I am very much obliged for all your trouble in writing me your long letter, which I will keep by me and ponder over. To answer it would require at least 200 folio pages! If you could see how often I have re-written some pages you would know how anxious I am to arrive as near as I can to the truth. I lay great stress on what I know takes place under domestication; I think we start with different fundamental notions on inheritance. I find it is most difficult, but not I think impossible, to see how, for instance, a few red feathers appearing on the head of a male bird, and which ARE AT FIRST TRANSMITTED TO BOTH SEXES, could come to be transmitted to males alone. It is not enough that females should be produced from the males with red feathers, which should be destitute of red feathers; but these females must have a LATENT TENDENCY to produce such feathers, otherwise they would cause deterioration in the red head-feathers of their male offspring. Such latent tendency would be shown by their producing the red feathers when old, or diseased in their ovaria. But I have no difficulty in making the whole head red if the few red feathers in the male from the first tended to be sexually transmitted. I am quite willing to admit that the female may have been modified, either at the same time or subsequently, for protection by the accumulation of variations limited in their transmission to the female sex. I owe to your writings the consideration of this latter point. But I cannot yet persuade myself that females ALONE have often been modified for protection. Should you grudge the trouble briefly to tell me whether you believe that the plainer head and less bright colours of a female chaffinch, the less red on the head and less clean colours of the female goldfinch, the much less red on the breast of the female bull-finch, the paler crest of golden-crested wren, etc., have been acquired by them for protection. I cannot think so any more than I can that the considerable differences between female and male house sparrow, or much greater brightness of the male Parus coeruleus (both of which build under cover) than of the female Parus, are related to protection. I even mis-doubt much whether the less blackness of the female blackbird is for protection. Again, can you give me reasons for believing that the moderate differences between the female pheasant, the female Gallus bankiva, the female black grouse, the pea-hen, the female partridge, [and their respective males,] have all special references to protection under slightly different conditions? I, of course, admit that they are all protected by dull colours, derived, as I think, from some dull-ground progenitor; and I account partly for their difference by partial transference of colour from the male and by other means too long to specify; but I earnestly wish to see reason to believe that each is specially adapted for concealment to its environment.