Evolution and Adaptation

by Thomas Hunt Morgan

In many of the lower animals in which the sexes are separated, and these alone, of course, can be supposed to come within the range of the theory, there are no striking differences between the sexes, in regard to ornamentation, although in other respects differences may exist.

“Moreover it is almost certain that these animals have too imperfect senses and much too low mental powers, to appreciate each other’s beauty or other attractions, or to feel rivalry.

“Hence in these classes or subkingdoms, such as the Protozoa, Cœlenterata, Echinodermata, Scolecida, secondary sexual characters, of the kind which we have to consider, do not occur; and this fact agrees with the belief that such characters in the higher classes have been acquired through sexual selection, which depends on the will, desire, and choice of either sex.”

There are some cases, however, in which animals low in the scale show a difference in the ornamentation of the two sexes. A few cases have been recorded in the roundworms, where different shades of the same tint distinguish the sexes. In the annelids the sexes are sometimes so different, that, as Darwin remarks, they have been placed in different genera and even families, “yet the differences do not seem to be of the kind which can be safely attributed to sexual selection.” In regard to the nemertian worms, although they “vie in variety and beauty of coloring with any other group in the invertebrate series,” yet McIntosh states that he “cannot discover that these colors are of any service.” In the copepods, belonging to the group of lower Crustacea, Darwin excludes those cases in which the males alone “are furnished with perfect swimming legs, antennæ, and sense-organs; the females being destitute of these organs, with their bodies often consisting of a mere distorted mass,” because these extraordinary differences between the two sexes are no doubt related to their widely different habits of life. Nevertheless, it is important to observe that such extreme differences may exist in cases where sexual selection cannot come in, because of the absence of eyes in the female.

In regard to another copepod, Saphirina, Darwin points out that the males are furnished with minute scales, which exhibit beautiful changing colors, and these are absent in the females; yet he states that it would be extremely rash to conclude that these curious organs serve to attract the females. Differences in the sexes are also found in one species of Squilla, and a species of Gelasimus. In the latter case Darwin thinks that the difference is probably due to sexual selection. In addition to these cases, recorded by Darwin, there may be added the two remarkable cases, shown in our Figure [2 A, B], of Calocalanus pavo, the female of which has a gorgeous tail worthy of a peacock, and of Calocalanus plumulosus, in which one of the setæ of the tail is drawn out into a long featherlike structure. In the former, the male is much more modestly adorned, as shown in Figure [2 C]; in the latter species the male is unknown.

Fig. 2.—A male of the copepod, Calocalanus plumulosus.
B and C, a male and a female of Calocalanus pavo. (After Giesbrecht.)

In spiders, where as a rule the sexes do not differ much from each other in color, the males are often of a darker shade than the females. “In some species, however, the difference is conspicuous; thus the female of Sparassus smaragdulus is dullish green, whilst the adult male has the abdomen of a fine yellow with three longitudinal stripes of rich red.” Darwin believes that sexual selection must take place in this group, because Canestrini has observed that the males fight for the possession of the females. He has also stated that the males pay court to the female, and that she rejects some of the males who court her, and sometimes devours them, until finally one is chosen. Darwin believed, on this evidence, that the difference in color of the sexes had been acquired by sexual selection, “though we have here not the best kind of evidence—the display by the male of his ornaments.” This evidence has, however, now been supplied through the interesting observations of Mr. and Mrs. Peckham. These accurate observers have studied the courtship of the male, and observed that during the process, he twists and turns his body in such a way as to show to best advantage his colors to the female. From their account this certainly appears to be the result of his movements, but whether this is really the case, and whether the female makes any choice amongst her suitors, according to whether they are more or less brilliantly marked, we are absolutely ignorant. The following account given by Darwin should not pass unnoticed:—

“The male is generally much smaller than the female, sometimes to an extraordinary degree, and he is forced to be extremely cautious in making his advances, as the female often carries her coyness to a dangerous pitch. De Geer saw a male that ‘in the midst of his preparatory caresses was seized by the object of his attentions, enveloped by her in a web and then devoured, a sight which, as he adds, filled him with horror and indignation.’ The Rev. O. P. Cambridge accounts in the following manner for the extreme smallness of the male in the genus Nephila. ‘M. Vinson gives a graphic account of the agile way in which the diminutive male escapes from the ferocity of the female, by gliding about and playing hide and seek over her body and along her gigantic limbs: in such a pursuit it is evident that the chances of escape would be in favor of the smallest males, while the larger ones would fall early victims; thus gradually a diminutive race of males would be selected, until at last they would dwindle to the smallest possible size compatible with the exercise of their generative functions,—in fact probably to the size we now see them, i.e. so small as to be a sort of parasite upon the female, and either beneath her notice, or too agile and too small for her to catch without great difficulty.’”

It is certainly surprising to find Darwin ascribing even this difference in size between the sexes to the action of selection. Is it not a little ludicrous to suppose that the females have reduced the males to a size too small for them to catch?