Dear Darwin

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.¹

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 sex^l. slectⁿ. has led to the modification of the nest for safety, as in the Australian finches which make domed nests while our European species make open ones.²

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.³ As you are going to treat fully the whole subject of “sexual selection” I hope you will not call it an “Essay on Man”.⁴ 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.⁵

Believe me Dear Darwin | Yours very faithfully | Alfred R. Wallace—