10. No single case is known of a male Papilio, Pieris, Diadema (or any other insect?) alone mimicking a Danais, etc. [pg 225] 11. But colour is more frequent in males, and variations always seem ready for purposes of sexual or other selection.
12. The fair inference seems to be that given in proposition 5 of the general argument, viz. that each species and each sex can only be modified by selection just as far as is absolutely necessary, not a step farther. A male, being by structure and habits less exposed to danger and less requiring protection than the female, cannot have more protection given to it by Natural Selection, but a female must have some extra protection to balance the greater danger, and she rapidly acquires it in one way or another.
13. An objection derived from cases like male fish, which seem to require protection, yet having brighter colours, seems to me of no more weight than is that of the existence of many white and unprotected species of Leptalis to Bates's theory of mimicry, that only one or two species of butterflies perfectly resemble leaves, or that the instincts or habits or colours that seem essential to the preservation of one animal are often totally absent in an allied species.
Down, Bromley, Kent. September 23, 1868.
My dear Wallace,—I am very much obliged for all your trouble in writing me your long letter, which I will keep by me and ponder over. To answer it would require at least 200 folio pages! If you could see how often I have rewritten some pages, you would know how anxious I am to arrive as near as I can to the truth. We differ, I think, chiefly from fixing our minds perhaps too closely on different points, on which we agree: I lay great stress on what I know takes place under domestication: I think we start with different fundamental notions on inheritance. I find [pg 226]it most difficult, but not, I think, impossible, to see how, for instance, a few red feathers appearing on the head of a male bird, and which are at first transmitted to both sexes, could come to be transmitted to males alone;[73] but I have no difficulty in making the whole head red if the few red feathers in the male from the first tended to be sexually transmitted. I am quite willing to admit that the female may have been modified, either at the same time or subsequently, for protection, by the accumulation of variations limited in their transmission to the female sex. I owe to your writings the consideration of this latter point. But I cannot yet persuade myself that females alone have often been modified for protection. Should you grudge the trouble briefly to tell me whether you believe that the plainer head and less bright colours of [female symbol][74] chaffinch, the less red on the head and less clean colours of [female symbol] goldfinch, the much less red on breast of [female symbol] bullfinch, the paler crest of goldencrest wren, etc., have been acquired by them for protection? I cannot think so; any more than I can that the considerable differences between [female symbol] and [male symbol] house-sparrow, or much greater brightness of [male symbol] Parus cæruleus (both of which build under cover) than of [female symbol] Parus are related to protection. I even misdoubt much whether the less blackness of blackbird is for protection.
Again, can you give me reason for believing that the merest differences between female pheasants, the female Gallus bankiva, the female of black grouse, the pea-hen, female partridge, have all special reference to protection [pg 227]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 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,