Dear Mr. Henslow,—I am still terribly ill and cannot write much. We must have a talk. Could you come to Oxford any day you like and be our guest? I think we might derive mutual benefit. I shall be there from the middle of April till I do not know when. Why not come on May 2, to hear Weismann give his lecture in the afternoon?
I much wish you would save seed of any fixed local varieties of plants you may find to be in seed, while you are in Malta (or bulbs), in order to see whether plants grown from them in England will or will not prove fully fertile. This is in relation to my own theory of physiological selection, according to which isolation produces segregation of type; in the same way as it does that of a language—viz. by prevention of intercourse with the parent type and consequently with an independent history of variation. Where the isolation is due to physical barriers (as at Malta) there is no need for any sexual differentiation to originate a species. But on common areas, sexual differentiation is the only means of securing the isolation. Therefore (I say) we can see why Jordan's French varieties all prove sterile with their parent forms, and I should expect your Malta varieties to prove fertile with theirs elsewhere.
G. J. R.
Costebelle: April 15, 1894.
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,