Should you grudge the trouble briefly to tell me, whether you believe that the plainer head and less bright colours of female chaffinch, the less red on the head and less clean colours of female goldfinch, the much less red on the breast of the female bullfinch, the paler crest of golden-crested wren, etc., have been acquired by them for protection? I cannot think so, any more than I can that the considerable differences between female and male house-sparrow, or much greater brightness of male Parus caeruleus (both of which build under cover) than of female Parus, are related to protection. I even misdoubt much whether the less blackness of female blackbird is for protection.

Again, can you give me reasons for believing that the moderate differences between the female pheasant, the female Gallus bankiva, the female of black grouse, the pea-hen, the female partridge, have all special references to protection under slightly different conditions? I, of course, admit that they are all protected by dull colours, derived, as I think, from some dull-ground progenitor; and I account partly for their difference by partial transference of colour from the male, and by other means too long to specify; but I earnestly wish to see reason to believe that each is specially adapted for concealment to its environment.

I grieve to differ from you, and it actually terrifies me and makes me constantly distrust myself. I fear we shall never quite understand each other. I value the cases of bright-coloured, incubating male fisher, and brilliant female butterflies, solely as showing that one sex may be made brilliant without any necessary transference of beauty to the other sex; for in these cases I cannot suppose that beauty in the other sex was checked by selection.

I fear this letter will trouble you to read it. A very short answer about your belief in regard to the female finches and Gallinaceae would suffice.

LETTER 450. A.R. WALLACE TO CHARLES DARWIN. 9, St. Mark's Crescent, N.W., September 27th, 1868.

Your view seems to be that variations occurring in one sex are transmitted either to that sex exclusively or to both sexes equally, or more rarely partially transferred. But we have every gradation of sexual colours, from total dissimilarity to perfect identity. If this is explained solely by the laws of inheritance, then the colours of one or other sex will be always (in relation to the environment) a matter of chance. I cannot think this. I think selection more powerful than laws of inheritance, of which it makes use, as shown by cases of two, three or four forms of female butterflies, all of which have, I have little doubt, been specialised for protection.

To answer your first question is most difficult, if not impossible, because we have no sufficient evidence in individual cases of slight sexual difference, to determine whether the male alone has acquired his superior brightness by sexual selection, or the female been made duller by need of protection, or whether the two causes have acted. Many of the sexual differences of existing species may be inherited differences from parent forms, which existed under different conditions and had greater or less need of protection.

I think I admitted before, the general tendency (probably) of males to acquire brighter tints. Yet this cannot be universal, for many female birds and quadrupeds have equally bright tints.

To your second question I can reply more decidedly. I do think the females of the Gallinaceae you mention have been modified or been prevented from acquiring the brighter plumage of the male, by need of protection. I know that the Gallus bankiva frequents drier and more open situations than the pea-hen of Java, which is found among grassy and leafy vegetation, corresponding with the colours of the two. So the Argus pheasant, male and female, are, I feel sure, protected by their tints corresponding to the dead leaves of the lofty forest in which they dwell, and the female of the gorgeous fire-back pheasant Lophura viellottii is of a very similar rich brown colour.

Charles Darwin

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