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.
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