220 GEOEGE JOHN BOMANES 



which afterwards arose, I should have reiterated such 

 statements ad nauseam. 



Sorry you cannot come to the B.A., or to dine, 

 but certainly do not wonder. 



Yours very sincerely, 



G. J. ROMANES. 



Lastly, about species not being able to exist as 

 species without the physiological isolation of physio- 

 logical selection (p. 403), the statement of course only 

 applies to nearly allied species occupying common 

 areas (see p. 404). If this statement is wrong, no 

 one has yet shown me wherein it is so. I fancy you 

 do not quite appreciate that by l sterility ' I always 

 mean (unless otherwise expressly stated) sterility in 

 some degree, and this not only with regard to the 

 fertile hybrids. It is by no means enough to point to 

 natural and fertile hybrids as cases opposed to phy- 

 siological selection unless it has been shown by 

 experiment through a generation or two that these 

 hybrids are fully fertile i.e. as fertile as their parent 

 species. Now, experiments of this kind have rarely 

 been carried through. If you assume that the result 

 of carrying them through would be destructive of 

 physiological selection by proving that fertile hybrids 

 are, as a rule, fully fertile, and also (which is very 

 important) that in any cases where experiment may 

 show them to be so, further experiment would 

 fail to show that isolation has not been effected in 

 any other way (as by pre-potency, differences of 

 insect fertilisation, &c.) in short, if you assume 

 that fertility is as complete between the two asso- 



