Chap. IX.] WHEN CROSSED. 39 



with a widely distinct species. He experimented on 

 five forms which are commonly reputed to be varieties, 

 and which he tested by the severest trial, namely, by 

 reciprocal crosses, and he found their mongrel offspring 

 perfectly fertile. But one of these five varieties, when 

 used either as the father or mother, and crossed with 

 the Nicotiana glutinosa, always yielded hybrids not so 

 sterile as those which were produced from the four 

 other varieties when crossed with N". glutinosa. Hence 

 the reproductive system of this one variety must have 

 been in some manner and in some degree modified. 



From these facts it can no longer be maintained that 

 varieties when crossed are invariably quite fertile. 

 From the great difficulty of ascertaining the infertility 

 of varieties in a state of nature, for a supposed variety, 

 if proved to be infertile in any degree, would almost 

 universally be ranked as a species ; — from man attend- 

 ing only to external characters in his domestic varieties, 

 and from such varieties not having been exposed for 

 very long periods to uniform conditions of life ; — from 

 these several considerations we may conclude that 

 fertility does not constitute a fundamental distinction 

 between varieties and species when crossed. The 

 general sterility of crossed species may safely be looked 

 at, not as a special acquirement or endowment, but as 

 incidental on changes of an unknown nature in their 

 sexual elements. 



Hybrids and Mongrels compared, independently of their 



fertility. 



Independently of the question of fertility, the off- 

 spring of species and of varieties when crossed may be 



