34 FEETILITY OF VARIETIES [Chap. IX- 



this same conclusion by considering reciprocal crosses, 

 in which the male of one species cannot be united, 

 or can be united with great difficulty, with the female 

 of a second species, whilst 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 repro- 

 ductive systems. 



Fertility of Varieties ivhen Crossed, and of their 

 Mongrel Ojfsjrring, 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 tins 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 

 ther, they are at once ranked by most naturalists 

 as species. F r 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 in- 

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



