Evolution and Adaptation

by Thomas Hunt Morgan

Davenport and Castle carried out a series of experiments on the egg of the toad, in which they tried to acclimatize the eggs to a temperature higher than normal. Recently laid eggs were used; one lot kept at a temperature of 15 degrees C., the other at 24-25 degrees C. Both lots developed normally. At the end of four weeks the temperature point at which the tadpoles were killed was determined. Those reared at a temperature of 15 degrees C. died at 41 degrees C., or below; those reared at 24-25 degrees C. sustained a temperature 10 degrees higher; no tadpole dying in this set under 43 degrees C. “This increased capacity for resistance was not produced by the dying off of the less resistant individuals, for no death occurred in these experiments during the gradual elevation of the temperatures in the cultures.” The increased resistance was due, therefore, to a change in the protoplasm of the individuals. It was also determined that the acquired resistance was only very gradually lost (after seventeen days’ sojourn in cooler water). The explanation of this result may be due, in part, to the protoplasm containing less water at higher temperatures, for it is known that while the white of egg (albumen) coagulates at 56 degrees C. in aqueous solution; with only 18 per cent of water it coagulates between 80 degrees and 90 degrees C.; and with 6 per cent, at 145 degrees C.; and without water between 100 degrees and 170 degrees C.

It has long been known that organisms in the dry condition resist a much higher temperature. The damp uredospore is killed at 58.5 degrees to 60 degrees C.; but dry spores withstand 128 degrees C. It is also known that organisms may become acclimatized to cold through loss of water, but we lack exact experimental data to show to what extent this can be carried.

There are also some experiments that go to show that animals may become attuned to certain amounts of light, but the facts in this connection will be described in another chapter.

Some important results have been obtained by accustoming organisms to solutions containing various amounts of salts. A number of cases of this sort are given by De Varigny. It has been found that littoral marine animals that live where the water may become diluted by the rain, or by rivers, survive better when put into fresh water than do animals living farther from the shore. Thus the oyster, the mussel, and the snail, Patella, withstand immersion in fresh water better than other animals that live farther out at sea. The reverse is also true; fresh-water forms, such as Lymnæa, Physa, Paludina, and others may be slowly acclimatized to water containing more salt. The forms mentioned above could be brought by degrees into water containing 4 per cent of salt, which would have killed the animals if they had been brought suddenly into it. Similar results have been obtained for amœba.

It has been shown that certain rotifers and tardigrades, and also some unicellular animals, that live in pools and ponds that are liable to become dry, withstand desiccation, while other members of the same groups, living in the sea, do not possess this power of resistance. Cases of this sort are usually explained as cases of adaptation, but it has not been shown experimentally that resistance to drying can be acquired by a process of acclimatization to this condition. The case is also in some respects different from the preceding, since intermediate conditions are less likely to be met with, or to be of sufficiently long duration for the animal to become acclimatized to them. It seems more probable, in such cases, that these forms have been able to live in such precarious conditions from the beginning because they could resist the effects of drying, not that they have slowly acquired this power. Finally, there must be discussed the question of the acclimatization to poisons, to which an individual may be rendered partially immune. The point of special importance in this connection is that the animal may be said to respond adaptively to a large number of substances, which it has never met before in its individual history, or to which its ancestors have never been subjected. It may become slowly adapted to many different kinds of injurious substances. These cases are amongst the most important adaptive individual responses with which we are familiar, and the point cannot be too much emphasized that organisms have this latent capacity without ever having had an opportunity to acquire it through experience.

The preceding groups of phenomena, included under the general heading of individual acclimatization, have one striking thing in common, namely, that a physiological adaptation is brought about without a corresponding change in form, although we must suppose that the structure has been altered in certain respects at least. The form of the individual remains the same as before, but so far as its powers of resistance are concerned it is a very different being.

In regard to the perpetuation of the advantages gained by means of this power of adaptation, it is clear in those cases in which the young are nourished during their embryonic life by the mother, that, in this way, the young may be rendered immune to a certain extent, and there are instances of this sort recorded, especially in the case of some bacterial diseases. Whether this power can also be transmitted through the egg, in those instances in which the egg itself is set free and development takes place outside the body, has not been shown. In any case, the effect appears not to be a permanent one and will wear off when the particular poison no longer acts. It is improbable, therefore, that any permanent contribution to the race could be gained in this way. Adaptations of this sort, while of the highest importance to the individual, can have produced little direct effect on the evolution of new forms, although it may have been often of paramount importance to the individuals to be able to adapt themselves, or rather to become able to resist the effect of injurious substances. The important fact in this connection is the wonderful latent power possessed by all animals. So many, and of such different kinds, are the substances to which they may become immune, that it is inconceivable that this property of the organism could ever have been acquired through experience, no matter how probable it may be made to appear that this might have occurred in certain cases of fatal bacterial diseases. And if not, in so many other cases, why invent a special explanation for the few cases?

We may defer the general discussion of the rôle that external factors have played in the adaptation of organisms, until we have examined some of the theories which attribute changes to internal factors. The idea that something innate in the living substance itself has served as the basis for evolution has given rise to a number of different hypotheses. That of the botanist Nägeli is one of the most elaborately worked out theories of this sort that has been proposed, and may be examined by way of illustration.

Nägeli’s Perfecting Principle

Nägeli used the term completing principle (“Vervollkommungsprincip”) to express a tendency toward perfection and specialization. Short-sighted writers, he says, have pretended to see in the use of this principle something mystical, but on the contrary it is intended that the term shall be employed in a purely physical sense. It represents the law of inertia in the organic realm. Once set in motion, the developmental process cannot stand still, but must advance in its own direction. Perfection, or completion, means nothing else than the advance to complicated structure, “but since persons are likely to attach more meaning to the word perfection than is intended, it would perhaps be better to replace it with the less objectionable word progression.”