CHAPTER VI DARWIN'S THEORY OF SEXUAL SELECTION SEXUAL SELECTION THE theory of sexual selection was formulated by Darwin, even in the first edition of the "Origin of Species," but was developed at much greater length in "The Descent of Man." "This form of selection depends, not on a struggle for existence in relation to other organic beings or to external conditions, but on a struggle between the individuals of one sex, generally the males, for the possession of the other sex. The result is not death to the unsuccessful competitor, but few or no offspring. Sexual selection is, therefore, less rigorous than natural selection. Generally the most vigorous males, those which are best fitted for their place in nature, will leave most progeny. But in many cases victory depends, not so much on general vigor, as on having special weapons, confined to the male sex. A hornless stag or spurless cock would have a poor chance of leaving numerous offspring. Sexual selection, by always allowing the victor to breed, might surely give indomitable courage, length to the spur, and strength to the wing to strike in the spurred leg in nearly the same manner as the brutal cock-fighter by the careful selection of his best cocks." It is important to notice that the theory of sexual selection is admittedly an extension of the selection principle into a new field. Having accounted for domesticated animals and plants by artificial selection, and for the adaptations of wild species by natural selection, there remained only to account for the second ary sexual differences between the sexes by the principle of sexual selection. There are two ways in which Darwin supposes sexual selection to act: (1) through competition of the individuals of the same sex with each other, the strongest or best-equipped for fighting or for finding the individuals of the other sex gaining an advantage; (2) through selection by the individuals of one sex of certain preferred individuals of the other sex. The first category is natural selection applied to the members of one sex in competition with each other, although the result does not lead to the death of the unsuccessful individual, but excludes it from leaving progeny. In the second category a new element is introduced, namely, the selective power of the individuals of one sex, usually the female. It is this part that adds a distinctly new element to Darwin's other two theories of selection, and it is this part that we naturally think of as the theory of sexual selection par excellence. Darwin makes, however, no sharp distinction between these two sides of his theory, but includes both under the heading of sexual selection. In order to get the theory itself before us in as concrete form as possible, let us examine some of the cases that Darwin has given to show how he supposes the process to be carried out. their courage their contrivances and their glands "There are many other structures and instincts which must have been developed through sexual selection — such as the weapons of offence and the means of defence of the males for fighting with and driving away their rivals and pugnacity — their various ornaments for producing vocal or instrumental music for emitting odors, most of these latter structures serving only to allure or excite the female. It is clear that these characters are the result of sexual and not of ordinary selection, since unarmed, unornamented, or unattractive males would succeed equally well in the battle for life and in leaving a numerous progeny, but for the presence of better-endowed males. We may infer that this would be the case, because the females, which are unarmed and unornamented, are able to survive and procreate their kind. Secondary sexual characters of the kind just referred to will be fully discussed in the following chapters, as being in many respects interesting, but especially as depending on the will, choice, and rivalry of the individuals of either sex. When we behold two males fighting for the possession of the female, or several male birds. displaying their gorgeous plumage, and performing strange antics before an assembled body of females, we cannot doubt that, though led by instinct, they know what they are about, and consciously exert their mental and bodily powers." This general statement gives an idea of the class of phenomena that Darwin proposes to explain by the theory of sexual selection. The close resemblance between this process and that of artificial selection may be gathered from the following statement : — "Just as man can improve the breed of his game-cocks by the selection of those birds which are victorious in the cockpit, so it appears that the strongest and most vigorous males, or those provided with the best weapons, have prevailed under nature, and have led to the improvement of the natural breed or species. A slight degree of variability leading to some advantage, however slight, in reiterated deadly contests would suffice for the work of sexual selection; and it is certain that secondary sexual characters are eminently variable. Just as man can give beauty, according to his standard of taste, to his male poultry, or more strictly can modify the beauty originally acquired by the parent species, can give to the Sebright bantam a new and elegant plumage, an erect and peculiar carriage so it appears that female birds in a state of nature have, by a long selection of the more attractive males, added to their beauty or other attractive qualities. No doubt this implies powers of discrimination and taste on the part of the female which will at first appear extremely improbable; but by the facts to be adduced hereafter, I hope to be able to show that the females actually have these powers. When, however, it is said that the lower animals have a sense of beauty, it must not be supposed that such sense is comparable with that of a cultivated man, with his multiform and complex associated ideas. A more just comparison would be between the taste for the beautiful in animals, and that in the lowest savages, who admire and deck themselves with any brilliant, glittering, or curious object." Darwin did not close his eyes to the difficulties which the theory had to contend against. One of the most formidable of these objections is described in the following words: "Our difficulty in regard to sexual selection lies in understanding how it is that the males which conquer other males, or those which prove the most attractive to the females, leave a greater number of offspring to inherit their superiority than their beaten and less attractive rivals. Unless this result does follow, the characters which give to certain males an advantage over others could not be perfected and augmented through sexual selection. When the sexes exist in exactly equal numbers, the worst-endowed males will (except where polygamy prevails) ultimately find females, and leave as many offspring, as well fitted for their general habits of life, as the best-endowed males. From various facts and considerations, I formerly inferred that with most animals, in which secondary sexual characters are well developed, the males considerably exceeded the females in number; but this is not by any means always true. If the males were to the females as two to one, or as three to two, or even in a somewhat lower ratio, the whole affair would be simple; for the better-armed or more attractive males would leave the largest number of offspring. But after investigating, as far as possible, the numerical proportion of the sexes, I do not believe that any great inequality in number commonly exists. In most cases sexual selection appears "Let us take any species, a bird for instance, and divide. the females inhabiting a district into two equal bodies, the one consisting of the more vigorous and better-nourished individuals, and the other of the less vigorous and healthy. The former, there can be little doubt, would be ready to breed in the spring before the others; and this is the opinion of Mr. Jenner Weir, who has carefully attended to the habits of birds during many years. There can also be no doubt that the most vigorous, best-nourished and earliest breeders would on an average succeed in rearing the largest number of fine offspring. The males, as we have seen, are generally ready to breed before the females; the strongest, and with some species the best-armed of the males, drive away the weaker; and the former would then unite with the more vigorous and better-nourished females, because they are the first to breed. Such vigorous pairs would surely rear a larger number of offspring than the retarded females, which would be compelled to unite with the conquered and less powerful males, supposing the sexes to be numerically equal; and this is all that is wanted to add, in the course of successive generations, to the size, strength and courage of the males, or to improve their weapons." I shall comment later on the points here raised, but we should not let this opportunity pass without noticing, that even if the pairing were to follow according to the method here imagined, still the argument breaks down at the critical point, for there is no evidence that the more precocious females would rear a larger number of offspring than the more normal females, or even those that breed somewhat later. The greater eagerness of the males which has been observed in so many different classes of animals is accounted for as follows: |