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remarkable fact of the display by the male of each species of its peculiar beauties of plumage and colour,— a display which Mr. Darwin evidently considers his strongest argument in favour of conscious selection by the female. This display is, no doubt, a very interesting and important phenomenon ; but it may, I believe, be satisfactorily explained on the general principles here laid down, without calling to our aid a purely hypothetical choice exerted by the female bird.
At pairing-time, the male is in a state of excitement, and full of exuberant energy. Even unornamental birds flutter their wings or spread them out, erect their tails or crests, and thus give vent to the nervous excitability with which they are overcharged. It is not improbable that crests and other erectile feathers may be primarily of use in frightening away enemies, since they are generally erected when angry or during combat. Those individuals who were most pugnacious and defiant, and who brought these crertile plumes most frequently and most powerfully into action, would tend to increase them by use, and to leave them further developed in some of their descendants. If, in the course of this development, colour appeared—and we have already shown that such developments of plumage are a very probable cause of colour-we have every reason to believe it would be most vivid in these most pugnacious and energetic individuals; and as these would always have the advantage in the rivalry for mates (to which advantage the excess of colour and plumage might sometimes conduce), there seems nothing to prevent a progressive development of these ornaments in all clominant ruces; that is, wherever there was such a supilus of vitality, and such complete adaption to conditions, that the inconvenience or danger produced by such ornaments was so comparatively small as not to affect the superiority of the race over its nearest allies.
But if those portions of the plumage, which were originally crected under the influence of anger or fear, became largely developed and brightly coloured, the actual display, under the influence of jealousy or sexual excitement becomes quite intelligible. The males, in their rivalry with each other, would see what plumes were most effective; and each would endeavour to excel liis enemy as far as voluntary exertion would enable him, just as they endeavour to rival each other in song, even sometimes to the point of causing their own destruction.
Natural Selection as Neutralizing Sexual Selection. - There is also a general argument against Mr. Darwin's views on this question, founded on the nature and potency of "natural" as opposed to “sexual” selection, which appears to me to be of itself almost conclusive as to the whole matter at issue. Natural selection, or the survival of the fittest, acts perpetually and on an enormous scale. Taking the offspring of cach pair of birds as, on the average, only six annually, one-third of these at most will be preserved, while the two-thirds which are least fitted will die. At intervals of a few years, whenever unfavourable conditions occur, fivesixths, nine-tenths, or even a greater proportion of the whole yearly production are weeded out, leaving only thie most perfect and best adapted to survive. Now unless these survivors are, on the whole, the most ornamental, this rigid natural selection must neutralise and destroy any influence that may be exerted by
female selection. The utmost that can be claimed for the latter is, that a small fraction of the least ornamented do not obtain mates, while a few of the most ornamented may leave more than the average number of offspring. Unless, therefore, there is the strictest correlation between ornament and general perfection, the more brightly coloured or ornamented varieties can obtain no permanent advantage; and if there is (as I maintain) such a correlation, then the sexual selection of colour or ornament, for which there is little or no evidence, becomes needless, because natural selection which is an admitted vera causa, will itself produce all the results.
In the case of butterflies the argument becomes even stronger, because the fertility is so much greater than in birds, and the weeding-out of the unfit takes place, to a great extent, in the egg and larva state. Unless the eggs and larvæ which escaped to produce the next generation were those which would produce the more highly-coloured butterflies, it is difficult to perceive how the slight preponderance of colour sometimes selected by the females, should not be wholly neutralized by the extremely rigid selection for other qualities to which the offspring in every stage are exposed. The only way in which we can account for the observed facts is, by the supposition that colour and ornament are strictly corre. lated with health, vigour, and general fitness to survive. We have shown that there is reason to believe that this is the case, and if so, conscious sexual selection becomes as unnecessary as it would certainly be ineffective.
Greuter Brilliancy of some Female Birds.—There is one other very curious case of sexual colouring among
birds—that, namely, in which the female is decidedly brighter or more strongly marked than the male ; as in the fighting quails (Turnix), painted snipe (Rhynchæa), two species of phalarope (Phalaropus), and the common cassowary (Casuarius galeatus). In all these cases, it is known that the males take charge of and incubate the eggs, while the females are almost always larger and more puguacious.
In my “ Theory of Birds' Nests” (Natural Selection, p. 251), I imputed this difference of colour to the greater need for protection by the male bird while incubating; to which Mr. Darwin has objected that the difference is not suflicient, and is not always so distributed as to be most effective for this purpose; and he believes that it is due to reversed sexual selection, that is, to the female taking the usual rôle of the male, and being chosen for her brighter tints. We have already scen reason for rejecting this latter theory in every case ; and I also admit that Mr. Darwin's criticism is sound, and that my theory of protection is, in this case, only partially, if at all, applicable. But the theory now advanced, of intensity of colour being due to general vital energy, is quite applicable ; and the fact that the superiority of the female in this respect is quite exceptional, and is therefore probably not in any case of very ancient date, will account for the difference of colour thus produced being always very slight.
Colour-development us Illustrated by II umming-birds. -Of the mode of action of the general principles of colour-development among animals, we have an excellent example in the humming-birls. Of all birds these are at once the smallest, the most active, and the fullest of
vital energy. When poised in the air their wings are invisible, owing to the rapidity of their motion, and when startled they dart away with the rapidity of a flash of light. Such active creatures would not be an easy prey to any rapacious bird ; and if one at length was captured, the morsel obtained would hardly repay the labour. We may be sure, therefore, that they are practically unmolested. The immense variety they exhibit in structure, plumage, and colour, indicates a high antiquity for the race; while their general abundance in individuals shows that they are a dominant group, well adapted to all the conditions of their existence. Ilere we find everything necessary for the development of colour and accessory plumes. The surplus vital energy shown in their combats and excessive activity, has expended itself in ever-increasing developments of plumage, and greater and greater intensity of colour, regulated only by the need for specific identification which would be especially required in such small and mobile creatures. Thus may be explained those remarkable differences of colour between closely-allied species, one having a crest like the topaz, while in another it resembles the sapphire. The more vivid colours and more developed plumage of the males, I am now inclined to think may be wholly due to their greater vital energy, and to those general laws which lead to such superior developments even in domestic breeds; but in some cases the need of protection by the female while incubating, to which I formerly imputed the whole phemomenon, may have suppressed a portion of the ornament which she would otherwise have attained.
The extreme pugnacity of humming-birds has been