Hitherto very few experiments have been made on these comparative degrees of fertility, yet it is by such alone, it seems to me, that physiological selections can be tested. Thus, e.g., my point about the 'interlocking' species (p. 392) is that in such cases I should expect a higher degree of fertility in A × A and B × B than crosswise. Indeed, my fear is that when I shall have proved by experiment that such is the general rule in such cases, naturalists will turn round and say: 'Well, of course, on merely a priori grounds you might have known that such must have been the case; for otherwise the two interlocking species could never have existed as separate species, they would have hybridised freely along the whole frontier line and eventually blended over the whole area.' And still more may this be said in the case of allied species, not merely interlocking, but intermixed through common areas. Therefore, as a believing F.R.S. said to me the other day, 'Your letters in "Nature" will at least have the effect of blunting the edge of such possible criticism in the future.' Of course you will laugh at the robustness of my faith in thus forecasting the line of future opposition, but I would like to ask you this much: Supposing, for the sake of argument, that twenty years hence I publish one hundred instances of allied species which grow intermixed in common areas, proving by experiment that in all the cases there is some comparative degree of sterility between them (if only due to pre-potency of their own pollen), would you regard this as making in favour of physiological selection? Or are you already prepared to admit that such must be the case, since otherwise the species A and B could not exist without fusion into one? If you say that you are prepared to admit this, it seems to me that you have already accepted the theory of physiological selection on a priori grounds.
Again, if I should publish one hundred other instances of allied species topographically isolated from one another, all of which were proved by experiment to present no degree at all of mutual infertility (so that A × A and B × B are not more fertile than crosswise), would you allow that, taken in conjunction with the previous set of experiments, these finally prove the theory of physiological selection to be true? If not, I do not see how it is possible to verify the theory at all: it is only by means of these two complementary lines of research that, as it seems to me, the theory can be experimentally tested.
In the former case—i.e. where allied species intermix in common areas—sometimes they intercross freely (e.g. Primula vulgaris and veris, Geum urbanum and rivale, Rumex, Epilobium, &c.), while in other instances they don't (e.g. Ranunculus repens and bulbosus, Lepidium Smithii and campestre, Scrophularia nodosa and aquatica, &c.). Now, as regards the latter, I suppose you would not question that the 'physiological isolation' has to do with preventing the species from fusing? But, if so, by parity of reasoning, should we not expect to meet with some degree of the same thing in the other cases, which, although not here sufficiently pronounced to block off frequent hybridisation, is nevertheless sufficient to prevent the species from blending over their common area?
And here, I may say, I should not at all object to the charge of misunderstanding Darwin on any merely trivial point such as the one you mention. But in this instance it so happens that it is rather you who have misunderstood me. I know that 'a hybrid is not an intermediate form in his sense,' and this is just what constitutes my difficulty against his paragraphs quoted on p. 392 of my paper. For what I say is, these intermediate forms ought to be hybrids, unless physiological selection, (i.e. mutual sterility) has been at work. 'In his sense' I cannot conceive how such 'intermediate forms' can exist in the circumstances described, seeing that they are not hybrids, and yet that (in the absence of any hypothesis of physiological isolation for which I am contending) there is no reason given why the two interlocking species should not freely intercross.
Regarding sexual selection I certainly am very much in earnest about its parallel to p.s.[62] If you intend the meaning of n.s. so as to embrace s.s. it will at the same time embrace also p.s. For s.s., like p.s., has nothing to do with life-preserving characters; yet, also like p.s., it has to do with the differentiation of specific forms. (There is no distinction to be drawn between 'the species of a cock' and 'the plumage of a cock': plumage is the most favourite part of a bird with ornithologists on which to found specific diagnoses.) Therefore, if p.s. is true at all—which, of course, is another question—even my celebrated powers of 'dialectical subtlety' are completely unable to perceive any difference between p.s. and s.s. in respect of their relation to n.s.
Lastly, as regards Nägeli, no doubt he is an out-and-out Lamarckian, but I did not see that this should make any difference touching his opinion on a matter of fact not more connected with Lism. than Dism. I will look up 'Nature' for 1870.
With best Christmas wishes and many thanks for botanical hints.
December 26, 1888.
It has occurred to me that if you know Churchill's address, I might save time by writing to him before seeing him when he comes in spring.
It has also occurred to me that I might perhaps put the argument on pp. 801-4 better before you thus: