Evolution and Adaptation

by Thomas Hunt Morgan

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.”

While it may readily be granted that display of the male may have for its purpose the excitement of the female, it is another question as to whether she will be more excited by the more beautiful suitor. The attentions of the male may be supposed to have a purpose, even if the female does not choose the more beautiful of her suitors. It is this last proposition, so necessary for the theory of sexual selection, that seems improbable. But even if it were probable, there are, as we shall see, other difficulties to be overcome before we should be justified in accepting Darwin’s statement quoted above, concerning the results of sexual selection.

In regard to those species of fish in which both sexes are equally ornamented, Darwin returns once more to his hypothesis that the color of the male, acquired through sexual selection, may be transmitted to the other sex, and then, as if in doubt on this point, he adds, that it may be the result of the “nature of the tissues and of the surrounding conditions.” He even makes the suggestion, somewhat further on, that the colors may be warning, although it is confessedly unknown that these fish are distasteful to fish-devouring animals.

In amphibians the crest on the back of the male triton, which becomes colored along its edge, is described as a secondary sexual character. The vocal sacs, present in some species of frogs, are found sometimes in both sexes, but more highly developed in the males. In other species, as in the toad, it is the male alone that sings. In the reptiles we find that the two sexes of the turtles are colored alike, and this holds also for the crocodiles. Some male turtles make sounds at the breeding season, and the same is true for the crocodiles, the males of which are said to make a “prodigous display.” In snakes the males are smaller, as a rule, than the females, and the colors are more strongly pronounced, and although some snakes are very brilliantly colored, Darwin puts this down either to protective coloration, or to mimicry of other kinds of snakes. But surely the extremely brilliant colors of many snakes cannot be accounted for in any of these ways. The cause of the color of the venomous kinds, that are supposed to be imitated by the others, “remains to be explained and this may perhaps be sexual selection.”

“It does not, however, follow because snakes have some reasoning power, strong passions and mutual affection, that they should likewise be endowed with sufficient taste to admire brilliant colors in their partners, so as to lead to the adornment of the species through sexual selection. Nevertheless, it is difficult to account in any other manner for the extreme beauty of certain species; for instance, of the coral-snakes of South America, which are of a rich red with black and yellow transverse bands.”

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.