40 THE ORIGIN OF SPECIES 



second species, while the converse cross can be effected 

 with perfect facility. That excellent observer, 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 

 Offspri7ig^ 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 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 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. 



