Evolution and Adaptation

by Thomas Hunt Morgan

Weismann lays great emphasis on the case of the Indian leaf-butterfly, Kallima inachis; and points out that the leaf markings are executed “in absolute independence of the other uniformities governing the wing.”

“The venation of the wing is utterly ignored by the leaf markings, and its surface is treated as a tabula rasa upon which anything conceivable can be drawn. In other words, we are presented here with a bilaterally symmetrical figure engraved on a surface which is essentially radially symmetrical in its divisions.

“I lay unusual stress upon this point because it shows that we are dealing here with one of those cases which cannot be explained by mechanical, that is, by natural means, unless natural selection actually exists and is actually competent to create new properties; for the Lamarckian principle is excluded here ab initio, seeing that we are dealing with a formation which is only passive in its effects: the leaf markings are effectual simply by their existence and not by any function which they perform; they are present in flight as well as at rest, during the absence of a danger, as well as during the approach of an enemy.

“Nor are we helped here by the assumption of purely internal motive forces, which Nägeli, Askenasy, and others have put forward as supplying a mechanical force of evolution. It is impossible to regard the coincidence of an Indian butterfly with the leaf of a tree now growing in an Indian forest as fortuitous, as a lusus naturæ. Assuming this seemingly mechanical force, therefore, we should be led back inevitably to a teleological principle which produces adaptive characters and which must have deposited the directive principle in the very first germ of terrestrial organisms, so that after untold ages at a definite time and place the illusive leaf markings should be developed. The assumption of preëstablished harmony between the evolution of the ancestral line of the tree with its prefigurative leaf, and that of the butterfly with its imitating wing, is absolutely necessary here, as I pointed out many years ago, but as is constantly forgotten by the promulgators of the theory of internal evolutionary forces.”

Weismann concludes, therefore, that for his present purpose it suffices to show “that cases exist wherein all natural explanations except that of selection fail us,” and he then proceeds to point out that even the natural selection of Darwin and of Wallace also fail to give us a reasonable explanation of how, for example, the markings on the wings of the Kallima butterfly have come about. The main reason that he gives to show that this is the case rests on the difficulty of the assumption that the right variations should always be present in the right place. Here “is the insurmountable barrier for the explanatory power of the principle [natural selection] for who, or what, is to be our guarantee that the dark scales shall appear at the exact spots on the wing where the midrib of the leaf must grow? And that later dark scales shall appear at the exact spots to which the midrib must be prolonged? And that still later such dark scales shall appear at the places whence the lateral ribs start, and that here also a definite acute angle shall be preserved.” Thus the philosopher in his closet multiplies and magnifies the difficulties for which he is about to offer a panacea. Had the same amount of labor been spent in testing whether the life of this butterfly is so closely dependent on the exact imitation of the leaf, we might have been spared the pains of this elaborate exordium. There are at least some grounds for suspicion that the whole case of Kallima is “made up.” If this should prove true, it will be a bad day for the Darwinians, unless they fall back on Weismann’s statement that their theory is insufficient to prove a single case!

Weismann has used Kallima only as the most instructive illustration. The objections that are here evident are found not only in the cases of protective coloration, but “are applicable in all cases where the process of selection is concerned. Take, for example, the case of instincts that are called into action only once in life, as the pupal performances of insects, the fabrication of cocoons, etc. How is it that the useful variations were always present here?” Weismann concludes that “something is still wanting to the selection theory of Darwin and Wallace, which it is obligatory on us to discover, if we possibly can, and without which selection as yet offers no complete explanation of the phyletic processes of transformation.” Weismann’s first step in the solution of the difficulty is contained in the following statement:—

“My inference is a very simple one: if we are forced by the facts on all hands to the assumption that the useful variations which render selection possible are always present, then, some profound connection must exist between the utility of a variation and its actual appearance, or, in other words, the direction of the variation of a part must be determined by utility, and we shall have to see whether facts exist that confirm our conjecture.”

Weismann finds the solution in the method by which the breeder has obtained his results in artificial selection. For instance, the long-tailed variety of the domestic cock of Japan owes its existence, it is claimed, to skilful selection, and not at all to the circumstance that, at some period of the race’s history, a cock with tail-feathers six feet in length suddenly and spasmodically appeared.

Weismann continues: “Now what does this mean? Simply that the hereditary diathesis, the germinal constitution (the Anlage) of the breed was changed in the respect in question, and our conclusion from this and numerous similar facts of artificial selection runs as follows: by the selection alone of the plus or minus variations of a character is the constant modification of that character in the plus or minus direction determined. Obviously the hereditary diminution of a part is also effected by the simple selection of the individuals in each generation possessing the smallest parts, as is proved, for example, by the tiny bills and feet of numerous breeds of doves. We may assert, therefore, in general terms: a definitely directed progressive variation of a given part is produced by continued selection in that definite direction. This is no hypothesis, but a direct inference from the facts and may also be expressed as follows: by a selection of the kind referred to the germ is progressively modified in a manner corresponding with the production of a definitely directed progressive variation of the part.”

So far there is nothing essentially new offered, since Darwin often tacitly recognized that the standard of variation could be raised in this way, and in some places he has made definite statements that this will take place. Weismann thinks that after each selection, fluctuation will then occur around a higher average (mode). He says “that this is a fact,” and is proved by the case of the Japanese cock. It need scarcely be pointed out that it is an assumption, based on what is supposed to have taken place in this bird, and is not a “fact.”