August 30, 1889.] 



SCIENCE. 



153 



of sexual selection lapses altogether, and becomes but a special 

 case of natural selection. 



Once more, Mr. Wallace argues that the evidence collected by 

 Mr. Darwin himself proves that each bird finds a mate under any 

 circumstances, — a general fact which in itself must quite neutral- 

 ize any effect of sexual selection of color or ornament, since the less 

 highly colored birds would be at no disadvantage as regards the 

 leaving of healthy progeny. 



Lastly, he urges the high improbability that through thousands 

 of generations all the females of any particular species — possibly 

 spread over an enormous area — should uniformly and always have 

 displayed exactly the same taste with respect to every detail of 

 color to be presented by the males. 



Now, without any question, we have here a most powerful array 

 of objections against the theory of sexual selection. Each of them 

 is ably developed by Mr. Wallace himself in his work on tropical 

 nature; and although I have here space only to state them in the 

 most abbreviated of possible forms, I think it will be apparent how 

 formidable these objections appear. Unfortunately the work in 

 which they are mainly presented was published several years after 

 the second edition of the " Descent of Man," so that Mr. Darwin 

 never had a suitable opportunity of replying. But, if he had had 

 such an opportunity, as far as I can judge, it seems that his reply 

 would have been more or less as follows : — 



In the first place, Mr. Wallace fails to distinguish between brill- 

 iancy and ornamentation — or between color as merely " height- 

 ened," 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 ac- 

 company a general increase of vigor, 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 color, but also the particular disposition of 

 colors 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 sexual selection ; and therefore, by thus 

 virtually ignoring the only facts which that theory endeavors to ex- 

 plain, Mr. Wallace is not really criticizing the theory at all. By 

 representing that the theory has to do only with brilliancy of color, 

 as distinguished from disposition of colors, he is going off upon a 

 false issue which has never really been raised. Look, for example, 

 at a peacock'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 con- 

 ceivable reason why mere brilliancy of color, as an accidental con- 

 comitant of general vigor, should have run into so extraordinary, 

 so elaborate, and so beautiful a pattern of colors. Moreover, this 

 pattern 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 courtship. Obviously, therefore, 

 the design of the pattern, so to speak, is correlated with the act of 

 courtship, — it being only then, in fact, that the general design of 

 the whole structure, as well as the more special design of the pat- 

 tern, becomes revealed. Lastly, the fact of this whole structure 

 being so large, entailing not only a great amount of physiological 

 material in its production, btit also of physiological energy in carry- 

 ing about such a weight, as well as of increased danger from im- 

 peding locomotion and inviting capture, — all this is obviously in- 

 compatible with the supposition of the peacock's tail having been 

 produced by natural selection. 



And such a case does not stand alone. There are multitudes of 

 other instances of ornamental structures imposing a drain upon the 

 vital energies of their possessors, without conferring any compen- 

 sating benefit from a utilitarian point of view. Now, in all these 

 cases, without any exception, such structures are ornamental struc- 

 tures which present a plain and obvious reference to the relation- 

 ship of the sexes. Therefore it becomes almost impossible to doubt, 

 first, that they exist for the sake of urnament, and next, that the 

 ornament e.xists on account of that relationship. 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 rea- 

 son, either why they should be so highly ornamental, on the one 

 hand, or so exclusively connected with the sexual relationship, on 

 the other. 



For these reasons I think that Mr. Wallace's main objection 

 falls to the ground. Passing 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 cock, other than that of making 

 himself attractive, when he performs his various antics, displays 

 his ornamental plumes, or sings his melodious songs. Considera- 

 tions somewhat analogous apply to the difficulty of supposing so 

 much similarity and constancy of taste on the part of female ani- 

 mals 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 mem- 

 bers 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 un- 

 der any circumstances, 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 sexual 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 selec- 

 tion must necessarily swallow up those of sexual selection, as the 

 fat kine swallowed up the lean in the dream of Pharaoh. And this 

 consideration, I doubt not, lies at the root of all Mr. Wallace's op- 

 position to the supplementary theory of sexual selection. He is 

 self-consistent in refusing to entertain the evidence of sexual selec- 

 tion, 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 selec- 

 tion. But here, again, we must refuse to allow any merely ante- 

 cedent presumption to blind our eyes to the actual evidence of 

 other agencies having co-operated with natural selection in produc- 

 ing the observed results. And, as regards the particular case now 

 before us, I think I have shown, as far as space will permit, that in 

 the phenomena of decorative coloring, as distinguished from merely 

 brilliant coloring, 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 sexual 

 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 be- 

 lieved 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 sexual selection, 

 I remain firmly convinced of its truth." 



Inherited Effects of Use, Disuse, and Direct Action of 

 Environment. 

 We have just seen that one of Mr. Wallace's strongest argu- 

 ments against sexual selection consists in representing a priori 



