Alfred Russel Wallace 



certainly been an immense relief to my mind; for I have 

 been stumbling over the subject for years, dimly seeing that 

 some relation existed between the various classes of facts. 

 I now hear from H. Spencer that his views quoted in my 

 footnote refer to something quite distinct, as you seem to 

 have perceived. 



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 con- 

 sider the case under the following point of view. If 

 sterility is caused or accumulated through Natural Selec- 

 tion, then, as every degree exists up to absolute barren- 

 ness. 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 cer- 

 tain, that the degree of 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. 



198 



