Darwin, and After Darwin Volume Ii Part 9

SECTION II

_UTILITY_

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 princ.i.p.al function of Darwin's work was held to be what the t.i.tle 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 princ.i.p.al 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 const.i.tuting it the t.i.tle 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, cla.s.ses, 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 cla.s.sify as species are nothing more than p.r.o.nounced--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 c.u.mulative 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 const.i.tute 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 cla.s.ses, 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--a.s.suredly natural selection is a theory of the origin (i.e. c.u.mulative development) of adaptations; and, no less a.s.suredly, 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 cla.s.s 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 cla.s.s. So it is with other cla.s.s 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 advent.i.tious 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 pa.s.s 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 const.i.tute 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 a.s.sumptions, 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 acc.u.mulate, 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 a.s.signed, and also stated that Darwin himself had expressly given utterance to the same opinion. When these statements were made, I did not antic.i.p.ate 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 a.s.sociation 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, Nageli, Kerner, Sachs, De Vries, Focke, Henslow, Haeckel, Kolliker, 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 cla.s.ses 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 cla.s.ses. 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 cla.s.ses which Mr. Wallace has expressly chosen to const.i.tute 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 cla.s.ses 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, ent.i.tled _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 acc.u.mulation 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, const.i.tutes 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.

a.n.a.logous ill.u.s.trations 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 acc.u.mulation 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 st.u.r.dy 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 astonis.h.i.+ng 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 s.e.xes 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.