FERTILITY OF VARIETIES 323 



exceptions, presently to be given, I fully admit that this is 

 the rule. But the subject is surrounded by difficulties, for, 

 looking to varieties produced under nature, if two forms 

 hitherto reputed to be varieties be found in any degree sterile 

 together, they are at once ranked by most naturalists as 

 species. For instance, the blue and red pimpernel, which 

 are considered by most botanists as varieties, are said by 

 Gartner to be quite sterile when crossed, and he conse- 

 quently ranks them as undoubted species. If we thus argue 

 in a circle, the fertility of all varieties produced under 

 nature will assuredly have to be granted. 



If we turn to varieties, produced, or supposed to have been 

 produced, under domestication, we are still involved in some 

 doubt. For when it is stated, for instance, that certain South 

 American indigenous domestic dogs do not readily unite with 

 European dogs, the explanation which will occur to every 

 one, and probably the true one, is that they are descended 

 from aboriginally distinct species. Nevertheless the perfect 

 fertility of so many domestic races, differing widely from 

 each other in appearance, for instance those of the pigeon, 

 or of the cabbage, is a remarkable fact ; more especially when 

 we reflect how many species there are, which, though re- 

 sembling each other most closely, are utterly sterile when 

 intercrossed. Several considerations, however, render the 

 fertility of domestic varieties less remarkable. In the first 

 place, it may be observed that the amount of external differ- 

 ence between two species is no sure guide to their degree of 

 mutual sterility, so that similar differences in the case of 

 varieties would be no sure guide. It is certain that with 

 species the cause lies exclusively in differences in their sex- 

 ual constitution. Now the varying conditions to which do- 

 mesticated animals and cultivated plants have been subjected, 

 have had so little tendency towards modifying the repro- 

 ductive system in a mr.nner leading to mutual sterility, that 

 we have good grounds for admitting the directly opposite 

 doctrine of Pallas, namely, that such conditions generally 

 eliminate this tendency ; so that the domesticated descendants 

 of species, which in their natural state probably would have 

 been in some degree sterile when crossed, become perfectly 

 fertile together. With plants, so far is cultivation from giving 



