DARWIN, AND AFTER DARWIN
AN EXPOSITION OF THE DARWINIAN THEORY AND A DISCUSSION OF POST-DARWINIAN QUESTIONS
GEORGE JOHN ROMANES, M.A., LL.D., F.R.S. Honorary Fellow of Gonville and Caius College, Cambridge
I THE DARWINIAN THEORY
Chicago THE OPEN COURT PUBLISHING COMPANY 1910
The Illustrations of this book (with the exception of the Frontispiece and the colored plate facing page 332) are copyrighted under the title "Darwinism Illustrated."
THE OPEN COURT PUBLISHING CO.
PRESS OF THE BLAKELY-OSWALD PRINTING CO. CHICAGO
[Illustration: Letter to Mr. Hegeler
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,
Yours very faithfully,
George J. Romanes
To Edward C. Hegeler Esq. La Salle, Ill. U. S.]
Several years ago Lord Rosebery founded, in the University of Edinburgh, a lectureship on "The Philosophy of Natural History," and I was invited by the Senatus to deliver the lectures. This invitation I accepted, and subsequently constituted the material of my lectures the foundation of another course, which was given in the Royal Institution, under the title "Before and after Darwin." Here the course extended over three years—namely from 1888 to 1890. The lectures for 1888 were devoted to the history of biology from the earliest recorded times till the publication of the "Origin of Species" in 1859; the lectures for 1889 dealt with the theory of organic evolution up to the date of Mr. Darwin's death, in 1882; while those of the third year discussed the further developments of this theory from that date till the close of the course in 1890.
It is from these two courses—which resembled each other in comprising between thirty and forty lectures, but differed largely in other respects—that the present treatise has grown. Seeing, however, that it has grown much beyond the bulk of the original lectures, I have thought it desirable to publish the whole in the form of three separate works. Of these the first—or that which deals with the purely historical side of biological science—may be allowed to stand over for an indefinite time. The second is the one which is now brought out and which, as its sub-title signifies, is devoted to the general theory of organic evolution as this was left by the stupendous labours of Darwin. As soon as the translations shall have been completed, the third portion will follow (probably in the Autumn season), under the sub-title, "Post-Darwinian Questions."
As the present volume is thus intended to be merely a systematic exposition of what may be termed the Darwinism of Darwin, and as on this account it is likely to prove of more service to general readers than to professed naturalists, I have been everywhere careful to avoid assuming even the most elementary knowledge of natural science on the part of those to whom the exposition is addressed. The case, however, will be different as regards the next volume, where I shall have to deal with the important questions touching Heredity, Utility, Isolation, &c., which have been raised since the death of Mr. Darwin, and which are now being debated with such salutary vehemence by the best naturalists of our time.
My obligations to the Senatus of the University of Edinburgh, and to the Board of Management of the Royal Institution, have already been virtually expressed; but I should like to take this opportunity of also expressing my obligations to the students who attended the lectures in the University of Edinburgh. For alike in respect of their large numbers, their keen intelligence, and their generous sympathy, the members of that voluntary class yielded a degree of stimulating encouragement, without which the labour of preparing the original lectures could not have been attended with the interest and the satisfaction 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 in executing most of the original drawings with which this volume is illustrated; and likewise to Messrs. Macmillan and Co. for kindly allowing me to reprint—without special acknowledgment in every case—certain passages from an essay which they published for me many years ago, under the title "Scientific Evidences of Organic Evolution." Lastly, I must mention that I am indebted to the same firm for permission to reproduce an excellent portrait of Mr. Darwin, which constitutes the frontispiece.
G. J. R.
CHRIST CHURCH, OXFORD, April 19th, 1892.
CHAPTER I. INTRODUCTORY 1
CHAPTER II. CLASSIFICATION 23
CHAPTER III. MORPHOLOGY 50
CHAPTER IV. EMBRYOLOGY 98
CHAPTER V. PALAEONTOLOGY 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
LIST OF ILLUSTRATIONS
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 Protozooen 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 gastraea-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 Archaeopteryx 171
72. Tail of modern Bird 171
73. Archaeopteryx 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 primaevus 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. Palaeotherium. (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
Among the many and unprecedented changes that have been wrought by Mr. Darwin's work on the Origin of Species, there is one which, although second in importance to no other, has not received the attention which it deserves. I allude to the profound modification which that work has produced on the ideas of naturalists with regard to method.
Having had occasion of late years somewhat closely to follow the history of biological science, I have everywhere observed that progress is not so much marked by the march of discovery per se, as by the altered views of method which the march has involved. If we except what Aristotle called "the first start" in himself, I think one may fairly say that from the rejuvenescence of biology in the sixteenth century to the stage of growth which it has now reached in the nineteenth, there is a direct proportion to be found between the value of work done and the degree in which the worker has thereby advanced the true conception of scientific working. Of course, up to a certain point, it is notorious that the revolt against the purely "subjective methods" in the sixteenth century revived the spirit of inductive research as this had been left by the Greeks; but even with regard to this revolt there are two things which I should like to observe.
In the first place, it seems to me, an altogether disproportionate value has been assigned to Bacon's share in the movement. At most, I think, he deserves to be regarded but as a literary exponent of the Zeitgeist of his century. Himself a philosopher, as distinguished from a man of science, whatever influence his preaching may have had upon the general public, it seems little short of absurd to suppose that it could have produced any considerable effect upon men who were engaged in the practical work of research. And those who read the Novum Organon with a first-hand knowledge of what is required for such research can scarcely fail to agree with his great contemporary Harvey, that he wrote upon science like a Lord Chancellor.
The second thing I should like to observe is, that as the revolt against the purely subjective methods grew in extent and influence it passed to the opposite extreme, which eventually became only less deleterious to the interests of science than was the bondage of authority, and addiction 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 up to the commencement of the present century the progress of science in general, and of natural history in particular, was seriously retarded by what may be termed the Bugbear of Speculation. Fully awakened to the dangers of web-spinning from the ever-fertile resources of their own inner consciousness, naturalists became more and more abandoned to the idea that their science ought to consist in a mere observation of facts, or tabulation of phenomena, without attempt at theorizing upon their philosophical import. If the facts and phenomena presented any such import, that was an affair for men of letters to deal with; but, as men of science, it was their duty to avoid the seductive temptations of the 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 absolute or universal; but there can be no question that it was both orthodox and general. Even Linnaeus was express in his limitations of true scientific work in natural history to the collecting and arranging of species of plants and animals. In accordance with this view, the status of a botanist or a zoologist was estimated by the number of specific names, natural habitats, &c., which he could retain in his memory, rather than by any evidences which he might give of intellectual powers in the way of constructive thought. At the most these powers might legitimately exercise themselves only in the direction of taxonomic work; and if a Hales, a Haller, or a Hunter obtained any brilliant results in the way of observation and experiment, their merit was taken to consist in the discovery of facts per se: not in any endeavours they might make in the way of combining their facts under general principles. Even as late in the day as Cuvier this ideal was upheld as the strictly legitimate one for a naturalist to follow; and although Cuvier himself was far from being always loyal to it, he leaves no doubt regarding the estimate in which he held the still greater deviations of his colleagues, St. Hilaire and Lamarck.
Now, these traditional notions touching the severance between the facts of natural history and the philosophy of it, continued more or less to dominate the minds of naturalists until the publication of the Origin of Species, in 1859. Then it was that an epoch was marked in this respect, as in so many other respects where natural history is concerned. For, looking to the enormous results which followed from a deliberate disregard of such traditional canons by Darwin, it has long since become impossible for naturalists, even of the strictest sect, not to perceive that their previous bondage to the law of a mere ritual has been for ever superseded by what verily deserves to be regarded as a new dispensation. Yet it cannot be said, or even so much as suspected, that Darwin's method in any way resembled that of pre-scientific days, the revolt against which led to the straight-laced—and for a long time most salutary—conceptions of method that we have just been noticing. Where, then, is the difference? To me it seems that the difference is as follows; and, if so, that not the least of our many obligations to Darwin as the great organizer of biological science arises from his having clearly displayed the true principle which ought to govern biological research.
To begin with, he nowhere loses sight of the primary distinction between fact and theory; so that, thus far, he loyally follows the spirit of revolt against subjective methods. But, while always holding this distinction clearly in view, his idea of the scientific use of facts is plainly that of furnishing legitimate material for the construction of theories. Natural history is not to him an affair of the herbarium or the cabinet. The collectors and the species-framers are, as it were, his diggers of clay and makers of bricks: even the skilled observers and the trained experimentalists are his mechanics. Valuable as the work of all these men is in itself, its principal value, as he has finally demonstrated, is that which it acquires in rendering possible the work of the architect. Therefore, although he has toiled in all the trades with his own hands, and in each has accomplished some of the best work that has ever been done, the great difference between him and most of his predecessors consists in this,—that while to them the discovery or accumulation of facts was an end, to him it is the means. In their eyes it was enough that the facts should be discovered and recorded. In his eyes the value of facts is due to their power of guiding the mind to a further discovery of principles. And the extraordinary success which attended his work in this respect of generalization immediately brought natural history into line with the other inductive sciences, behind which, in this most important of all respects, she has so seriously fallen. For it was the Origin of Species which first clearly revealed to naturalists as a class, that it was the duty of their science to take as its motto, what is really the motto of natural science in general,
Felix qui potuit rerum cognoscere causas.
Not facts, then, or phenomena, but causes or principles, are the ultimate objects of scientific quest. It remains to ask, How ought this quest to be prosecuted?
Well, in the second place, Darwin has shown that next only to the importance of clearly distinguishing between facts and theories on the one hand, and of clearly recognising the relation between them on the other, is the importance of not being scared by the Bugbear of Speculation. The spirit of speculation is the same as the spirit of science, namely, as we have just seen, a desire to know the causes of things. The hypotheses non fingo of Newton, if taken to mean what it is often understood as meaning, would express precisely the opposite spirit from that in which all scientific research must necessarily take its origin. For if it be causes or principles, as distinguished from facts or phenomena, that constitute the final aim of scientific research, obviously the advancement of such research can be attained only by the framing of hypotheses. And to frame hypotheses is to speculate.
Therefore, the difference between science and speculation is not a difference of spirit; nor, thus far, is it a difference of method. The only difference between them is in the subsequent process of verifying hypotheses. For while speculation, in its purest form, is satisfied to test her explanations only by the degree in which they accord with our subjective ideas of probability—or with the "Illative Sense" of Cardinal Newman,—science is not satisfied to rest in any explanation as final until it shall have been fully verified by an appeal to objective proof. This distinction is now so well and so generally appreciated that I need not dwell upon it. Nor need I wait to go into any details with regard to the so-called canons of verification. My only object is to make perfectly clear, first, that in order to have any question to put to the test of objective verification, science must already have so far employed the method of speculation as to have framed a question to be tested; and, secondly, that the point where science parts company with speculation is the point where this testing process begins.
Now, if these things are so, there can be no doubt that Darwin was following the truest method of inductive research in allowing any amount of latitude to his speculative thought in the direction of scientific theorizing. For it follows from the above distinctions that the danger of speculation does not reside in the width of its range, or even in the impetuosity of its vehemence. Indeed, the wider its reach, and the greater its energy, the better will it be for the interests of science. The only danger of speculation consists in its momentum being apt to carry away the mind from the more laborious work of adequate verification; and therefore a true scientific judgment consists in giving a free rein to speculation on the one hand, while holding ready the break of verification with the other. Now, it is just because Darwin did both these things with so admirable a judgment, that he gave the world of natural history so good a lesson as to the most effectual way of driving the chariot of science.
This lesson we have now all more or less learnt to profit by. Yet no other naturalist has proved himself so proficient in holding the balance true. For the most part, indeed, they have now all ceased to confound the process of speculation per se with the danger of inadequate verification; and therefore the old ideal of natural history as concerned merely with collecting species, classifying affinities, and, in general, tabulating facts, has been well-nigh universally superseded. But this great gain has been attended by some measure of loss. For while not a few naturalists have since erred on the side of insufficiently distinguishing between fully verified principles of evolution and merely speculative deductions therefrom, a still larger number have formed for themselves a Darwinian creed, and regard any further theorizing on the subject of evolution as ipso facto unorthodox.
Having occupied the best years of my life in closely studying the literature of Darwinism, I shall endeavour throughout the following pages to avoid both these extremes. No one in this generation is able to imitate Darwin, either as an observer or a generalizer. But this does not hinder that we should all so far endeavour to follow his method, as always to draw a clear distinction, not merely between observation and deduction, but also between degrees of verification. At all events, my own aim will everywhere be to avoid dogmatism on the one hand, and undue timidity as regards general reasoning on the other. For everything that is said justification will be given; and, as far as prolonged deliberation has enabled me to do so, the exact value of such justification will be rendered by a statement of at least the main grounds on which it rests. The somewhat extensive range of the present treatise, however, will not admit of my rendering more than a small percentage of the facts which in each case go to corroborate the conclusion. But although a great deal must thus be necessarily lost on the one side, I am disposed to think that more will be gained on the other, by presenting, in a terser form than would otherwise be possible, the whole theory of organic evolution as I believe that it will eventually stand. My endeavour, therefore, will be to exhibit the general structure of this theory in what I take to be its strictly logical form, rather than to encumber any of its parts by a lengthy citation of facts. Following this method, I shall in each case give only what I consider the main facts for and against the positions which have to be argued; and in most cases I shall arrange the facts in two divisions, namely, first those of largest generality, and next a few of the most special character that can be found.
As explained in the Preface, the present instalment of the treatise is concerned with the theory of evolution, from the appearance of the Origin of Species in 1859, to the death of its author in 1882; while the second part will be devoted to the sundry post-Darwinian questions which have arisen in the subsequent decade. To the possible criticism that a disproportionate amount of space will thus be allotted to a consideration of these post-Darwinian questions, I may furnish in advance the following reply.
In the first place, besides the works of Darwin himself, there are a number of others which have already and very admirably expounded the evidences, both of organic evolution as a fact, and of natural selection as a cause. Therefore, in the present treatise it seemed needless to go beyond the ground which was covered by my original lectures, namely, a condensed and connected, while at the same time a critical statement of the main evidences, and the main objections, which have thus far been published with reference to the distinctively Darwinian theory. Indeed while re-casting this portion of my lectures for the present publication, I have felt that criticism might be more justly urged from the side of impatience at a reiteration of facts and arguments already so well known. But while endeavouring, as much as possible, to avoid overlapping the previous expositions, I have not carried this attempt to the extent of damaging my own, by omitting any of the more important heads of evidence; and I have sought to invest the latter with some measure of novelty by making good what appears to me a deficiency which has hitherto obtained in the matter of pictorial illustration. In particular, there will be found a tolerably extensive series of woodcuts, serving to represent the more important products of artificial selection. These, like all the other original illustrations, have been drawn either direct from nature or from a comparative study of the best authorities. Nevertheless, I desire it to be understood that the first part of this treatise is intended to retain its original character, as a merely 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, or biological student as distinguished from those of a professed naturalist.
The case, however, is different with the second instalment, which will be published at no very distant date. Here I have not followed with nearly so much closeness the material of my original lectures. On the contrary, I have had in view a special class of readers; and, although I have tried not altogether to sacrifice the more general class, I shall desire it to be understood that I am there appealing to naturalists who are specialists in Darwinism. One must say advisedly, naturalists who are specialists in Darwinism, because, while the literature of Darwinism has become a department of science in itself, there are nowadays many naturalists who, without having paid any close attention to the subject, deem themselves entitled to hold authoritative opinions with regard to it. These men may have done admirable work in other departments of natural history, and yet their opinions on such matters as we shall hereafter have to consider may be destitute of value. As there is no necessary relation between erudition in one department of science and soundness of judgment in another, the mere fact that a man is distinguished as a botanist or zoologist does not in itself qualify him as a critic where specially Darwinian questions are concerned. Thus it happens now, as it happened thirty years ago, that highly distinguished botanists and zoologists prove themselves incapable as judges of general reasoning. It was Darwin's complaint that for many years nearly all his scientific critics either could not, or would not, understand what he had written—and this even as regarded the fundamental principles of his theory, which with the utmost clearness he had over and over again repeated. Now the only difference between such naturalists and their successors of the present day is, that the latter have grown up in a Darwinian environment, and so, as already remarked, have more or less thoughtlessly adopted some form of Darwinian creed. But this scientific creed is not a whit less dogmatic and intolerant than was the more theological one which it has supplanted; and while it usually incorporates the main elements of Darwin's teaching, it still more usually comprises gross perversions of their consequences. All this I shall have occasion more fully to show in subsequent parts of the present work; and allusion is made to the matter here merely for the sake of observing that in future I shall not pay attention to unsupported expressions of opinion from any quarter: I shall consider only such as are accompanied with some statement of the grounds upon which the opinion is held. And, even as thus limited, I do not think it will be found that the following exposition devotes any disproportional amount of attention to the contemporary movements of Darwinian thought, seeing, as we shall see, how active scientific speculation has been in the field of Darwinism since the death of Mr. Darwin.
* * * * *
Leaving, then, these post-Darwinian questions to be dealt with subsequently, I shall now begin a systematic resume of the evidences in favour of the Darwinian theory, as this was left to the world by Darwin himself.
There is a great distinction to be drawn between the fact of evolution and the manner of it, or between the evidence of evolution as having taken place somehow, and the evidence of the causes which have been concerned in the process. This most important distinction is frequently disregarded by popular writers on Darwinism; and, therefore, in order to mark it as strongly as possible, I will effect a complete separation between the evidence which we have of evolution as a fact, and the evidence which we have as to its method. In other words, not until I shall have fully considered the evidence of organic evolution as a process which somehow or another has taken place, will I proceed to consider how it has taken place, or the causes which Darwin and others have suggested as having probably been concerned in this process.
Confining, then, our attention in the first instance to a proof of evolution considered as a fact, without any reference at all to its method, let us begin by considering the antecedent standing of the matter.
* * * * *
First of all we must clearly recognise that there are only two hypotheses in the field whereby it is possible so much as to suggest an explanation of the origin of species. Either all the species of plants and animals must have been supernaturally created, or else they must have been naturally evolved. There is no third hypothesis possible; for no one can rationally suggest that species have been eternal.
Next, be it observed, that the theory of a continuous transmutation of species is not logically bound to furnish a full explanation of all the natural causes which it may suppose to have been at work. The radical distinction between the two theories consists in the one assuming an immediate action of some supernatural or inscrutable cause, while the other assumes the immediate action of natural—and therefore of possibly discoverable—causes. But in order to sustain this latter assumption, the theory of descent is under no logical necessity to furnish a full proof of all the natural causes which may have been concerned in working out the observed results. We do not know the natural causes of many diseases; but yet no one nowadays thinks of reverting to any hypothesis of a supernatural cause, in order to explain the occurrence of any disease the natural causation of which is obscure. The science of medicine being in so many cases able to explain the occurrence of disease by its hypothesis of natural causes, medical men now feel that they are entitled to assume, on the basis of a wide analogy, and therefore on the basis of a strong antecedent presumption, that all diseases are due to natural causes, whether or not in particular cases such causes happen to have been discovered. And from this position it follows that medical men are not logically bound to entertain any supernatural theory of an obscure disease, merely because as yet they have failed to find a natural theory. And so it is with biologists and their theory of descent. Even if it be fully proved to them that the causes which they have hitherto discovered, or suggested, are inadequate to account for all the facts of organic nature, this would in no wise logically compel them to vacate their theory of evolution, in favour of the theory of creation. All that it would so compel them to do would be to search with yet greater diligence for the natural causes still undiscovered, but in the existence of which they are, by their independent evidence in favour of the theory, bound to believe.
In short, the issue is not between the theory of a supernatural cause and the theory of any one particular natural cause, or set of causes—such as natural selection, use, disuse, and so forth. The issue thus far—or where only the fact of evolution is concerned—is between the theory of a supernatural cause as operating immediately in numberless acts of special creation, and the theory of natural causes as a whole, whether these happen, or do not happen, to have been hitherto discovered.
This much by way of preliminaries being understood, we have next to notice 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—that is to say, with the origin of species. The theory of descent starts from life as a datum already granted. How life itself came to be, the theory of descent, as such, is not concerned to show. Therefore, in the present discussion, I will take the existence of life as a fact which does not fall within the range of our present discussion. No doubt the question as to the origin of life is in itself a deeply interesting question, and although in the opinion of most biologists it is a question which we may well hope will some day fall within the range of science to answer, at present, it must be confessed, science is not in a position to furnish so much as any suggestion upon the subject; and therefore our wisdom as men of science is frankly to acknowledge that such is the case.
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We are now in a position to observe that the theory of organic evolution is strongly recommended to our acceptance on merely antecedent grounds, by the fact that it is in full accordance with what is known as the principle of continuity. By the principle of continuity is meant the uniformity of nature, in virtue of which the many and varied processes going on in nature are due to the same kind of method, i. e. the method of natural causation. This conception of the uniformity of nature is one that has only been arrived at step by step through a long and arduous course of human experience in the explanation of natural phenomena. The explanations of such phenomena which are first given are always of the supernatural kind; it is not until investigation has revealed the natural causes which are concerned that the hypotheses of superstition give way to those of science. Thus it follows that the hypotheses of superstition which are the latest in yielding to the explanations of science, are those which refer to the more recondite cases of natural causation; for here it is that methodical investigation is longest in discovering the natural causes. Thus it is only by degrees that fetishism is superseded by what now appears a common-sense interpretation of physical phenomena; that exorcism gives place to medicine; alchemy to chemistry; astrology to astronomy; and so forth. Everywhere the miraculous is progressively banished from the field of explanation by the advance of scientific discovery; and the places where it is left longest in occupation are those where the natural causes are most intricate or obscure, and thus present the greatest difficulty to the advancing explanations of science. Now, in our own day there are but very few of these strongholds of the miraculous left. Nearly the whole field of explanation is occupied by naturalism, so that no one ever thinks of resorting to supernaturalism except in the comparatively few cases where science has not yet been able to explore the most obscure regions 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 has been offered by science, it is but in accordance with all previous historical analogies that many minds should prove themselves unable all at once to adjust themselves to the new ideas, and thus still linger about the more venerable ideas of supernaturalism. But we are now in possession of so many of these historical analogies, that all minds with any instincts of science in their composition have grown to distrust, on merely antecedent grounds, any explanation which embodies a miraculous element. Such minds have grown to regard all these explanations as mere expressions of our own ignorance of natural causation; or, in other words, they have come to regard it as an a priori truth that nature is everywhere uniform in respect of method or causation; that the reign of law universal; the principle of continuity ubiquitous.
Now, it must be obvious to any mind which has adopted this attitude of thought, that the scientific theory of natural descent is recommended by an overwhelming weight of antecedent presumption, as against the dogmatic theory of supernatural design.
To begin with, we must remember that the fact of evolution—or, which is the same thing, the fact of continuity in natural causation—has now been unquestionably proved in so many other and analogous departments of nature, that to suppose any interruption of this method as between species and species becomes, on grounds of such analogy alone, well-nigh incredible. For example, it is now a matter of demonstrated fact that throughout the range of inorganic nature the principles of evolution have obtained. It is no longer possible for any one to believe with our forefathers that the earth's surface has always existed as it now exists. For the science of geology has proved to demonstration that seas and lands are perpetually undergoing gradual changes of relative positions—continents and oceans supplanting each other in the course of ages, mountain-chains being slowly uplifted, again as slowly denuded, and so forth. Moreover, and as a closer analogy, within the limits of animate nature we know it is the universal law that every individual life undergoes a process of gradual development; and that breeds, races, or strains, may be brought into existence by the intentional use of natural processes—the results bearing an unmistakeable resemblance to what we know as natural species. Again, even in the case of natural species themselves, there are two considerations which present enormous force from an antecedent point of view. The first is that organic forms are only then recognised as species when intermediate forms are absent. If the intermediate forms are actually living, or admit of being found in the fossil state, naturalists forthwith regard the whole series as varieties, and name all the members of it as belonging to the same species. Consequently it becomes obvious that naturalists, in their work of naming species, may only have been marking out the cases where intermediate or connecting forms have been lost to observation. For example, here we have a diagram representing a very unusually complete series of fossil shells, which within the last few years has been unearthed from the Tertiary lake basins of Slavonia. Before the series was completed, some six or eight of the then disconnected forms were described as distinct species; but as soon as the connecting forms were found—showing a progressive modification from the older to the newer beds,—the whole were included as varieties of one species.
Of course, other cases of the same kind might be adduced, and therefore, as just remarked, in their work of naming species naturalists may only have been marking out the cases where intermediate forms have been lost to observation. And this possibility becomes little less than a certainty when we note the next consideration which I have to adduce, namely, that in all their systematic divisions of plants and animals in groups higher than species—such as genera, families, orders, and the rest—naturalists have at all times recognised the fact that the one shades off into the other by such imperceptible gradations, that it is impossible to regard such divisions as other than conventional. It is important to remember that this fact was fully recognised before the days of Darwin. In those days the scientifically orthodox doctrine was, that although species were to be regarded as fixed units, bearing the stamp of a special creation, all the higher taxonomic divisions were to be considered as what may be termed the artificial creation of naturalists themselves. In other words, it was believed, and in many cases known, that if we could go far enough back in the history of the earth, we should everywhere find a tendency to mutual approximation between allied groups of species; so that, for instance, birds and reptiles would be found to be drawing nearer and nearer together, until eventually they would seem to become fused in a single type; that the existing distinctions between herbivorous and carnivorous mammals would be found to do likewise; and so on with all the larger group-distinctions, at any rate within the limits of the same sub-kingdoms. But although naturalists recognised this even in the pre-Darwinian days, they stoutly believed that a great exception was to be made in the case of species. These, the lowest or initial members of their taxonomic series, they supposed to be permanent—the miraculously created units of organic nature. Now, all that I have at present to remark is, that this pre-Darwinian exception which was made in favour of species to the otherwise recognised principle of gradual change, was an exception which can at no time have been recommended by any antecedent considerations. At all times it stood out of analogy with the principle of continuity; and, as we shall fully find in subsequent chapters, it is now directly contradicted by all the facts of biological science.
There remains one other fact of high generality to which prominent attention should be drawn from the present, or merely antecedent, point of view. On the theory of special creation no reason can be assigned why distinct specific types should present any correlation, either in time or in space, with their nearest allies; for there is evidently no conceivable reason why any given species, A, should have been specially created on the same area and at about the same time as its nearest representative, B,—still less, of course, that such should be a general rule throughout all the thousands and millions of species which have ever inhabited the earth. But, equally of course, on the theory of a natural evolution this is so necessary a consequence, that if no correlation of such a two-fold kind were observable, the theory would be negatived. Thus the question whether there be any indication of such a two-fold correlation may be regarded as a test-question as between the two theories; for although the vast majority of extinct species have been lost to science, there are a countless number of existing species which furnish ample material for answering the question. And the answer is so unequivocal that Mr. Wallace, who is one of our greatest authorities on geographical distribution, has laid it down as a general law, applicable to all the departments of organic nature, that, so far as observation can extend, "every species has come into existence coincident both in space and time with a pre-existing and closely allied species." As it appears to me that the significance of these words cannot be increased by any comment upon them, I will here bring this introductory chapter to a close.
The first line of direct evidence in favour of organic evolution which I shall open is that which may be termed the argument from Classification.
It is a matter of observable fact that different forms of plants and animals present among themselves more or less pronounced resemblances. From the earliest times, therefore, it has been the aim of philosophical naturalists to classify plants and animals in accordance with these resemblances. Of course the earliest attempts at such classification were extremely crude. The oldest of these attempts with which we are acquainted—namely, that which is presented in the books of Genesis and Leviticus—arranges the whole vegetable kingdom in three simple divisions of Grass, Herbs, and Trees; while the animal kingdom is arranged with almost equal simplicity with reference, first to habitats in water, earth, or air, and next as to modes of progression. These, of course, were what may be termed common-sense classifications, having reference merely to external appearances and habits of life. But when Aristotle laboriously investigated the comparative anatomy of animals, he could not fail to perceive that their entire structures had to be taken into account in order to classify them scientifically; and, also, that for this purpose the internal parts were of quite as much importance as the external. Indeed, he perceived that they were of greatly more importance in this respect, inasmuch as they presented so many more points for comparison; and, in the result, he furnished an astonishingly comprehensive, as well as an astonishingly accurate classification of the larger groups of the animal kingdom. On the other hand, classification of the vegetable kingdom continued pretty much as it had been left by the book of Genesis—all plants being divided into three groups, Herbs, Shrubs, and Trees. Nor was this primitive state of matters improved upon till the sixteenth century, when Gesner (1516-1565), and still more Caesalpino (1519-1603), laid the foundations of systematic botany.
But the more that naturalists prosecuted their studies on the anatomy of plants and animals, the more enormously complex did they find the problem of classification become. Therefore they began by forming what are called artificial systems, in contradistinction to natural systems. An artificial system of classification is a system based on the more or less arbitrary selection of some one part, or set of parts; while a natural classification is one that is based upon a complete knowledge of all the structures of all the organisms which are classified.
Thus, the object of classification has been that of arranging organisms in accordance with their natural affinities, by comparing organism with organism, for the purpose of ascertaining which of the constituent organs are of the most invariable occurrence, and therefore of the most typical signification. A porpoise, for instance, has a large number of teeth, and in this feature resembles most fish, while it differs from all mammals. But it also gives suck to its young. Now, looking to these two features alone, should we say that a porpoise ought to be classed as a fish or as a mammal? Assuredly as a mammal; because the number of teeth is a very variable feature both in fish and mammals, whereas the giving of suck is an invariable feature among mammals, and occurs nowhere else in the animal kingdom. This, of course, is chosen as a very simple illustration. Were all cases as obvious, there would be but little distinction between natural and artificial systems of classification. But it is because the lines of natural affinity are, as it were, so interwoven throughout the organic world, and because there is, in consequence, so much difficulty in following them, that artificial systems have to be made in the first instance as feelers towards eventual discovery of the natural system. In other words, while forming their artificial systems of classification, it has always been the aim of naturalists—whether consciously or unconsciously—to admit as the bases of their systems those characters which, in the then state of their knowledge, seemed most calculated to play an important part in the eventual construction of the natural system. If we were dealing with the history of classification, it would here be interesting to note how the course of it has been marked by gradual change in the principles which naturalists adopted as guides to the selection of characters on which to found their attempts at a natural classification. Some of these changes, indeed, I shall have to mention later on; but at present what has to be specially noted is, that through all these changes of theory or principle, and through all the ever-advancing construction of their taxonomic science, naturalists themselves were unable to give any intelligible reason for the faith that was in them—or the faith that over and above the artificial classifications which were made for the mere purpose of cataloguing the living library of organic nature, there was deeply hidden in nature itself a truly natural classification, for the eventual discovery of which artificial systems might prove to be of more or less assistance.
Linnaeus, for example, expressly says—"You ask me for the characters of the natural orders; I confess that I cannot give them." Yet he maintains that, although he cannot define the characters, he knows, by a sort of naturalist's instinct, what in a general way will subsequently be found to be the organs of most importance in the eventual grouping of plants under a natural system. "I will not give my reasons for the distribution of the natural orders which I have published," he said: "you, or some other person, after twenty or after fifty years, will discover them, and see that I was right."
Thus we perceive that in forming their provisional or artificial classifications, naturalists have been guided by an instinctive belief in some general principle of natural affinity, the character of which they have not been able to define; and that the structures which they selected as the bases of their classifications when these were consciously artificial, were selected because it seemed that they were the structures most likely to prove of use in subsequent attempts at working out the natural system.
This general principle of natural affinity, of which all naturalists have seen more or less well-marked evidence in organic nature, and after which they have all been feeling, has sometimes been regarded as natural, but more often as supernatural. Those who regarded it as supernatural took it to consist in a divine ideal of creation according to types, so that the structural affinities of organisms were to them expressions of an archetypal plan, which might be revealed in its entirety when all organisms on the face of the earth should have been examined. Those, on the other hand, who regarded the general principle of affinity as depending on some natural causes, for the most part concluded that these must have been utilitarian causes; or, in other words, that the fundamental affinities of structure must have depended upon fundamental requirements of function. According to this view, the natural classification would eventually be found to stand upon a basis of physiology. Therefore all the systems of classification up to the earlier part of the present century went upon the apparent axiom, that characters which are of most importance to the organisms presenting them must be characters most indicative of natural affinities. But the truth of the matter was eventually found to be otherwise. For it was eventually found that there is absolutely no correlation between these two things; that, therefore, it is a mere chance whether or not organs which are of importance to organisms are likewise of importance as guides to classification; and, in point of fact, that the general tendency in this matter is towards an inverse instead of a direct proportion. More often than not, the greater the value of a structure for the purpose of indicating natural affinities, the less is its value to the creatures presenting it.
Enough has now been said to show three things. First, that long before the theory of descent was entertained by naturalists, naturalists perceived the fact of natural affinities, and did their best to construct a natural system of classification for the purpose of expressing such affinities. Second, that naturalists had a kind of instinctive belief in some one principle running through the whole organic world, which thus served to bind together organisms in groups subordinate to groups—that is, into species, genera, orders, families, classes, sub-kingdoms, and kingdoms. Third, that they were not able to give any very intelligible reason for this faith that was in them; sometimes supposing the principle in question to be that of a supernatural plan of organization, sometimes regarding it as dependent on conditions of physiology, and sometimes not attempting to account for it at all.
Of course it is obvious that the theory of descent furnishes the explanation which is required. For it is now evident to evolutionists, that although these older naturalists did not know what they were doing when they were tracing these lines of natural affinity, and thus helping to construct a natural classification—I say it is now evident to evolutionists that these naturalists were simply tracing the lines of genetic relationship. The great principle pervading organic nature, which was seen so mysteriously to bind the whole creation together as in a nexus of organic affinity, is now easily understood as nothing more or less than the principle of Heredity. Let us, therefore, look a little more closely at the character of this network, in order to see how far it lends itself to this new interpretation.
The first thing that we have to observe about the nexus is, that it is a nexus—not a single line, or even a series of parallel lines. In other words, some time before the theory of descent was seriously entertained, naturalists for the most part had fully recognised that it was impossible to arrange either plants or animals, with respect to their mutual affinities, in a ladder-like series (as was supposed to be the type of classification by the earlier systematists), or even in map-like groups (as was supposed to be the type by Linnaeus). And similarly, also, with respect to grades of organization. In the case of the larger groups, indeed, it is usually possible to say that the members of this group as a whole are more highly organized than the members of that group as a whole; so that, for instance, we have no hesitation in regarding the Vertebrata as more highly organized than the Invertebrata, Birds than Reptiles, and so on. But when we proceed to smaller subdivisions, such as genera and species, it is usually impossible to say that the one type is more highly organized than another type. A horse, for instance, cannot be said to be more highly organized than a zebra or an ass; although the entire horse-genus is clearly a more highly organized type than any genus of animal which is not a mammal.
In view of these facts, therefore, the system of classification which was eventually arrived at before the days of Darwin, was the system which naturalists likened to a tree; and this is the system which all naturalists now agreed upon as the true one. According to this system, a short trunk may be taken to represent the lowest organisms which cannot properly be termed either plants or animals. This short trunk soon separates into two large trunks, one of which represents the vegetable and the other the animal kingdom. Each of these trunks then gives off large branches signifying classes, and these give off smaller, but more numerous branches, signifying families, which ramify again into orders, genera, and finally into the leaves, which may be taken to represent species. Now, in such a representative tree of life, the height of any branch from the ground may be taken to indicate the grade of organization which the leaves, or species, present; so that, if we picture to ourselves such a tree, we may understand that while there is a general advance of organization from below upwards, there are many deviations in this respect. Sometimes leaves growing on the same branch are growing at a different level—especially, of course, if the branch be a large one, corresponding to a class or sub-kingdom. And sometimes leaves growing on different branches are growing at the same level: that is to say, although they represent species belonging to widely divergent families, orders, or even classes, it cannot be said that the one species is more highly organized than the other.
Now, this tree-like arrangement of species in nature is an arrangement for which Darwin is not responsible. For, as we have seen, the detecting of it has been due to the progressive work of naturalists for centuries past; and even when it was detected, at about the commencement of the present century, naturalists were confessedly unable to explain the reason of it, or what was the underlying principle that they were engaged in tracing when they proceeded ever more and more accurately to define these ramifications of natural affinity. But now, as just remarked, we can clearly perceive that this underlying principle was none other than Heredity as expressed in family likeness,—likeness, therefore, growing progressively more unlike with remoteness of ancestral relationship. For thus only can we obtain any explanation of the sundry puzzles and apparent paradoxes, which a working out of their natural classifications revealed to botanists and zoologists during the first half of the present century. It will now be my endeavour to show how these puzzles and paradoxes are all explained by the theory that natural affinities are merely the expression of genetic affinities.
First of all, and from the most general point of view, it is obvious that the tree-like system of classification, which Darwin found already and empirically worked out by the labours of his predecessors, is as suggestive as anything could well be of the fact of genetic relationship. For this is the form that every tabulation of family pedigree must assume; and therefore the mere fact that a scientific tabulation of natural affinities was eventually found to take the form of a tree, is in itself highly suggestive of the inference that such a tabulation represents a family tree. If all species were separately created, there can be no assignable reason why the ideas of earlier naturalists touching the form which a natural classification would eventually assume should not have represented the truth—why, for example, it should not have assumed the form of a ladder (as was anticipated in the seventeenth century), or of a map (as was anticipated in the eighteenth), or, again, of a number of wholly unrelated lines, circles, &c. (as certain speculative writers of the present century have imagined). But, on the other hand, if all species were separately and independently created, it becomes virtually incredible that we should everywhere observe this progressive arborescence of characters common to larger groups into more and more numerous, and more and more delicate, ramifications of characters distinctive only of smaller and smaller groups. A man would be deemed insane if he were to attribute the origin of every branch and every twig of a real tree to a separate act of special creation; and although we have not been able to witness the growth of what we may term in a new sense the Tree of Life, the structural relations which are now apparent between its innumerable ramifications bear quite as strong a testimony to the fact of their having been due to an organic growth, as is the testimony furnished by the branches of an actual tree.
Or, to take another illustration. Classification of organic forms, as Darwin, Lyell, and Haeckel have pointed out, strongly resembles the classification of languages. In the case of languages, as in the case of species, we have genetic affinities strongly marked; so that it is possible to some extent to construct a Language-tree, the branches of which shall indicate, in a diagrammatic form, the progressive divergence of a large group of languages from a common stock. For instance, Latin may be regarded as a fossil language, which has given rise to a group of living languages—Italian, Spanish, French, and, to a large extent, English. Now what would be thought of a philologist who should maintain that English, French, Spanish, and Italian were all specially created languages—or languages separately constructed by the Deity, and by as many separate acts of inspiration communicated to the nations which now speak them—and that their resemblance to the fossil form, Latin, must be attributed to special design? Yet the evidence of the natural transmutation of species is in one respect much stronger than that of the natural transmutation of languages—in respect, namely, of there being a vastly greater number of cases all bearing testimony to the fact of genetic relationship.
But, quitting now this most general point of view—or the suggestive fact that what we have before us is a tree—let us next approach this tree for the purpose of examining its structure more in detail. When we do this, the fact of next greatest generality which we find is as follows.
In cases where a very old form of life has continued to exist unmodified, so that by investigation of its anatomy we are brought back to a more primitive type of structure than that of the newer forms growing higher up upon the same branch, two things are observable. In the first place, the old form is less differentiated than the newer ones; and, in the next place, it is seen much more closely to resemble types of structure belonging to some of the other and larger branches of the tree. The organization of the older form is not only simpler; but it is, as naturalists say, more generalized. It comprises within itself characters belonging to its own branch, and also characters belonging to neighbouring branches, or to the trunk from which allied branches spring. Hence it becomes a general rule of classification, that it is by the lowest, or by the oldest, forms of any two natural groups that 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 the theory of descent with divergent modification; while, upon the alternative theory of special creation, no reason can be assigned why the lowest or the oldest types should thus combine the characters which afterwards become severally distinctive of higher or newer types.
Again, I have already alluded to the remarkable fact that there is no correlation between the value of structures to the organisms which present them, and their value to the naturalist for the purpose of tracing natural affinity; and I have remarked that up to the close of the last century it was regarded as an axiom of taxonomic science, that structures which are of most importance to the animals or plants possessing them must likewise prove of most importance in any natural system of classification. On this account, all attempts to discover the natural classification went upon the supposition that such a direct proportion must obtain—with the result that organs of most physiological importance were chosen as the bases of systematic work. And when, in the earlier part of the present century, De Candolle found that instead of a direct there was usually an inverse proportion between the functional and the taxonomic value of a structure, he was unable to suggest any reason for this apparently paradoxical fact. For, upon the theory of special creation, no reason can be assigned why organs of least importance to organisms should prove of most importance as marks of natural affinity. But on the theory of descent with progressive modification the apparent paradox is at once explained. For it is evident that organs of functional importance are, other things equal, the organs which are most likely to undergo different modifications in different lines of family descent, and therefore in time to have their genetic relationships in these different lines obscured. On the other hand, organs or structures which are of no functional importance are never called upon to change in response to any change of habit, or to any change in the conditions of life. They may, therefore, continue to be inherited through many different lines of family descent, and thus afford evidence of genetic relationship where such evidence fails to be given by any of the structures of vital importance, which in the course of many generations have been required to change in many ways according to the varied experiences of different branches of the same family. Here, then, we have an empirically discovered rule in the science of classification, the raison d'etre of which we are at once able to appreciate upon the theory of evolution, whereas no possible explanation of why it should ever have become a rule could be furnished upon the theory of special creation.
Here, again, is another empirically determined rule. The larger the number, as distinguished from the importance, of structures which are found common to different groups, the greater becomes their value as guides 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 Linnaeus, namely, that the characters do not give the genus, but the genus gives the characters; for this seems founded on the appreciation of many trifling points of resemblance, too slight to be defined."
 Origin of Species, p. 367.
Now it is evident, without comment, of how much value aggregates of characters ought to be in classification, if the ultimate meaning of classification be that of tracing lines of pedigree; whereas, if this ultimate meaning were that of tracing divine ideals manifested in special creation, we can see no reason why single characters are not such sure tokens of a natural arrangement as are aggregates of characters, even though the latter be in every other respect unimportant. For, on the special creation theory, we cannot explain why an assemblage, say of four or five trifling characters, should have been chosen to mark some unity of plan, rather than some one character of functional importance, which would have served at least equally well any such hypothetical purpose. On the other hand, as Darwin remarks, "we care not how trifling a character may be—let it be the mere inflection of 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 prevail throughout many and different species, especially those having very different habits of life, it assumes high value; for we can account for its presence in so many forms, with such different habits, only by inheritance from a common parent. We may err in this respect in regard to single points of structure, but when several characters, let them be ever so trifling, concur throughout a large group of beings having different habits, we may feel almost sure, on the theory of descent, that these characters have been inherited from a common ancestor; and we know that such aggregated characters have especial value in classification."
 Origin of Species, p. 372.
It is true that even a single character, if found common to a large number of forms, while uniformly absent from others, is also regarded by naturalists as of importance for purposes of classification, although they recognise it as of a value subordinate to that of aggregates of characters. But this also is what we should expect on the theory of descent. If even any one structure be found to run through a number of animals presenting different habits of life, the readiest explanation of the fact is to be found in the theory of descent; but this does not hinder that if several such characters always occur together, the inference of genetic relationship is correspondingly confirmed. And the fact that before this inference was ever drawn, naturalists recognised the value of single characters in proportion to their constancy, and the yet higher value of aggregates of characters in proportion to their number—this fact shows that in their work of classification naturalists empirically observed the effects of a cause which we have now discovered, to wit, hereditary transmission of characters through ever-widening groups of changing species.
There is another argument which appears to tell strongly in favour of the theory of descent. We have just seen that non-adaptive structures, not being required to change in response to change of habits or conditions of life, are allowed to persist unchanged through many generations, and thus furnish exceptionally good guides in the science of classification—or, according to our theory, in the work of tracing lines of pedigree. But now, the converse of this statement holds equally true. For it often happens that adaptive structures are required to change in different lines of descent in analogous ways, in order to meet analogous needs; and, when such is the case, the structures concerned have to assume more or less close resemblances to one another, even though they have severally descended from quite different ancestors. The paddles of a whale, for instance, most strikingly resemble the fins of a fish as to their outward form and movements; yet, on the theory of descent, 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 an homologous (or anatomical) resemblance—in all such cases it is observable that the similarities do not extend further into the structure of the parts than it is necessary that they should extend, in order that the structures should both perform the same functions. The whole anatomy of the paddles of a whale is quite unlike that of the fins of a fish—being, in fact, that of the fore-limb of a mammal. The change, therefore, which the fore-limb has here undergone to suit it to the aquatic habits of this mammal, is no greater than was required for that purpose: the change has not extended to any one feature of anatomical significance. This, of course, is what we should expect on the theory of descent with modification of ancestral characters; but on the theory of special creation it is not intelligible why there should always be so marked a distinction between resemblances as analogical or adaptive, and resemblances as homological or of meaning in reference to a natural classification. To take another and more detailed instance, the Tasmanian wolf is an animal separated from true wolves in a natural system of classification. Yet its jaws and teeth bear a strong general resemblance to those of all the dog tribe, although there are differences of anatomical detail. In particular, while the dogs all have on each side of the upper jaw four pre-molars and two molars, the Tasmanian wolf has three pre-molars and four molars. Now there is no reason, so far as their common function of dealing with flesh is concerned, why the teeth of the Tasmanian wolf should not have resembled homologically as well as analogically the teeth of a true wolf; and therefore we cannot assign any intelligible reason why, if all the species of the dog genus were separately created with one pattern of teeth, the unallied Tasmanian wolf should have been furnished with what is practically the same pattern from a functional point of view, while differing from a structural point of view. But, of course, on the theory of descent with modification, we can well understand why similarities of habit should have led to similarities of structural appearance of an adaptive kind in different lines of descent, without there being any trace of such real or anatomical similarities as could possibly point to genetic relationship.
Lastly, to adduce the only remaining argument from classification which I regard as of any considerable weight, naturalists have found it necessary, while constructing their natural classifications, to set great store on what Mr. Darwin calls "chains of affinities." Thus, for instance, "nothing can be easier than to define a number of characters common to all birds; but with crustaceans any such definition has hitherto been found impossible. There are crustaceans at the opposite ends of the series, which have hardly a character in common; yet the species at both ends, from being plainly allied to others, and these to others, and so onwards, can be recognised as unequivocally belonging to this, and to no other class of the articulata." Now it is evident that this progressive modification of specific types—where it cannot be said that the continuity of resemblance is anywhere broken, and yet terminates in modification so great that but for the connecting links no one could divine a natural relationship between the extreme members of the series,—it is evident that such chains of affinity speak most strongly in favour of a transmutation of the species concerned, while it is impossible to suggest any explanation of the fact in terms of the rival theory. For if all the links of such a chain were separately forged by as many acts of special creation, we can see no reason why B should resemble A, C resemble B, and so on, but with ever slight though accumulating differences, until there is no resemblance at all between A and Z.
 Origin of Species, pp. 368-9.
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I hope enough has now been said to show that all the general principles and particular facts appertaining to the natural classification of plants and animals, are precisely what they ought to be according to the theory of genetic descent; while no one of them is such as might be—and, indeed, used to be—expected upon the theory of special creation. Therefore, the only possible way in which all this uniform body 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, it may be said, what hidden reasons there may have been for following all these general principles in the separate creation of specific types. Now, it is evident that this is a form of argument which admits of being brought against all the actual—and even all the possible—lines of evidence in favour of evolution. Therefore I deem it desirable thus early in our proceedings to place this argument from ignorance on its proper logical footing.
If there were any independent evidence in favour of special creation as a fact, then indeed the argument from ignorance might be fairly used against any sceptical cavils regarding the method. In this way, for example, Bishop Butler made a legitimate use of the argument from ignorance when he urged that it is no reasonable objection against a revelation, otherwise accredited, to show that it has been rendered in a form, or after a method, which we should not have antecedently expected. But he could not have legitimately employed this argument, except on the supposition that he had some independent evidence in favour of the revelation; for, in the absence of any such independent evidence, appeal to the argument from ignorance would have become a mere begging of the question, by simply assuming that a revelation had been made. And thus it is in the present case. A man, of course, may quite legitimately say, Assuming that the theory of special creation is true, it is not for us to anticipate the form or method of the process. 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 the argument from ignorance, or to represent in effect that there is no question to be discussed. And if, when the form or method is investigated, it be found everywhere charged with evidence in favour of the theory of descent, the case becomes the same as that of a supposed revelation, which has been discredited by finding that all available evidence points to a natural growth. In short, the argument from ignorance is in any case available only as a negative foil against destructive criticism: in no case has it any positive value, or value of a constructive kind. Therefore, if a theory on any subject is destitute of positive evidence, while some alternative theory is in possession of such evidence, the argument from ignorance can be of no logical use to the former, even though it maybe of such use to the latter. For it is only the possession of positive evidence which can furnish a logical justification of the argument from ignorance: in the absence of such evidence, even the negative value of the argument disappears, and it then implies nothing more than the gratuitous assumption of a theory.
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I will now sum up the various considerations which have occupied us during the present chapter.
First of all we must take note that the classification of plants and animals in groups subordinate to groups is not merely arbitrary, or undertaken only for a matter of convenience and nomenclature—such, for instance, as the classification of stars in constellations. On the contrary, the classification of a naturalist differs from that of an astronomer, in that the objects which he has to classify present structural resemblances and structural differences in numberless degrees; and it is the object of his classification to present a tabular statement of these facts. Now, long before the theory of evolution was entertained, naturalists became fully aware that these facts of structural resemblances running through groups subordinate to groups were really facts of nature, and not merely poetic imaginations of the mind. No one could dissect a number of fishes without perceiving that they were all constructed on one anatomical pattern, which differed considerably from the equally uniform pattern on which all mammals were constructed, even although some mammals bore an extraordinary resemblance to fish in external form and habits of life. And similarly with all the smaller divisions of the animal and vegetable kingdoms. Everywhere investigation revealed the bonds of close structural resemblances between species of the same genus, resemblance less close between genera of the same family, resemblance still less close between families of the same order, resemblance yet more remote between orders of the same class, and resemblance only in fundamental features between classes of the same sub-kingdom, beyond which limit all anatomical resemblance was found to disappear—the different sub-kingdoms being formed on wholly different patterns. Furthermore, in tracing all these grades of structural relationship, naturalists were slowly led to recognise that the form which a natural classification must eventually assume would be that of a tree, wherein the constituent branches would display a progressive advance of organization from below upwards.
Now we have seen that although this tree-like arrangement of natural groups was as suggestive as anything could well be of all the forms oL life being bound together by the ties of genetic relationship, such was not the inference which was drawn from it. Dominated by the theory of special creation, naturalists either regarded the resemblance of type subordinate to type as expressive of divine ideals manifested in such creation, or else contented themselves with investigating the facts without venturing to speculate upon their philosophical import. But even those naturalists who abstained from committing themselves to any theory of archetypal plans, did not doubt that facts so innumerable and so universal must have been due to some one co-ordinating principle—that, even though they were not able to suggest what it was, there must have been some hidden bond of connexion running through the whole of organic nature. Now, as we have seen, it is manifest to evolutionists that this hidden bond can be nothing else than heredity; and, therefore, that these earlier naturalists, although they did not know what they were doing, were really tracing the lines of genetic descent as revealed by degrees of structural resemblance,—that the arborescent grouping of organic forms which their labours led them to begin, and in large measure to execute, was in fact a family tree of life.
Here, then, is the substance of the argument from classification. The mere fact that all organic nature thus incontestably lends itself to a natural arrangement of group subordinate to group, when due regard is paid to degrees of anatomical resemblance—this mere fact of itself tells so weightily in favour of descent with progressive modification in different lines, that even if it stood alone it would be entitled to rank 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 fact of all the innumerable forms of life being thus united in a tree-like system by an unquestionable relationship of some kind, to those smaller details in the science of classification which have been found most useful as guides for this kind of research, then we find that all these details, or empirically discovered rules, are exactly what we should have expected them to be, supposing the real meaning of classification to have been that of tracing lines of pedigree.
In particular, we have seen that the most archaic types are both simpler in their organization and more generalized in their characters than are the more recent types—a fact of which no explanation can be given on the theory of special creation. But, upon the theory of natural evolution, we can without difficulty understand why the earlier forms should have been the simpler forms, and also why they should have been the most generalized. For it is out of the older forms that the newer must have grown; and, as they multiplied, they must have become more and more differentiated.
Again, we have seen that there is no correlation between the importance of any structure from a classificatory point of view, and the importance of that structure to the organism which presents it. On the contrary, it is a general rule that "the less any part of the organization is concerned with special habits, the more important it becomes for classification." Now, from the point of view of special creation it is unintelligible why unity of ideal should be most manifested by least important structures, whereas from the point of view of evolution it is to be expected that these life-serving structures should have been most liable to divergent modification in divergent lines of descent, or in adaptation to different conditions of life, while the trivial or less important characters should have been allowed to remain unmodified. Thus we can now understand why all primitive classifications were wrong in principle when they went upon the assumption that divine ideals were best exhibited by resemblances between life-serving (and therefore adaptive) structures, with the result that whales were classed with fishes, birds with bats, and so on. Nevertheless, these primitive naturalists were quite logical; for, from the premises furnished by the theory of special creation, it is much more reasonable to expect that unity of ideal should be shown in plainly adaptive characters than in trivial and more or less hidden anatomical characters. Moreover, long after biological science had ceased consciously to follow any theological theory, the apparent axiom continued to be entertained, that structures of most importance to organisms must also be structures of most importance to systematists. And when at last, in the present century, this was found not to be the case, no reason could be suggested why it was not the case. But now we are able fully to explain this apparent anomaly.