efinite conditions of life, so that the sexual constitution of
species A is attuned, as it were, to a condition different from that to
which B is attuned, and this leads to sterility. But domestic varieties
are not strictly adapted by Natural Selection to definite conditions,
and thus have less specialised sexual constitutions.) This elimination,
though admitted by many authors, rests on very slight evidence, yet I
think is very probably true, as may be inferred from the case of
dogs. Under nature it seems improbable that the differences in the
reproductive constitution, on which the sterility of any two species
when crossed depends, can be acquired directly by Natural Selection; for
it is of no advantage to the species. Such differences in reproductive
constitution must stand in correlation with some other differences; but
how impossible to conjecture what these are! Reflect on the case of
the variations of Verbascum, which differ in no other respect whatever
besides the fluctuating element of the colour of the flower, and yet it
is impossible to resist Gartner's evidence, that this difference in the
colour does affect the mutual fertility of the varieties.

The whole case seems to me far too mysterious to rest (92/4. The word
"rest" seems to be used in place of "to serve as a foundation for.") a
valid attack on the theory of modification of species, though, as you
say, it offers excellent ground for a mere advocate.

I am surprised, considering how ignorant we are on very many points,
[that] more weak parts in my book have not as yet been pointed out to
me. No doubt many will be. H.C. Watson founds his objection in MS. on
there being no limit to infinite diversification of species: I have
answered this, I think, satisfactorily, and have sent attack and answer
to Lyell and Hooker. If this seems to you a good objection, I would
send papers to you. Andrew Murray "disposes of" the whole theory by an
ingenious difficulty from the distribution of blind cave insects (92/5.
See "Life and Letters, Volume II., page 265. The reference here is to
Murray's address before the Botanical Society, Edinburgh. Mr. Darwin
seems to have read Murray's views only in a separate copy reprinted from
the "Proc. R. Soc. Edin." There is some confusion about the date of the
paper; the separate copy is dated January 16th, while in the volume of
the "Proc. R. Soc." it is February 20th. In the "Life and Letters," II.,
page 261 it is erroneously state