“June 9th [1871].
“Down.
“Dear Sir,—I am greatly obliged by your note. I have read with much interest and carefully perused your letter in Nature, and am looking out for a paper announced for Linn. Soc. Your remarks shall all be in due time fully considered. With respect to the separation of the sexes, I have often reflected on the subject; but there is much difficulty, as it seems to me and as Nageli has insisted, inasmuch as a strong case can be made out in favour of the view that with plants at least the sexes were primordially distinct, then became in many cases united, and in not a few cases re-separated. I have during the last 5 or 6 years been making a most laborious series of experiments, by which I shall be able, I think, to demonstrate the wonderful good derived from crossing, and I am almost sure but shall not know till the end of the summer that I shall be able to prove that the good is precisely of the same kind which the adult individual derives from slight changes of conditions.
“With my sincere thanks for your interest in my work, I remain, dear Sir, Yours very faithfully,
“Ch. Darwin.”
The following letter is of great interest in relation to many problems of sexual selection, protective resemblance, mimicry, etc.:—
“Jan. 23, 1872.
“Down.
“Dear Sir—The point to which you refer seems to me a very difficult one. 1st the comparison of the amount of variability in itself would be difficult. 2ndly of all characters, colour seems to be the most variable, as we see in domesticated productions. (3) I fully agree that selection if long continued gives fixity to characters. We see the reverse of this in the great variability of fancy races, now being selected by man. But to give fixity, selection must be continued for a very long period: pray consider on this head what I have said in the Origin about the variability of characters developed in an extraordinary manner, in comparison with the same characters in allied species. The selection must also be for a definite object, and not for anything so vague as beauty, or for the superiority of one male in its weapons over another male, which can in like manner be modified. This at least seems to me partly to account for the general variability of secondary sexual characters. In the case of mimetic insects, there is another element of doubt, as the imitated form may be undergoing change which will be followed by the imitating form. This latter consideration seems to me, as remarked in my ‘Descent of Man,’ to throw much light on how the process of imitation first began.
“I enclose a letter from Fritz Müller which I think is well worth reading, and which please to return to me.