Evolution and Adaptation

by Thomas Hunt Morgan

“But it is difficult to understand why the males of species, of which the progenitors were primordially free, should invariably have acquired the habit of approaching the females, instead of being approached by them. But in all cases, in order that the males should seek efficiently, it would be necessary that they should be endowed with strong passions; and the acquirement of such passions would naturally follow from the more eager leaving a larger number of offspring than the less eager.”

Thus we are led to the rather complex conclusion, that the more eager males will leave more descendants, and those that are better endowed with ornaments will be the ones selected. But unless it can be shown that there is some connection between greater eagerness and better ornamentation, it might often occur that the less ornamented were the more eager individuals, in which case there would be an apparent conflict between the two acquirements.

After giving some cases of the greater variability of the males, in respect to characters that are not connected with sexual selection, and presumably not the result of any kind of selection, Darwin concludes: “Through the action of sexual and natural selection male animals have been rendered in very many instances widely different from their females; but independently of selection the two sexes, from differing constitutionally, tend to vary in a somewhat different manner. The female has to expend much organic matter in the formation of her ova, whereas the male expends much force in fierce contests with his rivals, in wandering about in search of the female, in exerting his voice, pouring out odoriferous secretions, etc.: and this expenditure is generally concentrated within a short period. The great vigor of the male during the season of love seems often to intensify his colors, independently of any marked difference from the female. In mankind, and even as low down in the organic scale as in the Lepidoptera, the temperature of the body is higher in the male than in the female, accompanied in the case of man by a slower pulse. On the whole, the expenditure of matter and force by the two sexes is probably nearly equal, though effected in very different ways and at different rates.”

Again: “From the causes just specified, the two sexes can hardly fail to differ somewhat in constitution, at least during the breeding season; and although they may be subjected to exactly the same conditions, they will tend to vary in a different manner. If such variations are of no service to either sex, they will not be accumulated and increased by sexual or natural selection. Nevertheless, they may become permanent if the exciting cause acts permanently; and in accordance with a frequent form of inheritance they may be transmitted to that sex alone in which they first appeared. In this case, the two sexes will come to present permanent, yet unimportant, differences of character. For instance, Mr. Allen shows that with a large number of birds inhabiting the northern and southern United States, the specimens from the south are darker-colored than those from the north; and this seems to be the direct result of the difference in temperature, light, etc., between the two regions. Now, in some few cases, the two sexes of the same species appear to have been differently affected; in the Agelæus phœniceus the males have had their colors greatly intensified in the south; whereas with Cardinalis virginianus it is the females which have been thus affected: with Quiscalus major the females have been rendered extremely variable in tint, whilst the males remain nearly uniform.”

The admissions contained in this statement would seem to jeopardize the entire question, for, if it is admitted that, on account of the difference in the constitution of the two sexes, the influence of the surrounding conditions would produce a different effect on them, it would seem that there is no need whatsoever for the theory of sexual selection. What Darwin is probably attempting to show is that the material for the further action of sexual selection is already given; but the question may well be asked, if the external conditions have done so much, why may they not have gone farther and produced the entire result?

Darwin makes the following suggestion to account for those cases in which the female is the more highly colored:—

“A few exceptional cases occur in various classes of animals, in which the females instead of the males have acquired well-pronounced secondary sexual characters, such as brighter colors, greater size, strength, or pugnacity. With birds there has sometimes been a complete transposition of the ordinary characters proper to each sex; the females having become the more eager in courtship, the males remaining comparatively passive, but apparently selecting the more attractive females, as we may infer from the results. Certain hen birds have thus been rendered more highly colored or otherwise ornamented, as well as more powerful and pugnacious than the cocks; these characters being transmitted to the female offspring alone.”

Then follows immediately the discussion as to whether a double process of sexual selection may not be supposed to go on at the same time. “It may be suggested that in some cases a double process of selection has been carried on; that the males have selected the more attractive females, and the latter the more attractive males. This process, however, though it might lead to the modification of both sexes, would not make the one sex different from the other, unless indeed their tastes for the beautiful differed; but this is a supposition too improbable to be worth considering in the case of any animal, excepting man. There are, however, many animals in which the sexes resemble each other, both being furnished with the same ornaments, which analogy would lead us to attribute to the agency of sexual selection. In such cases it may be suggested with more plausibility, that there has been a double or mutual process of sexual selection; the more vigorous and precocious females selecting the more attractive and vigorous males, the latter rejecting all except the more attractive females. But from what we know of the habits of animals, this view is hardly probable, for the male is generally eager to pair with any female. It is more probable that the ornaments common to both sexes were acquired by one sex, generally the male, and then transmitted to the offspring of both sexes. If, indeed, during a lengthened period the males of any species were greatly to exceed the females in number, and then during another lengthened period, but under different conditions, the reverse were to occur, a double but not simultaneous process of sexual selection might easily be carried on, by which the two sexes might be rendered widely different.”

The improbability of such a process is so manifest that the suggestion can scarcely be looked upon as anything more than pure speculation. We shall have occasion later to return to the same subject, and point out its bearing more explicitly.

Nearly the whole animal kingdom is passed in review by Darwin from the point of view of the sexual selection theory. There is brought together a large number of extremely interesting facts, and if the theory did no more than merely hold them together, it has served, in this respect, a useful end. We may select some of the most instructive cases by way of illustrating the theory.