Dear Mr. Henslow,--Yes, please write when you get back, suggesting any time you may find convenient for spending a day or two with us at 94 St. Aldate's, Oxford (immediately opposite Christ Church). I cannot talk long at a time, but I think the meeting will be of use to both. Of course 'Isolation _produces_ segregation of type,' is only a short-hand expression, meaning--_indiscriminate variation being supposed_--isolation supplies a necessary condition to segregation of type by upsetting the previous stability that was due to free inter-crossing. I quite agree that Darwin _very greatly_ over-estimated the _benefit_ of inter-crossing, as I am showing in my forthcoming book on 'Physiological Selection.' But this is quite a different thing from his having _made too much of inter-crossing_ as a condition to stability of type; I do not think that this _can_ be made too much of. Indeed, how is it conceivable that there ever can be _divergence_ of type without _isolation_ of some kind having first occurred at the origin, and throughout the growth of every branch? Moreover, I agree with you about self-fertilisation, but see in it a form of physiological selection; it is one kind of sexual isolation, or prevention of inter-crossing with neighbouring individuals. So that the more perfectly it obtains in any given type, the better chance there is for that type to become a new species by independent variability--and this whether or not the _independent_ variability is likewise _indiscriminate_ (or in _your_ terminology 'indefinite'). In my last letter I referred to the works of Jordan and Nägeli for any number of '_facts_ in _Nature_ of varieties _arising among_ the type forms.' I will show you the passages when we meet. But even in cases of 'local varieties,' where a variety has a habitat of its own _surrounded by the type-form_, I should expect experiment would often (though by no means always) show some degree of _cross-infertility_ between the two, pointing to pre-potency (_i.e._ early stages of physiological selection) being the origin of the divergence. Before we meet I wish you would try to think of any plants which can be propagated by cuttings (or otherwise asexually) which are known to be modifiable by changed conditions of life in the first generation. I understand you that in some cases the seed of such a plant _will not revert_--when sown in its natural environment, though, of course, the rule is that it does. Well, in either case, I should much like to try whether a cutting &c. from the transplanted (and therefore modified) tubers &c. would revert to its ancestral character. When retransplanted to its natural environment, much would follow from result of such an experiment as regards Weismannism. Yours very and always truly, G. J. ROMANES.