80 'VARIATION UNDER DOMESTICATION.' [1868. 



say exactly and fully expresses my feeling, viz. that it 

 is a relief to have some feasible explanation of the various 

 facts, which can be given up as soon as any better hypo- 

 thesis is found. It has 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 foot-note refer to something quite distinct, 

 as you seem to have perceived. 



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

 cisms on the "causes of variability." Indeed I feel sure that 

 I am right about sterility and natural selection. ... 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 sterility of the individuals A 

 and B will vary, but any such extra-sterile individuals of, we 

 will say A, if they should hereafter breed with other indi- 

 viduals 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. 



. . . Hearty thanks for your letter. You have indeed 

 pleased me, for I had given up the great god Pan as a still- 

 born deity. I wish you could be induced to make it clear, 



