Evolution and Adaptation

by Thomas Hunt Morgan

Butterflies that have just emerged from their pupa case exhibit a marked negative geotropic reaction, and this appears to be connected with the necessity of unfolding their wings at this time. Loeb says that the same cause that determines the direction of the falling stone and the paths of the planets, namely, gravity, also directs the actions of the butterfly that has just left its pupa case. The geotropic response is especially strong at first. The animal wanders around until it reaches a vertical wall, which it immediately ascends, straight upward, and remains hanging at the top until its wings have unfolded. A similar response occurs in the final stage of the larva of the May-fly, which leaves the water and crawls up a blade of grass, or other vertical support, and there, bursting the pupa skin, it dries its wings and flies away. That this is a reaction to gravity and not to light is shown by Loeb’s observation, that their empty skins are sometimes observed under a bridge where the light does not come from above. “This observation on the larva of the May-fly contradicts the assumption that the ‘purpose’ of the geotropic response of the butterfly is that it may the better unfold its new wings, for in the ephemerid larva the negative geotropism appears at a time when no wings are present.” On the other hand, it should not be overlooked that the reaction is important for the May-fly larva in other ways, because it leads the larva to leave the water at the right period, and come out into the air, where the flying insect can more safely emerge.

It is not without interest to find that caterpillars exhibit some of the same reaction shown by butterflies. Loeb has made numerous experiments with the caterpillars of Porthesia chrysorrhœa. The caterpillars of this moth collect together in the autumn and spin a web or nest in which they pass the winter. If they are taken from the nest and brought into a warm room, they will orientate themselves to the light, and also crawl toward it. If placed in a tube, they crawl to the upper side of the glass and then along this side toward the light. If a covering is placed over the end of the tube that is turned toward the window, the caterpillars will crawl only as far as the edge of the cloth. They also react negatively to gravity. If kept in a dark room, they will crawl upward to the top of the receptacle in which they are enclosed. If subjected to the influences of both light and gravity, they respond more strongly to the light. The caterpillars also show a contact reaction. They tend to collect on convex sides or on corners and angles of solid bodies. They may even pile up one on top of the other in response to this reaction; the convex side of a quiescent animal acting on another animal crawling over it as any convex surface would do and holding the animal fast.

These three kinds of reactions determine the instincts of these caterpillars. In the spring, when they become warm, they leave the nest. Positive heliotropism and negative geotropism compel them to crawl upward to the tops of the branches of the trees, and there the contact reaction with the small buds holds them fast in this place. That they are not attracted to the end of the branches by the food that they find there is shown by placing buds in the bottom of the tubes in which the caterpillars are contained. The caterpillars remain at the top of the tube, although food is within easy reach. If, however, they are placed directly on the buds, the contact reaction will hold them there, and they will not crawl farther upward. Curiously enough, as soon as the caterpillars have fed and the time for shedding approaches, the responsiveness to light and to gravity decreases, and at the time of shedding they do not respond at all to these agents. These same caterpillars react also to warmth above a certain point. In a dark tube placed near a stove, the caterpillars collect at the end farthest away from the source of the heat. They react to light best at a temperature between 20 and 30 degrees C., and above this temperature point they become restless and wander about.

The very close connection between the reactions of this caterpillar and its mode of life is perfectly obvious. The entire series of changes seems to have for its “purpose” the survival of the individual by bringing it to the place where it will find its food. It may seem natural to conclude that these responses have been acquired for this very purpose, but let us not too quickly jump at this obvious conclusion until the whole subject has been more fully examined.

The upward and downward movements of some pelagic animals have been shown to depend on certain tropic responses. Every student of marine zoology is familiar with the fact that many animals come to the surface at night, and go down at the approach of daylight. It has been shown that this migration is due largely to a response to light. Light can penetrate to only about four hundred metres in sea-water, and there is complete darkness below this level. It has been shown that the swimming larvæ of one of the barnacles is positively heliotropic in a weak light, but negatively heliotropic in a stronger light. Animals having responses like these will come to the surface as the light fades away in the evening and remain there until the light becomes too bright in the following morning. They will then become negatively heliotropic and begin to go down. When they reach a level where the intensity of the light is such that they become positively heliotropic, they will turn and start upward again. Thus during the day they will keep below the surface, remaining in the region where they change from positive to negative, and vice versa.

It would not be difficult to imagine that this upward and downward migration of pelagic animals is useful to them, but, on the other hand, it may be equally well imagined that the response may be injurious to them. Thus it might be supposed that certain forms could procure their food by coming to the surface at night, and avoid their enemies by going down during the day. But it is difficult to see why organisms that serve as prey should not have acquired exactly the opposite tropisms in order to escape.

Some of these marine forms are also geotropic. Loeb has determined that “the same circumstances that make the animals negatively heliotropic also make them positively geotropic, and vice versa.” It was found, for instance, that the larva of the marine worm Polygordius is negatively geotropic at a low temperature, while at a higher temperature it is positively geotropic. This response would drive the animals upward when the water becomes too cold, and back again if the surface water becomes too warm; but whether the response is so adjusted that the animals keep, as far as possible, in water of that temperature that is best for their development, we do not know. We can easily imagine that within wide limits this is the case.

The change from positive to negative can also be brought about in other ways. One of the most striking cases of this sort is that described by Towle in one of the small crustaceans, Cypridopsis vidua. It was found that after an animal had been picked up in a pipette its response was always positive; that is, it swam toward the light, no matter what its previous condition had been. The disturbance caused by picking the animal up induced always a positive response towards light. If the light were moved, the Cypridopsis followed the light. In this way it could be kept positive for some time, but if it came to rest, or if it came into contact with the sides or end of the trough, it became, after a short time, negatively heliotropic, and remained negative as long as it could be kept in motion, without being disturbed, or coming into contact with a solid object. If when positive it were allowed to reach the glass at the end of the trough, it would swim about there, knocking against the glass, and then soon turn and swim away from the light. If the light were shifted while the negative animal was in the middle of the trough, it would turn and swim directly away, as before, from the source of light. It could be kept in this negative state as long as it did not come into contact with the ends.

It appears that the positive condition in Cypridopsis is of short duration, and ceases after a while either as a response to contact or without any observable external factor causing the change.

This crustacean lives at the bottom of pools, amongst water-plants, and here also, no doubt, the same change from one to the other reaction takes place. What possible advantage it may be to the animal to be kept continually changing in this way is not at all obvious, nor, in fact, are we obliged to assume that this reaction may be of any special use to it. Indeed, it is far from obvious how the change that causes the animal to swim toward the light when it is disturbed could be of the least advantage to it.