"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.
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