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“I believe that heredity depends upon the fact that a small portion of the effective substance of the germ, the germ-plasm, remains unchanged during the development of the ovum into an organism, and that this part of the germ-plasm serves as a foundation from which the germ-cells of the new organism are produced. There is, therefore, continuity of the germ- plasm from one generation to another. One might represent the germ-plasm by the metaphor of a long creeping root-stock from which plants arise at intervals, these latter representing the individuals of successive generations.” {31}

Mr. Wallace, who does not appear to have read Professor Weismann’s essays themselves, but whose remarks are, no doubt, ultimately derived from the sequel to the passage just quoted from page 266 of Professor Weismann’s book, contends that the impossibility of the transmission of acquired characters follows as a logical result from Professor Weismann’s theory, inasmuch as the molecular structure of the germ-plasm that will go to form any succeeding generation is already predetermined within the still unformed embryo of its predecessor; “and Weismann,” continues Mr. Wallace, “holds that there are no facts which really prove that acquired characters can be inherited, although their inheritance has, by most writers, been considered so probable as hardly to stand in need of direct proof.” {32}

Professor Weismann, in passages too numerous to quote, shows that he recognises this necessity, and acknowledges that the non-transmission of acquired characters “forms the foundation of the views” set forth in his book, p. 291.

Professor Ray Lankester does not commit himself absolutely to this view, but lends it support by saying (Nature, December 12, 1889): “It is hardly necessary to say that it has never yet been shown experimentally that anything acquired by one generation is transmitted to the next (putting aside diseases).”

Mr. Romanes, writing in Nature, March 18, 1890, and opposing certain details of Professor Weismann’s theory, so far supports it as to say that “there is the gravest possible doubt lying against the supposition that any really inherited decrease is due to the inherited effects of disuse.” The “gravest possible doubt” should mean that Mr. Romanes regards it as a moral certainty that disuse has no transmitted effect in reducing an organ, and it should follow that he holds use to have no transmitted effect in its development. The sequel, however, makes me uncertain how far Mr. Romanes intends this, and I would refer the reader to the article which Mr. Romanes has just published on Weismann in the Contemporary Review for this current month.

The burden of Mr. Thiselton Dyer’s controversy with the Duke of Argyll (see Nature, January 16, 1890, et seq.) was that there was no evidence in support of the transmission of any acquired modification. The orthodoxy of science, therefore, must be held as giving at any rate a provisional support to Professor Weismann, but all of them, including even Professor Weismann himself, shrink from committing themselves to the opinion that the germ-cells of any organisms remain in all cases unaffected by the events that occur to the other cells of the same organism, and until they do this they have knocked the bottom out of their case.

From among the passages in which Professor Weismann himself shows a desire to hedge I may take the following from page 170 of his book:–

“I am also far from asserting that the germ-plasm which, as I hold, is transmitted as the basis of heredity from one generation to another, is absolutely unchangeable or totally uninfluenced by forces residing in the organism within which it is transformed into germ-cells. I am also compelled to admit it as conceivable that organisms may exert a modifying influence upon their germ-cells, and even that such a process is to a certain extent inevitable. The nutrition and growth of the individual must exercise some influence upon its germ-cells . . . “