The Evolution Theory, Vol. 2 of 2

by August Weismann

I may name first Ludwig Zehnder, a physicist who has wandered into the domain of biology. In regard to the very facts which I have adduced as evidence against the existence of such inheritance, he has endeavoured to show how we might conceive of them as having by this very means arisen[19].

[19] Zehnder, Die Entstehung des Lebens, Freiburg-i.-Br., 1899.

He deals with the case of ants, that is to say, with the differentiation of the sterile workers into several castes, in the following interesting manner.

The task of the workers is to procure all the food necessary for all the individuals of the colony in quantity and quality corresponding to the demand; failing this the whole colony would perish. Now the different persons of the colony need different food, according to their constitution and their functions. Soldiers, for instance, are more powerful than ordinary workers, since they are adapted for fighting, and they therefore require a different kind of food from the weaker workers who are adapted only for other duties. Since the soldiers have evolved from the latter by selection, what we may, for the sake of brevity, call the soldier-food in the common stores of the ants would be drawn upon more lavishly than before, and would therefore disappear more quickly, and, whenever this occurred, those workers which had already brought in this kind of food would be impelled to bring more and more to satisfy the demand for it. But in order to do this they would require to exert themselves more, and would therefore require a larger quantity of food—not of course soldier-food, but the particular kind which their particular qualities demanded. Probably this second importation of food was undertaken by a second kind of worker, for, according to Zehnder, each worker does not carry all the kinds of food; they are divided into legions, each of which has its particular task of food-collecting to fulfil.

In the end the storehouse of the ant-colony must contain a provision in which the different kinds of food are in exact proportion to the necessities of the different kinds of persons in the colony. It must alter in its composition again as soon as, in the course of time, one or other kind of person acquires new characters, for these presuppose a new kind of diet.

But how are these acquired characters to be transmitted since neither soldiers nor workers reproduce? Zehnder answers this by pointing out that the sexual animals eat all the kinds of nourishment which are accumulated in the stores, that is to say, all the different kinds of food exactly in the proportion in which they have been imported—the proportion in which the different kinds of persons are represented in the colony. Thus the kinds of nourishment which caused the appearance of the newly acquired characters in the non-sexual animals also reached the sexual animals and their sex-cells, and there gave rise to substances which evoke the relevant qualities in their descendants, for instance, in the soldiers, or in the still more modified workers, and so on; and thus we have an 'inheritance of acquired characters.'

This is certainly ingeniously and cleverly thought out, and it reads even better and more smoothly in the original than in my brief summary, but it will hardly be regarded as a refutation of my position; the hypotheses are all too daring for that. We have no knowledge that particular modifications in form can be produced and conditioned by particular kinds of food, and, indeed, the contrary has been proved, namely, that the two or three different castes of polymorphic species have precisely similar diet. I need only recall the six forms of female in Papilio merope, of which at least three have been obtained from the same set of eggs, and by feeding with the same plant.

It is true that there are ants which lay in stores of nourishment, but these consist, for the most part, of one kind of seeds, or of honey, not of different substances, and we have no knowledge that the different persons use different food, or even that there is any diversity in the mode of feeding the helpless larvæ. The feeding in some species takes place from mouth to mouth, and therefore cannot be precisely investigated, and we can only suppose from analogy with bees that the larvæ of the males and females frequently receive not only more abundant, but qualitatively different food. They are fed from the crop unless the food consists of the pith of a tree in which the larvæ are imbedded, as Dahl informs us is the case with some tropical ants of the Bismarck Archipelago.

But even if we assume that the soldiers take different food from the ordinary workers, and different again from that of the sexual animals, is it by virtue of the quality of their food that they have become what they are? Have our breeds of pigeons or hens been produced by different diet, or do we know anything in the whole range of animal life of such a parallelism between food and bodily structure as Zehnder here assumes? And if, in reality, let us say, the breeds of pigeon had arisen through specific dieting, and we were to feed one pair with the specific food-stuffs of three different breeds, would the descendants of this pair exhibit the form of these three breeds? Or would they exhibit them in precisely the proportion in which the food-stuffs had been mixed? It seems to me that Zehnder's assumptions diverge so far from what we are accustomed to regard as solid ground in biology that they hardly require consideration, and yet he not only uses them for the explanation of the case of the ants, but bases upon them the whole of his theory of the inheritance of acquired characters.