ountries being sterile is no
difficulty, for either they diverged from a common ancestor in contact,
and Natural Selection increased the sterility, or they were isolated,
and have varied since, in which case they have been for ages influenced
by distinct conditions which may well produce sterility.

If the difficulty of _grafting_ was as great as the difficulty of
_crossing_, and as _regular_, I admit it would be a most serious
objection. But it is not. I believe many distinct species can be grafted
while others less distinct cannot. The regularity with which natural
species are sterile together, even when _very much alike_, I think is an
argument in favour of the sterility having been generally produced by
Natural Selection for the good of the species.

The other difficulty, of unequal sterility of reciprocal crosses, seems
none to me; for it is a step to more complete sterility, and as such
would be useful and would be increased by selection.

I have read Sir C. Lyell's second volume with great pleasure. He is, as
usual, very cautious, and hardly ever expresses a positive opinion, but
the general effect of the whole book is very strong, as the argument is
all on our side.

I am in hopes it will bring in a new set of converts to Natural
Selection, and will at all events lead to a fresh ventilation of the
subject.--Believe me yours very faithfully,

ALFRED R. WALLACE.

* * * * *

_4 Chester Place, Regent's Park, N.W. March 27, 1868._

My dear Wallace,--My son has failed in your problem, and says that it is
"excessively difficult": he says you will find something about it in
Thomson and Tait, "Natural Philosophy" (art. 649). He has, however, sent
the solution, if the plate rested on a square rim, but he supposes this
will not answer your purpose; nevertheless, I have forwarded it by this
same post. It seems that the rim being round makes the problem much more
difficult.

I enclose my photograph, which I have received from Down. I sent your
answer to George on his objection to your argument on sterility, but
have not yet heard from him. I dread beginning to think over this
fearful problem, which I believe beats the plate on the circular rim;
but I will sometime. I foresee, however, that there are so many doubtful
points that we shall never agree. As far as a glance serves it seems to
me, perhaps falsely, that you sometimes argue that hybrids have an
advantage from greater vigour,