The argument had often been repeated in the nineteenth century; and in the period under consideration we find Spencer observing that
The supporters of the Development Hypothesis. . . can show that the degrees of difference so produced [through structural changes under altered conditions] are often, as in dogs, greater than those on which distinctions of species are in other cases founded. They can show that it is a matter of dispute whether some of these modified forms are varieties or separate species.[2]
This argument, it is true, if taken by itself, suffered from two serious limitations. One has already been adverted to, in another connection: the absence of evidence that variation can produce varieties sterile inter se, as species are sterile. But we have already seen that this difficulty, upon the testimony of Huxley himself, was not removed in 1859. The other limitation of the argument was that, before the promulgation of the hypothesis of natural selection, it was commonly associated with a belief in the inheritance of acquired characters. But this association was not logically necessary; and in any case, the wholesale denial of such inheritance is a doctrine of neo-Darwinism unknown to the pre-Darwinian period and to Darwin himself; and was in that period, therefore, not a ground of difficulty. In a subsequent instalment of this inquiry it will remain to consider, somewhat more minutely, four more of the principal general arguments for evolutionism, three of these being, in 1844, of a much less venerable age than the two last mentioned.
(To be concluded)
