The expression, “Sexual Selection,” was first used by Mr. Darwin. Besides natural selection, which depends on the success of both sexes, at all ages, in relation to the general conditions of life, he introduced another principle, sexual selection, which depends on the success of certain individuals over others of the same sex, in relation to the propagation of the species. According to the former principle, those individuals who are most successful in the struggle for existence survive the others, and characters useful to the species are thus inherited; according to the latter, those individuals who have the greatest success in the struggle for mates have the most numerous offspring, and the characters which gave them the preference pass on to the new generation, and are afterwards intensified by the operation of like causes. The sexual struggle is of two kinds. In both it is carried on by individuals of the same sex; but in one these individuals, generally the males, try to drive away or kill their rivals; in the other, they seek to excite or charm those of the opposite sex, generally the females, who select the most attractive males for their partners. Therefore, the characters acquired through sexual selection, and transmitted chiefly to offspring of the same sex, generally the males, are, on the one hand, weapons for battle, vigour and courage; on the other hand, certain colours, forms, ornaments, sounds, or odours, which are felt to be pleasant. The secondary sexual characters of the latter sort are thus due to the taste of the females. They have been acquired because they are beautiful or otherwise agreeable, whereas the characters resulting from natural selection have been acquired because they are useful. How are we to explain the origin of this wonderful æsthetic faculty? “The senses of man and of the lower animals,” says Mr. Darwin, “seem to be so constituted that brilliant colours and certain forms, as well as harmonious and rhythmical sounds, give pleasure and are called beautiful; but why this should be so we know not.”[1486] According to Mr. Darwin, natural and sexual selection are two different sources from which animal characters have arisen. There is some truth in the statement of one of his critics, “Mr. Darwin, in fact, has so far abandoned his former belief in the efficacy of ‘natural selection’ as an agent in producing the differences which separate different species of animals, as to admit that some supplementary cause must, in some cases at any rate, be looked for; and this he thinks is to be found in the action, through long periods, of ‘sexual selection.’”[1487]