I send Sedgwick's letter; it is terribly muddled, and really the first page seems almost childish. I am sadly over-worked, so will not write to you. I have worked in a number of your invaluable corrections--indeed, all as far as time permits. I infer from a letter from Huxley that Ramsay (91/2. See a letter to Huxley, November 27th, 1859, "Life and Letters," II., page 282.) is a convert, and I am extremely glad to get pure geologists, as they will be very few. Many thanks for your very pleasant note. What pleasure you have given me. I believe I should have been miserable had it not been for you and a few others, for I hear threatening of attacks which I daresay will be severe enough. 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!