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 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
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, 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.
* * * * *
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.
[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 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.
CHAPTER II.
CLASSIFICATION.
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[1]."
[1] _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[2]."
[2] _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[3]." 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.
[3] _Origin of Species_, pp. 368-9.
* * * * *
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.
* * * * *
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.
Once more, we have seen that aggregates of characters presenting
resemblances to one another have always been found to be of special
importance as guides to classification. This, of course, is what we
should have expected, if the real meaning of classification be that of
tracing lines of pedigree; but on the theory of special creation no
reason can be assigned why single characters are not such sure tokens of
a natural arrangement as are aggregates of characters, however trivial
the latter may be. For it is obvious that unity of ideal might have been
even better displayed by everywhere maintaining the pattern of some one
important structure, than by doing so in the case of several unimportant
structures. Take an analogous instance from human contrivances. Unity of
ideal in the case of gun-making would be shown by the same principles of
mechanism running through all the different sizes and shapes of
gun-locks, rather than by the ornamental patterns engraved upon the
outside. Yet it must be supposed that in the mechanisms assumed to have
been constructed by special creation, it was the trivial details rather
than the fundamental principles of these mechanisms which were chosen by
the Divinity to display his ideals.
And this leads us to the next consideration--namely, that when in two
different lines of descent animals happen to adopt similar habits of
life, the modifications which they undergo in order to fit them for
these habits often induces striking resemblances of structure between
the two animals, as in the case of whales and fish. But in all such
instances it is invariably found that the resemblance is only
superficial and apparent: not anatomical or real. In other words, the
resemblance does not extend further than it is necessary that it
should, if both sets of organs are to be adapted to perform the same
functions. Now this, again, is just what one would expect to find as the
universal rule on the theory of descent, with modification of ancestral
characters. But, on the opposite theory of special creation, I know not
how it is to be explained that among so many instances of close
superficial resemblance between creatures belonging to different
branches of the tree of life, there are no instances of any real or
anatomical resemblance. So far as their structures are adapted to
perform a common function, there is in all such cases what may be termed
a deceptive appearance of some unity of ideal; but, when carefully
examined, it is always found that two apparently identical structures
occurring on different branches of the classificatory tree are in fact
fundamentally different in respect of their structural plan.
Lastly, we have seen that one of the guiding principles of
classification has been empirically found to consist in setting a high
value on "chains of affinities." That is to say, naturalists not
unfrequently meet with a long series of progressive modifications of
type, which, although it cannot be said that the continuity is anywhere
broken, at last leads to so much divergence of character that, but for
the intermediate links, the members at each end of the chain could not
be suspected of being in any way related. Well, such cases of chains of
affinity obviously tell most strongly in favour of descent with
continuous modification; while it is impossible to suggest why, if all
the links were separately forged by as many acts of special creation,
there should have been this gradual transmutation of characters carried
to the point where the original creative ideal has been so completely
transformed that, but for the accident of the chain being still
complete, no one of nature's interpreters could possibly have discovered
the connexion. For, as we have seen, this is not a case in which any
appeal can be logically made to the argument from ignorance of divine
method, unless some independent evidence could be adduced in favour of
special creation. And that no such independent evidence exists, it will
be the object of future chapters to show.
CHAPTER III.
MORPHOLOGY.
The theory of evolution supposes that hereditary characters admit of
being slowly modified wherever their modification will render an
organism better suited to a change in its conditions of life. Let us,
then, observe the evidence which we have of such adaptive modifications
of structure, in cases where the need of such modification is apparent.
We may begin by again taking the case of the whales and porpoises. The
theory of evolution infers, from the whole structure of these animals,
that their progenitors must have been terrestrial quadrupeds of some
kind, which gradually became more and more aquatic in their habits. Now
the change in the conditions of their life thus brought about would have
rendered desirable great modifications of structure. These changes would
have begun by affecting the least typical--that is, the least strongly
inherited--structures, such as the skin, claws, and teeth. But, as time
went on, the adaptation would have extended to more typical structures,
until the shape of the body would have become affected by the bones and
muscles required for terrestrial locomotion becoming better adapted for
aquatic locomotion, and the whole outline of the animal more fish-like
in shape. This is the stage which we actually observe in the seals,
where the hind legs, although retaining all their typical bones, have
become shortened up almost to rudiments, and directed backwards, so as
to be of no use for walking, while serving to complete the fish-like
taper of the body. (Fig. 2.) But in the whales the modification has gone
further than this so that the hind legs have ceased to be apparent
externally, and are only represented internally--and even this only in
some species--by remnants so rudimentary that it is difficult to make
out with certainty the homologies of the bones; moreover, the head and
the whole body have become completely fish-like in shape. (Fig. 3.) But
profound as are these alterations, they affect only those parts of the
organism 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 the
arm, which is used as a fin, still retains the bones of the shoulder,
fore-arm, wrist, and fingers, although they are all enclosed in a
fin-shaped sack, so as to render them useless for any purpose other than
swimming (Fig. 4.) Similarly, the head, although it so closely resembles
the head of a fish in shape, still retains the bones of the mammalian
skull in their proper anatomical relations to one another; but modified
in form so as to offer the least possible resistance to the water. In
short, it may be said that all the modifications have been effected with
the least possible divergence from the typical mammalian type, which is
compatible with securing so perfect an adaptation to a purely aquatic
mode 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 they
offer so extreme an example of profound modification of structure in
adaptation to changed conditions of life. But the same thing may be seen
in hundreds and hundreds of other cases. For instance, to confine our
attention to the arm, not only is the limb modified in the whale for
swimming, but in another mammal--the bat--it is modified for flying, by
having the fingers enormously elongated and overspread with a membranous
web.
In birds, again, the arm is modified for flight in a wholly different
way--the fingers here being very short and all run together, while the
chief expanse of the wing is composed of the shoulder and fore-arm. In
frogs and lizards, again, we find hands more like our own; but in an
extinct species of flying reptile the modification was extreme, the wing
having been formed by a prodigious elongation of the fifth finger, and a
membrane spread over it and the rest of the hand. (Fig. 5.) Lastly, in
serpents 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 wonderful
are the modifications which it is seen to undergo, although never losing
its typical character. Everywhere we find the distinction between
homology and analogy which was explained in the last chapter--the
distinction, that is, between correspondence of structure and
correspondence of function. On the one hand, we meet with structures
which are perfectly homologous and yet in no way analogous: the
structural elements remain, but are profoundly modified so as to perform
wholly different functions. On the other hand, we meet with structures
which are perfectly analogous, and yet in no way homologous: totally
different structures are modified to perform the same functions. How,
then, are we to explain these things? By design manifested in special
creation, or by descent with adaptive modification? If it is said by
design manifested in special creation, we must suppose that the Deity
formed an archetypal plan of certain structures, and that he determined
to adhere to this plan through all the modifications which those
structures exhibit. But, if so, why is it that some structures are
selected as typical and not others? Why should the vertebral skeleton,
for instance, be tortured into every conceivable variety of modification
in order to subserve as great a variety of functions; while another
structure, such as the eye, is made in different sub-kingdoms on
fundamentally different plans, notwithstanding that it has throughout to
perform the same function? Will any one have the hardihood to assert
that in the case of the skeleton the Deity has endeavoured to show his
_ingenuity_, by the manifold functions to which he has made the same
structure subservient; while in the case of the eye he has endeavoured
to show his _resources_, by the manifold structures which he has adapted
to serve the same function? If so, it becomes a most unfortunate
circumstance that, throughout both the vegetable and animal kingdoms,
all cases which can be pointed to as showing ingenious adaptation of the
same typical structure to the performance of widely different
functions--or cases of homology without analogy,--are cases which come
within the limits of the same natural group of plants and animals, and
therefore admit of being equally well explained by descent from a common
ancestry; while all cases of widely different structures performing the
same function--or cases of analogy without homology,--are to be found in
different groups of plants or animals, and are therefore suggestive of
independent variations arising in the different lines of hereditary
descent.
To take a specific illustration. The octopus, or devil-fish, belongs to
a 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 of
the theory of evolution by natural selection, because it must clearly be
a most improbable thing that so complicated a structure as the eye of a
fish should happen to be arrived at through each of two totally
different lines of descent. And this difficulty would, indeed, be a
formidable one to the theory of evolution, if the similarity were not
only analogical but homological. Unfortunately for the objection,
however, Darwin clearly showed in his reply that in no one anatomical or
homologous feature do the two structures resemble one another; so that,
in point of fact, the two organs do not resemble one another in any
particular further than it is necessary that they should, if both are to
be 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 what
force might it have then been urged,--Your hypothesis of hereditary
descent with progressive modification being here excluded by the fact
that the animals compared belong to two widely different branches of the
tree of life, how are we to explain the identity of type manifested by
these two complicated organs of vision? The only hypothesis open to us
is intelligent adherence to an ideal plan or mechanism. But as this
cannot now be urged in any comparable case throughout the whole organic
world, 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 of
life we constantly find the same typical structures modified so as to
perform very different functions, we never find any of these particular
types of structure in other large branches of the tree. That is to say,
we never find typical structures appearing except in cases where their
presence may be explained by the hypothesis of hereditary descent; while
in thousands of such cases we find these structures undergoing every
conceivable variety of adaptive modification.
Consequently, special creationists must fall back upon another position
and say,--Well, but it may have pleased the Deity to form a certain
number of ideal types, and never to have allowed the structures
occurring in one type to appear in any of the others. We
answer,--Undoubtedly such may have been the case; but, if so, it is a
most unfortunate thing for your theory, because the fact implies that
the Deity has planned his types in such a way as to suggest the
counter-theory of descent. For instance, it would seem most capricious
on the part of the Deity to have made the eyes of an innumerable number
of fish on exactly the same ideal type, and then to have made the eye of
the octopus so exactly like these other eyes in superficial appearance
as to deceive so accomplished a naturalist as Mr. Mivart, and yet to
have taken scrupulous care that in no one ideal particular should the
one type resemble the other. However, adopting for the sake of argument
this great assumption, let us suppose that God did lay down these
arbitrary rules for his own guidance in creation, and then let us see to
what the assumption leads. If the Deity formed a certain number of ideal
types, and determined that on no account should he allow any part of one
type to appear in any part of another, surely we should expect that
within the limits of the same type the same typical structures should
always be present. Thus, remember what efforts, so to speak, have been
made to maintain the uniformity of type in the case of the fore-limb as
previously explained, and should we not expect that in other and similar
cases a similar method should have been followed? Yet we repeatedly find
that this is not the case. Even in the whale, as we have seen, the
hind-limbs are either altogether absent or dwindled almost to nothing;
and it is impossible to see in what respect the hind-limbs are of any
less ideal value than the fore-limbs--which are carefully preserved in
all vertebrated animals except the snakes, and the extinct _Dinornis_,
where again we meet in this particular with a sudden and sublime
indifference to the maintenance of a typical structure. (Fig. 6.)[4] Now
I say that if the theory of ideal types is true, we have in these facts
evidence of a most unreasonable inconsistency. But the theory of descent
with continued adaptive modification fully explains all the known cases;
for in every case the degree of divergence from the typical structure
which an organism presents corresponds, in a general way, with the
length of time during which the divergence has been going on. Thus we
scarcely ever meet with any great departure from the typical form with
respect to one of the organs, without some of the other organs being so
far modified as of themselves to indicate, on the supposition of
descent with modification, that the animal or plant must have been
subject to the modifying influences for an enormously long series of
generations. And this combined testimony of a number of organs in the
same 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 supposed
ideal type, some of the other organs in the same organism should tend to
keep 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 been
mentioned, I may draw attention to what seems to me a particularly
suggestive case. So-called soldier-or hermit-crabs, are crabs which have
adopted the habit of appropriating the empty shells of mollusks. In
association with this peculiar habit, the structure of these animals
differs very greatly from that of all other crabs. In particular, the
hinder part of the body, which occupies the mollusk-shell, and which
therefore has ceased to require any hard covering of its own, has been
suffered to lose its calcareous integument, and presents a soft fleshy
character, quite unlike that of the more exposed parts of the animal.
Moreover, this soft fleshy part of the creature is specially adapted to
the particular requirements of the creature by having its lateral
appendages--i. e. appendages which in other crustacea perform the
function of legs--modified so as to act as claspers to the inside of the
mollusk-shell; while the tail-end of the part in question is twisted
into the form of a spiral, which fits into the spiral of the
mollusk-shell. Now, in Keeling Island there is a large kind of crab
called _Birgus latro_, which lives upon land and there feeds upon
cocoa-nuts. The whole structure of this crab, it seems to me,
unmistakeably resembles the structure of a hermit-crab (see drawings on
the next page, Fig. 7). Yet this crab neither lives in the shell of a
mollusk, nor is the hinder part of its body in the soft and fleshy
condition just described: on the contrary, it is covered with a hard
integument like all the other parts of the animal. Consequently, I think
we may infer that the ancestors of _Birgus_ were hermit-crabs living in
mollusk-shells; but that their descendants gradually relinquished this
habit as they gradually became more and more terrestrial, while,
concurrently with these changes in habit, the originally soft posterior
parts acquired a hard protective covering to take the place of that
which was formerly supplied by the mollusk-shell. So that, if so, we now
have, within the limits of a single organism, evidence of a whole series
of morphological changes in the past history of its species. First,
there must have been the great change from an ordinary crab to a
hermit-crab in all the respects previously pointed out. Next, there must
have 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 the
existing structure of _Birgus_ a morphological record of all these
changes, and one which gives us a reasonable explanation of why the
animal presents the extraordinary appearance which it does. But, on the
theory of special creation, it is inexplicable why this land-crab should
have been formed on the pattern of a hermit-crab, when it never has need
to enter the shell of a mollusk. In other words, its peculiar
structure is not specially in keeping with its present habits, although
so curiously allied to the similar structure of certain other crabs of
totally different habits, in relation to which the peculiarities are of
plain 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 another
branch of the argument from morphology, to which the case of _Birgus_
serves as a suitable introduction: I mean the argument from rudimentary
structures.
Throughout both the animal and vegetable kingdoms we constantly meet
with dwarfed and useless representatives of organs, which in other and
allied kinds of animals and plants are of large size and functional
utility. Thus, for instance, the unborn whale has rudimentary teeth,
which are never destined to cut the gums; and throughout its life this
animal retains, in a similarly rudimentary condition, a number of organs
which never could have been of use to any kind of creature save a
terrestrial quadruped. The whole anatomy of its internal ear, for
example, has reference to hearing in air--or, as Hunter long ago
remarked, "is constructed upon the same principle as in the quadruped";
yet, as Owen says, "the outer opening and passage leading therefrom to
the tympanum can rarely be affected by sonorous vibrations of the
atmosphere, and indeed they are reduced, or have degenerated, to a
degree which makes it difficult to conceive how such vibrations can be
propagated to the ear-drum during the brief moments in which the opening
may 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? of
course the theory of descent with adaptive modification has a simple
answer to supply--namely, that when, from changed conditions of life, an
organ which was previously useful becomes useless, it will be suffered
to dwindle away in successive generations, under the influence of
certain natural causes which we shall have to consider in future
chapters. On the other hand, the theory of special creation can only
maintain that these rudiments are formed for the sake of adhering to an
ideal type. Now, here again the former theory appears to be triumphant
over the latter; for, without waiting to dispute the wisdom of making
dwarfed and useless structures merely for the whimsical motive assigned,
surely if such a method were adopted in so many cases, we should expect
that in consistency it would be adopted in all cases. This reasonable
expectation, however, is far from being realized. We have already seen
that in numberless cases, such as that of the fore-limbs of serpents, no
vestige of a rudiment is present. But the vacillating policy in the
matter of rudiments does not end here; for it is shown in a still more
aggravated form where within the limits of the same natural group of
organisms a rudiment is sometimes present and sometimes absent. For
instance, although in nearly all the numerous species of snakes there
are no vestiges of limbs, in the python we find very tiny rudiments of
the 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 nearly
all the numerous species of snakes, he should have added a tiny rudiment
in the case of the python--and even in that case should have maintained
his ideal very inefficiently, inasmuch as only two limbs, instead of
four, are represented? how much more reasonable is the naturalistic
interpretation; for here the very irregularity of their appearance in
different species, which constitutes rudimentary structures one of the
crowning difficulties to the theory of special design, furnishes the
best possible evidence in favour of hereditary descent; seeing that this
irregularity then becomes what may be termed the anticipated expression
of progressive dwindling due to inutility. Thus, for example, to return
to the case of wings, we have already seen that in an extinct genus of
bird, _dinornis_, these organs were reduced to such an extent as to
leave it still doubtful whether so much as the tiny rudiment
hypothetically supplied to fig. 6 (p. 61) was present in all the
species. And here is another well-known case of another genus of still
existing bird, which, as was the case with _dinornis_, occurs only in
new zealand. (Fig. 9.) Upon this island there are no four-footed
enemies--either existing or extinct--to escape from which the wings of
birds would be of any service. Consequently we can understand why on
this island we should meet with such a remarkable dwindling away of
wings.
[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 along
the surface of the water, having its wings considerably reduced though
less so than the _Apteryx_ of New Zealand. But here the interesting fact
is that the young birds are able to fly perfectly well. Now, in
accordance with a general law to be considered in a future chapter, the
life-history of an individual organism is a kind of condensed
recapitulation of the life-history of its species. Consequently, we can
understand why the little chickens of the logger-headed duck are able to
fly like all other ducks, while their parents are only able to flap
along the surface of the water.
Facts analogous to this reduction of wings in birds which have no
further use for them, are to be met with also in insects under similar
circumstances. Thus, there are on the island of Madeira somewhere
between 500 and 600 species of beetles, which are in large part
peculiar to that island, though related to other--and therefore
presumably parent--species on the neighbouring continent. Now, no less
than 200 species--or nearly half the whole number--are so far deficient
in wings that they cannot fly. And, if we disregard the species which
are not peculiar to the island--that is to say, all the species which
likewise occur on the neighbouring continent, and therefore, as
evolutionists conclude, have but _recently_ migrated to the island,--we
find this very remarkable proportion. There are altogether 29 peculiar
genera, and out of these no less than 23 have _all_ their species in
this condition.
Similar facts have been recently observed by the Rev. A. E. Eaton with
respect to insects inhabiting Kerguelen Island. All the species which he
found on the island--viz. a moth, several flies, and numerous
beetles--he found to be incapable of flight; and therefore, as Wallace
observes, "as these insects could hardly have reached the islands in a
wingless state, even if there were any other known land inhabited by
them, which there is not, we must assume that, like the Madeiran
insects, they were originally winged, and lost their power of flight
because its possession was injurious to them"--Kerguelen Island being
"one of the stormiest places on the globe," and therefore a place where
insects could rarely afford to fly without incurring the danger of being
blown 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 the
following question. In a certain dark subterranean cave, called the
Mammoth cave, there are found some peculiar species of blind fishes. Now
the editors of _Silliman's Journal_ wished to know whether Prof. Agassiz
would 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 from
other species, but, having got into the dark cave, gradually lost their
eyes through disuse. Prof. Agassiz, who was a believer in special
creation, allowed that this ought to constitute a crucial test as
between the two theories of special design and hereditary descent. "If
physical circumstances," he said, "ever modified organized beings, it
should be easily ascertained here." And eventually he gave it as his
opinion, that these fish "were created under the circumstances in which
they now live, within the limits over which they now range, and with the
structural peculiarities which now characterise them."
Since then a great deal of attention has been paid to the fauna of this
Mammoth cave, and also to the faunas of other dark caverns, not only in
the New, but also in the Old World. In the result, the following general
facts have been fully established.
(1) Not only fish, but many representatives of other classes, have been
found 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 some
degree of light, they may have large and lustrous eyes.
(4) In all cases the species of blind animals are closely allied to
species inhabiting the district where the caves occur; so that the blind
species inhabiting American caves are closely allied to American
species, while those inhabiting European caves are closely allied to
European species.
(5) In nearly all cases structural remnants of eyes admit of being
detected, in various degrees of obsolescence. In the case of some of the
crustaceans 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 agreement
with the theory of evolution, while they offer serious difficulties to
the theory of special creation. As Darwin remarks, it is hard to imagine
conditions of life more similar than those furnished by deep limestone
caverns under nearly the same climate in the two continents of America
and Europe; so that, in accordance with the theory of special creation,
very close similarity in the organizations of the two sets of faunas
might have been expected. But, instead of this, the affinities of these
two sets of faunas are with those of their respective continents--as of
course they ought to be on the theory of evolution. Again, what would
have been the sense of creating useless foot-stalks for the imaginary
support of absent eyes, not to mention all the other various grades of
degeneration in other cases? So that, upon the whole, if we agree with
the late Prof. Agassiz in regarding these cave animals as furnishing a
crucial test between the rival theories of creation and evolution, we
must further conclude that the whole body of evidence which they now
furnish is weighing on the side of evolution.