1864— 1869] STERILITY OF HYBRIDS 289 



To A. R. Wallace. Letter 210 



Feb. 27th [1868]. 



I shall be very glad to hear, at some future day, your 

 criticisms on the " causes of variability." Indeed, I feel sure 

 that I am right about sterility and Natural Selection. Two 

 of my grown-up children who are acute reasoners have two 

 or three times at intervals tried to prove me wrong ; and 

 when your letter came they had another try, but ended by 

 coming back to my side. I do not quite understand your 

 case, and we think that a word or two is misplaced. I wish 

 some time you would consider the case under the following 

 point of view. If sterility is caused or accumulated through 

 Natural Selection, then, as every degree exists up to absolute 

 barrenness, Natural Selection must have the power of 

 increasing it. Now take two species A and B, and assume 

 that they are (by any means) half-sterile, i.e., produce half the 

 full number of offspring. Now try and make (by Natural 

 Selection) A and B absolutely sterile when crossed, and you 

 will find how difficult it is. I grant, indeed it is certain, that 

 the degree of the sterility of the individuals of A and B will 

 vary ; but any such extra-sterile individuals of, we will say A, 

 if they should hereafter breed with other individuals of A, will 

 bequeath no advantage to their progeny, by which these 

 families will tend to increase in number over other families of 

 A, which are not more sterile when crossed with B. But I do 

 not know that I have made this any clearer than in the 

 chapter in my book. It is a most difficult bit of reasoning, 

 which I have gone over and over again on paper with 

 diagrams. 1 



A. R. Wallace to C. Darwin. Letter 211 



March 1st, 1868. 



I beg to enclose what appears to me a demonstration on 

 your own principles, that Natural Selection could produce 

 sterility of hybrids. If it does not convince you, I shall be 

 glad if you will point out where the fallacy lies. I have taken 

 the two cases of a slight sterility overcoming perfect fertility, 

 and of a perfect sterility overcoming a partial fertility, — the 

 beginning and end of the process. You admit that variations 

 in fertility and sterility occur, and I think you will also admit 



1 This letter appeared in Life and Letters ; III., p. 80. 



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