I fear we shall never quite understand each other. I value the cases of bright-coloured, incubating male fishes--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 symbol] finches and Gallinaceæ would suffice.--Believe me, my dear Wallace, yours very sincerely, CH. DARWIN. * * * * * _9 St. Mark's Crescent, S.W. September 27, 1868._ Dear Darwin,--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 their 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 who 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. I think the case of [female symbol] _Pieris pyrrha_ proves that females alone can be greatly modified for protection.