1890 ON PHTSIOLOGICAL SELECTION 205 



species sterility does occur in all degrees, from there 

 being no such sterility at all in very many cases, to 

 there being absolute steiility in other cases. There- 

 fore, in stating this fact as a fact, I am not playing 

 at ' heads I mn and tails you lose,' nor ' begging the 

 whole question at the outset.' Any ' question ' really 

 arises only with regard to the second expectation-\dz. 

 whether there is a general correlation between de- 

 grees of mutual fertihty and degrees of topographical 

 isolation. 



Xow, this question I have not begged, but, on the 

 contrary, stated as the question by an experimental 

 answer to which my theory must stand or fall. 



Thus, the cases which you mention obviously go 

 to support the theory, inasmuch as they conform to 

 the expectation above mentioned. TVliat I want to 

 do is to find as many genera as possible like binchona 

 and begonia, where the constituent species are 

 separated geographically or topographically, and (? in 

 consequence) easily hybridise with one another. 



Therefore, as a mere matter of method, I cannot 

 see that I have begged any question ; for the only 

 question is not about the facts which I state, but 

 about my suggested explanation of them. And this 

 question can only be answered by ascertaining whether 

 there is in nature any such general correlation be- 

 tween isolation and capabOity of hybridising (also, of 

 course, between the absence of isolation and the 

 absence of such capability) as my theory would 

 require. 



Yours very sincerely, 



G. J. EoilAXES. 



