From A. R. Wallace   19 March 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.¹

I am sorry I shall not see you in town but shall look forward with pleasure to paying you a visit in the summer.²

I am sorry about the Primulas, but I feel sure some equally good case will some day be discovered, for it seems impossibible to understand how all natural species whatever should have acquired sterility.³ Closely allied forms from adjacent islands would, I should think, offer the best chance of finding good species fertile “inter se”; since even if nat. select. 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 nat. select to work with,—could be sexually selected. We seem to require a series of bold and abrupt variations. How can we imagine than an inch in the tail of the peacock, or $\frac{1}{4}$ inch in that of the Bird of Paradise, would be noticed and preferred by the female?⁴

Pray let me see what your son says about the “Sterility selection” question.⁵ I am deeply interested in all that concerns the powers of Nat. 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?⁶

Believe me | Yours very faithfully | Alfred R. Wallace—