Evolution and Adaptation

by Thomas Hunt Morgan

It seems to me that we shall not go far wrong if we assume that it is largely a matter of indifference whether an individual snail is a right-handed or a left-handed form, as far as its relation to the environment is concerned. One form would have as good a chance for existing as the other. If this is granted, we may conclude that, while in most species a perfectly definite type is found, a right or a left spiral, yet neither the one nor the other has been acquired on account of its relation to the environment. This conclusion does not, of course, commit us in any way as to whether the spiral form of the visceral mass has been acquired in relation to the environment, but only to the view that, if a spiral form is to be produced, it is indifferent which way it turns. From the evolutionary point of view this conclusion is of some importance, since it indicates that one of the alternatives has been adopted and has become practically constant in most cases without selection having had anything to do with it.

Somewhat similar conditions are found in the flounders and soles. As is well known, these fishes lie upon one side of the body on the bottom of the ocean. Some species, with the rarest exceptions to be mentioned in a moment, lie always on the right side, others on the left side. A few species are indifferently right or left. At rare intervals a left-sided form is found in a right-sided species, and conversely, a right-sided form in a left-sided species. In such cases the reversed type is as perfectly developed in all respects as the normal form, but with a complete reversal of its right and left sides.

When the young flounders leave the egg, they swim in an upright position, as do ordinary fishes, with both sides equally developed. There cannot be any doubt that the ancestors of these fish were bilaterally symmetrical. Therefore, within the group, both right-handed and left-handed forms have appeared. It seems to me highly improbable that if a right-handed form had been slowly evolved through the selection of favorable variations in this direction, the end result could be suddenly reversed, and a perfect left-sided form appear. Moreover, as has been pointed out, the intermediate stages would have been at a great disadvantage as compared with the parent, and this would lead to their extermination on the selection theory. If, however, we suppose that a variation of this sort appeared at once, and was fixed,—a mutation in other words,—and that whether or not it had an advantage over the parent form, it could still continue to exist, and propagate its kind, then we avoid the chief difficulty of the selection theory. Moreover, we can imagine, at least, that if this variation appeared in the germ and was, in its essential nature, something like the relation seen in the snail, the occasional reversal of the relations of the parts presents no great difficulty.

In this same connection may be mentioned a curious fact first discovered by Przibram and later confirmed by others. If the leg carrying the large claw of a crustacean be removed, then, at the next moult, the leg of the other side that had been the smaller first leg becomes the new big one; and the new leg that has regenerated from the place where the big one was cut off becomes the smaller one.

Wilson has suggested that both claws in the young crustacean have the power to become either sort. We do not know what decides the matter in the adult, after the removal of one of the claws. Some slight difference may turn the balance one way or the other, so that the smaller claw grows into the larger one. At any rate, there is seen a latent power like that in the egg of the snail. Zeleny has found a similar relation to exist for the big and the little opercula of the marine worm, Hydroides.

Let us consider now the more general questions involved in these symmetrical and asymmetrical relations between the organism and its environment. In what sense, it may be asked, is the symmetry of a form an adaptation to its environment? That the kind of symmetry gives to the animal in many cases a certain advantage in relation to its environment is so evident that I think it will not be questioned. The main question is how this relation is supposed to have been attained. Three points of view suggest themselves: First, that the form has resulted directly from the action of the environment upon the organism. This is the Lamarckian point of view, which we rejected as improbable. Second, that the form has been slowly acquired by selecting those individual variations that best suited it to a given set of surrounding conditions. This is the Darwinian view, which we also reject. The third, that the origin of the form has had nothing to do with the environment, but appeared independently of it. Having, however, appeared, it has been able to perpetuate itself under certain conditions.

It should be pointed out that the Darwinian view does not suppose that the environment actually produces any of the new variations which it selects after they have appeared, but in so far as the environment selects individual differences it is supposed to determine the direction in which evolution takes place. On the theory that evolution has taken place independently of selection, this latter is not supposed to be the case; the finished products, so to speak, are offered to the environment; and if they pass muster, even ever so badly, they may continue to propagate themselves.

The asymmetrical form of certain animals living in a symmetrical environment might be used as an argument to show that the relation of symmetry between an animal and its environment can easily be overstepped without danger. The enormous claw of the fiddler-crab must throw the animal out of all symmetrical relation with its environment, and yet the species flourishes. The snail carries around a spiral hump that is entirely out of symmetrical relation with the surroundings of a snail.

These facts, few though they are, yet suffice to show, I believe, that the relation of symmetry between the organism and its environment may be, and is no doubt in many cases, more perfect than the requirements of the situation demand. The fact that animals made unsymmetrical through injuries (as when a crab loses several legs on one side, or a worm its head) can still remain in existence in their natural environment, is in favor of the view that I have just stated. By this I do not mean to maintain that a symmetrical form does not have, on the whole, an advantage over the same form rendered asymmetrical, but that this relation need not have in all forms a selective value, and if not, then it cannot be the outcome of a process of natural selection.

To sum up: it appears probable that the laws determining the symmetry of a form are the outcome of internal factors, and are not the result either of the direct action of the environment, or of a selective process. The finished products and not the different imperfect stages in such a process, are what the inner organization offers to the environment. While the symmetry or asymmetry may be one of the numerous conditions which determine whether a form can persist or not, yet we find that the symmetrical relations may be in some cases more perfect than the environment actually demands; and in other cases, although the form may place the organism at a certain disadvantage, it may still be able to exist in certain localities.