Alfred Russel Wallace 



him in 1899 : ** When writing my ^ Darwinism ' and coming 

 again to the consideration of the problem of the effect of 

 Natural Selection in accumulating variations in the amount 

 of sterility between varieties or incipient species, twenty 

 years later, I became more convinced than I was when 

 discussing with Darwin, of the substantial accuracy of 

 my argument. Recently a correspondent who is both a 

 naturalist and a mathematician hr>^. pointed out to me a 

 slight error in my calculation at p. 183 (which does not, 

 however, materially affect the result) disproving the phy- 

 siological selection of the late Dr. Romanes, but he can 

 see no fallacy in my argument as to the power of Natural 

 Selection to increase sterility between incipient species, 

 nor, so far as I am aware, has anyone shown such fallacy 

 to exist. 



*' On the other points on which I differed from Mr. 

 Darwin in the foregoing discussion — the effect of high fer- 

 tility on population of a species, etc. — I still hold the 

 views I then expressed, but it would be out of place to 

 attempt to justify them here.'' — A. R. W. 



9 >S'/. Mark's Crescent, N,W. August 16, [1868 ?]. 



Dear Darwin, — I ought to have written before to thank 

 you for the copies of your paper on "Primula" and on 

 " Cross Unions of Dimorphic Plants, etc.'*' The latter is 

 particularly interesting, and the conclusion most impor- 

 tant ; but I think it makes the difficulty of how these forms, 

 with their varying degres of sterility, originated, greater 

 than ever. If Natural Selection could not accumulate 

 varying degrees of sterility for the plant's benefit, then 

 how did sterility ever come to be associated with one cross 

 of a trimorphic plant rather than another ? The difficulty 

 seems to be increased by the consideration that the advan- 

 tage of a cross with a distinct individual is gained just 

 as well by illegitimate as by legitimate unions. By what 

 means, then, did illegitimate unions ever become sterile ? 

 It would seem a far simpler way for each plant's pollen 



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