On the problem of want of sterility in crosses of domestic varieties. Refers to discussion in Origin, pp. 267–72 ["Fertility of varieties when crossed"]. We do not know precise cause of sterility in species.
Andrew Murray has attacked Origin [see 2647].
H. C. Watson objects to natural selection on grounds of limitless diversification of species.
Down Bromley Kent
My dear Huxley
I fully agree that the difficulty is great, & might be made much of by a mere advocate. Will you oblige me by reading again slowly from p. 267--272.— 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.— 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 G¨artner'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 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. 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.
My dear Huxley | Yours most sincerely | C. Darwin
- f1 2649.f1Huxley was preparing a lecture on CD's theory to be delivered as one of the Friday evening lectures at the Royal Institution on 10 February 1860. In his lecture, Huxley stated his general approval of CD's work, but he nonetheless said that it `falls short of being a satisfactory theory', primarily because `there is as yet no proof that, by selection, modifications having the physiological character of species (i.e., whose offspring are incapable of propagation, inter se) have ever been produced from a common stock.' (T. H. Huxley 1860a, p. 198). A note in CD's hand in DAR 205.7 (2): 149 discusses Huxley's point:
Jan 11. 1860. Huxley objects to domestic vars. not being sterile. I answer in addition to my Book. Too ignorant of causes of sterility in species.— *Reflect on what slight causes in conditions render pure species sterile [interl] Reflect on unequal reciprocity, on Hippeastrum case—on Verbascum case. Probably want of *adaptation to [interl] uniform [altered from `uniformity'] conditions in domestic productions important; for this seems to cause elimination of sterility. The Verbascum case shows how ignorant we are on what differences [interl] the difference in reproductive condition are correlated: it is probably correlation.—The `addition to my Book' is a reference to Variation, in which both Hippeastrum and Verbascum are discussed ( Variation 2: 138--9, 105--7, 136--7).
- f2 2649.f2These pages of Origin discuss the `Fertility of varieties when crossed, and of their mongrel offspring'. CD contended that the sterility of interspecific hybrids, when contrasted with the fertility of crosses between different varieties, does not imply an essential distinction between species and varieties. He argued that the sterility of interspecific hybrids is not a special endowment but is gradually acquired through divergent modifications in the reproductive systems of the forms that are crossed (Origin, p. 272).
- f3 2649.f3CD intended to publish a comprehensive work on natural selection giving all his references and sources and a more considered view of the subject. His plan was never realised beyond the publication of Variation, which was the first part of the planned `larger book'. In Variation 2, chap. 18, the beneficial and detrimental effects of changed conditions of life on fertility are discussed in detail.
- f4 2649.f4G¨artner 1849, pp. 215--16. CD's annotated copy is in the Darwin Library--CUL. These pages were heavily marked by CD. Karl Friedrich von G¨artner's case of the incipient sterility of different varieties of Verbascum according to the colour of the flowers had interested CD greatly and was cited in Origin, pp. 270--1. See also Correspondence vol. 5, letter to M. J. Berkeley, 7 April , and letter from C. C. Babington, [c. June 1855].
- f5 2649.f5Letter from H. C. Watson, [3? January 1860], and letter to H. C. Watson, [5--11 January 1860]. The letters to Charles Lyell and to Joseph Dalton Hooker have not been located.
- f6 2649.f6See preceding letter and nn. 15 and 16.