FERTILITY OF VARIETIES WHEN CROSSED. 283 



Gartner, likewise concluded that species when crossed are 

 sterile owing to differences confined to their reproductive 

 systems. 



FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR 

 MONGREL OFFSPRING, NOT UNIVERSAL. 



It may be urged as an overwhelming argument that there 

 must be some essential distinction between species and 

 varieties, inasmuch as the latter, however much they may 

 differ from each other in external appearance, cross with 

 perfect facility, and yield perfectly fertile offspring. _ With 

 some exceptions, presently to be given, I fully admit that 

 this is the rule. But the subject is surrounded by difficul- 

 ties, 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 pim- 

 pernel, which are considered by most botanists as varieties, 

 are said by Gartner to be quite sterile when crossed, and he 

 consequently 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 resembling 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 difference 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 exclu- 

 sively in differences in their sexual constitution. Now the 

 varying conditions to which domesticated animals and cul' 



