1S64-1S69] STERILITY OF HYBRIDS 293 



p.S. — In answer to the objection as to the unequal sterility Letter 211 

 of reciprocal crosses {Variation, etc., Vol. II., p. 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. 



To A. R. Wallace. Letter 212 



4, Chester Place, March 17th, 1868." 

 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. 1 suppose that you do 

 not care enough about the subject to like to see what he 

 has written. 



A. R. Wallace to C. Darwin. Letter 



Hurstpierpoint, March 24th [1868]. 



I return your son's notes with my notes on them. With- 

 out 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. 



therefore the cause of sterility between species, is during the process of 

 differentiation. When species are fully formed, the occasional occurrence 

 of hybrids is of comparatively small importance, and can never be a 

 danger to the existence of the species. A. R. W. (1899). 



1 Mr. Darwin had already written a short note to Mr. Wallace 

 expressing a general dissent from his view. 



