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9 St. Mark’s Crescent, N.W.  October 4, 1868.

Dear Darwin,—­I should have answered your letter before, but have been very busy reading over my MSS. the last time before going to press, drawing maps, etc. etc.

Your first question cannot be answered, because we have not, in individual cases of slight sexual difference, sufficient evidence to determine how much of that difference is due to sexual selection acting on the male, how much to natural selection (protective) acting on the female, or how much of the difference may be due to inherited differences from ancestors who lived under different conditions.  On your second question I can give an opinion.  I do think the females of the Gallinaceae you mention have been either modified; or prevented from acquiring much of the brighter plumage of the males, by the need of protection.  I know that Gallus bankiva frequents drier and more open situations than Pavo muticus, which in Java is found among grassy and leafy vegetation corresponding with the colours of the two females.  So the Argus pheasants, male and female, are, I feel sure, protected by their tints corresponding to dead leaves of the dry lofty forests in which they dwell; and the female of the gorgeous fire-back pheasant, Lophura viellottii, is of a very similar rich brown colour.

These and many other colours of female birds seem to me exactly analogous to the colours of both sexes in such groups as the snipes, woodcocks, plovers, ptarmigan, desert birds, Arctic animals, greenbirds.

[The second page of this letter has been torn off.  This letter and that of September 27 appear both to answer the same letter from Darwin.  The last page of this or of another letter was placed with it in the portfolio of letters; it is now given.]

I am sorry to find that our difference of opinion on this point is a source of anxiety to you.

Pray do not let it be so.  The truth will come out at last, and our difference may be the means of setting others to work who may set us both right.

After all, this question is only an episode (though an important one) in the great question of the origin of species, and whether you or I are right will not at all affect the main doctrine—­that is one comfort.

I hope you will publish your treatise on Sexual Selection as a separate book as soon as possible, and then while you are going on with your other work, there will no doubt be found someone to battle with me over your facts, on this hard problem.

With best wishes and kind regards to Mrs. Darwin and all your family, believe me, dear Darwin, yours very faithfully,

ALFRED R. WALLACE.

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Down, Bromley, Kent, S.E.  October 6, 1868.

My dear Wallace,—­Your letter is very valuable to me, and in every way very kind.  I will not inflict a long answer, but only answer your queries.  There are breeds (viz.  Hamburgh) in which both sexes differ much from each other and from both sexes of G. bankiva; and both sexes are kept constant by selection.