I have been working hard in collecting facts on sexual selection every morning in London, and have done a good deal; but the subject grows more and more complex, and in many respects more difficult and doubtful. I have had grand success this morning in tracing gradational steps by which the peacock tail has been developed: I quite feel as if I had seen a long line of its progenitors. I do not feel that I shall grapple with the sterility argument till my return home; I have tried once or twice and it has made my stomach feel as if it had been placed in a vice. Your paper has driven three of my children half-mad--one sat up to twelve o'clock over it. My second son, the mathematician, thinks that you have omitted one almost inevitable deduction which apparently would modify the result. He has written out what he thinks, but I have not tried fully to understand him. I suppose that you do not care enough about the subject to like to see what he has written? I hope your book progresses. I am intensely anxious to see your paper in _Murray's Journal_.--My dear Wallace, yours very sincerely, CH. DARWIN. * * * * * _Hurstpierpoint. March 19, 1868._ Dear Darwin,--I should very much value a _large_ photograph of you, and also a carte for my album, though it is too bad to ask you for both, as you must have so many applicants. I am sorry I shall not see you in town, but shall look forward with pleasure to paying you a visit in the summer. I am sorry about the Primulas, but I feel sure some such equally good case will some day be discovered, for it seems impossible to understand how all natural species whatever should have acquired sterility. Closely allied forms from adjacent islands would, I should think, offer the best chance of finding good species fertile _inter se_; since even if Natural Selection induces sterility I do not see how it could affect them, or why they should _always_ be sterile, and varieties _never_. I am glad you have got good materials on sexual selection. It is no doubt a difficult subject. One difficulty to me is, that I do not see how the constant _minute_ variations, which are sufficient for Natural Selection to work with, could be _sexually_ selected. We seem to require a series of bold and abrupt variations. How can we imagine that an inch in the tail of a peacock, or a quarter of an inch in that of the bird of paradise, would be noticed and preferred by the female? Pray let me see what your son says about the sterility selection question. I am deeply interested in all that concerns the powers of Natural Selection, but, though I admit there are a few things it cannot do, I do not yet believe sterility to be one of them. In case your son has turned his attention to mathematical physics, will you ask him to look at the enclosed question, which I have vainly attempted to get an answer to?--Believe me yours very faithfully, ALFRED R. WALLACE. * * * * *