Evolution and Adaptation

by Thomas Hunt Morgan

Organs of Little Use to the Individual

In every organism there are parts of the body whose presence cannot be of vital importance to the individual. We may leave out of consideration the reproductive organs, since their presence, as has just been stated, is connected with the continuation of the race. The rudimentary organs, so-called, furnish many examples of structures whose presence may be of little or of no use to the individual; in fact, as in the case of the appendix in man, the organs may be a source of great danger to the individual. In this respect the organism is a structure not perfectly adapted to its conditions of life, since it contains within itself parts that are of little or of no use, which may even lead to its destruction, and may often expose it to unnecessary danger. Nevertheless such parts are surprisingly infrequent, and their presence is usually accounted for on the supposition that in the past these organs have been of use, and have only secondarily come to play an insignificant part in the functions of the organism. Another example of the same thing is found in the rudimentary eyes of animals living in the dark, such as the mole and several cave animals, fishes, amphibia, and insects.

There are still other organs, which cannot be looked upon as rudimentary, yet whose presence can scarcely be considered as essential to the life of the individual. It is with this class that we are here chiefly concerned. For instance, the electric organs in some of the rays and fish can hardly protect the animal from enemies, even when as highly developed as in the torpedo; and we do not know of any other essential service that they can perform. Whether the same may be also said of the phosphorescent organs of many animals is perhaps open in some cases to doubt, but there can be little question that the light produced by most of the small marine organisms, such as noctiluca, jellyfish, ctenophores, copepods, pyrosoma, etc., cannot be of use to these animals in protecting them from attack. In the case of certain bacteria it seems quite evident that the production of light can be of no use as such to them. The production of light may be only a sort of by-product of changes going on in the organism, and have no relation to outside conditions. In certain cases, as in the glowworm, it has been supposed that the display may serve to bring the sexes together; but since the phosphorescent organs are also present in the larval stages of the glowworm, and since even the egg itself is said to be phosphorescent, it is improbable, in these stages at least, that the presence of the light is of service to the organism.

It has been pointed out that the colors of certain animals may serve to conceal them and may be regarded as an adaptation; but it is also true that in many cases the color of the whole animal or the color of special parts can be of little if any direct use. While it is difficult to show that the wonderful patterns and magnificent coloration of many of the larger animals are not of service to the animal, however sceptical we may be on the subject, yet in the case of many microscopical forms that are equally brilliantly colored there can be little doubt that the coloration can be of no special service to them. If it be admitted that in these small forms the color and the color patterns are not protective, we should at least be on our guard in ascribing off-hand to larger forms a protective value in their coloration, unless there is actual proof that it serves some purpose.

We also see in other cases that the presence of color need not be connected with any use that it bears as such to the animal. For instance, the beautiful colors on the inside of the shells of many marine snails and of bivalve mollusks, can be of no use to the animal that makes the shell, because as long as the animal is alive this color cannot be seen from the outside. This being the case let us not jump too readily to the conclusion that when other shells are colored on the outer surface that this must be of use to the mollusk.

In regard to the colors of plants, there are many cases of brilliant coloration, which so far as we can see can be of no service to the organism. In such forms as the lichens and the toadstools, many of which are brilliantly colored, it is very doubtful if the color, as such, is of any use to the plant. The splendid coloring of the leaves in the autumn is certainly of no service to the trees.

It should not pass unnoticed in this connection that the stems and the trunks of shrubs and of trees and also many kinds of fruits and nuts are sometimes highly colored. It is true that some of the latter have been supposed to owe their color to its usefulness in attracting birds and other animals which, feeding on the fruit, swallow the seeds, and these, passing through the digestive tract and falling to the ground, may germinate. The dissemination of the seeds of such plants is supposed to be brought about in this way; and since they may be widely disseminated it may be supposed that it is an advantage to the plant to have attracted the attention of the fruit-eating birds. On the other hand one of the most brilliantly colored seeds, the acorn, is too large to pass through the digestive tracts of birds, and is, in fact, ground to pieces in the gizzard, and in the case of several mammals that feed on the acorns, the acorn is crushed by the teeth. It would seem, therefore, that its coloration is injurious to it rather than the reverse, as it leads to its destruction. It has been suggested by Darwin that since the acorns are for a time stored up in the crop of the bird, the passenger pigeon for example, and since the birds may be caught by hawks and killed, the seeds in the crop thus become scattered. Consequently it may be, after all, of use to the oak to produce colored acorns that attract the attention of these pigeons. This suggestion seems too far-fetched to consider seriously. In the case of the horse-chestnut the rich brown color is equally conspicuous, but the nut is too large to be swallowed by any of the ordinary seed-feeding birds or mammals. Shall we try to account for its color on the grounds of the poisonous character of the seed? Has it been acquired as a warning to those animals that have eaten it once, and been made sick or have died in consequence? I confess to a personal repugnance to imaginative explanations of this sort, that have no facts of experience to support them.