Evolution and Adaptation

by Thomas Hunt Morgan

9. What has just been said applies also with almost equal force to the development of such structures as the horns of deer, bison, antelopes, and the brilliant colors of many insects and birds. If in nature, competition between species takes place on the scale that the Darwinian theory of natural selection postulates, such forms, if they are much exposed, would be needlessly reduced in numbers in the process of acquiring these structures. So many individuals would have been at such a disadvantage in breeding, that if competition is as severe as the theory of natural selection postulates, these species could hardly be expected to compete successfully with other species in which sexual selection was not taking place.

10. Darwin admits that, in certain cases, external conditions may have acted directly to produce the colors in certain forms, and if these were not injurious he thinks they might have become constant. Such cases are left unexplained in the sense that they are not supposed to be adaptations to anything in particular. That colors produced in this way might afterward be found useful, irrespective of how they arose, is admitted as one of the ways in which sexual differences may have arisen.

11. It is baffling to find Darwin resorting to the Lamarckian explanation in those cases in which the improbability of the hypothesis of sexual selection is manifest. If either principle is true, we should expect it to apply to all phenomena of the same sort; yet Darwin makes use of the Lamarckian principle, in the hypothesis of sexual selection, only when difficulties arise.

12. In attempting to explain the development of the musical sense in man, it is clear that the hypothesis of sexual selection fails to give a satisfactory explanation. To suppose that the genius of a Beethoven or of a Mozart could have been the result of a process of sexual selection is too absurd to discuss. Neither the power of appreciation nor of expression in music could possibly have been the outcome of such a process, and it does not materially help the problem to refer it back to a troop of monkeys making the woods hideous with their cries.

We come now to some of the special cases to which Darwin’s hypothesis has been applied.

13. In one case at least, it is stated that a bird living on the ground might have acquired the color of the upper surface of the body through natural selection, while the under surface of the males of the same species might have become ornamented through the action of sexual selection. Thus in one and the same individual the two processes are supposed to have been at work, and it does not lessen the difficulty very much by supposing the two processes to have been carried out at different times, because it is evident that what had been gained at one time by one process might become lost while the color of certain parts was being acquired through the other process.

14. Darwin points out that “the plumage of certain birds goes on increasing in beauty during many years after they are fully mature,” as in the peacock, and in some of the birds of paradise, and with the plumes and crests of some herons. This is explained as possibly merely the result of “continued growth.” The improbability of selection is manifest in these cases, but if “continued growth” can accomplish this much, why may not the whole process be also the outcome of such growth? At any rate, whatever the explanation is, it is important to find a case of a secondary sexual character that the hypothesis obviously is insufficient to explain.

15. It is admitted in a number of cases, as in the stag for instance, that, although the larynx of the male is enlarged, this is not, in all probability, the outcome of sexual selection, but in other forms this same enlargement is ascribed to the selection process.

16. It is admitted that in none of the highly colored British moths is there much difference according to sex, although when a difference of color is found in butterflies this is put down to the action of sexual selection. If such wonderful colors as those of moths can arise without the action of selection, why make a special explanation for those cases in which this difference is associated with sex?

17. It is well known that birds sing at other times of the year than at the breeding season, and an attempt is made to account for this in that birds take pleasure in practising those instincts that they make use of at other times, as the cat plays with the captive mouse. Does not this suggest that, if they had certain instincts, they would be more likely to employ them at the times when their vitality or excitement is at its highest without regard to the way in which they have come by them?