Sexual selection is usually unisexual, acting only upon one sex. In many cases, however (courtship decorations in herons, grebes, etc.), it is mutual and affects both sexes similarly. Rarely, as in phalaropes, the male broods and cares for the young, and the fe male does most of the courting. We may here speak of reversed sexual selection, falling mainly upon the females, which are accordingly the more brightly coloured.
A. R. Wallace wished to discard the whole sexual selection theory, and to ascribe the development of male weapons and dis play characters to the hypothetical action of "male vigour." This view as it stands is undoubtedly erroneous. However, it will often be true that greater prowess in battle and greater efficiency in song or display will be associated with high vigour ; and in this way sexual selection will have, as an important by-product, the selection, as fathers of the next generation, of males above the average in vigour.
The new definition above given obviates one great difficulty in the theory as originally stated. It was difficult to see how intra sexual selection could be very effective without a preponderance of males, or polygamy; and in the great majority of animals there is no polygamy, and the proportion of the sexes is nearly equal. (See, however, Darwin, Descent of Man, 2nd ed., p. 329; and E. Howard, Territory in Bird Life, p. 35.) If, however, sexual selec tion is primarily designed for securing efficient and speedy mating, this difficulty is removed.
The other objection which is commonly raised, that the theory of sexual selection ascribed too much taste and conscious discrimi nation to the female, rests upon a pure misconception. So long as the courtship display stimulates the female, a selective result will be obtained, whether she exercise taste and judgment or be stimu lated in the most automatic way.
A few examples will go to show how entangled are natural and intra-sexual selection.
In the courtship of the little fly Drosophila, the male ap proaches the female while waving his wings in a peculiar manner. Sturtevant made up three series of bottles. In each of series A was placed a normal male and female; in B a female with a male whose wings had been cut off ; and in C a female with one normal and one wing-clipped male. Successful mating was accomplished by the wing-clipped males in series B, but in a much longer aver age time than in series A; i.e., the wing-waving courtship appar ently has the effect of stimulating the female to sexual response more quickly than does the mere presence of a male. In series C, the average time of mating was only a trifle longer than in series A; but the wing-clipped males were almost as successful as their normal rivals. This can only be explained as meaning that, once the courtship has excited and stimulated the female, she will be ready to accept any male. Courtship is, in this case, almost entirely an affair of natural selection; like a copulatory organ, it renders fertilization more certain, and so benefits the race, and is of little benefit to one male in competition with another.
The same appears definitely to hold good in newts, in which courtship is the preliminary to the male's depositing his sperm in a packet on the pond-bottom. Even should two or more males be courting simultaneously, and should the courtship of one be more effective than that of the rest, it would be impossible that the female should pick up his sperm packet in preference to another. In certain polygamous bird's, on the other hand, although
the display of the males presumably has some function in merely securing fertilization, yet intra-sexual selection of one male in preference to others is predominant. This occurs in birds where there are special assembly-places for the males' display in the breeding season : the females visit these places only for the pur pose of mating, and the males take no share in caring for eggs and young. Black grouse, certain birds of paradise, and the ruff are examples from different orders. Owing to the unique variability of male ruffs in the breeding-plumage, every male on a "hill" (as the ruff's assembly places are called) can be individually recog nized. Edmund Selous took advantage of this fact. He made observations on one "hill" throughout several weeks of the breed ing season, and was able to establish the fact that whereas some of the cocks never succeeded in mating at all while he was watching, and others but rarely, one or two secured a large num ber of mates. Not only that, but the most successful cock had an exceptionally well-developed ruff, and one that to our eyes at least was very striking in its colour contrasts. In this species, then, by no means every male secures a mate, and courtship-dis play is concerned with the securing of mates. There is a real "struggle for reproduction" among males, with resultant intra sexual selection.
In man it is obvious that sexual selection plays, and has played an important role, but in a more complex way than in most species, since not only does some degree of mutual sexual selec tion exist, but also some degree of intra-sexual selection in both sexes separately. Sexual selection has undoubtedly helped to diversify as well as to improve human appearance.
Sexual selection has exercised a considerable secondary effect in evolution, through characters which have arisen in one sex under the influence of sexual selection, being often partially or completely transferred to the other sex under thz influence of heredity. A good example is afforded by the finch family (Fringi llidae). In general, male finches are brighter-coloured than the females, but their characteristic patterns are usually reproduced in duller colours in the females. It is probable that the bright colours originated in the males through sexual selection; but their partial transference to the females (though of no functional significance in them) has resulted in the females of the different species being more different from each other than they otherwise would have been.
When, e.g., the females nest in holes, bright colours can be transferred to them more completely without endangering their safety; this is so, for instance, in the European robin and the tits. When, however, as in most pheasants, the female nests in exposed situations, natural selection inhibits the transference.
Admittedly many of these conclusions are based solely on indirect evidence ; but in view of the experimental work on Drosophila and newts, and the natural experiment annually staged with ruffs, which decisively point to a stimulating effect of display on the female, the cumulative circumstantial evidence must be presumed correct unless it is actually shown false. (See COURT SHIP OF ANIMALS ; SELECTION : Natural Selection; BIRD : Repro ductive Habits.)