( f^iv ) 



paired were unproductive, yet neither impotent. For instance, 

 I had this morning a letter with a case of a Hereford 

 heifer, which seemed to be, after repeated trials, sterile with 

 one particular and far from impotent bull, but not with 

 another bull. But it is too long a story — it is to attempt to 

 make two strains, both fertile, and yet sterile when one of one 

 strain is crossed with one of the other strain. But the 

 difliculty . . . would be beyond calculation." * 



The experiment was evidently unsuccessful, — perhaps was 

 never seriously undertaken, — and a few years later Darwin 

 added the following postscript to a letter to Huxley, January 7 

 [1867]. 



" P.tS. — Nature never made species mutually sterile by 

 .selection, nor will men." t 



This was probably only an offhand expression of opinion, 

 not intended to be taken serioiisly. An altogether hopeless 

 attitude would not be reasonable until the suofgested scheme 

 had been applied many times, and in sevei-al parts of the 

 animal and vegetable kingdoms. 



But the positive results demanded by Huxley, even if 

 obtained, woidd by no means justify his far-reaching 

 conclusions. If the barrier of sterility were thus artificially 

 produced, we should be very far from the proof that its exist- 

 ence in nature is due to the same kind of cause, viz. selection. 

 If Darwin was right in his controversy with Wallace, if 

 " Natui'e never made species mutually sterile by selection," 

 the suggested experiment would merely do by artificial selection 

 what is not done by natural selection. 



It is by no means difiicult to understand the mutual sterility 

 which is usual between natural species as an incidental result 

 of their separation by asyngamy for a long period of time. 

 In the process of fertilisation a portion of a single cell nucleus 

 from one individual fuses with a portion from another in- 

 dividual, the two combining to form the complete nucleus of 

 tlie first cell of the offspring, from which all the countless 

 cells of the future individual will arise by division. Each 

 pai't-nucleus contains the whole of the hereditary qualities 



* "More Letters," vol. i, pp. 225, 226, Letter 154. 

 + Ibid. vol. i, p. 277, Letter 197. 



