Evolution and Adaptation

by Thomas Hunt Morgan

Nägeli says that Darwin, having in view only the condition of adaptation, designates that as more complete which gives its possessor an advantage in the battle for existence. Nägeli claims that this is not the only criterion that applies to organisms, and it leaves out the most important part of the phenomenon. There are two kinds of completeness which we should keep distinctly apart: (1) the completeness of organization characterized by the complication of the structure and the most far-reaching specialization of the parts; (2) the completeness of the adaptation, present at each stage in the organization, which consists in the most advantageous development of the organism (under existing conditions) that is possible with a given complication of structure and a given division of functions.

The first of these conceptions Nägeli always calls “completeness” (Vollkommenheit), for want of a simpler and better expression; the second he calls adaptation. By way of illustrating the difference between the two, the following examples may be given. The unicellular plants and the moulds are excellently adapted each to its conditions of life, but they are much less complete in structure than an apple tree, or a grape vine. The rotifers and the leeches are well adapted to their station, but in completeness of structure they are much simpler than the vertebrates.

If we consider only organization and division of labor as the work of the completing principle, and leave for the moment adaptation out of account, we may form the following picture of the rise of the organic world. From the inorganic world there arose the simplest organic being thinkable, being little more than a drop of substance. If this underwent any change at all, it would have been necessarily in the direction of greater complication of structure; and this would constitute the first step in the upward direction. In this way Nägeli imagines the process once begun would continue. When the movement has reached a certain point, it must continue in the same direction. The organic kingdom consists, therefore, of many treelike branches, which have had a common starting-point. Not only does he suppose that organisms were once spontaneously generated, and began their first upward course of development, but the process has been repeated over and over again, and each time new series have been started on the upward course. The organic kingdom is made up, therefore, of all degrees of organization, and all these have had their origins in the series of past forms that arose and began their upward course at different times in the past. Those that are the highest forms at the present time represent the oldest series that successfully developed; the lowest forms living at the present time are the last that have appeared on the scene of action.

Organisms, as has been said, are distinguished from one another, not only in that one is simpler and another more complicated, but also in that those standing at the same stage of organization are unequally differentiated in their functions and in their structure, which is connected primarily with certain external relations which Nägeli calls adaptations.

Adaptation appears at each stage of the organization, which stage is, for a given environment, the most advantageous expression of the main type that was itself produced by internal causes. For this condition of adaptation, a sufficient cause is demanded, and this is, as Nägeli tries to show later, the result of the inherited response to the environment. In many cases this cause will continue to act until complete adaptation is gained; in other cases, the external conditions give a direction only, and the organism itself continues the movement to its more perfect condition.

The difference between the conception of the organic kingdom as the outcome of mechanical causes on the one hand, or of competition and extermination on the other hand, can be best brought out, Nägeli thinks, by the following comparison of the two respective methods of action. There might have been no competition, and no consequent extermination in the plant kingdom, if from the beginning the surface of the earth had continually grown larger in proportion as living things increased in numbers, and if animals had not appeared to destroy the plants. Under these conditions each germ could then have found room and food, and have unfolded itself without hinderance. If now, as is assumed to be the case on the Darwinian theory, individual variations had been in all directions, the developmental movement could not have gone beyond its own beginnings, and the first-formed plants would have remained swinging now on one side and now on another of the point first reached. The whole plant kingdom would have remained in its entirety at its first stage of evolution, that is, it would never have advanced beyond the stage of a naked drop of plasma with or without a membrane. But, according to the further Darwinian conception, competition, leading to extermination, is capable of bringing such a condition to a higher stage of development, since it is assumed that those individuals which vary in a beneficial direction would have an advantage over those that have not taken such a step, or have made a step backward.

If, on the other hand, under the above-mentioned conditions of unrestricted development, without competition, variations were determined by “mechanical principles,” then, according to Nägeli’s view, all plant forms that now exist would still have evolved, and would be found living at the present time, but along with all those that now exist there would be still other forms in countless numbers. These would represent those forms which have been suppressed. On Nägeli’s view competition and suppression do not produce new forms, but only weed out the intermediate forms. He says without competition the plant kingdom would be like the Milky Way; in consequence of competition the plant kingdom is like the firmament studded with bright stars.

The plant kingdom may also be compared to a branched tree, the ends of whose branches represent living species. This tree has an inordinate power of growth, and if left to itself it would produce an impenetrable tangle of interwoven branches. The gardener prevents this crowding by cutting away some of the parts, and thus gives to the tree distinct branches and twigs. The tree would be the same without the watchful trimming of the gardener, but without definite form.

Nägeli states: “From my earlier researches I believe that the external influences are small in comparison to the internal ones. I shall speak here only of the influences of climate and of food, which are generally described as the causes of change, without however any one’s having really determined whether or not a definite result can be brought about by these factors. Later I shall speak of a special class of external influences which, according to my view, bring forth beyond a doubt adaptive changes.”

The external influence of climate and of food act only as transitory factors. A rich food supply produces fat, lack of food leads to leanness, a warm summer makes a plant more aromatic, and its fruit sweeter; a cold year means less odor and sour fruit. Of two similar seeds the one sown in rich soil will produce a plant with many branches and abundance of flowers; the other, planted in sandy soil, will produce a plant without branches, with few flowers, and with small leaves. The seeds from these two plants will behave in exactly the same way; they have inherited none of the differences of their parents. Influences of this sort, even if extending over many generations, have no permanent effect. Alpine plants that have lived since the ice age under the same conditions, and have the characters of true high-mountain plants, lose these characters completely during the first summer, if transplanted to the plains. Moreover, it makes no difference whether the seed or the whole plant itself be transferred. In place of the dwarfed, unbranched growth, and the reduced number of organs, the plant when transferred to the plains shoots up in height, branches strongly, and produces numerous leaves and flowers. The plants retain their new characters as long as they live in the plain without any other new variation being observed in them.