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 increased by selection.

LETTER 213. TO A.R. WALLACE. Down, April 6th [1868].

I have been considering the terrible problem. Let me first say that
no man could have more earnestly wished for the success of Natural
Selection in regard to sterility than I did; and when I considered a
general statement (as in your last note) I always felt sure it could be
worked out, but always failed in detail. The cause being, as I
believe, that Natural Selection cannot effect what is not good for the
individual, including in this term a social community. It would take a
volume to discuss all the points, and nothing is so humiliating to me
as to agree with a man like you (or Hooker) on the premises and disagree
about the result.

I agree with my son's argument and not with the rejoinder. The cause of
our difference, I think, is that I look at the number of offspring as an
important element (all circumstances remaining the same) in keeping up
the average number of individuals within any area. I do not believe that
the amount of food by any means is the sole determining cause of number.
Lessened fertility is equivalent to a new source of destruction. I
believe if in one district a species produced from any cause fewer
young, the deficiency would be supplied from surrounding districts. This
applies to your Paragraph 5. (213/1. See Letter 211.) If the species
produced fewer young from any cause in every district, it would become
extinct unless its fertility were augmented through Natural Selection
(see H. Spencer).

I demur to probability and almost to possibility of Paragraph 1., as
you start with two forms within the same area, which are not mutually
sterile, and which yet have supplanted the parent-form.

(Paragraph 6.) I know of no ghost of a fact supporting belief that
disinclination to cross accompanies sterility. It cannot hold with
plants, or the lower fixed aqua