Evolution and Adaptation

by Thomas Hunt Morgan

Adjustments of the Individual to Changes in the Environment

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.

Not only have most organisms the power of repairing injuries, but many animals have also the closely related power of regenerating new parts if the old ones are lost. If a crab loses its leg, a new one is regenerated. If a fresh-water worm (Lumbriculus) is cut into pieces, each piece makes a new head at its anterior end and a new tail at the posterior end. In this way as many new worms are produced as there are pieces. And while in a strict sense it cannot be claimed that this power of regeneration is of any use to the original worm, since the original worm, as such, no longer exists, yet since it has not died but has simply changed over into several new worms, the process is of use inasmuch as by this means the pieces can remain in existence.

We need not discuss here the relative importance to different animals of this power of regeneration, but it may be stated, that, while in some cases it may be necessary to replace the lost part if the animal is to remain in existence, as when a new head is formed on an earthworm after the old one was cut off, in other cases the replacement of the lost part appears to be of minor importance, as in the case of the leg of the crab. While we are not, for the moment, concerned with the relative importance of the different adaptations, this question is one of much importance in other connections and will be considered later.

The protective coloration of some animals, which is the direct result of a change in color of the animal in response to the surroundings, furnishes us with some most striking cases of adaptive coloration. A change of this sort has been recorded in a number of fishes, more especially in the flounders. The individuals found living on a dark background are darker than those living on a lighter background; and when the color of the background is changed it has been observed that the color of the fish also changes in the same direction. I have observed a change of this sort from dark to light, or from light to dark, in the common minnow (Fundulus) in accordance with a change of its background, and the same sort of change appears to take place in many other fishes.

The change from green to brown and from brown to green in certain tree frogs and in the lizard (Anolis), which is popularly supposed to take place according to whether the background is green or brown, is not after all, it appears, connected with the color of the background, but depends on certain other responses of the animals that have not yet been satisfactorily made out. If it be claimed that in summer the animal would generally be warm, and therefore, often green, and that this color would protect it at this time of year when the surroundings are green, and in winter brown, when this color is the prevailing one in temperate regions, then it might appear that the change is of use to the animal; but if it is true that the same change takes place in some of the lizards that live in the tropics, where the prevailing color is always green, it would appear that the result may have no direct relation with the surroundings. It has been shown in a number of well-authenticated cases that the pupæ of certain butterflies vary in color within certain limits in response to the color of the background. When the caterpillar fixes itself to some surface, and there throws off the outer skin, and acquires a new one, the color of the latter is influenced by the background. The result is a better protection to the pupa. The change is not brought about through the ocelli or eyes, but through the general surface of the skin, for the same change takes place when the eyes have been previously covered with a dark pigment.

The growth of plants toward the light may be looked upon as an adaptive process, since only in the light can they find the conditions necessary for their life. The extraordinary elongation of shoots and young plants when grown in the dark may also be considered an adaptation for finding the light, since in this way a plant, deeply embedded in the ground, may ultimately reach the surface. Thus while the actual process of elongation in the dark is not in itself of any use, yet under the ordinary conditions of its life, this response may be of great benefit to the plant.

The closing together of the leaves of some plants has been supposed to protect them from too rapid radiation of heat, and incidentally this purpose may be fulfilled; but since some tropical plants also close their leaves during the night, it can hardly be maintained that the closing has been acquired for this purpose. It has been suggested that the opening of certain flowers under certain conditions of light is connected with the visits of insects that bring about cross-fertilization.

The preceding examples will suffice to give a general idea of what is meant by adaptation in organisms. That the term includes a large number of phenomena of very different kinds is apparent. When we have examined these phenomena further we shall find, I think, that it will be necessary to put some of them into different categories and treat them differently. It is probably incorrect to suppose that all processes useful to the organism have been acquired in the same way, nevertheless, for the present the term adaptation is sufficiently general, even if vague, to cover these different groups of cases.

It may be asked, in what respects are these structures and processes of adaptation different from the ordinary structures and changes that go on in the organism? Why is the leg of the mole more of an adaptation than that of a dog? The one is of as much use as the other to its possessor. What reason can we give for citing the poison of the snake, and not mentioning in the same connection the other glands of the body? In fact, the poison gland of the snake is supposed to be a modified superior labial gland. Why, in short, are not the processes of digestion, excretion, secretion, the beating of the heart, the ordinary reflex acts of the nervous system, and the action of the sense-organs, as truly adaptations as the special cases that have been selected for illustration. The answer is simply that we are more impressed by those cases of adaptation that are more unusual, as when an animal departs in the use of certain structures from the rest of the group to which it belongs. For example, if all mammals lived underground, ourselves included, and the fore-legs or arms were used for burrowing, we should not think this unusual; but if we found an animal using all four legs to support the body and for purposes of progression, we should, most likely, think this was an excellent illustration of adaptation.

In other instances the condition is somewhat different. The color of certain animals may unquestionably be of use to them in concealing them from their enemies. In other cases the color may not serve this purpose, or any purpose at all. Thus while in the former case we speak of the color as an adaptation to the surroundings, in the latter we do not think of it as having any connection at all with the environment. Even in the same animal the color of different parts of the body may appear under this twofold relation. For example, the green color of the skin of the frog renders it less conspicuous amongst the green plants on the edge of the stream, but the brilliant orange and black pigment in the body-cavity cannot be regarded as of any use to the animal.