“Let us take any species,” he writes, on page 329 of _The Descent of Man_ (Ed. 1901), “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.” From this competition among the males there arise, firstly, contests between the males for mates; secondly, the preference of the females for favoured males. It is a matter of common knowledge that at the breeding season the males of nearly all, if not all, species are very pugnacious. Two males often engage in desperate fights for one or more females; the victor drives away his foe and secures the harem. In such contests the stronger male wins, and thus emerges that particular form of sexual selection which Darwin termed “the law of battle.” “There are,” writes Darwin, on page 324 of _The Descent of Man_, “many other structures and instincts which must have 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—their courage and pugnacity—their various ornaments—their contrivances for producing vocal or instrumental music—and their glands for emitting odours.” The former characters have, according to Darwin, been developed by the law of battle, and the latter, since they serve only to allure or excite the female, by the preference of the female.