The Evolution Theory, Vol. 2 of 2

by August Weismann

I abstain from analysing the assumptions here made; their improbability and their contradictions to established facts are so great that it is not necessary to emphasize them; the theory shows plainly that it is necessary to have recourse to very improbable assumptions, if an attempt is to be made to find a theoretical basis for the transmission of acquired (somatogenic) characters. Even when Darwin formulated his theory of Pangenesis his assumptions were hardly reconcileable with what was known of cell-multiplication; now they are above all irreconcileable with the fact that the germ-substance never arises anew, but is always derived from the preceding generation—that is, with the continuity of the germ-plasm.

If we were now to try to think out a theoretical justification we should require to assume that the conditions of all the parts of the body at every moment, or at least at every period of life, were reflected in the corresponding primary constituents of the germ-plasm and thus in the germ-cells. But, as these primary constituents are quite different from the parts themselves, they would require to vary in quite a different way from that in which the finished parts had varied; which is very like supposing that an English telegram to China is there received in the Chinese language.

In spite of this almost insuperable theoretical obstacle, various authors have worked out the idea that the nervous system, which connects all parts of the body with the brain and thus also with each other, communicates these conditions to the reproductive organs, and that thus variations may arise in the germ-cells corresponding to those which have taken place in remote parts of the body.

Even supposing it were proved that every germ-cell in ovary or testis was associated with a nerve-fibre, what could be transmitted to it by the nerves, except a stronger or weaker nerve-current? There is no such thing as qualitative differences in the current; how then could the primary constituents of the germ be influenced by the nerve-current, either individually or in groups, in harmony with the organs and parts of the body corresponding to them, much less be caused to vary in a similar manner? Or are we to imagine that a particular nerve-path leads to every one of the countless primary constituents? Or does it make matters more intelligible if we assume that the germ-plasm is without primary constituents, and suppose that, after each functional variation of a part, telegraphic notice is sent to the germ-plasm by way of the brain as to how it has to alter its 'physico-chemical constitution,' so that the descendants may receive some benefit from the acquired improvement?

I am not of the number of those who believe that we already know all, or at least nearly all, that is essential, but am rather convinced that whole regions of phenomena are still sealed to us, and I consider it probable that the nervous system in particular is not yet exhaustively known to us, either in regard to its functioning or in regard to its finest structural architecture, although I gratefully recognize the advances in this domain that the last decades have brought about. In any case, such assumptions as I have just indicated, or similar ones, seem to me quite too improbable to furnish any foothold for progress. Yet we must always remain conscious that we cannot decide as to the possibility or impossibility of any biological process whatever from a purely theoretical standpoint, because we can only guess at, not discern, the fundamental nature of biological processes. At the close of this lecture I shall return to the question of the theoretical conceivability of an inheritance of functional adaptations; but first of all we must consider the facts and be guided by them alone. If they prove, or even make it seem probable, that such inheritance exists, then it must be possible, and our task is no longer to deny it, but to find out how it can come about.

Let us therefore investigate the question whether an inheritance of acquired characters, that is, in the first place, of functional adaptations, is demonstrable from experience. We shall speak later on of the effect of climatic and similar influences in causing variation; the case in regard to them is quite different, because they undoubtedly affect not only the parts of the body but the germ-cells as well.

When we inquire into the facts which have been brought forward by the modern adherents of the Lamarckian principle as proofs of the inheritance of acquired characters in this restricted sense, we shall find that none of them can withstand criticism.

First, there are the numerous reputed cases of the inheritance of mutilations and losses of whole parts of the body.

It is not without interest to note here how opinion in regard to this point has altered in the course of the debate.

At the beginning of the discussion they were all brought forward as evidence of undoubted value for the Lamarckian principle.