54 Darwin, and after Darwin. 



For the theory does not assert that all varieties 

 have been due to physiological selection. There are 

 doubtless many other causes of the origin of varieties 

 besides cross-infertility with parent forms; but, as 

 a general rule, it does not appear that they are by 

 themselves capable of carrying divergence beyond 

 a merely varietal stage. In order to carry divergence 

 to the stage of producing species, it appears to be 

 a general condition that, sooner or later, cross-infertility 

 should arise seeing that, when varieties do succeed 

 in becoming species, we almost invariably find that, 

 as a matter of fact, cross-infertility has arisen. Hence, 

 if cross-infertility has thus usually been a necessary 

 condition to a varietal divergence becoming specific, 

 it can make no material difference when the incipient 

 infertility arose. 



It may be asked, however, whether I suppose that, 

 when the physiological change is subsequent, it is 

 directly caused by change of structure, size, colour, &c., 

 or that it arises, so to speak, accidentally, from other 

 causes which may have affected the sexual system in 

 the required way. To this question I may briefly 

 reply, that, looking to the absence of any influence 

 exercised on the reproductive systems of our domesti- 

 cated plants and animals by the great and varied 

 changes which so many of these forms present, it 

 would seem that among natural varieties such closely 

 analogous changes are presumably not the usual causes 

 of the physiological change, even where the latter are 

 subsequent to the former. Nevertheless, I do not 

 deny that in some of these cases changes of structure, 

 size, colour, &c., may be the causes of the physiological 

 change by reacting on the sexual system in the re- 



