The third line of evidence against this theory—i.e., the evidence in favour of the transmission of acquired characters—is to constitute the subject-matter of future chapters. Therefore it will here be sufficient to adduce only one fact of this kind. And I select it because it is one that has been dealt with by Weismann himself. In one of his more recent statements he says:—
The distinguished botanist De Vries has proved that certain constituents of the cell body—e. g., the chromatophores of Algae—pass directly from the maternal ovum to the daughter organism, while the male germ-cells generally contain no chromatophores. Here it appears possible that a transmission of somatogenetic variation has occurred[24].
Now although, as Weismann goes on to observe, “in these lower plants, the separation between somatic and reproductive cells is slight,” in the facts to which he alludes we appear to have good evidence of an influence exercised by somatic cells upon the germinal contents of reproductive cells. And if such an influence is capable of being exercised in the case of “these lower plants,” it follows that there is no such absolute separation between somatic tissues and germ-plasm as Weismann’s theory requires. Moreover it follows that, if the essential distinction between germ-plasm and somato-plasm (or “somatic idio-plasm”) is thus violated at the very foundation of the multicellular organisms, there ceases to be any a priori reason for drawing arbitrary limits, either as to the level of organization at which such “transmission of somatogenetic variation has occurred,” or as to the degree of detail into which it may extend. Both these matters then stand to be tested by observation; and the burden of proof lies with the school of Weismann to show at what level of organization, and at what degree of representation, somatogenetic changes cease to reproduce themselves by heredity.
Passing on, then, to higher levels of organization, and therefore to higher degrees of representation, I shall endeavour to show that this burden of proof cannot be discharged. For I shall endeavour to show, not merely, as just shown, that there ceases to be any a priori reason for drawing arbitrary limits with respect either to levels of organization or to degrees of representation, but that, as a matter of fact, there are no such limits as the passage above quoted assigns. On the contrary, I believe there is as good evidence to prove the not unfrequent transmission of acquired (“somatogenetic”) characters among the higher plants—and even among the higher animals—as there is of the occurrence of this phenomenon in the case of the Alga just mentioned. But in order to do this evidence justice, I shall have to take a new point of departure and consider as a separate question the transmissibility of acquired characters. Meanwhile, and as far as Weismann’s theory of heredity is concerned, it is enough to have shown,—if I have been successful in doing so,—that not only is there no evidence to sustain his fundamental postulate touching the material of heredity having always occupied a separate “sphere” of its own “since the first origin of life”; but that there is good evidence to prove the contrary. For whether or not the reciprocal action of “somato-plasm” and “germ-plasm” can ever proceed to the extent of causing acquired characters to be inherited (so as to produce “representative congenital changes”), all that is distinctive in this theory must be regarded as barren speculation, unless it can be shown that the foregoing facts have failed to prove such a reciprocal action as ever occurring in any lower degree (so as to produce “specialized congenital changes”).
CHAPTER IV.
Examination of Weismann’s Theory of Evolution (1891).
Having now considered germ-plasm as perpetually continuous, we have next to regard it as unalterably stable.
First, let it be noted that these two fundamental and distinctive postulates of the whole Weismannian system are so intimately connected as to be in large measure mutually dependent. For, on the one hand, if germ-plasm has not been perpetually continuous since the first origin of life, it cannot have been absolutely stable “since the first origin of sexual propagation”: every time that its hereditary characters are modified by its containing soma (whether or not representatively so), its stability has been so far upset. On the other hand, if germ-plasm has not been absolutely stable, it cannot have been perpetually continuous “since the first origin of life.” As often as its stability has been upset, its “molecular structure” has been modified by causes ab extra, as distinguished from mixtures of germ-plasms in sexual unions. Therefore, it can no longer have been continuous in the sense of having borne an ineffaceable record of all congenital variations, due to sexual unions, throughout the entire phylogeny of the Metaphyta and Metazoa. At most it can have been continuous only in the attenuated sense, that however much and however often its hereditary characters may have been modified by somatic changes on the one hand or by changes in the external conditions of life on the other, they can never have been thus modified representatively, as supposed by the theory of pangenesis.