e, my dear Wallace, with many thanks, yours very
sincerely,

CH. DARWIN.

* * * * *

Wallace's more recent views on the question of Natural Selection and
Sterility may be found in a note written by him in 1899: "When writing
my 'Darwinism' and coming again to the consideration of the problem of
the effect of Natural Selection in accumulating variations in the amount
of sterility between varieties or incipient species, twenty years later,
I became more convinced than I was when discussing with Darwin, of the
substantial accuracy of my argument. Recently a correspondent who is
both a naturalist and a mathematician has pointed out to me a slight
error in my calculation at p, 183 (which does not, however, materially
affect the result) disproving the physiological selection of the late
Dr. Romanes, but he can see no fallacy in my argument as to the power of
Natural Selection to increase sterility between incipient species, nor,
so far as I am aware, has anyone shown such fallacy to exist.

"On the other points on which I differed from Mr. Darwin in the
foregoing discussion--the effect of high fertility on population of a
species, etc.--I still hold the views I then expressed, but it would be
out of place to attempt to justify them here."--A.R.W.

* * * * *

_9 St. Mark's Crescent, N.W. August 16, [1868?]._

Dear Darwin,--I ought to have written before to thank you for the copies
of your paper on "Primula" and on "Cross Unions of Dimorphic Plants,
etc." The latter is particularly interesting, and the conclusion most
important; but I think it makes the difficulty of _how_ these forms,
with their varying degrees of sterility, originated, greater than ever.
If Natural Selection could not accumulate varying degrees of sterility
for the plant's benefit, then how did sterility ever come to be
associated with _one cross_ of a trimorphic plant rather than another?
The difficulty seems to be increased by the consideration that the
advantage of a cross with a _distinct individual_ is gained just as well
by illegitimate as by legitimate unions. By what means, then, did
illegitimate unions ever become sterile? It would seem a far simpler way
for each plant's pollen to have acquired a prepotency on another
individual's stigma over that of the same individual, without the
extraordinary complication of three differences of structure and
eighteen different unions with vary