Evolution and Adaptation

by Thomas Hunt Morgan

Paramœcia, as well as other protozoans, show a contact response. They fix themselves to certain kinds of solid bodies. If, for example, a small bit of bacterial slime is put into the water, the paramœcia collect around it in crowds, and eat the bacteria; but they will collect in the same way around almost any solid. On coming in contact with bodies having a certain physical texture, the cilia covering the paramœcium stop moving, only those in the oral groove continuing to strike backward. The animal comes to rest, pressed against the solid body. If one or more paramœcia remain in the same place, they set free carbon dioxide, as a result of their respiratory processes. There is formed around them a region containing more of this acid than does the surrounding water. If other moving paramœcia swim, by chance, into this region, they are caught, and as a result an accumulation of individuals will take place. The more that collect the larger will the area become, and thus large numbers may be ultimately entrapped in a region where there is formed a substance that, from analogy with other animals, we should expect to be injurious.

The question as to how far these responses of the unicellular forms are of advantage to them is difficult to decide, for while, as in the above case, the response appears to be injurious rather than useful, yet under other conditions the same response may be eminently advantageous. In other cases, as when the paramœcia back away, and then swim forward again, only to repeat the process, the act appears to be such a stupid way of avoiding an obstacle that the reaction hardly appears to us in the light of a very perfect adaptation. If we saw a higher animal trying to get around a wall by butting its head into it until the end was finally reached, we should probably not look upon that animal as well adapted for avoiding obstacles.

Bacteria, which are generally looked upon as unicellular plants, appear, despite the earlier statements to the contrary, to react in much the same way as do the protozoans, according to the recent work of Rothert, and of Jennings and Crosby. The bacteria do not seem to turn toward or away from chemical substances, but they collect in regions containing certain substances in much the same way as do the protozoans. The collecting of bacteria in regions where oxygen is present has been known for some time, but it appears from more recent results that they are not attracted toward the oxygen, but by accidentally swimming into a region containing more oxygen they are held there in the same way as is paramœcium in a drop of acid. On the other hand bacteria do not enter a drop of salt solution, or of acids, or of alkalies. They react negatively to all such substances. Some kinds of bacteria have a flagellum at each end, and swim indifferently in either direction. If they meet with something that stimulates them, as they move forward, they swim away in the opposite direction, and continue to move in the new direction until something causes again a reversal of their movement. In this respect their mode of reaction seems of greater advantage than that followed by paramœcium.

Another instinct, that appears to be due to a tropic response, is the definite time of day at which some marine animals deposit their eggs. The primitive fish, Amphioxus, sets free its eggs and sperm only in the late afternoon. A jellyfish, Gonionema, also lays its eggs as the light begins to grow less in the late afternoon, and in this case it has been found that the process can be hastened if the animals are placed in the dark some hours before their regular time of laying. There is no evidence that this habit is of any advantage to the animal. We may imagine, if we like, that the early stages may meet with less risk at night, but this is not probable, for it is at this time that countless marine organisms come to the surface, and it would seem that the chance of the eggs being destroyed would then be much greater. It is more probable that the response is of no immediate advantage to the animals that exhibit it, although in particular cases it may happen to be so.

This response recalls the diurnal opening and closing of certain flowers. The flowers of the night-blooming cereus open only in the dusk of evening, and then emit their strong fragrance. Other flowers open only in the daytime, and some only in bright sunlight. It is sometimes pointed out that it is of advantage to some of these flowers to open at a certain time, since the particular insects that are best suited to fertilize them may then be abroad. This may often be the case, but we cannot but suspect that in other cases it may be a matter of little importance. In special instances it may be that the time of opening of the flowers is of importance to the species; but even if this is so, there is no need to assume that the response has been gradually acquired for this particular purpose. If it were characteristic of a new form to open at a particular time, and there were insects in search of food at this time that would be likely to fertilize the plant, then the plant would be capable of existing; but this is quite different from supposing that the plant developed this particular response, because this was the most advantageous time of day for the fertilization of its flowers.

We can apply this same point of view, I believe, to many of the remarkable series of tropisms shown by plants, whose whole existence in some cases is closely connected with definite reactions to their environment. Let us examine some of these cases.

When a seed germinates, the young stem is negatively geotropic, and, in consequence, as it elongates it turns upward towards the light that is necessary for its later growth. The root, on the contrary, is positively geotropic, and, in consequence, it is carried downward in the ground. Both responses are in this case of the highest importance to the seedling, for in this way its principal organs are carried into that environment to which they are especially adapted. It matters very little how the seed lies in the ground, since the stem when it emerges will grow upward and the root downward. The young stem, when it emerges from the soil, will turn toward the light if the illumination comes from one side, and this also may often be of advantage to the plant, since it turns toward the source from which it gets its energy. The leaves also turn their broad surfaces toward the light, and as a result they are able to make use of a greater amount of the energy of the sunlight. The turning is due to one side of the stem growing more slowly than the opposite side, and it is true, in general, that plants grow faster at night than in the daylight. Very bright light will in some cases actually stop all growth for a time. Thus we see that this bending of the stem toward the light and the turning of the leaves to face the light are only parts of the general relation of the whole plant toward the light.

Negative heliotropism is much less frequent in plants. It has been observed in aërial roots, in many roots that are ordinarily buried in the ground, in anchoring tendrils that serve as holdfasts, and even in the stems of certain climbers. In all of these cases, and more especially in the case of the climbers, the reaction is obviously of advantage to the plant; and it is significant to find, in plants that climb by tendrils carrying adhering disks, that there is a reversal of the ordinary heliotropism shown by homologous organs in other plants. There is an obvious adaptation in the behavior of the tendril, since its growth away from the more illuminated side is just the sort of reaction that is likely to bring it into contact with a solid body.

In this connection it is important to observe that these reactions to light are perfectly definite, being either positive or negative under given conditions, and therefore there is at present nothing to indicate that there has been a gradual transformation from positive to negative, or vice versa. It seems to me much more probable that when the structural change took place, that converted the plant into a climber, there appeared a new heliotropic response associated with the other change. In other words, both appeared together in the new organ, and neither was gradually acquired by picking out fluctuating variations.

The leaves of plants also show a sort of transverse heliotropic response. It has been found, for example, that the leaves of Malva will turn completely over if illuminated by a mirror from below. A curious case of change of heliotropism is found in the flower stalks of Linaria. They are at first positively heliotropic, but after the flower has been fertilized the stalk becomes negatively heliotropic. As the stalks continue to grow longer, they push the fruits into the crevices of the rocks on which the plants grow, and in this way insure the lodgement of the seeds. Here we have an excellent example showing that the negative heliotropism of the flower stalk could scarcely have been acquired by slight changes in the final direction, for only the complete change is useful to the plant. Intermediate steps would have no special value.