But I am sure that I can now bear them.
LETTER 92. TO T.H. HUXLEY.
(92/1. The point here discussed is one to which Mr. Huxley attached great, in our opinion too great, importance.)
Down, January 11th [1860?].
I fully agree that the difficulty is great, and might be made much of by a mere advocate. Will you oblige me by reading again slowly from pages 267 to 272. (92/2. The reference is to the "Origin," Edition I.: the section on "The Fertility of Varieties when crossed, and of their Mongrel Offspring" occupies pages 267-72.) 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 the precise cause of sterility in species.
Reflect for a moment on how small and on what very peculiar causes the unequal reciprocity of fertility in the 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, and shall be given in my larger book (independently of hybridity) of very slight changes of conditions causing one species to be quite sterile and not affecting a closely allied species. How profoundly ignorant we are on the intimate relation between conditions of life and 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 the want of uniformity, and changes in the conditions of life, seem the only cause of the elimination of sterility (when crossed) under domestication. (92/3. The meaning which we attach to this obscure sentence is as follows: Species in a state of nature are closely adapted to definite conditions of life, so that the sexual constitution of species A is attuned, as it were, to a condition different from that to which B is attuned, and this leads to sterility. But domestic varieties are not strictly adapted by Natural Selection to definite conditions, and thus have less specialised sexual constitutions.) 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 the case of the variations of Verbascum, which differ in no other respect whatever besides the fluctuating element of the colour of the flower, and yet it is impossible to resist Gartner's evidence, that this difference in the colour does affect the mutual fertility of the varieties.
The whole case seems to me far too mysterious to rest (92/4. The word "rest" seems to be used in place of "to serve as a foundation for.") a valid attack on the theory of modification of species, though, as you say, it offers excellent ground for a 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 his objection in MS. on there being no limit to infinite diversification of species: I have answered this, I think, satisfactorily, and have sent attack and answer to Lyell and Hooker. If this seems to you a good objection, I would send papers to you. Andrew Murray "disposes of" the whole theory by an ingenious difficulty from the distribution of blind cave insects (92/5. See "Life and Letters, Volume II., page 265. The reference here is to Murray's address before the Botanical Society, Edinburgh. Mr. Darwin seems to have read Murray's views only in a separate copy reprinted from the "Proc.