Dear Darwin

I have lately hit upon a generalization connected with sexual characters which pleases me very much, and I make no doubt will interest you. I have been writing a popular article on “Mimicry” & allied phenomena in which I have tried to bring together all the groups of facts which bear upon the subject.² While doing this I have become more than ever convinced of the powerful effect of “protective resemblances” in determining and regulating the development of colour,—more especially in the females of insects. Following out this view I impute the absence of brilliant or conspicuous tints in the female of Birds (when it exists in the male) almost entirely to this protective adaptation because in Birds, the female while sitting is much more exposed to attack than the male. This is I think pretty well demonstrated by the wonderful case of the Phalaropus fulicarius. ³ In this bird the sexes are alike in winter plumage, but in the summer plumage the female is much the most gaily coloured, having a black head, dark wings & reddish back, while the male is uniform brownish with dusky spots; the usual sexual differences being exactly reversed— Now strange to say the sexual habits are exactly reversed also, the male alone sitting on the eggs!!

The genus Turnix is another example, the males being always smaller, & often less brightly coloured than the females;—and the males certainly do also sit on the eggs if they do not do it exclusively.

Now these facts led me to consider why it is that in a number of groups of conspicuously coloured birds the sexes are alike, or at least equally conspicuous, contrary to the more general rule; and I was immediately led to the very simple reason that in these cases sexual selection had acted unchecked in both sexes, because the habits of the species were such that the female was not more exposed during incubation than the male— Hence the law,—that where birds nidificate in holes in the ground, or in holes in trees, or build covered nests, the females will generally be as gaily coloured as the males.

This is very generally true. For example

Kingfishers—nests in holes in the earth

sexes alike,—or females quite as conspicuous.

Bee Eaters… do. do.

Rollers … do. do.

Woodpeckers nests in holes in trees.

females very conspicuous, head often wh. spotted.

Parrots and Perroquets. ⁴ nests in holes.

sexes generally alike and very conspicuous Fam.

Icteridæ. Hungnests.⁵ covered nests

sexes, generally alike, and very conspicuous,

black red and yellow colours. Genus.

Pardalotus .. nests in holes, or dome shaped.

sexes alike, or females equally conspicuous.

Tits .. nests concealed in holes or covered,

sexes alike— very ornamental & conspicuous.

Wagtails— nests well concealed

sexes nearly alike.

and there are many more equally curious examples which I shall bring together perhaps in a paper for the Linnæan Soc^y.⁶

I think this proves that the primary action of sexual selection is to produce colour pretty equally in both sexes, but that it is checked in the females by the immense importance of protection, and the danger of conspicuous colouring. Of course this rule will not apply always, as there are many unknown causes affecting both the habits & the colouration of animals. When a bird is strong and has few enemies or is too large and numerous to build covered nests, as in rooks, jays & such like birds, the females remain conspicuous and unprotected.

The case of female birds being in several cases more brightly coloured than the males when they do not incubate, and almost always as brightly coloured when incubation is performed in perfect concealment proves I think that the male admires gay colours in the female as well as the female in the male,—and that the direct cause of the prevalent dull colours in the female, is solely their danger; & does not at all shew that the males have no taste in colour, which would be the natural inference if sexual selection alone had produced all the phenomena.

This case of the birds is exactly parallel to that of insects. The objects of mimicry have the sexes alike or equally bright coloured, for both are equally protected; witness Heliconidæ, Danaidæ, Carabidæ, Malacoderms,⁷ Eumorphidæ⁸ Bees & wasps &c. And Coccinellidæ, which though not mimicked are certainly protected since they are refused by birds, have the sexes alike.

I shall be glad to hear how far you agree with these views and what objections occur to you.

Lyell has some splendid facts about the swimming powers of pigs at sea, which quite explains their wide diffusion in the East beyond all other placental land Mammals.⁹

Hoping you are well   Believe me | Yours very sincerely | Alfred R Wallace

C. Darwin Esq.