Evolution and Adaptation

by Thomas Hunt Morgan

Nägeli discusses a question in this connection, which, he says, has been unnecessarily confused in the descent theory. Since we are entirely in the dark as to how much time has been required for the formation of phyla, so also are we ignorant as to how long it may have taken for each step in advance. We may err equally in ascribing too much and too little time to the process. It is, moreover, not necessary that for every step the same amount of time should have been required. On the contrary, the probability is that recognizable changes may at times follow each other rapidly, and then for a time come to a standstill,—just as in the development of the individual there are periods of more rapid and others of less rapid change.

A more difficult problem than that relating to the sort of changes the external influences bring about in the organism, is the question as to how they effect the organism, or how they act on it mechanically. This, as is well known, was answered by Darwin, who regards all organization as a problem of adaptation: only those chance variations surviving which are capable of existence, the others being destroyed. On this theory external influences have only a negative or a passive action, namely, in setting aside the unadapted individuals. Nägeli, on the other hand, looks upon some kinds of external conditions as directly giving rise to the adaptive characters of the organism. This is accomplished, he supposes, in the following ways: two kinds of influence are recognized; the direct action, which, as in inorganic nature, comes to an end when the external influences come to an end, as when cold diminishes the chemical actions in the plant; and the indirect action, generally known as a stimulus, which starts a series of molecular motions, invisible to us, but which we recognize only in their effects. Very often the stimulus starts only a reflex action, usually at the place of application.

A stimulus acting for but a short time produces no lasting effect on the idioplasm. A person stung by a wasp suffers no permanent effect from the injury. But if a stimulus acts for a long time, and through a large number of generations, then it may, even if of small strength, so change the idioplasm, that a tendency or disposition capable of being seen may be the result. This appears to be the case in regard to the action of light, which causes certain parts of the plant to turn toward it and others away from it; also for the action of gravity, which determines the downward direction of the roots. It may be claimed, perhaps, that these are the results of direct influence and not of an internal response, but this is not the case; for some plants act in exactly the opposite way, and send a stem downward, as in the case of the cleistogamous flowers of Cardamine chenopodifolia; and other plants turn away from the light. This means that the idioplasm behaves differently in different plants in response to the same stimulus.

Concerning the more visible effects of adaptation, Nägeli states that in regard to some of them there can be no question as to how they must have arisen. Protection against cold, by the formation of a thick coat of hair, is the direct result of the action of the cold on the skin of the animal. The different weapons of offence and of defence, horns, spurs, tusks, etc., have arisen, he maintains, through stimulus to those parts of the body where these structures arise.

The causes of the other adaptations, especially of those occurring in plants, are less obvious. Land plants protect themselves from drying by forming a layer of cork over the surface. The most primitive plants were water plants, which acclimated themselves little by little to moist, and then to dry, air. When they first emerged from the water the drying acted as a stimulus on the surface, and caused it to harden in the same way as a drop of glue hardens. This hardening in turn acted as a stimulus, causing a chemical transformation of the surface into a corky substance. This effect was inherited, and in this way the power to form cork originated.

Land plants have, in addition to the soft parts, the hard bast and wood which serves the mechanical purpose of supporting the soft tissues and protecting them from being injured. The arrangement of the hard parts is such as to suggest that they are the result of the action of pressures and tensions on the plant, for the strongest cells are found where there is most need for them. It is easy to imagine, Nägeli adds, that this important arrangement of the tissues is the result of external forces which brought about the result in these parts.

Nägeli accounts for the origin of twining plants as follows. Being overshadowed by other plants, the stem will grow rapidly in the damp air. Coming in contact with the stems of other plants, the delicate stem is stimulated on one side, and grows around the point of contact. This tendency becomes inherited, and the habit to twine is ultimately established.

The difference in the two sides of leaves is explained by Nägeli as the result of the difference in the illumination of the two sides. This influence of light on the leaf has been inherited. The formation of the tubular corolla that is seen in many plants visited by insects is explained as the result of the stimulus produced by the insects in looking for the pollen. The increase in the length of the proboscis of the insect is the result of the animal straining to reach the bottom of the ever elongating tube of the corolla. “The tubular corolla and the proboscis of the insect appear as though made for each other. Both have slowly developed to their present condition, the long tube from a short tube and the long proboscis from a short one.” Thus, by purely Lamarckian principles, Nägeli attempts to account for many of the adaptations between the organism and the outer world. But if this takes place, where is there left any room for the action for his so-called perfecting principle? Nägeli proceeds to show how he supposes that the two work together.

As a result of inner causes the organism would pass through a series of perfectly definite stages, J, J¹, J². But if, at any stage, external influences produced an effect on the organism so that the arrangement of the idioplasm changes in response, a new adaptation is produced. In this way new characters, not inherent in the idioplasm, may be added, and old ones be changed or lost. “In order not to be misunderstood in regard to the completing or perfecting principle I will add, that I ascribe to it no determinate action in the organism, neither in producing the long neck of the giraffe, nor the prehensile tail of the ape, neither the claws of the crab, nor the decoration of the bird of paradise. These structures are the outcome of both factors. I cannot picture to myself how external causes alone, and just as little how internal causes alone, could have changed a monad into a man.” But Nägeli goes on to say, that if at any stage of organization one of the two causes should cease to act, the other could only produce certain limited results. Thus, if external causes alone acted, the organization would remain at the same stage of completeness, but might become adapted to all kinds of external conditions—a worm, for instance, would not develop into a fish, but would remain a worm forever, although it might change its worm structure in many ways in response to external stimuli. If, on the other hand, only the completing principle acted, then without changing its adaptations the number of the cells and the size of the organs might be increased, and functions that were formerly united might become separated. Thus, without altering the character of the organism, a more highly developed (in the sense of being more specialized) organism would appear.

Nägeli, as we have just seen, has attempted to build up a conception of nature based on two assumptions, neither of which has been demonstrated to be an actual principle of development. His hypothesis appears, therefore, entirely arbitrary and speculative to a high degree. Even if it were conceivable that two such principles as these control the evolution of organisms, it still requires a good deal of imagination to conceive how the two go on working together. Moreover, it is highly probable that whole groups have evolved in the direction of greater simplification, as seen especially in the case of those groups that have become degenerate. To what principle can we refer processes of this sort?