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: _If_ phys. sel. is true, it would follow that as between allied species, mutual sterility ought to occur in all degrees (from zero to absolute), and that there ought to be a correlation between these degrees of sterility and degrees of non-separation, topographically. Now, you cannot possibly doubt that the first expectation is realised in nature; as between allied species sterility does occur in all degrees, from there being no such sterility at all in very many cases, to there being absolute sterility in other cases. Therefore, in stating this fact as a fact, I am not playing at 'heads I win and tails you lose,' nor 'begging the whole question at the outset.' Any 'question' really arises only with regard to the second expectation--viz. whether there is a general correlation between degrees of mutual fertility and degrees of topographical isolation. Now, this question I have not begged, but, on the contrary, stated as the question by an experimental answer to which my theory must stand or fall. Thus, the cases which you mention obviously go to support the theory, inasmuch as they conform to the expectation above mentioned. What I want to do is to find as many genera as possible like binchona and begonia, where the constituent species are separated geographically or topographically, and (? in consequence) easily hybridise with one another. Therefore, as a mere matter of method, I cannot see that I have begged any question: for the only question is not about the facts which I state, but about my suggested explanation of them. And this question can only be answered by ascertaining whether there is in nature any such general correlation between isolation and capability of hybridising (also, of course, between the absence of isolation and the absence of such capability) as my theory would require. Yours very sincerely, G. J. ROMANES. 18 Cornwall Terrace, Regent's Park, N.W.: December 27, 1888.