I have been greatly interested by your letter, but your view is not new to me. (429/1. We have not been able to find Mr. Wallace's letter to which this is a reply. It evidently refers to Mr. Wallace's belief in the paramount importance of protection in the evolution of colour. This is clear from the P.S. to the present letter and from the passages in the "Origin" referred to. The first reference, Edition IV., page 240, is as follows: "We can sometimes plainly see the proximate cause of the transmission of ornaments to the males alone; for a pea-hen with the long tail of the male bird would be badly fitted to sit on her eggs, and a coal-black female capercailzie would be far more conspicuous on her nest, and more exposed to danger, than in her present modest attire." The passages in Edition I. (pages 89, 101) do not directly bear on the question of protection.) If you will look at page 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 page 101, or at page 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 caterpillars, which you have so admirably (as I believe it will prove) explained. (429/2. See a letter of February 26th, 1867, to Mr. Wallace, "Life and Letters" III., page 94.) I have got one capital case (genus forgotten) of a [Australian] bird in which the female has long tail-plumes, and which consequently builds a different nest from all her allies. (429/3. Menura superba: see "Descent of Man" (1901), page 687. Rhynchoea, mentioned a line or two lower down, is discussed in the "Descent," page 727. The female is more brightly coloured than the male, and has a convoluted trachea, elsewhere a masculine character. There seems some reason to suppose that "the male undertakes the duty of incubation.") With respect to certain female birds being more brightly coloured than the males, and the latter incubating, I have gone a little