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.

* * * * *

_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