Chapter VII: Characters as Adaptive and Specific

One of the great changes which has been wrought in biological science by the Darwinian theory of natural selection, consists in its having furnished an intelligible explanation of the phenomena of _adaptation_. Indeed, in my opinion, this is the most important function which this theory has had to perform; and although we still find systematic zoologists and systematic botanists who hold that the chief merit of Darwin's work consists in its having furnished an explanation of the origin of _species_, a very little consideration is enough to show that such an idea is but a survival, or a vestige, of an archaic system of thought. So long as species were regarded as due to separate acts of creation, any theory which could explain their production by a process of natural evolution became of such commanding importance in this respect, that we cannot wonder if in those days the principal function of Darwin's work was held to be what the title of that work--_The Origin of Species by means of Natural Selection_--itself serves to convey. And, indeed, in those days this actually was the principal function of Darwin's work, seeing that in those days the _fact_ of evolution itself, as distinguished from its _method_, had to be proved; and that the whole proof had to stand or fall with the evidence which could be adduced touching the mutability of species. Therefore, without question, Darwin was right in placing this issue as to the stability or instability of species in the forefront of his generalizations, and hence in constituting it the title of his epoch-making book. But nowadays, when the fact of evolution has been sufficiently established, one would suppose it self-evident that the theory of natural selection should be recognized as covering a very much larger field than that of explaining the origin of _species_--that it should be recognized as embracing the whole area of organic nature in respect of _adaptations_, whether these happen to be distinctive of species only, or of genera, families, orders, classes, and sub-kingdoms. For it follows from the general fact of evolution that species are merely arbitrary divisions, which present no deeper significance from a philosophical point of view than is presented by well-marked varieties, out of which they are in all cases believed to have arisen, and from which it is often a matter of mere individual taste whether they shall be separated by receiving the baptism of a specific name. Yet, although naturalists are now unanimously agreed that what they classify as species are nothing more than pronounced--and in some greater or less degree permanent--varieties, so forcible is the influence of traditional modes of thought, that many zoologists and botanists still continue to regard the origin of species as a matter of more importance than the origin of adaptations. Consequently, they continue to represent the theory of natural selection as concerned, primarily, with explaining the origin of species, and denounce as a "heretic" any one who regards the theory as primarily a theory of the origin and cumulative development of adaptations--whether structural or instinctive, and whether the adaptations are severally characteristic of species only or of any of the higher taxonomic divisions. Indeed, these naturalists appear to deem it in some way a disparagement of the theory to state that it is, primarily, a theory of adaptations, and only becomes secondarily a theory of species in those comparatively insignificant cases where the adaptations happen to be distinctive of the lowest order of taxonomic division--a view of the matter which may fitly be compared to that of an astronomer who should define the nebular hypothesis as a theory of the origin of Saturn's rings. It is indeed a theory of the origin of Saturn's rings; but only because it is a theory of the origin of the entire solar system, of which Saturn's rings form a part. Similarly, the theory of natural selection is a theory of the entire system of organic nature in respect of adaptations, whether these happen to be distinctive of particular species only, or are common to any number of species.

Now the outcry which has been raised over this definition of the theory of natural selection is a curious proof of the opposition which may be furnished by habitual modes of thought to an exceedingly plain matter of definition. For, I submit, that no one can deny any of the following propositions; nor can it be denied that from these propositions the foregoing definition of the theory in question follows by way of necessity. The propositions are, first, that natural selection is taken to be the agency which is mainly, if not exclusively, concerned in the evolution of adaptive characters: secondly, that these characters, when evolved, are in some cases peculiar to single species only, while in other cases, and in process of time, they become the common property of many species: thirdly, that in cases where they are peculiar to single species only, they constitute at all events one of the reasons (or even, as the ultra-Darwinians believe, the only reason) why the particular species presenting them have come to be species at all. Now, these being the propositions on which we are all agreed, it obviously follows, of logical necessity, that the theory in question is primarily one which explains the existence of adaptive characters wherever these occur; and, therefore, whether they happen to be restricted to single species, or are common to a whole group of species. Of course in cases where they are restricted to single species, the theory which explains the origin of these particular adaptations becomes also a theory which explains the origin of these particular species; seeing that, as we are all agreed, it is in virtue of such particular adaptations that such particular species exist. Yet even in these cases the theory is, primarily, a theory of the adaptations in virtue of which the particular species exists; for, _ex hypothesi_, it is the adaptations which condition the species, not the species the adaptations. But, as just observed, adaptations may be the common property of whole groups of species; and thus the theory of natural selection becomes a theory of the origin of genera, of families, of orders, and of classes, quite as much as it is a theory of the origin of species. In other words, it is everywhere a theory of adaptations; and it is only where the adaptations happen to be restricted to single species that the theory therefore and incidentally becomes also a theory of the particular species which presents them. Hence it is by no means the same proposition to affirm that the theory of natural selection is a theory of the origin of species, and that it is a theory of the origin of adaptations, as some of my critics have represented it to be; for these two things are by no means conterminous. And in as far as the two propositions differ, it is perfectly obvious that the latter is the true one.

Possibly, however, it may be said--Assuredly natural selection is a theory of the origin (i.e. cumulative development) of adaptations; and, no less assuredly, although species owe their origin to such adaptations, there is now no common measure between these two things, seeing that in numberless cases the same adaptations are the common property of numberless species. But, allowing all this, we must still remember that in their _first beginnings_ all these adaptations must have been distinctive of, or peculiar to, some one particular species, which afterwards gave rise to a whole genus, family, order, or class of species, all of which inherited the particular adaptations derived from this common ancestor, while progressively gaining additional adaptive characters severally distinctive of their subsequently diverging lines of descent. So that really all adaptive characters must originally have been specific characters; and therefore there is no real distinction to draw between natural selection as a theory of species and as a theory of adaptations.

Well, if this objection were to be advanced, the answer would be obvious. Although it is true that every adaptive character which is now common to a group of species must originally have been distinctive of a single parent species, it by no means follows that in its first beginning as a specific character it appeared in the fully developed form which it now presents as a generic, family, ordinal, or yet higher character. On the contrary, it is perfectly certain that in the great majority of instances such cannot possibly have been the case; and the larger the group of species over which any particular adaptive character now extends, the more evidently do we perceive that this character must itself have been the product of a gradual evolution by natural selection through an innumerable succession of species in branching lines. The wing of a bird, for example, is an adaptive structure which cannot possibly have ever appeared suddenly as a merely specific character: it must have been slowly elaborated through an incalculable number of successive species, as these branched into genera, families, and orders of the existing class. So it is with other class distinctions of an adaptive kind; and so, in progressively lessening degrees, is it with adaptive characters of an ordinal, a family, or a generic value. That is to say, in _all_ cases where an adaptive structure is common to any considerable group of species, we meet with clear evidence that the structure has been the product of evolution through the ancestry of those species; and this evidence becomes increasingly cogent the higher the taxonomic value of the structure. Indeed, it may be laid down as a general rule, that the greater the _degree_ of adaptation the greater is its _diffusion_--both as regards the number of species which present it now, and the number of extinct species through which it has been handed down, in an ever ramifying extension and in an ever improving form. Species, therefore, may be likened to leaves: successive and transient crops are necessary for the gradual building up of adaptations, which, like the woody and permanent branches, grow continuously in importance and efficiency through all the tree of life. Now, in my view, it is the great office of natural selection to see to the growth of these permanent branches; and although natural selection has likewise had an enormously large share in the origination of each successive crop of leaves--nay, let it be granted to the ultra-Darwinians for the sake of argument, an exclusive prerogative in this respect--still, in my view, this is really the least important part of its work. Not as an explanation of those merely permanent varieties which we call species, but as an explanation of the adaptive machinery of organic nature, which has led to the construction both of the animal and vegetable kingdoms in all their divisions do I regard the Darwinian theory as one of the greatest generalizations in the history of science.

* * * * *

I have dwelt thus at some length upon a mere matter of definition because, as we shall now find, although it is but a matter of definition, it is fraught with consequences of no small importance to the general theory of descent. Starting from an erroneous definition of the theory of natural selection as primarily a theory of the origin of species, both friends and foes of the theory have concluded that the principle of utility must by hypothesis be of universal occurrence so far as species are concerned; whereas, if once these naturalists were to perceive that their definition of the theory is erroneous, they would likewise perceive that their conclusion cannot follow deductively from the theory itself. If such a conclusion is to be established at all, it can only be by other and independent evidence of the inductive kind--to wit, by actual observation.

Hence we see the importance of starting with an accurate definition of the theory before proceeding to examine the doctrine of utility as of universal application to species--a doctrine which, as just stated, has been habitually and expressly deduced from the theory. This doctrine occurs in two forms; or, more correctly, there are with reference to this subject two distinct doctrines, which partly coincide and partly exclude one another. First, it is held by some naturalists that all species must necessarily owe their origin to natural selection. And secondly, it is held by other naturalists, that not only all species, but likewise all specific characters must necessarily do the same. Let us consider these two doctrines separately.

The first, and less extensive doctrine, rests on the deduction that every species must owe its differentiation as a species to the evolution of at least one adaptive character, which is peculiar to that species. Although, when thus originated, a species may come to present any number of other peculiar characters of a non-adaptive kind, these merely indifferent peculiarities are supposed to hang, as it were, on the peg supplied by the one adaptive peculiarity; it is the latter which conditions the species, and so furnishes an opportunity for any number of the former to supervene. But without the evolution of at least one adaptive character there could have been no distinct species, and therefore no merely adventitious characters as belonging to that species. I will call this the Huxleyan doctrine, because Professor Huxley is its most express and most authoritative supporter.

The second and more extensive doctrine I will call, for the same reason, the Wallacean doctrine. This is, as already stated, that it follows deductively from the theory of natural selection, that not only all species, but even all the distinctive characters of every species, must necessarily be due to natural selection; and, therefore, can never be other than themselves useful, or, at the least, correlated with some other distinctive characters which are so.

Here, however, I should like to remark parenthetically, that in choosing Professor Huxley and Mr. Wallace as severally representative of the doctrines in question, I earnestly desire to avoid any appearance of discourtesy towards such high authorities.

I am persuaded--as I shall hereafter seek to show Darwin was persuaded--that the doctrine of utility as universal where species are concerned, is, in both the above forms, unsound. But it is less detrimental in its Huxleyan than in its Wallacean form, because it does not carry the erroneous deduction to so extreme a point. Therefore let us first consider the doctrine in its more restricted form, and then proceed, at considerably greater length, to deal with it in its more extended form.

* * * * *

The doctrine that all _species_ must necessarily be due to natural selection, and therefore must severally present at least one adaptive character, appears to me doubly erroneous.

In the first place, it is drawn from what I have just shown to be a false premiss; and, in the second place, the conclusion does not follow even from this premiss. That the premiss--or definition of the theory as primarily a theory of the origin of species--is false, I need not wait again to argue. That the conclusion does not follow even from this erroneous premiss, a very few words will suffice to prove. For, even if it were true that natural selection is primarily a theory of the origin of species, it would not follow that it must therefore be a theory of the origin of _all_ species. This would only follow if it were first shown that the theory is not merely _a_ theory of the origin of species, but _the_ theory of the origin of species--i.e. that there can be no further theory upon this subject, or any cause other than natural selection which is capable of transforming any single specific type.

Needless to say, this cannot be shown by way of deduction from the theory of natural selection itself--which, nevertheless, is the only way whereby it is alleged that the doctrine is arrived at[86].

[86] For a full treatment of Professor Huxley's views upon this
subject, see Appendix II.

* * * * *

From the doctrine of utility as advocated by Professor Huxley, we may now pass on to consider it in the much more comprehensive form advocated by Mr. Wallace. Of course it is obvious that if the doctrine is erroneous in its Huxleyan form, much more must it be so in its Wallacean; and, therefore, that having shown its erroneousness in its less extended application, there is little need to consider it further in its more extended form. Looking, however, to its importance in this more extended application, I think we ought to examine it independently as thus presented by Mr. Wallace and his school. Let us therefore consider, on its own merits, the following statement:--It follows directly from the theory of natural selection that not only all species, but likewise all specific characters, must be due to natural selection, and, therefore, must all be of use to the species which present them, or else correlated with other characters which are so.

It seems worth while to observe, _in limine_, that this doctrine is contradicted by that of Professor Huxley. For supposing natural selection to be the only principle concerned in the origin of all species, it by no means follows that it is the sole agency concerned in the origin of all specific characters. It is enough for the former proposition if only some of the characters distinctive of any given species--nay, as he very properly expresses it, if only one such character--has been due to natural selection; for it is clear that, as he adds, "any number of indifferent [specific] characters" may thus have been furnished with an opportunity, so to speak, of being produced by causes other than natural selection. Hence, as previously remarked, the Huxleyan doctrine, although coinciding with the Wallacean up to the point of maintaining utility as the only principle which can be concerned in the origin of species, designedly excludes the Wallacean doctrine where this proceeds to extend any similar deduction to the case of specific characters[87].

[87] Professor Huxley's views upon this matter are quoted _in
extenso_ in Appendix II.

In the next place, and with special reference to the Wallacean doctrine, it is of importance to observe that, up to a certain point there is complete agreement between Darwinists of all schools. We all accept natural selection as a true cause of the origin of species (though we may not all subscribe to the Huxleyan deduction that it is necessarily a cause of the origin of _all_ species). Moreover, we agree that specific characters are often what is called rudimentary or vestigial; and, once more, that our inability to detect the use of any given structure or instinct is no proof that such a structure or instinct is actually useless, seeing that it may very probably possess some function hitherto undetected, or possibly undetectable. Lastly, we all agree that a structure which is of use may incidentally entail the existence of some other structure which is not of use; for, in virtue of the so-called principle of correlation, the useless structure may be an indirect consequence of natural selection, since its development may be due to that of the useful structure, with the growth of which the useless one is correlated.

Nevertheless, while fully conceding all these facts and principles to the Wallacean party, those who think with Professor Huxley--and still more, of course, those few naturalists who think as I do--are unable to perceive that they constitute any grounds for holding the doctrine that all specific _characters_ are, or formerly have been, directly or indirectly due to natural selection. My own reasons for dissenting from this Wallacean doctrine are as follows.

* * * * *

From what has just been said, it will be apparent that the question in debate is not merely a question of fact which can be settled by a direct appeal to observation. If this were the case, systematic naturalists could soon settle the question by their detailed knowledge of the structures which are severally distinctive of any given group of species. But so far is this from being the case, that systematic naturalists are really no better qualified to adjudicate upon the matter than are naturalists who have not devoted so much of their time to purely diagnostic work. The question is one of general principles, and as such cannot be settled by appeals to special cases. For example, suppose that the rest of this chapter were devoted to a mere enumeration of cases where it appears impossible to suggest the utility of certain specific characters, although such cases could be adduced by the thousand, how should I be met at the end of it all? Not by any one attempting to suggest the utility, past or present, of the characters named; but by being told that they must all present some _hidden_ use, must be _vestigial_, or else must be due to _correlation_. By appealing to one or other of these assumptions, our opponents are always able to escape the necessity of justifying their doctrine in the presence of otherwise inexplicable facts. No matter how many seemingly "indifferent characters" we may thus accumulate, Mr. Wallace and his followers will always throw upon us the impossible burden of proving the negative, that these apparently useless characters do _not_ present some hidden or former use, are _not_ due to correlation, and therefore have _not_ been produced by natural selection. It is in vain to retort that the burden of proof really lies the other way, or on the side of those who affirm that there is utility where no man can see it, or that there is correlation where no one can detect it. Thus, so far as any appeal to particular facts is concerned, it does not appear that there is any _modus vivendi_. Our opinions upon the question are really determined by the views which we severally take on matters of general principle. The issue, though it has a biological bearing, is a logical issue, not a biological one: it turns exclusively on those questions of definition and deduction with which we have just been dealing.

But although it thus follows that we cannot determine in fact what proportion of apparently useless characters are or are not really useful, we may very easily determine in fact what proportion of specific characters _fail to present any observable evidences of utility_. Yet, even upon this question of observable fact, it is surprising to note the divergent statements which have of late years been made by competent writers; statements in fact so divergent that they can only be explained by some want of sufficient thought on the part of those naturalists who are antecedently persuaded that all specific characters must be either directly or indirectly due to natural selection. Hence they fail to give to apparently useless specific characters the attention which, apart from any such antecedent persuasion, they deserve. For example, a few years ago I incidentally stated in a paper before the Linnaean Society, that "a large proportional number of specific characters" are of a trivial and apparently unmeaning kind, to which no function admits of being assigned, and also stated that Darwin himself had expressly given utterance to the same opinion. When these statements were made, I did not anticipate that they would be challenged by anybody, except perhaps, by Mr. Wallace. And, in order now to show that my innocence at that time was not due to ignorance of contemporary thought on such matters, a sentence may here be quoted from a paper which was read at the meeting of the British Association of the same year, by a highly competent systematic naturalist, Mr. Henry Seebohm, and soon afterwards extensively republished. Criticizing adversely my then recently published paper, he said:--

"I fully admit the truth of this statement; and I presume that few
naturalists would be prepared to deny that 'distinctions of
specific value frequently have reference to structures which are
without any utilitarian significance[88].'"

[88] _Geographical Distribution of the Family Charadriidae_, p. 19.

But since that time the course of Darwinian speculation has been greatly influenced by the writings of Weismann, who, among other respects in which he out-darwins Darwin, maintains the doctrine of utility as universal. In consequence of the influence which these writings have exercised, I have been more recently and extensively accused of "heresy" to Darwinian principles, for having stated that "a large proportional number of specific characters" do not admit of being proved useful, or correlated with other characters that are useful. Now, observe, we have here a simple question of fact. We are not at present concerned with the question how far the argument from ignorance may be held to apply in mitigation of such cases; but we are concerned only with the question of fact, as to what proportional number of cases actually occur where we are _unable to suggest_ the use of specific characters, or the useful characters with which these apparently useless ones are correlated. I maintain, as a matter of fact, that the cases in question embrace "a large proportional number of specific characters." On the other hand, I am accused of betraying ignorance of species, and of the work of "species-makers," in advancing this statement; and have been told by Mr. Wallace, and others of his school, that there is absolutely no evidence to be derived from nature in support of my views. Well, in the first place, if this be the case, it is somewhat remarkable that a large body of competent naturalists, such as Bronn, Broca, Naegeli, Kerner, Sachs, De Vries, Focke, Henslow, Haeckel, Koelliker, Eimer, Giard, Pascoe, Mivart, Seebohm, Lloyd Morgan, Dixon, Beddard, Geddes Gulick, and also, as we shall presently see, Darwin himself, should have fallen into the same error. And it is further remarkable that the more a man devotes himself to systematic work in any particular department--whether as an ornithologist, a conchologist, an entomologist, and so forth--the less is he disposed to accept the dogma of specific characters as universally adaptive characters. But, in the second place, and quitting considerations of mere authority, I appeal to the facts of nature themselves; and will now proceed, as briefly as possible, to indicate the result of such an appeal.

For the following reasons, that birds and mammals seem to furnish the best field for testing the question by direct observation. First, these classes present many genera which have been more carefully worked out than is usually the case with genera of invertebrates, or even of cold-blooded vertebrates. Secondly, they comprise many genera each including a large number of species, whose habits and conditions of life are better known than is the case with species belonging to large genera of other classes. Thirdly, as birds and mammals represent the highest products of evolution in respect of organization, a more severe test is imposed than could be imposed elsewhere, when the question is as to the utility of specific characters; for if these highest products of organization fail to reveal, in a large proportional number of cases, the utility of their specific characters, much more is this likely to be the case among organic beings which stand lower in the scale of organization, and therefore, _ex hypothesi_, are less elaborate products of natural selection. Fourthly, and lastly, birds and mammals are the classes which Mr. Wallace has expressly chosen to constitute his ground of argument with regard to the issue on which we are now engaged.

It would take far too long to show, even in epitome, the results of this inquiry. Therefore I will only state the general upshot. Choosing genera of birds and mammals which contain a large number of species whose diagnostic characters have been worked out with most completeness, I restricted the inquiry to specific distinctions of colour, not only for the sake of having a uniform basis for comparisons, but still more because it seemed that the argument from our ignorance of possibly unknown uses could be more successfully met in the case of slight differences of colour or of shading, than in that of any differences of structure or of form. Finally, after tabulating all the differences of colour which are given as diagnostic of each species in a genus, and placing in one column those which may conceivably be useful, while placing in another column those of which it appeared inconceivable that any use could be suggested, I added up the figures in the two columns, and thus obtained a grand total of all the specific characters of the genus in respect of colours, separated into the two classes of conceivably useful and apparently useless. Now, in all cases the apparently useless characters largely preponderated over the conceivably useful ones; and therefore I abundantly satisfied myself regarding the accuracy of my previous statement, that a large proportional number--if not an actual majority--of specific characters belong to the latter category.

The following is a brief abstract of these results.

With respect to Birds, a large number of cases were collected wherein the characters of allied species differ from one another in such minute respects of colour or shading, that it seemed unreasonable to suppose them due to any selective value to the birds in question. It is needless--even if it were practicable on the present occasion--to adduce this evidence in detail, since an exceedingly good sample of it may be found in a small book which is specially devoted to considering the question in its relation to birds. I allude to an essay by Mr. Charles Dixon, entitled _Evolution without Natural Selection_ (1885). In this work Mr. Dixon embodies the results of five years' "careful working at the geographical distribution and variations of plumage of Palaearctic birds and their allies in various other parts of the world"; and shows, by a large accumulation of facts, not only that there is no utility to be suggested in reference to the minute or trivial differences of colouration which he describes; but also that these differences are usually correlated with isolation on the one hand, or with slight differences of climate on the other. Now it will be shown later on that both these agents can be proved, by independent evidence, capable of inducing changes of specific type without reference to utility: therefore the correlation which Mr. Dixon unquestionably establishes between apparently useless (because utterly trivial) specific distinctions on the one hand, and isolation or climatic change on the other, constitutes additional evidence to show that the uselessness is not only apparent, but real. Moreover I have collected a number of cases where such minute differences of colour between allied species of birds happen to affect parts of the plumage which are _concealed_--as for instance, the breast and abdomen of creepers. In such cases it seems impossible to suggest how natural selection can have operated, seeing that the parts affected are not exposed to the view either of enemies or of prey.

Analogous illustrations to any amount may be drawn from Mammals. For instance, I have worked through the Marsupials with the aid of Mr. Oldfield Thomas' diagnostic description of their numerous species. Now, let us take any one of the genera, such as the kangaroos. This comprises 23 species living on an island continent of high antiquity, and not exposed to the depredations of any existing carnivorous enemies; so that there is here no present need to vary colour for purposes of protection. Moreover, in all cases the diagnostic distinctions of colour are so exceedingly trivial, that even if large carnivora were recently abundant in Australia, no one could reasonably suggest that the differences in question would then have been protective. On an average, each of the 23 species presents rather more than 20 peculiarities of shading, which are quoted as specifically diagnostic. Altogether there are 474 of these peculiarities distributed pretty evenly among the 23 species; and in no case can I conceive that utility can be suggested.

* * * * *

Hitherto we have been considering the question of fact, as to whether "a large proportional number of specific characters" do or do not admit of having their utility demonstrated, or even so much as plausibly suggested. In the result, I can only conclude that this question of fact is really not an open one, seeing that it admits of an abundantly conclusive answer by any naturalist who will take the trouble to work through the species of any considerable number of genera in the way above indicated. But although the question of fact is thus really closed, there remains a more ultimate question as to its theoretical interpretation. For, as already pointed out, no matter how great an accumulation of such facts may be collected, our opponents are always able to brush them aside by their _a priori_ appeal to the argument from ignorance. In effect they say--We do not care for any number of thousands of such facts; it makes no difference to us what "proportional number" of specific characters fail to show evidence of utility; you are merely beating the air by adducing them, for we are already persuaded, on antecedent grounds, that _all_ specific characters _must_ be either themselves useful, or correlated with others that are, whether or not we can perceive the utility, or suggest the correlation.

To this question of theoretical interpretation, therefore, we must next address ourselves. And here, first of all, I should like to point out how sturdy must be the antecedent conviction of our opponents, if they are to maintain it in the face of such facts as have just been adduced. It must be remembered that this antecedent conviction is of a most uncompromising kind. By its own premisses it is committed to the doctrine that _all_ specific characters, without a single exception, _must_ be either useful, vestigial, or correlated. Well, if such be the case, is it not somewhat astonishing that out of 474 differences of colour which are distinctive of the 23 species of the genus Macropus, no single one appears capable of having any utility demonstrated, or indeed so much as suggested? For even the recent theory that slight differences of colour, which cannot be conceived as serving any other purpose, may enable the sexes of the same species quickly to recognize each other, is not here available. The species of the genus Macropus are more conspicuously distinguished by differences of size and form than by these minute differences of colour; and therefore no such use can be attributed to the latter. And, as previously stated, even within the order Marsupialia the genus Macropus is not at all exceptional in this respect; so that by including other genera of the order it would be easy to gather such apparently indifferent specific characters by the hundred, without any one of them presenting evidence--or even suggestion--of utility. How robust therefore is the faith of an _a priori_ conviction which can stand against such facts as these! What, then, are the _a priori_ grounds on which it stands? Mr. Wallace, the great leader of this school of thought, says:--

"It is a necessary deduction from the theory of natural selection,
that none of the definite facts of organic nature, no special
organ, no characteristic form or marking, no peculiarities of
instinct or of habit, no relations between species or between
groups of species, can exist, but which must now be, or once have
been, _useful_ to the individuals or the races which possess
them[89]."

[89] _Contributions to the Theory of Natural Selection_, p. 47
(1870); republished in 1892.

Here, then, we have in brief compass the whole essence of our opponents' argument. It is confessedly an argument _a priori_, a deduction from the theory of natural selection, a supposed consequence of that theory which is alleged to be so necessary that to dispute the consequence is tantamount to denying the theory from which it is derived. In short, as before stated, it is a question of theory, not a question of fact: our difference of opinion is logical, not biological: it depends on our interpretation of principles, not on our observation of species. It will therefore be my endeavour to show that the reasoning in question is fallacious: that it is _not_ a necessary deduction from the theory of natural selection that no characteristic form or marking, no peculiarities of instinct or of habit, can exist, but which must now be, or once have been, useful, or correlated with some other peculiarity that is useful.

"The tuft of hair on the breast of a wild turkey-cock _cannot be of any use_, and it is doubtful whether it can be ornamental in the eyes of the female bird;--indeed, had the tuft appeared under domestication, it would have been called a monstrosity[90]."

[90] _Origin of Species_, p. 70: italics mine.

As a matter of common sense, unprejudiced by dogma, this appears to be a perfectly sound judgement; but if Wallace had asked Darwin to prove such a negative, Darwin could only have replied that it was for Wallace to prove the affirmative--and thus the issue would have been thrown back upon a discussion of general principles. Then Wallace would have said--"The assertion of inutility in the case of any organ or peculiarity which is not a rudiment or a correlation _is not, and can never be_, the statement of a fact, but _merely an expression of our ignorance of its purpose or origin_[91]." Darwin, however, would have replied:--"Our ignorance of the laws of variation is profound"; and while, on this account, we ought "to be extremely cautious in _pretending to decide what structures are now, or have formerly been, of use to each species_," in point of fact "there can be little doubt that the tendency to vary in the same manner has _often_ been so strong, that _all_ individuals of the same _species_ have been similarly modified _without the aid of any form of selection_[92]."

[91] _Darwinism_, p. 137: italics mine.

[92] _Origin of Species_, p. 72: Mr. Wallace himself quotes this
passage (_Darwinism_, p. 141); but says with regard to it "the
important word 'all' is probably an oversight." In the Appendix
(II), on Darwin's views touching the doctrine of utility I
adduce a number of precisely equivalent passages, derived from
all his different works on evolution, and _every one of them_
presenting "the important word 'all.'"

It will be my endeavour in the following discussion to show that Darwin would have had an immeasurable advantage in this imaginary debate.

To begin with, Wallace's deductive argument is a clear case of circular reasoning. We set out by inferring that natural selection is a cause from numberless cases of observed utility as an effect: yet, when "in a large proportional number" of cases we fail to perceive any imaginable utility, it is argued that nevertheless utility must be there, since otherwise natural selection could not have been the cause.

Be it observed, in any given case we may properly anticipate utility as _probable_, even where it is not perceived; because there are already so enormous a number of cases where it is perceived, that, if the principle of natural selection be accepted at all, we must conclude with Darwin that it is "the _main_ means of modification." Therefore, in particular cases of unperceived utility we may take this antecedent probability as a guide in our biological researches--as has been done with such brilliant success both by Darwin and Wallace, as well as by many of their followers. But this is a very different thing from laying down the universal maxim, that in _all_ cases utility _must_ be present, whether or not we shall ever be able to detect it[93]. For this universal maxim amounts to an assumption that natural selection has been the "_exclusive_ means of modification." That it has been "the main means of modification" is proved by the generality of the observed facts of adaptation. That it has been "the exclusive means of modification," with the result that these facts are universal, cannot be thus proved by observation. Why, then, is it alleged? Confessedly it is alleged by way of deduction from the theory of natural selection itself. Or, as above stated, after having deduced the theory from the facts, it is sought to deduce the facts from the theory.

[93] See Introductory Chapter, p. 20.

Thus far I have been endeavouring to show that the universality of adaptation cannot be inferred from its generality, or from the theory of natural selection itself. But, of course, the case would be quite different if there were any independent evidence--or rather, let us say, any logical argument--to show that natural selection is "the exclusive means of modification." For in this event it would no longer involve circular reasoning to maintain that all specific characters are likewise adaptive characters. It might indeed appear antecedently improbable that no other principle than natural selection can possibly have been concerned in the differentiation of those relatively permanent varieties which we call species--that in all the realm of organic nature, and in all the complexities of living processes, there is no room for any other influence in the production of change, even of the most trivial and apparently unmeaning kind. But if there were any good evidence or logical argument to the contrary, this antecedent presumption would have to give way; and the certainty that all specific characters are likewise adaptive characters would be determined by the cogency of such evidence or argument as could be adduced. In short, we are not entitled to conclude--and still less does it follow "as a necessary deduction from the theory of natural selection"--that all the details of specific differentiation must in every case be either useful, vestigial, or correlated, _unless it has been previously shown, by independent evidence, or accurate reasoning, that there is no room for any other principle of specific change_.

This, apparently, is the central core of the question. Therefore I will now proceed to consider such arguments as have been adduced to prove that, other than natural selection, there _can_ have been no "means of modification." And, after having exhibited the worthlessness of these arguments, I will devote the next chapter to showing that, as a matter of observable fact, there _are_ a considerable number of other principles, which can be proved to be capable of producing such minute differences of form and colour as "in a large proportional number" of cases constitute diagnostic distinctions between species and species.

First, then, for the reasons _a priori_--and they are confessedly _a priori_--which have been adduced to prove that natural selection has been what in Darwin's opinion it has not been,--"the _exclusive_ means of modification." Disregarding the Lamarckian factors--which, even if valid, have but little relation to the present question, seeing that they are concerned, almost exclusively, with the evolution of _adaptive_ characters--it is alleged that natural selection must occupy the whole field, because no other principle of change can be allowed to operate in the presence of natural selection. Now, I fully agree that this statement may hold as regards any principle of change which is deleterious; but clearly it does not hold as regards any principle which is merely neutral. If any one were to allege that specific characters are frequently detrimental to the species presenting them, he would no doubt lay himself open to the retort that natural selection could not allow such characters to persist; or, which amounts to the same thing, that it _does_ "necessarily follow from the theory of natural selection" that specific characters can never be in any large number, or in any large measure, _harmful_ to the species presenting them. But where the statement is that specific characters are frequently _indifferent_--again to use Professor Huxley's term--the retort loses all its relevancy. No reason has ever been shown why natural selection should interfere with merely indifferent characters, supposing such to have been produced by any of the agencies which we shall presently have to consider. Therefore this argument--or rather assertion--goes for nothing.

The only other argument I have met with on this side of the question is one that has recently been adduced by Mr. Wallace. He says:--

"One very weighty objection to the theory that _specific_
characters can ever be wholly useless appears to have been
overlooked by those who have maintained the frequency of such
characters, and that is, their almost necessary instability[94]."

[94] _Darwinism_, p. 138.

This argument he proceeds to elaborate at considerable length, but fails to perceive what appears to me the obvious answer. Provided that the cause of the useless character is constant, there is no difficulty in understanding why the character is stable. Utility is not the only principle that can lead to stability: any other principle must do the same, provided that it acts for a sufficient length of time, and with a sufficient degree of uniformity, on all the individuals of a species. This is a consideration the cogency of which was clearly recognized by Darwin, as the following quotations will show. Speaking of unadaptive characters, he says they may arise as merely

"fluctuating variations, which sooner or later become _constant_
through the nature of the organism and of surrounding conditions,
_but not through natural selection_[95]."

[95] _Origin of Species_, p. 176: italics mine, as also in the
following.

Elsewhere we read:--

"Each of the endless variations which we see in the plumage of our
fowls must have had some efficient cause; and if the _same_ cause
were to act _uniformly_ during a long series of generations on
_many_ individuals, _all_ probably would be modified in the same
manner."

As special illustrations of this fact I may quote the following cases from Darwin's works.

"Dr. Bachman states that he has seen turkeys raised from the eggs
of wild species, lose their metallic tints, and become spotted in
the third generation. Mr. Yarrell many years ago informed me that
the wild ducks bred in St James' Park lost their true plumage
after a few generations. An excellent observer (Mr. Hewitt) ...
found that he could not breed wild ducks true for more than five or
six generations, as they proved so much less beautiful. The white
collar round the neck of the mallard became broader and more
irregular, and white feathers appeared in the duckling's wings
&c.[96]"

[96] _Var._ vol. ii. p. 250.

Now, such cases--to which numberless others might be added--prove that even the subtle and inconspicuous causes incidental to domestication are capable of inducing changes of specific character quite as great, and quite as "stable," as any that in a state of nature are taken to constitute specific distinctions. Yet there can here be no suggestion of utility, inasmuch as the change takes place in the course of a few generations, and therefore without leaving time for natural selection to come into play--even if it ever could come into play among the sundry domesticated birds in question.

But the facts of domestication also make for the same conclusion in another way--namely, by proving that when time enough _has_ been allowed for the production of useless changes of greater magnitude, such changes are not infrequently produced. And the value of this line of evidence is that, great as are the changes, it is impossible that either natural or artificial selection can have been concerned in their production. It will be sufficient to give two examples--both with regard to structure.

The first I will render in the words whereby it has already been stated in my own paper on _Physiological Selection_, because I should like to take this opportunity of answering Mr. Wallace's objection to it.

"Elsewhere (_Origin of Species_, p. 158) Mr. Darwin points out that
modifications which appear to present obvious utility are often
found on further examination to be really useless. This latter
consideration, therefore, may be said to act as a foil to the one
against which I am arguing, namely, that modifications which appear
to be useless may nevertheless be useful. But here is a still more
suggestive consideration, also derived from Mr. Darwin's writings.
Among our domesticated productions changes of structure--or even
structures wholly new--not unfrequently arise, which are in every
way analogous to the apparently useless distinctions between wild
species. Take, for example, the following most instructive case:--

"'Another curious anomaly is offered by the appendages described by
M. Eudes-Deslongchamps as often characterizing the Normandy pigs.
These appendages are always attached to the same spot, to the
corners of the jaws; they are cylindrical, about three inches in
length, covered with bristles, and with a pencil of bristles rising
out of a sinus on one side; they have a cartilaginous centre with
two small longitudinal muscles; they occur either symmetrically on
both sides of the face, or on one side alone. Richardson figures
them on the gaunt old Irish Greyhound pig; and Nathusius states
that they occasionally appear in all the long-eared races, but are
not strictly inherited, for they occur or fail in the animals of
the same litter. As no wild pigs are known to have analogous
appendages, we have at present no reason to suppose that their
appearance is due to reversion; and if this be so, we are forced to
admit that a somewhat complex, though apparently useless, structure
may be suddenly developed without the aid of selection[97].'"

[97] _Variation_, &c. vol. i. pp. 78-79.

To this case Mr. Wallace objects:--

"But it is expressly stated that they are not constant; they appear
'frequently' or 'occasionally,' they are 'not strictly inherited,
for they occur or fail in animals of the same litter'; and they are
not always symmetrical, sometimes appearing on one side of the face
alone. Now, whatever may be the cause or explanation of these
anomalous appendages, they cannot be classed with 'specific
characters,' the most essential features of which are, that they
_are_ symmetrical, that they _are_ inherited, and that they _are_
constant[98]."

[98] _Darwinism_, pp. 139-40.

But, to begin with, I have not classed these appendages with "specific characters," nor maintained that Normandy pigs ought to be regarded as specifically distinct on account of them. What I said was:--

"Now, if any such structure as this occurred in a wild species, and
if any one were to ask what is the use of it, those who rely on the
argument from ignorance would have a much stronger case than they
usually have; for they might point to the cartilage supplied with
muscles, and supporting a curious arrangement of bristles, as much
too specialized a structure to be wholly meaningless. Yet we happen
to know that this particular structure is wholly meaningless[99]."

[99] Mr. Wallace deems the concluding words "rather confident." I
was not, however, before aware that he extended his _a priori_
views on utility to domesticated varieties which are bred for
the slaughter-house. If he now means to indicate that these
appendages are possibly due to natural selection, he is surely
going very far to save his _a priori_ dogma; and in the case
next adduced will have to go further still.

In the next place, is it either fair or reasonable to expect that a varietal character of presumably very recent origin should be as strongly inherited--and therefore as constant both in occurrence and symmetry--as a true specific character, say, of a thousand times its age? Even characters of so-called "constant varieties" in a state of nature are usually less constant than specific characters; while, again, as Darwin says, "it is notorious that specific characters are more variable than generic,"--the reason in both cases being, as he proceeds to show, that the less constant characters are characters of more recent origin, and therefore less firmly fixed by heredity[100]. Hence I do not understand how Mr. Wallace can conclude, as he does, "that, admitting that this peculiar appendage is wholly useless and meaningless, the fact would be rather an argument against specific characters being also meaningless, because the latter never have the characteristics [i.e. inconstancy of occurrence, form, and transmission] which this particular variation possesses[101]." Mr. Wallace can scarcely suppose that when specific characters first arise, they present the three-fold kind of constancy to which he here alludes. But, if not, can it be denied that these peculiar appendages appear to be passing through a phase of development which all "specific characters" must have passed through, before they have had time enough to be firmly fixed by heredity[102]?

[100] _Origin of Species_, pp. 122-3.

[101] _Darwinism_, p. 140.

[102] In the next paragraph Mr. Wallace says that the appendages in
question "are apparently of the same nature as the 'sports'
that arise in our domesticated productions, but which, as Mr.
Darwin says, without the aid of selection would soon
disappear." But I cannot find that Mr. Darwin has made any
such statement: what he does say is, that whether or not a
useless peculiarity will soon disappear without the aid of
selection depends upon the nature of the causes which produce
it. If these causes are of a merely transitory nature, the
peculiarity will also be transitory; but if the causes be
constant, so will be the result. Again, the point to be
noticed about this "sport" is, that, unlike what is usually
understood by a "sport," it affects a whole race or breed, is
transmitted by sexual propagation, and has already attained so
definite a size and structure, that it can only be reasonably
accounted for by supposing the continued operation of _some
constant_ cause. This cause can scarcely be correlation of
growth, since closely similar appendages are often seen in so
different an animal as a goat. Here, also, they run in breeds
or strains, are strongly inherited, and more "constant," as
well as more "symmetrical" than they are in pigs. This, at all
events, is the account I have received of them from
goat-breeders in Switzerland.

If, however, even this should be denied, what will be said of the second case, that of the niata cattle?

"I saw two herds on the northern bank of the Plata.... The forehead
is very short and broad, with the nasal end of the skull, together
with the whole plane of the upper molar-teeth, curved upwards. The
lower jaw projects beyond the upper, and has a corresponding upward
curvature.... The skull which I presented to the College of
Surgeons has been thus described by Professor Owen. 'It is
remarkable from the stunted development of the nasals,
premaxillaries, and fore part of the lower jaw, which is unusually
curved upwards to come into contact with the premaxillaries. The
nasal bones are about one-third the ordinary length, but retain
almost their normal breadth. The triangular vacuity is left between
them and the frontal and lachrymal, which latter bone articulates
with the premaxillary, and thus excludes the maxillary from any
junction with the nasal.' So that even the connexion of some of
the bones is changed. Other differences might be added: thus the
plane of the condyles is somewhat modified, and the terminal edge
of the premaxillaries forms an arch. In fact, on comparison with
the skull of a common ox, scarcely a single bone presents the same
exact shape, and the whole skull has a wonderfully different
appearance[103]."

[103] Darwin, _Variation_, &c., vol. i. pp. 92-4.

As I cannot find that this remarkable skull has been figured before, I have had the accompanying woodcut made in order to compare it with the skull of a Charsley Forest ox; and a glance is sufficient to show what "a wonderfully different appearance" it presents.

* * * * *

Now the important points in the present connexion with regard to this peculiar race of cattle are the following.

Their origin is not known; but it must have been subsequent to the year 1552, when cattle were first introduced to America from Europe, and it is known that such cattle have been in existence for at least a century. The breed is very true, and a niata bull and cow invariably produce niata calves. A niata bull crossed with a common cow, and the reverse cross, yield offspring having an intermediate character, but with the niata peculiarities highly conspicuous[104].

[104] _Ibid._ p. 94.

Here, then, we have unquestionable evidence of a whole congeries of very distinctive characters, so unlike anything that occurs in any other cattle, that, had they been found in a state of nature, they would have been regarded as a distinct species. And the highly peculiar characters which they present conform to all "the most essential features of specific characters," as these are stated by Mr. Wallace in his objection to the case of the pig's appendages. That is to say, "they _are_ symmetrical, they _are_ inherited, and they _are_ constant." In point of fact, they are _always_ "constant," both as to occurrence and symmetry, while they are so completely "inherited" that not only does "a niata bull and cow _invariably_ produce niata calves"; but even when crossed with other cattle the result is a _hybrid_, "with the niata character _strongly_ displayed."

Hence, if we were to follow Mr. Wallace's criteria of specific characters, which show that the pig's appendages "cannot be classed with specific characters" (or with anything of the nature of specific characters), it would follow that the niata peculiarities _can_ be so classed. This, therefore, is a case where he will find all the reasons which in other cases he takes to justify him in falling back upon the argument from ignorance. The cattle are half wild, he may urge; and so the three-fold constancy of their peculiar characters may very well be due, either directly or indirectly, to natural selection--i.e. they may either be of some hidden use themselves, or correlated with some other modifications that are of use: it is, he may say, as in such cases he often does say, for us to disprove both these possibilities.

Well, here we have one of those rare cases where historical information, or other accidents, admit of our discharging this burden of proving a negative. Darwin's further description shows that this customary refuge in the argument from ignorance is most effectually closed. For--

"When the pasture is tolerably long, these cattle feed as well as
common cattle with their tongue and palate; but during the great
droughts, when so many animals perish on the Pampas, the niata
breed lies under a great disadvantage, and would, if not attended
to, become extinct; for the common cattle, like horses, are able to
keep alive by browsing with their lips on the twigs of trees and on
reeds; this the niatas cannot so well do, as their lips do not
join, and hence they are found to perish before the common cattle.
This strikes me as a good illustration of how little we are able to
judge from the ordinary habits of an animal, on what circumstances,
occurring only at long intervals of time, its rarity or extinction
may depend. It shows us, also, how natural selection would have
determined the rejection of the niata modification, had it arisen
in a state of nature[105]."

[105] Darwin, _Variation_, &c. vol. i. p. 94.

Hence, it is plainly _impossible_ to attribute this modification to natural selection, either as acting directly on the modified parts themselves, or indirectly through correlation of growth. And as the modification is of specific magnitude on the one hand, while it presents all "the most essential features of specific characters" on the other, I do not see any means whereby Mr. Wallace can meet it on his _a priori_ principles. It would be useless to answer that these characters, although conforming to all his tests of specific characters, differ in respect of being deleterious, and would therefore lead to extermination were the animals in a wholly wild state; because, considered as an argument, this would involve the assumption that, apart from natural selection, only deleterious characters can arise under nature--i. e. that merely "indifferent" characters can never do so, which would be absurd. Indeed, I have chosen this case of the niata cattle expressly because their strongly marked peculiarities _are_ deleterious, and therefore exclude Mr. Wallace's appeal to the argument from ignorance of a possible utility. But if even these pronounced and deleterious peculiarities can arise and be perpetuated with such constancy and fidelity, much more is this likely to be the case with less pronounced and merely neutral peculiarities.

It may, however, be further objected that these cattle are not improbably the result of _artificial_ selection. It may be suggested that the semi-monstrous breed originated in a single congenital variation, or "sport," which was isolated and multiplied as a curiosity by the early settlers. But even if such be the explanation of this particular case, the fact would not weaken our illustration. On the contrary, it would strengthen our general argument, by showing an additional means whereby indifferent specific characters can arise and become fixed in a state of nature. As it seems to me extremely probable that the niata cattle did originate in a congenital monstrosity, which was then isolated and multiplied by human agency (as is known to have been the case with the "ancon sheep"), I will explain why this tends to strengthen our general argument.

It is certain that if these animals were ever subject to artificial isolation for the purpose of establishing their breed, the process must have ceased a long time ago, seeing that there is no memory or tradition of its occurrence. Now this proves that, however the breed may have originated, it has been able to maintain its many and highly peculiar characters for a number of generations without the help of selection, either natural or artificial. This is the first point to be clear upon. Be its origin what it may, we know that this breed has proved capable of perpetuating itself with uniform "constancy" for a number of generations after the artificial selection has ceased--supposing such a process ever to have occurred. And this certain fact that artificial selection, even if it was originally needed to establish the type, has not been needed to perpetuate the type, is a full answer to the supposed objection. For, in view of this fact, it is immaterial what the origin of the niata breed may have been. In the present connexion, the importance of this breed consists in its proving the subsequent "stability" of an almost monstrous form, continued through a long series of generations by the force of heredity alone, without the aid of any form of selection.

The next point is, that not only is a seeming objection to the illustration thus removed, but that, if we do entertain the question of origin, and if we do suppose the origin of these cattle to have been in a congenital "sport," afterwards multiplied by artificial isolation, we actually strengthen our general argument by increasing the importance of this particular illustration. For the illustration then becomes available to show how indifferent specific characters may sometimes originate in merely individual sports, which, if not immediately extinguished by free intercrossing, will perpetuate themselves by the unaided force of heredity. But this is a point to which we shall recur in the ensuing chapter.

In conclusion, it is worth while to remark, with regard to Mr. Wallace's argument from constancy, that, as a matter of fact, utility does not seem to present any greater power in securing "stability of characters" than any other cause of like constancy. Thus, for instance, whatever the causes may have been which have produced and perpetuated the niata breed of cattle, they have certainly produced a wonderful "stability" of a great modification in a wonderfully short time. And the same has to be said of the ducks in St. James' Park, as well as sundry other cases. On the other hand, when, as in the case of numberless natural species, modification has been undoubtedly produced by natural selection, although the modification must have had a very much longer time in which to have been fixed by heredity, it is often far from being stable--notwithstanding that Mr. Wallace regards stability as a criterion of specific characters. Indeed--and this is more suggestive still--there even seems to be a kind of _inverse_ proportion between the utility and the stability of a specific character. The explanation appears to be (_Origin of Species_, pp. 120-2), that the more a specific character has been forced on by natural selection on account of its utility, the less time will it have had to become well fixed by heredity before attaining a full development. Moreover, as Darwin adds, in cases where the modification has not only been thus "comparatively recent," but also "extraordinarily great," the probability is that the parts so modified must have been very variable in the first instance, and so are all the more difficult to render constant by heredity. Thus we see that utility is no better--even if it be so good--a cause of stability in specific characters, as are the unknown causes of stability in many varietal characters[106].

[106] Should it be objected that useless characters, according to my
own view of the Cessation of Selection, ought to disappear,
and therefore cannot be constant, the answer is evident. For,
by hypothesis, it is only those useless characters which were
at one time useful that disappear under this principle.
Selection cannot cease unless it was previously present--i.e.
save in cases where the now useless character was originally
due to selection. Hence, in all cases where it was due to any
other cause, the useless character will persist at least as
long as its originating cause continues to operate. And even
after the latter (whatever it may be) has ceased to operate,
the useless character will but slowly degenerate, until the
eventual failure of heredity causes it to disappear _in
toto_--long before which time it may very well have become a
genetic, or some higher, character.