Evolution and Adaptation

by Thomas Hunt Morgan

In another crustacean, one of the marine copepods, Labidocera æstiva, it has been shown by Parker that the male and female react in a somewhat different way both to light and to gravity. The females are strongly negatively geotropic, and this sends them up to the top of the water. The males are very slightly negatively geotropic. The females are strongly positively heliotropic toward light of low intensity; the males show the same response to a less degree. To strong light the females are negative and the males are indifferent. On the other hand, the males are attracted to the females, probably in response to some chemical substance diffusing from the females, since the males show the same reaction when the females are enclosed in an opaque tube through whose ends a diffusion of substances may take place. This crustacean frequents the surface of the ocean from sunset to sunrise. During the day it retires to deeper water. Its migrations can be explained as follows: The females come to the surface at night, because they are positively heliotropic to weak light, and also because they are negatively geotropic. They go down during the day, because they react to bright light more strongly than to gravity. The males follow the females, largely because they react positively chemotactically toward the females.

Some other animals respond in a somewhat different way to light, as shown by the fresh-water planarians. These animals remain during the day under stones, where the amount of light is relatively less than outside. If they are placed in a dish in the light in front of a window, they crawl away from the light, but when they reach the back of the dish they do not come to rest, but continue to crawl around the sides of the dish even toward the light. The light makes the worms restless, and while they show a negative response as long as they are perfectly free to move away from the light, they will not come to rest when they come to the back of the dish if they are there still in the light, because the irritating action of the light on them is stronger than its directive action. If, however, in crawling about they come accidentally into a place less bright than that in which they have been, they stop, and will not leave this somewhat darker spot for a brighter one, although they might leave the newly found spot for one still less bright.

At night the planarians come out and wander around, which increases their chance of finding food, although it would not be strictly correct to say that they come out in search of food. If, however, food is placed near them, a piece of a worm, for example, they will turn toward it, being directed apparently by a sense of smell, or rather of taste.

The heliotropic responses of the planarians appear to be of use to them, causing them to hide away in the daytime, and to come out only after dark, when their motions will not discover them to possible enemies. But some of the planarians are protected in other ways, so that they will not be eaten by fish, probably owing to a bad taste; so that it is not so apparent that they are in real need of the protection that their heliotropic response brings to them. Their turning towards their food is, however, beyond question of great advantage to them, for in this way they can find food that they cannot detect in any other way.

The unicellular plants were amongst the first organisms whose tropic responses were studied, and the classical work of Strasburger gave the impetus to much of the later work. In recent years the unicellular animals, the protozoans, have been carefully studied, more especially by Jennings. His results show that the reactions in these animals are different in some important respects from those met with in higher forms. For instance, most of the free-swimming infusoria are unsymmetrical, as are also many of the flagellate forms, and as they move forward they rotate freely on a longitudinal axis. It is therefore impossible that they could orientate themselves as do the higher animals that have been described above, and we should not expect these Protozoa to react in the same way. In fact, Jennings shows that they exhibit a different mode of response. Paramœcium offers a typical case. As it moves forward it rotates toward the aboral side of the body. As a result of the asymmetry of the body, the path followed, as it revolves on its own axis, is that of a spiral. Did the animal not rotate, as it swims forward, its asymmetrical form would cause it to move in a circle, but its rotation causes, as has been said, the course to be that of a spiral, and the general direction of movement is forward.[^[31]] The rotation of a paramœcium on its axis is in turn caused by the oblique stroke of the cilia that cover the surface of the body. Their action when reversed causes the animal to rotate backward.

[31]. The same result is attained by a bullet that is caused by the rifling to rotate as it moves forward.

If a drop of weak acid be put into the water in which the paramœcia are swimming,—for instance, in the water between a cover-slip and a slide,—it will be found, after a time, that many individuals have collected in the drop. It was at first supposed that the paramœcia are attracted by the diffusion of the acid in the water, and turn toward the source of the chemical stimulus; but Jennings has shown that this is not the way in which the aggregation is brought about. If the individuals are watched, it will be found that they swim forward in a spiral path without regard to the position of the drop of acid. If one happens, by chance, to run into the drop, there is no reaction as it enters, but when it reaches the other side of the drop, and comes into contact with the water on this side, it suddenly reacts. It stops, backs into the middle of the drop, rotates somewhat toward the aboral side (i.e. away from the vestibule), and then starts forward again, only to repeat the action on coming into contact with the edge of the drop again. The paramœcium has been caught in a veritable trap. All paramœcia that chance to swim into the drop will also be caught, until finally a large number will accumulate in the region. The result shows, that, in passing from ordinary water into a weak acid, no reaction takes place; but having once entered the acid, the animal reacts on coming into contact with the water again.

On the other hand, there are some substances to which the paramœcium may be said to be negatively chemotropic. If a drop of a weak alkaline solution be put into water in which paramœcium is swimming, an individual that happens to run against it reacts at once. It stops instantly, backs off, revolving in the opposite direction, turns somewhat to one side, and swims forward again. The chances are that it will again hit the drop, in which case it repeats the same reaction, turning again to one side. If it continues to react in this way, it will, in the course of time, turn so far that when it swims forward it will miss the edge of the drop, and then continue on its way. If an individual were put into an alkaline drop, it would leave it, because it would not react when it passed from inside the drop into the surrounding water.

Unicellular animals react to other things besides differences in the chemical composition of different parts of a solution. In many cases they react to light, swimming toward or away from it according to whether they are positively or negatively heliotropic. If they are positively heliotropic, and while swimming run into a shadow, they react as they would on coming into contact with a drop of acid. Since they rotate as they swim forward, we cannot explain their orientation as in the case of other animals that hold a fixed vertical position. If we assume that the two ends of the body are differently affected by the light, for which there is some evidence, we can perhaps in this way account for their turning toward, or away from, the source of light.

Changes in the osmotic pressure of the different parts of the fluid, mechanical stimulation produced by jarring, extremes of heat and of cold, all cause this same characteristic reaction in Paramœcium; and this accounts for their behavior toward these agents that are so different in other respects.