In lizards the erectile crests of the male _Anolis_, the brilliant throat patches of _Sitaria minor_, which is colored blue, black, and red, the skinny appendages present on the throat of the little lizards of the genus Draco, which in the beauty of their colors baffle description, are given as cases of sexual adornment. In the last case cited the ornaments are present, however, in both sexes. The remarkable horns in the males of different species of chameleons are imagined to have been acquired through the battle of the males with each other. In the group of birds we find some of the most striking cases of secondary sexual differences. The spurs, combs, wattles, horns, air-filled sacs, topknots, feathers with naked shafts, plumes, and greatly elongated feathers are all secondary sexual characters. The songs of the males, the rattling together of the quills of the peacock, the drumming of the grouse, and the booming sounds made by the night jars while on the wing, are further examples of secondary sexual differences. The odor of the male of the Australian musk duck is also put in the same category. The pugnacity of many male birds is well known, and it is imagined that one of the results of the competition of the individuals of the same sex with each other has led to the development of the organs of defence and offence. The males that have been successful in these battles are then supposed to mate with the best females. In this way those secondary sexual differences, connected with the encounters of the males, are supposed to have been formed. Darwin states in this connection:— “Even with the most pugnacious species it is probable that the pairing does not depend exclusively on the mere strength and courage of the male; for such males are generally decorated with various ornaments, which often become more brilliant during the breeding season, and which are sedulously displayed before the females. The males also endeavor to charm or excite their mates by love-notes, songs, and antics; and the courtship is, in many instances, a prolonged affair. Hence it is not probable that the females are indifferent to the charms of the opposite sex, or that they are invariably compelled to yield to the victorious males.” Thus a double process of selection is imagined to take place; one, the outcome of a competition of the males with each other, and the other, through a choice of the more successful males by the females, the more beautiful being supposed to be chosen.