“DEAR SIR--The point to which you refer seems to me a very difficult one. 1^{st} the comparison of the amount of variability in itself would be difficult. 2^{ndly} of all characters, colour seems to be the most variable, as we see in domesticated productions. (3) I fully agree that selection if long continued gives fixity to characters. We see the reverse of this in the great variability of fancy races, now being selected by man. But to give fixity, selection must be continued for a very long period: pray consider on this head what I have said in the Origin about the variability of characters developed in an extraordinary manner, in comparison with the same characters in allied species. The selection must also be for a definite object, and not for anything so vague as beauty, or for the superiority of one male in its weapons over another male, which can in like manner be modified. This at least seems to me partly to account for the general variability of secondary sexual characters. In the case of mimetic insects, there is another element of doubt, as the imitated form may be undergoing change which will be followed by the imitating form. This latter consideration seems to me, as remarked in my ‘Descent of Man,’ to throw much light on how the process of imitation first began. “I enclose a letter from Fritz Müller which I think is well worth reading, and which please to return to me. “You will see he lays much stress on the difficulty of several remotely allied forms all imitating some one species. Mr. Wallace did not think that there was so much weight in this objection as I do. It is, however, possible that a few species in widely different groups, before they had diverged much, should have accidentally resembled, to a certain extent, some one species. You will also see in this letter a strange speculation, which I should not dare to publish, about the appreciation of certain colours being developed in those species which frequently behold other forms similarly ornamented. I do not feel at all sure that this view is as incredible as it may at first appear. Similar ideas have passed through my mind when considering the dull colours of all the organisms which inhabit dull-coloured regions, such as Patagonia and the Galapagos Is. I suppose you know Mr. Riley’s excellent essay on mimicry in the last report on the noxious insects of Missouri or some such title. “I hope your work may be in every way successful. “I remain, dear Sir, yours faithfully, “CHARLES DARWIN.” The next letter deals with mimetic resemblance:-- “_Mar. 28, 1872._ “Down.