Evolution and Adaptation

by Thomas Hunt Morgan

Other characteristics also, which arise from different kinds of external influences due to different localities, such as dampness and shade, a swampy region, or different geological substrata, last only so long as the external conditions last.

These transient peculiarities make up the characters of local varieties. That they have no permanency is intelligible, since they exhibit no new characters, but the change consists mainly in the over- or under-development of those peculiarities that are dependent on external influences. The effect of these influences may be compared to an elastic rod, which, however much it may be distorted by external circumstances, returns again to its original form as soon as released.

Besides these temporary changes, due to external influences, there are many cases known in which the same plant lives under very diverse conditions and yet remains exactly the same. For example, the species of Rhododendron ferragineum lives on archæan mountains and especially where the soil is poor in calcium. Another species, Rhododendron hirsutum is found especially on soil rich in calcium. The difference in the two species has been supposed to depend on differences in the soil, and if so, we would imagine that, if transplanted for a long time, the one should change in the direction of the other. Yet it is known that the rusty rhododendron may be found in all sorts of localities, even on dry, sunny, calcareous rocks of the Apennines and of the Jura, and despite its residence in these localities, since the glacial epoch, no change whatever has taken place.

Single varieties of the large and variable genus of Hieracium have lived since the glacial period in the high regions of the Alps, Carpathians, and in the far north, and also in the plains of different geological formations, but these varieties have remained exactly the same, although on all sides there are transitional forms leading from these to other varieties.

Some parasitic species also furnish excellent illustrations of the same principle. Besides the several species of Orobanchia and of the parasitic moulds, the mistletoe deserves special mention. It lives on both birch and apple trees and on both presents exactly the same appearance; and even if it is true that mistletoe growing on conifers presents certain small deviations in its character, it is still doubtful whether, if transferred to the birch or apple tree, it would not lose these differences, thus indicating that they are not permanent.

It is a fact of general observation that, on the one hand, the same variety occurs in different localities and under different surroundings, and, on the other hand, that slightly different varieties live together in the same place and therefore under the same external conditions. It is evident, then, that food conditions have neither originated the differences nor kept them up. The rarer cases in which in different localities different varieties exist show nothing, because competition and suppression keep certain varieties from developing where it would be possible otherwise for them to exist.

Nägeli says his conclusion may be tested from another point of view. If food conditions, as is generally supposed, have a definite, i.e. a permanent, effect on the organism, then all organisms living under the same conditions should show the same characters. Indeed, it has been claimed in some instances that this is actually the case. Thus it is stated that dry localities cause plants to become hairy, and that absence of hairiness is met with in shady localities. This may apply to certain species, but in other cases exactly the reverse is true, and even the same species behaves differently in different regions, as in Hieracium. And so it is with all characteristics which are ascribed to external influences. As soon as it is supposed a discovery has been made in this direction, we may rest assured that in other cases the reverse will be found to hold. We have had, in respect to the influence of the outer world on organisms, the same experience as with the rules for the weather,—when we come to examine the facts critically there are found to be as many exceptions as confirmations of the rule.

If climatic influence has a definite effect, the entire flora of a special locality ought to have the same peculiarities, but this stands in contradiction to all the results of experience. The character of the vegetation is not determined by the environment of the plants but by their prehistoric origin, and as the result of competition. Nägeli concludes his discussion with the statement that all of our experience goes to show that the effects of external influences (climate and food) appear at once, and their results last only as long as the influences themselves last, and are then lost, leaving nothing permanent behind. This is true even when the external influences have lasted for a long time,—since the glacial epoch, for instance. We find, he claims, nothing that supports the view that such influences are inherited.

If we next examine the question of changes from internal causes, Nägeli claims that here also observation and research fail to show the origin of a new species, or even of a new variety from external causes. In the organic world little change has taken place, he believes, since the glacial epoch. Many varieties have even remained the same throughout the whole intervening time; and while it cannot be doubted that new varieties have also been formed, yet the cause of their origin cannot be empirically demonstrated. The permanent, hereditary characters, of whose origin we know something from experience, belong to the individual changes which have appeared under cultivation in the formation of domestic races. These are for the most part the result of crossing. So far as we have any definite information as to the origin of the changes, they are the result of inner, and never of external, causes. We recognize that this must be the case, since under the same external conditions individuals behave differently—in the same flower-bud some seeds give rise to plants like the parent, others to altered ones. The strawberry with a single leaflet, instead of three, arose in the last century in a single individual amongst many other ordinary plants. From the ten seeds of a pear Van Mons obtained as many different kinds of pears. The most conclusive proof of the action of inner causes is most clearly seen when the branches of the same plant differ. In Geneva a horse-chestnut bore a branch with “filled” flowers, and from this branch, by means of cuttings, this variation has been carried over all Europe. In the Botanic Garden at Munich there is a beech with small divided leaves; but one of its branches produces the common broad undivided leaves. Many such examples have been recorded which can only be explained by assuming that a cell, or a group of cells, like those from which the other branches arose, have become changed in some unknown way as the result of inner causes. The properties that are permanent and inherited are contained in the idioplasm, which the parent transmits to its offspring. A cause that permanently transforms the organism must also transform the idioplasm. How powerless, in comparison to internal causes, the external causes are is shown most conclusively in grafting. The graft, although it receives its nourishment through the stock, which may be another species, remains itself unchanged.

Nägeli makes the following interesting comparison between the development of the individual from an egg, and the evolution, or development, of the phylum. No one will doubt that the egg during the entire time of its process of transformation is guided by internal factors. Each successive stage follows with mechanical necessity from the preceding. If an animal can develop from inner causes from a drop of plasma, why should not the entire evolutionary process have also been the outcome of developmental inner causes? He admits that there is a difference in the two cases in that the plasma that forms the egg has come from another animal, and contains all the properties of the individual in a primordial condition. In the other case we must suppose that the original drop of plasma did not contain at first the primordium of definite structures, but only the ability to form such. Logically the difference is unimportant. The main point is that in the primordium of the germ a special peculiarity of the substance is present which by forming new substances grows, and changes as it grows, and the one change of necessity excites the next until finally a highly organized being is the result.