In many species of fish the males are brighter in color than the females. In the case of _Callionymus lyra_, Darwin states:— “When fresh caught from the sea the body is yellow of various shades, striped and spotted with vivid blue on the head; the dorsal fins are pale brown with dark longitudinal bands, the ventral, caudal, and anal fins being bluish black. The female, or sordid dragonet, was considered by Linnæus, and by many subsequent naturalists, as a distinct species; it is of a dingy reddish brown, with the dorsal fin brown and the other fins white. The sexes differ also in the proportional size of the head and mouth, and in the position of the eyes; but the most striking difference is the extraordinary elongation in the male of the dorsal fin. Mr. W. Saville Kent remarks that this ‘singular appendage appears from my observations of the species in confinement, to be subservient to the same end as the wattles, crests, and other abnormal adjuncts of the male in gallinaceous birds, for the purpose of fascinating their mates.’” In the case of another fish, _Cottus scorpius_, there is also a great difference between the sexes, and here the males become very brilliant only at the breeding season. In other fishes, in which the sexes are colored alike, the males may become more brilliant during the breeding season. This, too, is explained by Darwin on the assumption that those males that have varied at the breeding season, so as to become more brightly colored, have been chosen in preference to the other males. A few cases are cited in which it has been observed that the males appear to exhibit themselves before the females, as in the following case of the Chinese Macropus:— “The males are most beautifully colored, more so than the females. During the breeding season they contend for the possession of the females; and, in the act of courtship, expand their fins, which are spotted and ornamented with brightly colored rays, in the same manner, according to M. Carbonnier, as the peacock. They then also bound about the females with much vivacity, and appear by ‘l’étalage de leurs vives couleurs chercher à attirer l’attention des femelles, lesquelles ne paraissaient indifférentes à ce manége, elles nageaient avec une molle lenteur vers les mâles et semblaient se complaire dans leur voisinage.’” In this connection Darwin makes the following general statement:— “The males sedulously court the females, and in one case, as we have seen, take pains in displaying their beauty before them. Can it be believed that they would thus act to no purpose during their courtship? And this would be the case, unless the females exert some choice and select those males which please or excite them most. If the female exerts such choice, all the above facts on the ornamentation of the males become at once intelligible by the aid of sexual selection.”