"We might almost reserve our belief that the structural [i. e. somatic] cells can react on the sexual elements at all, and we may be confident that at most they do so in a very faint degree; in other words, that acquired modifications are barely, if at all, _inherited_, in the correct sense of that word[27]."
[27] _Theory of Heredity_ (Journ. Anthrop. Inst. 1875, p. 346).
So far Mr. Galton; but for Weismann's further theory of evolution, &c., it is necessary to postulate the additional doctrine in question; and it makes a literally immeasurable difference to any theory of evolution whether or not we entertain this additional postulate. For no matter how faintly or how fitfully the substance of heredity may be modified by somatic tissues, the Lamarckian principles are hypothetically allowed some degree of play. And although this is a lower degree than Darwin supposed, their influence in determining the course of organic evolution may still have been enormous; seeing that their action in any degree must always have been _directive_ of variation on the one hand, and _cumulative_ on the other.
Thus, by merely laying this theory side by side with Weismann's we can perceive at a glance how a _pure_ theory of _heredity_ admits of being based on the postulate of Continuity alone, without cumbering itself by any further postulate as to this Continuity being _absolute_. And this, in my opinion is the truly scientific attitude of mind for us to adopt as preliminary to the following investigation. For the whole investigation will be concerned--and concerned only--with this question of Continuity as absolute, or as admitting of degrees. There is, without any question, abundant evidence to prove that the substance of heredity is at least partly continuous (Gemmules). It may be that there is also abundant evidence to prove this substance much more _largely_ continuous than Darwin supposed (Stirp); but be this as it may, it is certain that any such question as to the _degree_ of continuity differs, _toto caelo_, from that as to whether there can ever be any continuity at all.
How, then, we may well ask, is it that so able a naturalist and so clear a thinker as Weismann can have so far departed from the inductive methods as to have not merely propounded the question touching Continuity and its degrees, or even of Continuity as absolute; but to have straightway assumed the latter possibility as a basis on which to run a system of branching and ever-changing speculations concerning evolution, variation, the ultimate structure of living material, the intimate mechanism of heredity, or, in short, such a system of deductive conjectures as has never been approached in the history of science? The answer to this question is surely not far to seek. Must it not be the answer already given? Must it not have been for the sake of rearing this enormous structure of speculation that Weismann has adopted the assumption of Continuity as absolute? As we have just seen, Galton had well shown how a theory of heredity could be founded on the general doctrine of Continuity, without anywhere departing from the inductive methods--even while fully recognizing the possibility of such continuity as absolute. But Galton's theory was a "_Theory of Heredity_," and nothing more. Therefore, while clearly perceiving that the Continuity in question _may_ be absolute, he saw no reason, either in fact or in theory, for concluding that it _must_ be. On the contrary, he saw that this question is, for the present, necessarily unripe for profitable discussion--and, _a fortiori_, for the shedding of clouds of seed in all the directions of "Weismannism."