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, [pg 204]which I have vainly attempted to get an answer to?—Believe me yours very faithfully,
ALFRED R. WALLACE.
4 Chester Place, Regent's Park, N.W. March 19-24, 1868.
My dear Wallace,—I have sent your query to Cambridge to my son. He ought to answer it, for he got his place of Second Wrangler chiefly by solving very difficult problems. I enclose his remarks on two of your paragraphs: I should like them returned some time, for I have not studied them, and let me have your impression.
I have told E. Edwards to send one of my large photographs to you addressed to 76-1/2 Westbourne Grove, not to be forwarded. When at home I will send my carte.
The sterility is a most [? puzzling] problem. I can see so far, but I am hardly willing to admit all your assumptions, and even if they were all admitted, the process is so complex and the sterility (as you remark in your note) so universal, even with species inhabiting quite distinct countries (as I remarked in my chapter), together with the frequency of a difference in reciprocal unions, that I cannot persuade myself that it has been gained by Natural Selection, any more than the difficulty of grafting distinct genera and the impossibility of grafting distinct families. You will allow, I suppose, that the capacity of grafting has not been directly acquired through Natural Selection.
I think that you will be pleased with the second volume or part of Lyell's Principles, just out.