240 ON THE ORIGIN OF SPECIES 



is crosses between the males and females of the same 

 species which have varied and become slightly dif- 

 ferent, give vigour and fertility to the offspring. But 

 we have seen that greater changes, or changes of a 

 particular nature, often render organic beings in some 

 degree sterile ; and that greater crosses, that is crosses 

 between males and females which have become widely 

 or specifically different, produce hybrids which are 

 generally sterile in some degree. I cannot persuade 

 myself that this parallelism is an accident or an illusion. 

 Both series of facts seem to be connected together by 

 some common but unknown bond, which is essentially 

 related to the principle of life. 



Fertility of Varieties when crossed, and of their Mongrel 

 offspring. — It may be urged, as a most forcible argu- 

 ment, that there must be some essential distinction 

 between species and varieties, and that there must be 

 gome error in all the foregoing remarks, inasmuch as 

 varieties, however much they may differ from each 

 other in external appearance, cross with perfect facility, 

 and yield perfectly fertile offspring. I fully admit that 

 this is almost invariably the case. But if we look to 

 varieties produced under nature, we are immediately 

 involved in hopeless difficulties ; for if two hitherto re- 

 puted 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, the primrose 

 and cowslip, which are considered by many of our best 

 botanists as varieties, are said by Gartner not to be 

 quite fertile 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 doubt. For when it is stated, for instance, 

 that the German Spitz dog unites more easily than 

 other dogs with foxes, or that certain South American 

 indigenous domestic dogs do not readily cross with 



