The second difficulty is one that lies against the theory itself, and has always seemed to me most formidable. But as nobody else has ever mentioned it, I have not hitherto done so, as I want to work it out quietly. I allude to your remark about the extraordinary differences that obtain among different genera with regard to the capability of intercrossing exhibited by their constituent species. This, I confess, has from the first appeared a tremendous objection to my theory. On the other hand, I have taken comfort from the consideration that besides being a tremendous objection, it is also a tremendous mystery. For, as it must admit of some explanation, and as this explanation must almost certainly have to do with the sexual system, it becomes not improbable that when found the explanation may square with p.s. That the difference in question is functional and not structural (or physiological as distinguished from morphological) seems to be proved by the fact that in some cases it obtains as between the most closely allied genera, being, e.g., most strongly pronounced of all between Geranium and Pelargonium. Even quite apart from my own theory, it seems to me that this is a subject of the highest importance to investigate.
As regards sexual selection I allow, of course, that the 'law of battle' is a form of natural selection. But where the matter is merely a pleasing of æsthetic taste, and the resulting structures therefore only ornamental, I can see nothing 'advantageous' in the sense of life-preserving. On the contrary, in most cases such structures entail considerable expenditure of physiological energy in their production. On this account Darwin says that nat. sel. must impose a check on sexual selection running beyond a certain point of injuriousness ('D. of M.,' p. 227). Now, physiological selection is never thus injurious; and although it is a 'form of isolation,' the isolation is neither so extreme nor of such long continuance as the ones you compare it with. Moreover, the environment (therefore all other or external conditions of life) remain the same, which is not the case under the other forms of isolation. Provided that the physiological change is not in itself injurious, I do not see why physiologically isolated forms should be less fit than those from which they have been separated, though I can very well see why this should be the case with such geographically isolated forms as you mention, for there the schooling is different. Lastly, physiological selection, if not in itself injurious, does not require that its children should be 'protected against the struggle for existence.' On the contrary, as I say in my paper, it is calculated to give this struggle a better chance than ever to develope adaptive character in the sexually isolated forms, because the swamping effects of intercrossing are diminished.
But I really did not intend to afflict you with another jaw of this kind. I am, however, very glad that we now understand each other better than we did. At all events on my side I think I now know exactly the points which I have to make good if Nature is so constituted as to admit of my theory. One thing only I have forgotten to say, viz. that nothing can be argued against the theory from the fact of hybridisation occurring in cases where, according to the theory, it ought not to occur. This argument only becomes valid where it is found that the resulting hybrids are fertile. In relation to the theory, a sterile hybrid is all the same as a failure to cross.
Yours very sincerely,
G. J. Romanes.
P.S.—I forgot to ask you if there would be any facilities in spring at Kew for repeating Adam's graft of purple on yellow laburnum. I want to try this experiment in budding on a large scale because of its importance on Weismannism, should the result of any of the grafts go to corroborate Adam's account of the way in which he produced the hybrid. If you agree to the experiments being tried at Kew, perhaps you might let me know whether there are any purple laburnums already in the gardens, or whether I should get the material over from France. But in that case you might also let me know to whom in France or elsewhere I had best apply. However, do not bother to answer any other parts of this tremendous letter, these we can discuss in conversation hereafter. A postcard to answer this postscript, however, is desirable, as then it might be possible to get matters in train for next budding season.
G. J. R.
I should much like to meet Churchill. Will you remember to tell me when he comes?
To F. Darwin, Esq.
18 Cornwall Terrace, Regent's Park, N.W.: January 20, 1889.