Never imagined that the facts about sexual selection could be new to CD. Thought fact that brightly coloured females build concealed nests and almost all those in which sexes differ remarkably build exposed nests might be new to him. Some problems remain. Sends his notes for CD to use if he wants.
9, St. Mark's Crescent
I was afraid you had rather misunderstood my letter on first reading it; for I assure you I never for a moment imagined that any of the more obvious facts connected with sexual selection (which is altogether your own subject) could have been new to you. The remarkable coincidence, of so many of the birds which have females coloured as gayly as the males, making their nests so that the female is concealed during incubation, while almost all in which the female differs remarkably from the male in colour build exposed and uncovered nests;—appeared to me to get over one great difficulty in the way of explaining the origin of colour in birds; and as it was so new & interesting to me I thought it might not possibly have occurred to you.
There are some exceptions which I cannot yet explain, but this is to be expected, for we cannot but suppose that many different causes have favoured or checked the developement of colour at different times. The exceptions are not I think numerous enough to upset the rule.
This view is I think also interesting as explaining the absence of much sexual difference of colour among mammals or reptiles, in which the sexes are not very differently situated as regards danger from enemies.
The mode of nidification in birds is no doubt primarily dependent on their
structural peculiarities and their general habits (on which subject I have a paper
written ten years ago,) and we may therefore conclude that the mode of nidification of
Kingfishers Toucans &c. has been the acting cause in determining or
permitting the action of sexual selection on the female bird. In other cases however it
is quite possible, that the colour being first produced by
On powerful and pugnacious birds, such as crows and hawks I do not expect the principle of protection has acted much in modifying colour.
I enclose you a copy of my notes on the subject, which I beg you to make what use of you like, in your proposed essay. I will merely allude to the subject in my paper on ``mimicry'', which is finished & sent to the ``Westminster'' to see if they will publish it. As you are going to treat fully the whole subject of ``sexual selection'' I hope you will not call it an ``Essay on Man''. I had thought of a short paper on ``The connexion between the colours of female birds & their mode of nidification'',—but had rather leave it for you to treat as part of the really great subject of ``sexual selection'' which combined with ``protective resemblances'' and ``differences'' will I think when thoroughly worked out explain the whole colouring of the Animal Kingdom.
Believe me Dear Darwin | Yours very faithfully | Alfred R. Wallace—
- f1 5522.f1See letter from A. R. Wallace, 26 April , and letter to A. R. Wallace, 29 April . Wallace had suggested that the dull colouring of many female birds was related to their need for protection while sitting on exposed nests.
- f2 5522.f2Wallace discussed how the structure and habits of birds, as opposed to their instincts, determined how they built their nests in A. R. Wallace 1867b; in A. R. Wallace 1868--9 he went on to argue that whether a nest was open or covered influenced the colour of the female bird. In A. R. Wallace 1868--9, p. 83, he wrote: `When the confirmed habit of a group of birds was to build their nests in holes of trees, like the toucans, or in holes in the ground, like the kingfishers, the protection the female thus obtained, during the important and dangerous time of incubation, placed the two sexes on an equality as regards exposure to attack, and allowed ``sexual selection'' to act unchecked in the development of gay colours and conspicuous markings in both sexes.'
- f3 5522.f3Wallace refers to [A. R. Wallace] 1867a, which was published in the Westminster Review in July 1867. The need for protection of birds sitting on eggs is discussed in ibid., pp. 33--4. There is a heavily annotated copy of the paper in DAR 133: 13. CD discussed [A. R. Wallace] 1867a extensively in the second volume of Descent.
- f4 5522.f4See letter to A. R. Wallace, 29 April  and n. 5.
- f5 5522.f5Wallace published two papers on birds' nests, A. R. Wallace 1867b and A. R. Wallace 1868--9 (see n. 2, above). There is a lightly annotated copy of A. R. Wallace 1867b in DAR 133: 12. CD discussed Wallace's arguments in Descent 2: 166--80.