Evolution and Adaptation

by Thomas Hunt Morgan

The most familiar cases of adjustments of the individual to the environment are those that we recognize in our own bodies. After violent exercise we breathe more rapidly, and take deeper inspirations. Since during exercise our blood loses more oxygen and takes in more carbon dioxide from the muscles, it is clear that one result of more rapid breathing is to get more oxygen into the blood and more carbon dioxide out of it. The process of sweating, that also follows exercise, may be also looked upon as an adaptive process, since by evaporation the skin is kept cooler, and, in consequence, the blood, which at this time flows in larger quantities to the skin, is cooled also.

More permanent adaptive changes than these also take place as the result of prolonged use of certain parts. If the muscles work against powerful resistance, they become larger after several days or weeks, and are capable of doing more work than at first. Conversely, when any group of muscles is not used, it becomes smaller than the normal and capable of doing less work. It would be a nice point to decide whether this latter change is also an adaptation. If so it is one in a somewhat different sense from that usually employed. The result is of no direct advantage to the animal, except possibly in saving a certain amount of food, but since the same change will take place when an abundance of food is consumed, the result is, under these conditions, of no use.

The thickening of the skin on those parts of the body where continued pressure is brought to bear on it is a change in a useful direction. The thickening on the soles of the feet and on the palms of the hands is a case in point. Not only is the skin thicker at birth in these parts, but it becomes thicker through use. In other parts of the body also, the skin hardens and becomes thicker if pressure is brought to bear on it. We may regard this as a general property of the skin, which is present even in those parts where, under ordinary circumstances, it can rarely or never be brought into use.

Even as complicated and as much used an organ as the eye can become adaptively improved. It is said that the lateral region of the field of vision can be trained to perceive more accurately; and every one who has used a microscope is familiar with the fact that if one eye is habitually used it becomes capable of seeing more distinctly and better than the other eye. This seems to be due, in part at least, to the greater contraction of the iris.

Another phenomenon, which, I think, must be looked upon as an adaptation, is the immunity to certain poisons that can be gradually brought about by slowly increasing the amount introduced into the body. Nicotine is a most virulent poison, and yet by slowly increasing the dose an animal can be brought into a condition in which an amount of nicotine, fatal to an ordinary individual, can be administered without any ill effects at all resulting.

The same phenomenon has been observed in the case of other poisons, not only in case of other alkaloids, such as morphine and cocaine, but also in the case of caffein, alcohol, and even arsenic. There is a curious phenomenon in regard to arsenic, which appears to be well established, viz., that a person who has gradually increased the dose to an amount great enough to kill ten ordinary men, will die if he suddenly ceases altogether to take arsenic. He can, however, be gradually brought back to a condition in which arsenic is not necessary for his existence, if the dose is gradually decreased. It is a curious case of adaptation that we meet with here, since the man becomes so thoroughly adjusted to a poison that if he is suddenly brought back to the normal condition of the race he will die.

Immunity to the poison of venomous snakes can also be acquired by slowly increasing the amount given to an animal. It is possible to make a person so immune to the poison of venomous snakes that he would become, in a sense, adapted to live amongst them without danger to himself. It is to be noted, moreover, that this result could be reached only by quite artificial means, for, under natural conditions it is inconceivable that the nicely graded series of doses of increasing strength necessary to bring about the immunity could ever be acquired. Hence we find here a case of response in an adaptive direction that could not have been the outcome of experience in the past. It is important to emphasize this capacity of organisms to adapt themselves to certain conditions entirely new to them.

These cases lead at once to cases of immunity to certain bacterial diseases. An animal may become immune to a particular disease in several ways. First, by having the disease itself, which renders it immune for a longer or a shorter period afterwards; or, second, by having a mild form of the disease as in the case of smallpox, where immunity is brought about by vaccination, i.e. by giving the individual a mild form of smallpox; or, third, by introducing into the blood an antidote, in the form, for example, of antitoxin, which has been made by another animal itself immune to the disease. The first two classes of immunity may be looked upon as adaptations which are of the highest importance to the organism; the last case can scarcely be looked upon as an adaptive process, since the injurious effect of the poison may as well be neutralized outside of the body by mixing it with the antitoxin. We may suppose, then, that in the body a similar process goes on, so that the animal itself takes no active part in the result.

When we consider that there are a number of bacterial diseases, in each of which a different poison is made by the bacteria, we cannot but ask ourselves if the animal really makes a counter-poison for each disease, or whether a single substance may not be manufactured that counteracts all alike? That the latter is not the case is shown by the fact that an animal made immune to one disease is not immune to others. When we recall that the animal has also the capacity to react in one way or another to a large number of organic and inorganic poisons, to which it or its ancestors can have had little or no previous experience, we may well marvel at this wonderful regulative power.

The healing of wounds, which takes place in all animals, forms another class of adaptive processes. The immense usefulness of this power is obvious when it is remembered how exposed most animals are to injuries. By repairing the injury the animal can better carry on its normal functions. Moreover, the presence of the wound would give injurious bacteria a ready means of entering the body. In fact, an intact skin is one of the best preventives to the entrance of bacteria.