Volume I Part 17 (1/2)

In the first place, Mr. Wallace fails to distinguish between brilliancy and ornamentation--or between colour as merely ”heightened,” and as distinctively decorative. Yet there is obviously the greatest possible difference between these two things. We may readily enough admit that a mere heightening of already existing coloration is likely enough--at all events in many cases--to accompany a general increase of vigour, and therefore that natural selection, by promoting the latter, may also incidentally promote the former, in cases where brilliancy is not a source of danger. But clearly this is a widely different thing from showing that not only a _general brilliancy of colour_, but also _the particular disposition of colours_, in the form of ornamental patterns, can thus be accounted for by natural selection. Indeed, it is expressly in order to account for the occurrence of such ornamental patterns that Mr. Darwin constructed his theory of s.e.xual selection; and therefore, by thus virtually ignoring the only facts which that theory endeavours to explain, Mr. Wallace is not really criticizing the theory at all. By representing that the theory has to do only with brilliancy of colour, as distinguished from disposition of colours, he is going off upon a false issue which has never really been raised[48]. Look, for example, at a peac.o.c.k's tail. No doubt it is sufficiently brilliant; but far more remarkable than its brilliancy is its elaborate pattern on the one hand, and its enormous size on the other. There is no conceivable reason why mere _brilliancy of colour_, as an accidental concomitant of general vigour, should have run into so extraordinary, so elaborate, and so beautiful a _design of colours_. Moreover, this design is only unfolded when the tail is erected, and the tail is not erected in battle (as Mr.

Wallace's theory of the erectile function in feathers would require), but in courts.h.i.+p; obviously, therefore, the purpose of the pattern, so to speak, is correlated with the act of courts.h.i.+p--it being only then, in fact, that the general purpose of the whole structure, as well as the more special purpose of the pattern, becomes revealed. Lastly, the fact of this whole structure being so large, entailing not only a great amount of physiological material in its production, but also of physiological energy in carrying about such a weight, as well as of increased danger from impeding locomotion and inviting capture--all this is obviously incompatible with the supposition of the peac.o.c.k's tail having been produced by natural selection. And such a case does not stand alone. There are mult.i.tudes of other instances of ornamental structures imposing a drain upon the vital energies of their possessors, without conferring any compensating benefit from a utilitarian point of view. Now, in all these cases, without any exception, such structures are ornamental structures which present a plain and obvious reference to the relations.h.i.+p of the s.e.xes. Therefore it becomes almost impossible to doubt--first, that they exist for the sake of ornament; and next, that the ornament exists on account of that relations.h.i.+p. If such structures were due merely to a superabundance of energy, as Mr. Wallace supposes, not only ought they to have been kept down by the economizing influence of natural selection; but we can see no reason, either why they should be so highly ornamental on the one hand, or so exclusively related to the s.e.xual relations.h.i.+p on the other.

[48] Note C.

Finally, we must take notice of the fact that where peculiar _structures_ are concerned for purposes of display in courts.h.i.+p, the _elaboration_ of these structures is often no less remarkable than that of patterns where colours are thus concerned. Take, for example, the case of the Bell-bird, which I select from an innumerable number of instances that might be mentioned because, while giving a verbal description of this animal, Darwin does not supply a pictorial representation thereof. The bird, which lives in South America, has a very loud and peculiar call, that can be heard at a distance of two or three miles. The female is dusky-green; but the adult male is a beautiful white, excepting the extraordinary structure with which we are at present concerned. This is a tube about three inches long, which rises from the base of the beak. It is jet black, and dotted over with small downy feathers. The tube is closed at the top, but its cavity communicates with the palate, and thus the whole admits of being inflated from within, when, of course, it stands erect as represented in one of the two drawings. When not thus inflated, it hangs down, as shown in the second figure, which represents the plumage of a young male. (Fig. 124.)

[Ill.u.s.tration: FIG. 124.--The Bell-bird (_Chasmorhynchus niveus_, 1/4 natural size). Drawn from nature (_R. Coll. Surg. Mus._). In the drawing of the adult male the ornamental appendage is represented in its inflated condition, during courts.h.i.+p; in the drawing of the young male it is shown in its flaccid condition.]

In another species of the genus there are three of these appendages--the two additional ones being mounted on the corners of the mouth. (Fig.

125.) In all species of the genus (four in number) the tubes are inflated during courts.h.i.+p, and therefore perform the function of s.e.xual embellishments. Now the point to which I wish to draw attention is, that so specialized and morphologically elaborate a structure cannot be regarded as merely advent.i.tious. It must have been developed by some definite cause, acting through a long series of generations. And as no other function can be a.s.signed to it than that of charming the female when it is erected in courts.h.i.+p, the peculiarity of form and mechanism which it presents--like the elaboration of patterns in cases where colour only is concerned--virtually compels us to recognise in s.e.xual selection the only conceivable cause of its production.

[Ill.u.s.tration: FIG. 125.--_C. tricarunculatus_, 1/4 natural size.

Copied from the _Ibis_. The ornamental appendages of the male are represented in a partly inflated condition.]

For these reasons I think that Mr. Wallace's main objection falls to the ground. Pa.s.sing on to his subsidiary objections, I do not see much weight in his merely negative difficulty as to there being an absence of evidence upon hen birds being charmed by the plumage, or the voice, of their consorts. For, on the one hand, it is not very safe to infer what sentiments may be in the mind of a hen; and, on the other hand, it is impossible to conceive what motive can be in the mind of a c.o.c.k, other than that of making himself attractive, when he performs his various antics, displays his ornamental plumes, or sings his melodious songs.

Considerations somewhat a.n.a.logous apply to the difficulty of supposing so much similarity and constancy of taste on the part of female animals as Mr. Darwin's theory undoubtedly requires. Although we know very little about the psychology of the lower animals, we do observe in many cases that small details of mental organization are often wonderfully constant and uniform throughout all members of a species, even where it is impossible to suggest any utility as a cause.

Again, as regards the objection that each bird finds a mate under any circ.u.mstances, we have here an obvious begging of the whole question.

That every feathered Jack should find a feathered Jill is perhaps what we might have antecedently expected; but when we meet with innumerable instances of ornamental plumes, melodious songs, and the rest, as so many witnesses to a process of s.e.xual selection having always been in operation, it becomes irrational to exclude such evidence on account of our antecedent prepossessions.

There remains the objection that the principles of natural selection must necessarily swallow up those of s.e.xual selection. And this consideration, I doubt not, lies at the root of all Mr. Wallace's opposition to the supplementary theory of s.e.xual selection. He is self-consistent in refusing to entertain the evidence of s.e.xual selection, on the ground of his antecedent persuasion that in the great drama of evolution there is no possible standing-ground for any other actor than that which appears in the person of natural selection. But here, again, we must refuse to allow any merely antecedent presumption to blind our eyes to the actual evidence of other agencies having co-operated with natural selection in producing the observed results.

And, as regards the particular case now before us, I think I have shown, as far as s.p.a.ce will permit, that in the phenomena of decorative colouring (as distinguished from merely brilliant colouring), of melodious song (as distinguished from merely tuneless cries), of enormous arborescent antlers (as distinguished from merely offensive weapons), and so forth--I say that in all these phenomena we have phenomena which cannot possibly be explained by the theory of natural selection; and, further, that if they are to be explained at all, this can only be done, so far as we can at present see, by Mr. Darwin's supplementary theory of s.e.xual selection.

I have now briefly answered all Mr. Wallace's objections to this supplementary theory, and, as previously remarked, I feel pretty confident that, at all events in the main, the answer is such as Mr.

Darwin would himself have supplied, had there been a third edition of his work upon the subject. At all events, be this as it may, we are happily in possession of unquestionable evidence that he believed all Mr. Wallace's objections to admit of fully satisfactory answers. For his very last words to science--read only a few hours before his death at a meeting of the Zoological Society--were:

I may perhaps be here permitted to say that, after having carefully weighed, to the best of my ability, the various arguments which have been advanced against the principle of s.e.xual selection, I remain firmly convinced of its truth[49].

[49] Since the above exposition of the theory of s.e.xual selection was written, Mr. Poulton has published his work on the _Colours of Animals_. He there reproduces some of the ill.u.s.trations which occur in Mr. and Mrs. Peckham's work on _s.e.xual Selection in Spiders_, and furnishes appropriate descriptions. Therefore, while retaining the ill.u.s.trations, I have withdrawn my own descriptions.

Mr. Poulton has also in his book supplied a _resume_ of the arguments for and against the theory of s.e.xual selection in general.

Of course in nearly all respects this corresponds with the _resume_ which is given in the foregoing pages; but I have left the latter as it was originally written, because all the critical part is reproduced _verbatim_ from a review of Mr. Wallace's _Darwinism_, of a date still earlier than that of Mr. Poulton's book--viz.

_Contemporary Review_, August, 1889.

_Concluding Remarks._

I will now conclude this chapter, and with it the present volume, by offering a few general remarks on what may be termed the philosophical relations of Darwinian doctrine to the facts of adaptation on the one hand, and to those of beauty on the other. Of course we are all aware that before the days of this doctrine the facts of adaptation in organic nature were taken to const.i.tute the clearest possible evidence of special design, on account of the wonderful mechanisms which they everywhere displayed; while the facts of beauty were taken as const.i.tuting no less conclusive evidence of the quality of such special design as beneficent, not to say artistic. But now that the Darwinian doctrine appears to have explained scientifically the former cla.s.s of facts by its theory of natural selection, and the latter cla.s.s of facts by its theory of s.e.xual selection, we may fitly conclude this brief exposition of the doctrine as a whole by considering what influence such naturalistic explanations may fairly be taken to exercise upon the older, or super-naturalistic, interpretations.

To begin with the facts of adaptation, we must first of all observe that the Darwinian doctrine is immediately concerned with these facts only in so far as they occur in organic nature. With the adaptations--if they can properly be so called--which occur in all the rest of nature, and which go to const.i.tute the Cosmos as a whole so wondrous a spectacle of universal law and perfect order, this doctrine is but indirectly concerned. Nevertheless, it is of course fundamentally concerned with them to the extent that it seeks to bring the phenomena of organic nature into line with those of inorganic; and therefore to show that whatever view we may severally take as to the kind of causation which is energizing in the latter we must now extend to the former. This is usually expressed by saying that the theory of evolution by natural selection is a mechanical theory. It endeavours to comprise all the facts of adaptation in organic nature under the same category of explanation as those which occur in inorganic nature--that is to say, under the category of physical, or ascertainable, causation. Indeed, unless the theory has succeeded in doing this, it has not succeeded in doing anything--beyond making a great noise in the world. If Mr. Darwin has not discovered a new mechanical cause in the selection principle, his labour has been worse than in vain.

Now, without unduly repeating what has already been said in Chapter VIII, I may remark that, whatever we may each think of the measure of success which has thus far attended the theory of natural selection in explaining the facts of adaptation, we ought all to agree that, considered as a matter of general reasoning, the theory does certainly refer to a _vera causa_ of a strictly physical kind; and, therefore, that no exception can be taken to the theory in this respect on grounds of _logic_. If the theory in this respect is to be attacked at all, it can only be on grounds of _fact_--namely, by arguing that the cause does not occur in nature, or that, if it does, its importance has been exaggerated by the theory. Even, however, if the latter proposition should ever be proved, we may now be virtually certain that the only result would be the relegation of all the residual phenomena of adaptation to other causes of the physical order--whether known or unknown. Hence, as far as the matter of _principle_ is concerned, we may definitely conclude that the great naturalistic movement of our century has already brought all the phenomena of adaptation in organic nature under precisely the same category of mechanical causation, as similar movements in previous centuries have brought all the known phenomena of inorganic nature: the only question that remains for solution is the strictly _scientific_ question touching the particular causes of the mechanical order which have been at work.

So much, then, for the phenomena of adaptation. Turning next to those of beauty, we have already seen that the theory of s.e.xual selection stands to these in precisely the same relation as the theory of natural selection does to those of adaptation. In other words, it supplies a physical explanation of them; because, as far as our present purposes are concerned, it may be taken for granted, or for the sake of argument, that inasmuch as psychological elements enter into the question the cerebral basis which they demand involves a physical side.