LETTERS OF CHARLES DARWIN. 409 



I fear that this letter can be of no use to you, but it will serve to 

 show my sympathy and good wishes in your researches. 



To A. R. Wallace. 



Down, April 29th [1867]. 



I have been greatly interested by your letter, but your view is not 

 new to me. If you will look at p. 240 of the fourth edition of the 

 Origin you will find it very briefly given with two extreme examples of 

 the peacock and black grouse. A more general statement is given at 

 p. 101, or at p. 89 of the first edition, for I have long entertained this 

 view, though I have never had space to develop it. But I had not 

 sufficient knowledge to generalise as far as you do about colouring and 

 nesting. In your paper perhaps you will just allude to my scanty 

 remark in the fourth edition, because in my Essay on Man I intend to 

 discuss the whole subject of sexual selection, explaining as I believe 

 it does much with respect to man. I have collected all my old notes, 

 and partly written my discussion, and it would be flat work for me to 

 give the leading idea as exclusively from you. But, as I am sure from 

 your greater knowledge of Ornithology and Entomology that you will 

 write a much better discussion than I could, your paper will be of great 

 use to me. Nevertheless I must discuss the subject fully in my Essay 

 on Man. When we met at the Zoological Society, and I asked you 

 about the sexual differences in kingfishers, I had this subject in view; 

 as I had when I suggested to Bates the difficulty about gaudy cater- 

 pillars, which you have so admirably (as I believe it will prove) ex- 

 plained. I have got one capital case (genus forgotten) of a [Aus- 

 tralian] bird in which the female has long tail-plumes, and which 

 consequently builds a different nest from all her allies. With respect 

 to certain female birds being more brightly coloured than the males, 

 and the latter incubating, I have gone a little into the subject, and 

 can not say that I am fully satisfied. I remember mentioning to you 

 the case of Rhynclicea, but its nesting seems unknown. In some other 

 cases the difference in brightness seemed to me hardly sufficiently ac- 

 counted for by the principle of protection. At the Falkland Islands 

 there is a carrion hawk in which the female (as I ascertained by dis- 

 section) is the brightest coloured, and I doubt whether protection will 

 here apply; but I wrote several months ago to the Falklands to make 

 enquiries. The conclusion to which I have been leaning is that in 

 some of these abnormal cases the colour happened to vary in the female 

 alone, and was transmitted to females alone, and that her variations 

 have been selected through the admiration of the male. 



It is a very interesting subject, but I shall not be able to go on 

 with it for the next five or six months, as I am fully employed in cor- 



