Evolution and Adaptation

by Thomas Hunt Morgan

Form and Symmetry

Almost without exception, animals and plants have definite and characteristic forms. In other words, they are not amorphous masses of substance. The members of each species conform, more or less, to a sort of ideal type. Our first problem is to examine in what sense the form itself may be looked upon as an adaptation to the surroundings.

It is a well-recognized fact that the forms of many animals appear to stand in a definite relation to the environment. For instance, animals that move in definite directions in relation to their structure have the anterior and the posterior ends quite different, and it is evident that these ends stand in quite different relations to surrounding objects; while, on the other hand, the two sides of the body which are, as a rule, subjected to the same influences are nearly exactly alike. The dorsal and the ventral surfaces of the body are generally exposed to very different external conditions, and are quite different in structure.

The relation is so obvious in most cases that it might lead one quite readily to conclude that the form of the animal had been moulded by its surroundings. Yet this first impression probably gives an entirely wrong conception of how such a relation has been acquired. Before we attempt to discuss this question, let us examine some typical examples.

A radial type of structure is often found in fixed forms, and in some floating forms, like the jellyfish. In a fixed form, a sea-anemone, for instance, the conditions around the free end and the fixed end of the body are entirely different, and we find that these two ends are also different. The free end contains the special sense-organs, the mouth, tentacles, etc.; while the fixed end contains the organ for attachment. It is evident that the free end is exposed to the same conditions in all directions, and it may seem probable that this will account for the radial symmetry of the anemone. There are also a few free forms, the sea-urchin for instance, that have a radial symmetry. Whether their ancestors were fixed forms, for which there is some evidence, we do not know definitely; but, even if this is true, it does not affect the main point, namely, that, although at present free to move, the sea-urchin is radially symmetrical. But when we examine its method of locomotion, we find that it moves indifferently in any direction over a solid surface; that is, it keeps its oral face against a solid object, and moves over the surface in any direction. Under these circumstances the same external conditions will act equally upon all sides of the body. In contrast to these common sea-urchins, there are two other related groups, in which, although traces of a well-marked radial symmetry are found, the external form has been so changed that a secondary bilateral form has been superimposed on it. These are the groups of the clypeasters and the spatangoids, and it is generally supposed that their forefathers were radially symmetrical forms like the ordinary forms of sea-urchins. These bilateral forms move in the direction of their plane of symmetry, but we have no means of knowing whether they first became bilateral and, in consequence, now move in the direction of the median plane, or whether they acquired the habit of moving in one direction, and in consequence acquired a bilateral symmetry. It seems more probable that the form changed first, for otherwise it is difficult to see why a change of movement in one direction should ever have taken place.

The radially symmetrical form is characteristic of many flowers that stand on the ends of their stalks. They also will be subjected to similar external influences in all directions. Many flowers, on the other hand, are bilaterally symmetrical. Some of these forms are of such a sort that they are generally interpreted as having been acquired in connection with the visits of insects. Be this as it may, it is still not clear why, if the flowers are terminal, insects should not approach them equally from every direction. If the flowers are not terminal, as, in fact, many of them are not, their relation to the surroundings is bilateral with respect to internal as well as to external conditions. The former, rather than the latter, may have produced the bilateral form of the flower. Here also we meet with the problem as to whether the flowers, being lateral in position, have assumed a bilateral form because their internal relations were bilateral; or whether an external relation, for example, the visits of insects, has been the principle cause of their becoming bilateral.

Fig. 4.—A, right and left claws of lobster;
B, of the fiddler-crab; and
C, of Alpheus.

In some bilateral forms the right and left sides may be unsymmetrical in certain organs. Right and left handedness in man is the most familiar example, although the structural difference on which this rests is not very obvious. More striking is the difference in the two big claws of the lobster (Fig. [4 A]). One of the two claws is flat and has a fine saw-toothed edge. The other is thicker and has rounded knobs instead of teeth. It is said that these two claws are used by the lobster for different purposes,—the heavy one for crushing and for holding on, and the narrower for cutting up the food. If this is true, then we find a symmetrical organism becoming unsymmetrical, and in consequence it takes advantage of its asymmetry by using its right and left claws for different purposes.

More striking still is the difference in the size of the right and left claws in a related form, Alpheus—a crayfish-like form that lives in the sea. With the larger claw (Fig. [4 C]) it makes a clicking sound that can be heard for a long distance. In some of the crabs the difference in the size of the two claws is enormous, as in the male fiddler-crab, for example (Fig. [4 B]). One of the claws is so big and unwieldy that it must put the animal at a distinct disadvantage. Its use is unknown, although it has been suggested that it is a secondary sexual character.

The asymmetry of the body of the snail is very conspicuous, at least so far as certain organs are concerned. The foot on which the animal crawls and the head have preserved their bilaterality; but the visceral mass of the animal, contained in the spirally wound shell, lying on the middle of the upper surface of the foot, is twisted into a spiral form. Many of the organs of one side of the body are atrophied. The gill, the kidney, the reproductive organ, and one of the auricles of the heart have completely, or almost completely, disappeared. The cause of this loss seems to be connected with the spiral twist of the visceral mass. One of the consequences of the twisting has been to bring the organs of the left side of the body around the posterior end until they come to lie on the right side, the organs of the original right side being carried forward and there atrophying.

There is another remarkable fact connected with the asymmetry of the snail. In some species, Helix pomatia, for example, the twist has been toward the right, i.e. in the direction which the hands of a watch follow when the face is turned upward toward the observer. Individuals twisted in this direction are called dextral. Occasionally there is found an individual with the spiral in the opposite direction (sinistral), and in this the conditions of the internal organs are exactly reversed. It is the left set of organs that is now atrophied, and the right set that is functional. Such changes appear suddenly. Organs of one side of the body that have not been functional for many generations may become fully developed. Moreover, Lang has shown that when a sinistral form breeds with a normal dextral form, or even when sinistral forms are bred with each other, the young are practically all of the ordinary type.

An attempt has been made to connect these facts with the mode of development of the mollusks. It is known that the eggs of a number of gasteropod mollusks segment in a perfectly definite manner. A sort of spiral cleavage is followed by the formation of a large mesodermal cell from the left posterior yolk-cell. From this mesodermal cell nearly all the mesodermal organs of the body are formed. Thus it may appear that the spiral form of the snail is connected with the spiral form of the cleavage. In a few species of marine and fresh-water snails the cleavage spiral is reversed, and the mesoderm arises from the right posterior yolk-cell. It has been shown in several cases that the snail coming from such an egg is twisted in the reverse direction from that of ordinary snails.

It has been suggested, therefore, that the occasional sinistral individual of Helix arises from an egg cleaving in the reverse direction, and there is nothing improbable in an assumption of this kind. No attempt has been made as yet to explain why, in some cases, the cleavage spiral is turned in one direction, and in other cases in the reverse direction; but even leaving this unaccounted for, the assumption of the unusual form of Helix being the result of a reversal of the cleavage throws some light as to how it is possible for the complete reversal of the organs of the adult to arise. If it is assumed that in the early embryo the cells on each side of the median line are alike, and at this time capable of forming adult structures, a simple change of the spiral from right to left might determine on which side of the middle line the mesodermal cell would lie, and its presence on one side rather than on the other might determine which side of the embryo would develop, and which would not. This possibility removes much of the mystery which may appear to surround a sudden change of this sort.

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.