Evolution and Adaptation

by Thomas Hunt Morgan

Although in beetles the sexes are generally colored alike, yet in some of the longicorns there are exceptions to the rule. “Most of these insects are large and splendidly colored. The males in the genus Pyrodes, which I saw in Mr. Bates’s collection, are generally redder but rather duller than the females, the latter being colored of a more or less splendid golden-green. On the other hand, in one species the male is golden-green, the female being richly tinted with red and purple. In the genus Esmeralda the sexes differ so greatly in color that they have been ranked as distinct species; in one species both are of a beautiful shining green, but the male has a red thorax. On the whole, as far as I could judge, the females of those Prionidæ, in which the sexes differ, are colored more richly than the males, and this does not accord with the common rule in regard to color, when acquired through sexual selection.”

The great horns that rise from the heads of many male beetles are very striking cases of sexual difference, and Darwin compares them to the horns of stags and of the rhinoceros. They “are wonderful from their size and shapes.” Darwin offers the following conjecture as to their meaning: “The extraordinary size of the horns, and their widely different structure in closely allied forms, indicate that they have been formed for some purpose; but their excessive variability in the males of the same species leads to the inference that this purpose cannot be of a definite nature. The horns do not show marks of friction, as if used for any ordinary work. Some authors suppose that as the males wander about much more than the females, they require horns as a defence against their enemies; but as the horns are often blunt, they do not seem well adapted for defence. The most obvious conjecture is that they are used by the males for fighting together; but the males have never been observed to fight; nor could Mr. Bates, after a careful examination of numerous species, find any sufficient evidence, in their mutilated or broken condition, of their having been thus used. If the males had been habitual fighters, the size of their bodies would probably have been increased through sexual selection, so as to have exceeded that of the females; but Mr. Bates, after comparing the two sexes in above a hundred species of the Copridæ, did not find any marked difference in this respect amongst well-developed individuals. In Lethrus, moreover, a beetle belonging to the same great division of the lamellicorns, the males are known to fight, but are not provided with horns, though their mandibles are much larger than those of the female.”

“The conclusion that the horns have been acquired as ornaments is that which best agrees with the fact of their having been so immensely, yet not fixedly, developed,—as shown by their extreme variability in the same species, and by their extreme diversity in closely allied species. This view will at first appear extremely improbable; but we shall hereafter find with many animals standing much higher in the scale, namely fishes, amphibians, reptiles and birds, that various kinds of crests, knobs, horns and combs have been developed apparently for this sole purpose.”

It is asking a great deal to suppose that animals, so dull and sluggish as these beetles, are endowed with a sufficient æsthetic discrimination to select in each generation those males whose horns are a little longer than the average. The resemblance of the horns to those of stags is, as Darwin points out, obvious, but in the latter case also it remains to be proven that they are the result of sexual selection, as Darwin believes to be the case; but the evidence for this belief is not much better, as we shall see in the case of the antlers of deer, than it is in these beetles.

In regard to butterflies, the males and females are both often equally brilliantly colored; in other species the differences in the sexes are very striking. Darwin states:—

“Even within the same genus we often find species presenting extraordinary differences between the sexes, whilst others have their sexes closely alike.” The fine colors of the wings of many moths are also supposed by Darwin to have arisen through sexual selection, although the colors are usually on the lower wings, which are covered during the day by the less ornamented upper wings. It is assumed that, since the moths often begin to fly at dusk, their colors might at this time be seen and appreciated by the other sex. It should not be overlooked, however, that, in the case of some of the most highly colored moths, it is known that the males find the females through the sense of smell. Moreover, although moths are often finely colored, Darwin points out that “it is a singular fact that no British moths which are brilliantly colored, and, as far as I can discover, hardly any foreign species, differ much in color according to sex; though this is the case with many brilliant butterflies.”

Yet Darwin does not hesitate to conclude: “From the several foregoing facts it is impossible to admit that the brilliant colors of butterflies, and of some few moths, have commonly been acquired for the sake of protection. We have seen that their colors and elegant patterns are arranged and exhibited as if for display. Hence I am led to believe that the females prefer or are most excited by the more brilliant males; for on any other supposition the males would, as far as we can see, be ornamented to no purpose. We know that ants and certain lamellicorn beetles are capable of feeling an attachment for each other, and that ants recognize their fellows after an interval of several months. Hence there is no abstract improbability in the Lepidoptera, which probably stand nearly or quite as high in the scale as these insects, having sufficient mental capacity to admire bright colors. They certainly discover flowers by color.”

So far as the evidence of ants having an attachment for each other is concerned, we may eliminate this part of the argument, since the evidence on which the statement is based is now regarded as only showing that ants recognize each other by their sense of smell, which resides in the antennæ. Hence the so-called fondling means only that the ants are trying by smell to determine the odor of the other individual.

Darwin points out a number of cases in which the females are more brightly colored than the males, and for such cases he reverses the process of selection, supposing that the males have been discriminating, and have not “gladly accepted any female.” No explanation is offered to account for this reversal of instinct, in fact, no evidence to show that such a reversal really exists. Darwin points out that in most cases the male insect carries the female during the period of union, while in two species of butterflies, Colias edusa and hyale, the females carry the males, and the females are here the more highly colored. He suggests that since in this case “the females take the more active part in the final marriage ceremony, so we may suppose that they likewise do so in the wooing; and in this case we can understand how it is that they have been rendered the more beautiful.”

A most significant fact in connection with the difference in sexual coloration of butterflies did not escape Darwin’s attention.