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 secondary 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.