vantageous even if the other cross was fertile: and just as characters
now co-ordinated may have been separately accumulated by Natural
Selection, so the reciprocal crosses may have become sterile one at a
time.

LETTER 212. TO A.R. WALLACE. 4, Chester Place, March 17th, 1868.

(212/1. Mr. Darwin had already written a short note to Mr. Wallace
expressing a general dissent from his view.)

I do not feel that I shall grapple with the sterility argument till my
return home; I have tried once or twice, and it has made my stomach feel
as if it had been placed in a vice. Your paper has driven three of my
children half mad--one sat up till 12 o'clock over it. My second son,
the mathematician, thinks that you have omitted one almost inevitable
deduction which apparently would modify the result. He has written out
what he thinks, but I have not tried fully to understand him. I suppose
that you do not care enough about the subject to like to see what he has
written.

LETTER 212A. A.R. WALLACE TO CHARLES DARWIN. Hurstpierpoint, March, 24th
[1868].

I return your son's notes with my notes on them. Without going into any
details, is not this a strong general argument?

1. A species varies occasionally in two directions, but owing to their
free intercrossing the varieties never increase.

2. A change of conditions occurs which threatens the existence of the
species; but the two varieties are adapted to the changing conditions,
and if accumulated will form two new species adapted to the new
conditions.

3. Free crossing, however, renders this impossible, and so the species
is in danger of extinction.

4. If sterility would be induced, then the pure races would increase
more rapidly, and replace the old species.

5. It is admitted that partial sterility between varieties does
occasionally occur. It is admitted [that] the degree of this sterility
varies; is it not probable that Natural Selection can accumulate these
variations, and thus save the species? If Natural Selection can NOT do
this, how do species ever arise, except when a variety is isolated?

Closely allied species in distinct countries being sterile is no
difficulty; for either they diverged from a common ancestor in contact,
and Natural Selection increased the sterility, or they were isolated,
and have varied since: in which case they have been for ages influenced
by distinct conditions which may well produce sterility.

If the difficulty of grafting was