Evolution and Adaptation

by Thomas Hunt Morgan

Some beetles also possess rasping organs in different parts of the body, but they cannot produce much noise by this means.

“We thus see that in the different coleopterous families the stridulating organs are wonderfully diversified in position, but not much in structure. Within the same family some species are provided with these organs, and others are destitute of them. This diversity is intelligible, if we suppose that originally various beetles made a shuffling or hissing noise by the rubbing together of any hard and rough parts of their bodies, which happened to be in contact; and that from the noise thus produced being in some way useful, the rough surfaces were gradually developed into regular stridulating organs. Some beetles as they move, now produce, either intentionally or unintentionally, a shuffling noise, without possessing any proper organs for the purpose.”

Darwin says that he expected from analogy to find in this group also differences in the sexes, but none such were found, although in some cases the males alone possess certain characters or have them more highly developed.

It is important not to forget, when considering all questions connected with sexual selection, that in order for the result to be successful it is not only necessary that the female respond to the noises and music of the other sex, but that she choose the suitor that makes the greatest, or the most pleasing, noise. If the stridulating organs are only used by the animals in finding each other, then the case might be considered as coming under the head of natural selection. If this be granted, then it may be claimed, and apparently Darwin is inclined to adopt this view, that those males that make the most noise will be more likely to be heard, and possibly approached. They will, therefore, be more likely to leave descendants. We have already considered this question when dealing with the theory of natural selection in the preceding chapter and need not go over the ground again. This much may, however, be said again, that even if it is probable that these organs are of use to the animals in finding each other, and this seems not improbable, it does not follow that the organs have been acquired through selection for this purpose.

Darwin finds his best examples of secondary sexual characters in the group of vertebrates, and since in this group the intelligence is of a higher order than in the other groups, the argument that the female chooses the more pleasing suitor is made to appear more plausible.

The elongation of the lower jaw that occurs in a few fishes at the breeding season is regarded as a secondary sexual character. On the other hand, Darwin recognizes the following difficulty in regard to the size of the males:—

“In regard to size, M. Carbonnier maintains that the female of almost all fishes is larger than the male; and Dr. Günther does not know of a single instance in which the male is actually larger than the female. With some cyprinodonts the male is not even half as large. As in many kinds of fishes the males habitually fight together, it is surprising that they have not generally become larger and stronger than the females through the effects of sexual selection. The males suffer from their small size, for, according to M. Carbonnier, they are liable to be devoured by the females of their own species when carnivorous, and no doubt by other species. Increased size must be in some manner of more importance to the females, than strength and size are to the males for fighting with other males; and this perhaps is to allow of the production of a vast number of ova.”

The last sentence implies that this particular case is to be explained by the females becoming larger on account of the number of eggs that they are to produce. But why was not the same explanation offered in the case of the spiders? It is this uncertain way of applying any explanation that suggests itself, that puts the whole method in an unfortunate light.

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.’”