and sometimes a disadvantage from not
being so well fitted to their conditions. Heaven protect my stomach
whenever I attempt following your argument!--Yours most sincerely,

C. DARWIN.

* * * * *

_Down, Bromley, Kent. April 6, 1868._

My dear Wallace,--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 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 produce _from any cause_ fewer
young, the deficiency would be supplied from surrounding districts. This
applies to your par. 5. 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 the probability and almost to the possibility of par. 1, as
you start with two forms, within the same area, which are not mutually
sterile, and which yet have supplanted the parent-form (par. 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
aquatic animals. I saw clearly what an immense aid this would be, but
gave it up. Disinclination to cross seems to have been independently
acquired, probably by Natural Selection; and I do not see why it would
not have sufficed to have prevented incipient species from blending to
have simply increased sexual disinclination to cross.

Par. 11: I demur to a certain extent to amount of sterility and
struc