Alleged Sterility of Hybrids 



all the difficulties in the way of accepting the 

 theory of natural selection as a complete expla- 

 nation of the origin of species, has been the 

 remarkable difference between varieties and 

 species in respect of fertility when crossed. 

 Generally speaking, it may be said that the 

 varieties of any one species, however different 

 they may be in external appearance, are per- 

 fectly fertile when crossed, and their mongrel 

 offspring are equally fertile when bred among 

 themselves ; while distinct species, on the other 

 hand, however closely they may resemble one 

 another externally, are usually infertile when 

 crossed, and their hybrid offspring absolutely 

 sterile. This used to be considered a fixed 

 law of nature, constituting the absolute test and 

 criterion of a species as distinct from a variety; 

 and so long as it was believed that species were 

 separate creations, or at all events had an origin 

 quite distinct from that of varieties, this law could 

 have no exceptions, because if any two species 

 had been found to be fertile when crossed and 

 their hybrid offspring to be also fertile, this fact 

 would have been held to prove them to be not 

 species but varieties. On the other hand, if two 

 varieties had been found to be infertile, or their 

 mongrel offspring to be sterile, then it would 

 have been said These are not varieties, but 

 true species. Thus the old theory led inevitably 

 to reasoning in a circle, and what might be 

 H 113 



