My dear Huxley

I fully agree that the difficulty is great, & might be made much of by a mere advocate.¹ Will you oblige me by reading again slowly from p. 267–272.—² I may add to what is there said, that it seems to me quite hopeless to attempt to explain why varieties are not sterile; until we know precise cause of sterility in species.—   Reflect for a moment on how small & on what very peculiar causes the unequal reciprocity of fertility in same two species must depend.— Reflect on the curious case of species more fertile with foreign pollen than their own. Reflect on many cases which could be given, & shall be given in my larger book (independently of hybridity) of very slight changes of conditions causing one species to be quite sterile & not affecting a closely allied species.—³ How profoundly ignorant we are on this intimate relation between conditions of life & impaired fertility in pure species.—

The only point which I might add to my short discussion on this subject, is that I think it probable that the want of adaptation to uniform conditions of life in our domestic varieties has played an important part in preventing their acquiring sterility when crossed. For this want of uniformity & changes in the conditions of life seems only cause of the elimination of sterility (When crossed) under domestication. This elimination though admitted by many authors rests on very slight evidence, yet I think is very probably true,—as may be inferred from the case of dogs.— Under nature it seems improbable that the differences in the reproductive constitution, on which the sterility of any two species when crossed depends, can be acquired directly by natural selection; for it is of no advantage to the species. Such differences in reproductive constitution must stand in correlation with some other differences; but how impossible to conjecture what these are!

Reflect on case of the vars. of Verbascum which differ in no other respects whatever besides the fluctuating element of colour of flower, & yet it is impossible to resist Gärtner’s evidence, that this difference in the colour does affect the mutual fertility of the varieties.⁴ The whole case seems to me far too mysterious to rest valid attack on the theory of modification of species, though, as you say, it offers excellent ground for mere advocate.—

I am surprised considering how ignorant we are on very many points, that more weak parts in my Book have not as yet been pointed out to me. No doubt many will be. H. C. Watson founds objection in M.S. on there being no limit to infinite diversification of species: I have answered this I think satisfactorily, & have sent attack & answer to Lyell & Hooker.⁵ If this seems to you good objection, I would send papers to you.—

Andrew Murray “disposes of” the whole theory by ingenious difficulty from distribution of blind cave insects; but it can, I think, be fairly answered.⁶

My dear Huxley | Yours most sincerely | C. Darwin