(1899).) P.S .-- In answer to the objection as to the unequal sterility of reciprocal crosses ("Variation, etc." Volume II., page 186) I reply that, as far as it went, the sterility of one cross would be advantageous 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.