Evolution and Adaptation

by Thomas Hunt Morgan

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.