294 EVOLUTION [CHAP. IV 



Letter 4. If sterility could be induced, then the pure races would 



2I2A 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 as great as the difficulty 

 of crossing, and as regular, I admit it would be a most 

 serious objection. But it is not. I believe many distinct 

 species can be grafted, while others less distinct cannot. 

 The regularity with which natural species are sterile together, 

 even when very much alike, I think is an argument in favour 

 of the sterility having been generally produced by Natural 

 Selection for the good of the species. 



The other difficulty, of unequal sterility of reciprocal 

 crosses, seems none to me ; for it is a step to more complete 

 sterility, and as such would be increased by selection. 



To A. R. Wallace. 



Letter 213 Down, April 6th [1868]. 



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 the rejoinder. 



