178. Beautiful, the, sense of, in animals, 380-385; standards of, 380-404; Darwin's explanation of, in organic nature, 379-411; facts of, in inorganic nature in relation to Darwin's theory of, in organic, 404; often determined by natural selection, 406, 407; absent in many plants and animals, 408; in nature often accidental, 409-411; does not exist in organic nature as an end _per se_, 410, 411. Bees, co-operative instincts of, 268. Beetles, wingless, 68-70; on oceanic islands, 224, 226, 229, 232. Bell, Dr. , on natural theology, 412. Bell-bird, 396-398. _Bembidium_, 233. Bermudas, 225-227. Biology, ideas of method in, 1-9. Birds, ovum of, 124; embryology of, 151-155; paleontology of, 163-165, 172, 173; brain of, 194-197; as carriers of seed, eggs, and small organisms, 217, 218; distribution of, 224-240; æsthetic sense of, 380-385; courtship of, 380-385. _Birgus latro_, 62-65. Blood, colour of arterial, 409. Boar, _see_ Pig. _Bombus lapidarius_, 331. Bower-birds, play-houses of, 381-383. Boyd-Dawkins, on flattening of early human tibiæ, 96. Brain, palæontology of, 194-197. British Isles, _see_ Islands. Broca, 363. Bronn, 363. Budding, _see_ Germination. Burdon-Sanderson, Prof. , on electric organ of skate, 366. Butler, Bishop, on argument from ignorance, 41. Butterflies, defensive colouring of, 321-329. C. Cæsalpino, on classification, 24. Calf, embryology of, 153. Camel, foot of, 187-191. Canadian stag, 196, 198, 199. Canaries, portraits of, 303; first mentioned by Gesner, 312, 313. Cape de Verde Archipelagoes, fauna of, 228. _Carcharias melanopterus_, 149. Carruthers, on evolution, 436-442. Caterpillars, colours and forms of, 319, 322-326. Cattle, portraits of, 311. Causation, natural, 402, 413, 414. Caves, faunas of dark, 70-72. Cell, physiological, and properties of the, 104-134. _Cerura vinula_, 325, 326. _Cervalces Americanus_, 196, 198, 199. _Cervus dicrocerus_, _issiodorensis_, _matheronis_, _pardinensis_, _Sedgwickii_, _tetraceros_, 168. Chalmers, Dr. , on natural theology, 412. Chameleons, 317. Characters, as adaptive, 273-276, 286-293, 349; as specific, 274-276, 286-295; as congenital and acquired, 274-276. _Chasmorhynchus niveus_, and _C. Tricarunculatus_, 396-398. _Chelydra serpentina_, anterior limb of, 179-181. Chick, embryology of, 153. Chimpanzee, _see_ Apes. Chlorophyll, 408. _Chondracanthus cornutus_, 122. Cirripedes, 430. Classification, 23-49; of organic nature by Genesis and Leviticus, 23; artificial and natural, 24-26; empirical rules of, 33-40; Darwin on, 35, 36, 39, 40; form of, a nexus or tree, 29-32; of organic forms like that of languages, 32; single characters in relation to, 37; aggregates of characters in relation to, 35-37; adaptive and non-adaptive characters in relation to, 34, 35, 38, 39; chains of affinities in relation to, 39-40; biological differs from astronomical, 43. Cockerell, on vegetable galls, 447, 448. Colours, of plants and animals in relation to the theory of natural selection, 317-332; in relation to the theory of sexual selection, 391, 392, 394-396, 408-410, 448-450. Colouring, _see_ Recognition marks, Protective, Seasonal, Warning, and Mimicry. Congenital characters, _see_ Characters. Conjugation, of Protozoa, 115-117. Continuity, principle of, in nature, 15-21. Contrivance, Darwin's use of the word, 281. Co-operation, mutual, of species alleged, 445-448. Co-operative instincts, due to natural selection, 267, 269. Cope, Professor, his table of geological formations, 163, 164; his table of palæontological development of feet, vertebral column, and brain, 197. Correlation of growth, 357-362. _Cossonidæ_, 233. Courtship, _see_ Sexual Selection. Crabs, 62-65, 139. Cuttle-fish, 317. Cuvier, on method in natural history, 3-4; on monkeys, 429. Cyst, _see_ Encystation. D. Darwin, Charles, his influence on ideas of method, 1-9; on classification, 35, 36, 39, 40; on vestigial characters in man, 77, 86, 87, 92; on imperfection of geological record, 165, and Appendix; on means of dispersal, 216, 218; on geographical distribution, 218, 219; on fauna of the Galapagos Archipelago, 227, 228; on natural selection, 252, 253, 255, 256, 286, 375, 376; his use of such words as 'accident, ' 'fortuitous, ' 'purpose, ' 'contrivance, ' &c. , 281, 334-340; on sexual selection, 379-400. Darwin, Erasmus, his theory of evolution, 253. De Blainville, on the theory of descent, 258. De Candolle, on classification, 34. Deer, 98, 99, 167-169, 187, 191, 196, 198, 199. Degeneration, 269, 270, 342. Delamination, 139. _Diadema euryta_, 330. Diaster, 129-133. Dingo, _see_ Dog. _Dinornis_, 60, 61. _Diptera_ mimicking _Hymenoptera_, 329. Dog, dentition of, 39; Dingo, 304; domesticated varieties of, 305, 307; hairless, 307; skulls of, 307. Duck, logger-headed, 68. Dugong, eye of, 75. E. Eagle, eye of, 75. Ear, of whales, 65; vestigial features of human, 76, 86-89; of man and apes compared, 88. Eaton, Rev. A. E. , on wingless insects, 70. _Echinodermata_, 125-127, 138, 155. Ectoderm, 137-142. Egg, _see_ Ovum. Eimer, 363. _Elaps fulvius_ imitated by non-venomous snakes, 330. Electric organs, 365-373. Elephant, foot of, 185, 186; rate of propagation of, 261, 262. Elk, 196-198, 199. Embryo, human, _see_ Man. Embryogeny, _see_ Ontogeny. Embryology, 98-155. Embryos, comparative series of, 152, 153. _Encyclopædia Britannica_, eighth ed. , on instinct, 289-291. Encystation of Protozoa, 115. Endoderm, 137-142. Equatorial plate, 129. _Equus_, _see_ Horse. _Erythrolamprus venustissimus_, 330. Evolution, organic, fact of, Section I; Method of, Section II; ideas upon, prior to Darwin, 253-258; divergent, 266, 267. Ewart, Professor Cossar, on electric organ of skate, 364, 367. Existence, _see_ Struggle for. Eye, of octopus, 57, 58, 347-350; absence of, in dark cave animals, 70-72; nictitating membrane of, 74, 75; development of, from cutaneous nerve-ending, 352-354. F. Feet, 51-59, 66, 77-80, 174-192, 197. Fertilization of ova, 127, 128; of flowers by insects, 406. Fish, embryology of, 143-155; palæontology of, 163, 165, 169-171; brain of, 194-197; distribution of, 224-246; flying, 355. Fission, reproduction by, 106, 107. Flat fish, 317. Float, _see_ Swim-bladder. Flowers, fertilization of, by insects, 406. Fly, imitating a wasp, 329. Flying-fish, and squirrels, 355. _Foraminifera_, 346. Forbes, H. O. , on scapulo-coracoid bones of _Dinornis_, 60. Fortuitous, Darwin's use of the word, 340. Fossils, _see_ Palæontology. Frogs, 317. G. Galapagos Islands, 227-231, 236, 237. _Galeus_, eye of, 75. Galls, vegetable, 293-295, 446-448. Gastræa, 137-140. _Gastrophysema_, 138. Gastrulation, 137, 140. Gegenbaur, 147, 181. Gemmation, reproduction by, 106, 107, 110, 111. Generalization, 5. Generalized types, 33. Genesis, classification of organic nature in, 23. Genial tubercle, 96. Geographical distribution, 204-248; _see_ Glacial period, Barriers Transport of organisms, Oceanic islands, &c. Geology, record of imperfect, 156-160, and Appendix; _see_ Palæontology. Germs, prophetic, 272, 351-362. Gesner, on classification, 24; on canaries, 313. Gill-arches, 146, 147, 150, 151. Gill-slits, 146, 147, 150-153. Gills, of young salamanders, 102; origin of, in embryo, 144; of fish, 150, 152. Giraffe, neck of, in relation to Lamarck's theory, 254. Glacial periods, effects of, on distribution of plants and animals, 209, 210, and Appendix. Goose, Frizzled, portrait of, 304. Gorilla, _see_ Apes. Gray, Professor Asa, 337 Great-toe, in man and apes, 79-81. Grouse, 317-319 Growth, correlation of, 357, 362. _Gymnotus_, 365, 367. H. Häckel, on analogy between species and languages, 32; on reproduction as discontinuous growth, 105, 106; his ideal primitive vertebrate, 143, 144. Hair, vestigial characters of, in man, 89-92. Hales, 3. Haller, 3. Hamilton, Sir William, 272. Hands, 51-55, 66, 80-82, 174-192. Hare, 318, 319. Hartmann, on flattening of early human tibiæ, 96. Harvey, on Lord Bacon's writings, 2. Heart, development of, 154. Heilprin, on skulls of deer, 198, 199; on fossil shells, 201, 202. Hen, ovum of, 122. Heredity, in relation to classification, 28-31; in relation to embryology, 98-102; chromatin-fibres in relation to, 134; in relation to theories of organic evolution, 253-255, 260-264, 377. Hermit-crabs, 62-65, 288, 289. _Heteromera_, 233. Hilgendorf, on shells of _Planorbis_, 201. _Hipparion_, 191, 192. Hippopotamus, foot of, 187. Hog, _see_ Pig. Homology, 38, 50-65, 176, 177, 347-350, 357-359. Homopterous insect, imitating leaf-cutting ants, 331, 332. Hooker, Sir Joseph, on flora of St. Helena, 234. Horns, 98-100, 167-169. Horse, eye of, 75; limb-bones of, 176, 177, 186, 188-192; teeth of, 189-191; portraits of domesticated breeds of, 309. Human, _see_ Man. _Humerus_, perforations of, in quadrumana and man, 94, 95. Humming-birds, restricted to the New World, 211. Hunter, 3; on ear of whale, 65. Huxley, Prof. , on mechanical selection, 283; on age of the earth, 435, 436; on Dr. Whewell, 243. Hyatt, on shells of _Planorbis_, 201. _Hydra_, 111, 122. _Hyrax_, foot of, 185, 186. I. Ignorance, argument from, 41, 42, 49. Illative Sense, 6. Imitative colours, 317-323, 326-332. Infant, feet of, 78, 79; grasping power of, 81. Infertility, inter-specific, in relation to natural selection, 374-376. Insects, wingless, 68-70; in primary formations, 163, Appendix; on oceanic islands, 224-238; in relation to galls, 293-295, 446-448; defensive colouring of, 321-332; fertilizing flowers, 406. Instincts, always of primary use to species presenting them, 286-293. Intercrossing, in relation to natural selection, 374-376. Inutility of specific characters, in relation to natural selection, 374-376. Islands, oceanic, 224-237; British, 238-241. J. Japan, hairless dog of, 101. Jelly-fish, 119, 120. K. _Kallima_, 323. Karyokinesis, 112-114, 128-134. Kepler, 272. Kerguelen Island, flightless insects of, 70. Kropotkin, Prince, on co-operative instincts, 269. L. _Lagopus mutus_, 317, 318. Lamarck, his method in natural history, 4; his theory of evolution, 253-256. Lamprey, 148. Languages, classification of, resembles that of organic forms, 32. Lankester, E. Ray, on karyokinesis, 129, 130. Leaf insect, 322. Le Conte, on geological succession of animal classes, 164, 165; on types of tails, 169-173; on fossil shells of _Planorbis_, 201; his work on the relation of the theory of evolution to religious thought, 412. _Leptalis_, 328. _Leuculmis echinus_, 122. Leviticus, classification of organic nature in, 23. Life, origin of, 15. Linnæus, on method in natural history, 3; on classification, 26, 35-40. Lion, skeleton of, 175; feet of, 178. Lizard, heart and gill-arches of, 150. Lloyd Morgan, 273, 449, 450. Lungs, development of, 154, 354. Lyell, Sir Charles, on classification, 32; on uniformitarianism, 258; on rational species, 344; on geological record, 420, 435, 439. M. Madeira, wingless beetles of, 68-70; peculiar beetles of, 226, 227. Mammals, ovum of, 120-124; embryology of, 151-155; palæontology of, 163, 165, 167, 180-199; limbs of, 174-178, 182-199; brain of, 194-199; of Australia and New Zealand, 204, 205; distribution of, on islands, 224-240. Mammoth cave, fauna of, 70-72. Man, nictitating membrane of, 75; vestigial muscles of, 76, 77, 82, 83; tail of, compared with that of apes, 82-84; hair of, compared with that of apes, 89-92; teeth of, compared with those of apes, 92-94; perforation of humerus of, 94, 95; flattening of ancient tibiæ of, 95, 96; embryology of, 119, 153; hand of, 54; arm of, 90, 91; limb-bones of, 176, 177; palæontology of, 163, 165; brain of, 194, 195; Mr. Syme on, 346, 347. Marsh, on palæontology of the horse, 188-190. Matthew, Patrick, on natural selection, 257. Mesoderm, 142. _Mesohippus_, 189, 192. Metaphyta, 104, 105. Metazoa, 104. Method, ideas of, in natural history, 1-9; of organic evolution, 252-261. Meyer, Professor Ludwig, on helix of the human ear, 86. Mimicry, 320-322. Ministration, mutual, of species alleged, 445, 446. _Miohippus_, 189. Mivart, St. George, on eye of octopus, 57, 58, 348, 349; on incipient organs, 362; on mutual ministration of species, 445, 446. Mollusca, shells of, 19, 199-203; eye of, 57, 58; embryology of, 155; palæontology of, 163, 165. Monkeys, why all, do not become men, 342-344. _Monotremata_, 205. Morgan, _see_ Lloyd Morgan. Morphology, 50-97. Mule, portrait of, 309. Multicellular organisms, 104. Multiplication, _see_ Reproduction. N. Nägeli, Prof. , 337, 367. Natural History, ideas of method in, 1-9. Natural, interpretations as opposed to super-natural, 13-15; causation, 13-15. Natural selection, 252-378, 401-410; Wells, Matthew, and Whewell on, 257, 258, 443-445; statement of theory of, 256-284; of evidences of, 285-332; of criticisms of, 333-378; relation of theory of, to religious thought, 401-410; preserves types, 264-267; cessation and reversal of, 270, 342; errors touching theory of, 270-284, 332-364; definition of, 275-376; antecedent standing of theory of, 277-284; Prof. Owen on, 333, 334; Duke of Argyll on, 334-362; Mr. Syme on, 340, 341, 345; need not always make for improvement, 341-347; homology and analogy in relation to, 347-350; often determines beauty, 406, 407; in relation to the formation of galls, 293-295. 446-448. Nature, organic, 17; inorganic, 1, 17, 18. _Nauplius_, 138. Neumayr, 19. New Zealand, fauna of, 68, 204, 205; thriving of exotic species in, 286. Newman, on the Illative Sense, 6. Newton, his idea of scientific method, 6. Nictitating membrane, 74, 75. Notochord, 146. Novum Organon, the, on scientific method, 2. Nucleus, 105, 112-134. Nucleus-spindle, 129. Nut-hatch, Syrian, ornamented nests of, 381. O. Objective methods, 6. Oceanic islands, _see_ Islands. Octopus, eye of, 57, 58, 348-350. _[OE]dicnemus crepitans_, 320. Ontogeny, as recapitulation of phylogeny, 98-104. Orang Outang, _see_ Apes. _Oredon Culbertsoni_, 167. Origin of Species, the, influence exercised by, on ideas of method, 1-9 _Orohippus_, 189. Otaria, eye of, 75. Ovum, 113-142; human, 120-133; amoeboid movements of young, 121-123; segmentation of, 134, 135. Owen, on ear of whale, 65; on natural selection, 333, 334. Owl, eye of, 75. P. Paddle, _see_ Whale, and _Baptanodon discus_. _Pagurus bernhardus_, 64. Pain, in relation to the theory of evolution, 417. Palæontology, 159-203; general testimony of, 156-165; testimony of, in particular cases, 165-203; consideration of objections to theory of evolution founded on grounds of, 156-165, and Appendix. _Palæotherium_, 190, 191. Paley, on natural theology, 98, 412. _Paludina_, successive forms of, 19. Panama, Isthmus of, 219. _Panniculus carnosis_, 77. _Papilio merope_, 330. Parasites, of animals, devoid of beauty, 408. Parsimony, law of, 272. Parthenogenesis, 119. Partridges, 319. Peacock, tail of, 378; courtship of, 383. Peckham, Mr. And Mrs. , on courtship of spiders, 388-390. Perissodactyls, 182-192. _Petromyzon marinus_, 148. _Phenacodus primævus_, 184, 185. Phylogeny, _see_ Ontogeny. Physiological selection, 376. Pig, embryology of, 153; feet of, 176, 187; portraits of wild and domesticated, 312. Pigeons, portraits of, 298, 299; feather-footed, 359. Pilot fish, 289. _Planorbis_, transmutations of, 200, 201. Pleasure and pain, in relation to the theory of evolution, 417. _Plica semilunaris_, 75. _Pliohippus_, 189. Polar bear, skeleton of, 174; feet of, 178. Polar bodies, 125, 126. Polar star, 129. Polyps, 114. Porpoises, 24, 25, 50. Poulton, E. B. , on warning colours, 325, 326; on mimicry, 331, 332; sexual selection, 400, 401, 449, 450. Poultry, portraits of, 300-302. Pronucleus, 126-128. Prophetic types, 272, 351-362. _Prophysema primordiale_, 140. Protective colouring, 317-323. _Protohippus_, 189. Protozoa, 104. Ptarmigan, 317, 318. _Pterodactyl_, wing of, 56. Purpose, Darwin's use of the word, 281, 340. Puss moth, larva of, 325, 326. Python, 66, 67. Q. Quadrumana, muscles of, 76, 82, 83; perforations of humeri of, 94, 95; hair on phalanges of, 91. R. Rabbit, embryology of, 153; multiplication of, in Australia, 286; portraits of wild and domesticated breeds of, 308; protective colouring of, 319, 320. Radiate form, beauty of, 408, 409. _Raia radiata_, and _batis_, 367-371. Rats, species of, restricted to Old and New Worlds, 212; British and Norwegian, 285, 286. Rattle-snake, tail of, 289. Recognition marks, 271-273. Religion, in relation to Darwinism, 401-418. Reproduction, different methods of, 106-117; essence of sexual, 110; foreshadowing of sexual in unicellular organisms, 115-117. Reptiles, wing of flying, 56; rudimentary limbs of, 67; nictitating membrane of, 75; branchial arches of, 150; embryology of, 152; palæontology of, 163, 165, 178-180; brain of, 194-197; distribution of, 224-240. _Rhinoceros_, foot of, 186. Robinson, Dr L. , on grasping power of an infant's hands, 80-82. Rudimentary organs, 65-97. Ruminants, palæontology of, 167, 168. S. Sacrum of man, compared with that of apes, 82-84. _Sagitta_, 138. Salamander, young of terrestrial, living in water, 102; embryology of, 152. Sandwich Islands, 234-237. Science, method of, 1-9. Sclater, W. L. , on a case of mimicry, 331, 332. Scorpion in Silurian formation, 163. Sea, lamprey, 148; destructive agency of the, 423, 424. Seal, 51, 52, 75. Seasonal changes of colour, 317-319. Selection, value, 275; by physical processes, 282, 283, 335. _See also_ Natural selection, Artificial selection, Sexual selection, Physiological selection. Sentiency, in relation to the theory of evolution, 417. Sex, difference of, restricted to Metazoa and Metaphyta, 105. Sexual reproduction, _see_ Reproduction. Sexual selection, theory of, 277, 378-410; statement and evidences of, 379-391; criticisms of, 391-400; includes law of battle with that of charming, 385, 386; in relation to religious thought, 411-418; Tylor's theory substituted for, by Wallace, 449, 450. Shark, eye of, 75; man-eating, 149; and pilot-fish, 289. Sheep, limb-bones of, 176, 177; portraits of, 310. Shells, of crabs, 62-64; palæontology of mollusks, 199-203; land on oceanic islands, 224-240. Silliman's Journal, on fauna of the Mammoth Cave, 70. Skate, electric organ of, 364-373. Skull, palæontology of, 194-199; of bull-dog compared with that of deer-hound, 307. Slavonia, Tertiary deposits of, 18, 19. Species, not eternal, but either created or evolved, 13; named as such through absence of intermediate forms, 18-20; groups of, in classification, 20, and appearing suddenly in geological formations, 427-432, 437-440; origin of, coincide in space and time with pre-existing and allied species, 22; geographical distribution of, 204-248; extinct and living allied on same areas, 213; life of, preserved by natural selection, 264-270; not room for more than one rational, 344; characters of, 274-276, 286-295, 374-376; inter-sterility of allied, 374-376; mutual ministration of alleged, 445, 446. Specific characters, _see_ Characters. Speculation, method of, 3-9. Spencer, Herbert, on reproduction as discontinuous growth, 105, 106; on use-inheritance, 253-256; his failure to conceive the idea of natural selection, 257. Spermatozoa, 123, 126-128. Spiders, in primary formations, 163; courtship of, 388, 389. Sponges, 122, 139, 140. Spontaneous, Darwin's use of the term, 340. Spores, 115. Squirrels, flying, 355. Sterility, _see_ Infertility. St. Helena, 231-234, 236-237. St. Hilaire, 4. Stick-insect, 322. Stoat, 318. _Strombus accipilrinus_, 201. _Strombus Leidy_, 201. Struggle for existence, 259-270. Subjective, methods, 6. Survival of the fittest, 335. _See also_ Natural selection. Swim-bladder of fish, 154, 354. Symbiosis, 269. Syme, David, on the theory of natural selection, 340, 341. T. Tail, types of, in fish and birds, 169-173. Tasmanian wolf, dentition of, 39. Teeth, of Tasmanian wolf, 39; molar, of man, compared with those of apes, 92-94; palæontology of horses', 189-191. Temperature, sense of, probable origin of that of sight, 353, 354. Tennyson, 266. Tibiæ, flattening of, 95, 96. Tissue-cells, _see_ Cell. Toes, 79, 80; _see also_ Feet. Tomes, C. S. , on molar teeth of man and apes, 94. _Torpedo_, 365, 367. Tortoise, embryology of, 152, 154. _Toxopneustes variegatus_, and _T. Lividus_, 122. Transport of organisms, means of, 207, 216-218. Tribal fitness, as distinguished from individual, 267-269. Trout, ovum of, 122. Turtle, eye of, 75. Tylor, Alfred, on colouration of animals, 448-450. Type, preserved by natural selection, 264-269; improvement of, by natural selection, 269, 270; prophetic, 272, 351-362. Types, as simple and generalized, 33. U. Unicellular organisms, 104. _Uraster_, 138. Utility, of specific characters, 274, 275; of incipient characters, 351-363; of electric organs, 365-373. V. Variation, in relation to natural selection, 263, 335-340, 377. Verification, 6-9. Vertebral column, embryology of 145, 146; palæontology of, 192, 193. Vertebrated animal, ideal primitive, 143, 144; embryology of, 143, 155. _Vespa vulgaris_, 331. Vestigial organs, 65-97. _Volucella inans_, and _V. Bombylans_, 329. W. Wagner, Moritz, on geographical distribution, 216. Wallace, A. R. , on origin of species as coincident in time and space with pre-existing and allied species, 22; on wingless insects, 70; on absence of hair from human back, and function of on arms of orang, 89; on geographical distribution, 207, 231, 232, 233, 243; on natural selection, 256; on recognition marks, 271-273; on alleged deductive consequences of the natural selection theory, 273-276; his theory of warning colours, 323, 324; on sexual selection, 391-400, 450; his principal defect in treating of animal colouration, 449, 450. Warning colours, 323-326. Wasp, imitated by a fly, 329. Water-cress, multiplication of, in New Zealand, 286. Weevils, on St. Helena, 232. Weismann, his theory of heredity, 130, 134. Wells, Dr. , on natural selection, 257. Wetterhan, Prof. , on vegetable galls, 448. Whales, 38, 50, 53, 54, 65, 180. Whewell, on natural selection, 257, 258, 443-445. Wings, 54-56, 60, 61, 68-70, 355. Wolf, Tasmanian, dentition of, 34. Wood, John, on vestigial muscles in man, 77. Woodward, on fossil cirripedes, 431. Woolner, on the human ear, 86. Worms, embryology of, 155. Wyman, Prof. , on the great toe of human embryo, 79, 80. Z. _Zona pellucida_, 121. +--------------------------------------------------------------+ | | | Transcriber’s Notes and Errata | | | | The following words were found in both hyphenated and | | unhyphenated forms in the text. The number of instances is | | given in parentheses after each word. | | | | |deer-hound (2) |deerhound (1) | | | |fresh-water (13) |freshwater (1) | | | |inter-relations (1) |interrelations (1) | | | |re-action (1) |reaction (1) | | | |sea-weed (7) |seaweed (1) | | | |super-natural (2) |supernatural (24) | | | |wood-cut (3) |woodcut (3) | | | |wood-cuts (4) |woodcuts (1) | | | | | There were 9 instances of 'larvae' and 3 instances of | | 'larvæ'. | | | | The following typographical errors were corrected: | | | | |Error |Correction | | | | | | | | |arboresent |arborescent | | | |the |The | | | |dicussion |discussion | | | | | In the index, the page entry for "Lyell, Sir Charles ... On | | geological record" was changed from '420' to '422'. | | | | Also, the page entry for "Natural selection ... Definition | | of" was changed from '275-376' to '275-276'. | +--------------------------------------------------------------+