34 FERTILITY OF VARIETIES [Chap. IX= 



a second species, whilst the converse cross can be efEected 

 with perfect facility. That excellent observer, Gart- 

 ner, likewise concluded that species when crossed are 

 sterile owing to diSerences confined to their reproduc- 

 tive 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 sub- 

 ject is surrounded by difficulties, for, looking to varie- 

 ties produced under nature, if two forms hitherto re- 

 puted to be varieties be found in any degree sterile to- 

 gether, 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 subsequently 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 in- 

 volved in some doubt. For when it is stated, for in- 

 stance, that certain South American indigenous domes- 

 tic dogs do not readily unite with European dogs, the 

 explanation which will occur to every one, and probably 



