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. 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 .
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.