36 EVOLUTION OF TO-DAY. 



half-breeds are fertile for an indefinite period, and 

 sometimes even more so than the pure breeds. 

 This subject has in the past become so much in- 

 volved in theological discussions as to make it some- 

 what difficult to know what is the true state of the 

 case. It is not possible, therefore, to say definitely 

 whether or not man can be cited as a case where 

 varieties have a certain amount of sterility. The 

 evidence for such a conclusion is at all events very 

 slight. 



Finding, then, no cases where domestic varieties 

 are sterile when crossed, naturalists have attempted 

 to meet the difficulty in another way. They have 

 questioned this definition of species by showing that 

 true species are not always sterile when crossed. 

 In accordance with the idea that sterility is a bar 

 which has been laid down to separate species, it is 

 difficult to see how there could be any grades of 

 sterility. We should expect either that all species 

 would be sterile when crossed, or they would all be 

 equally fertile. But investigations have certainly 

 shown that sterility is no such rigid bar as this. It 

 is true that species are usually sterile when crossed, 

 but the amount of fertility is a variable quantity. 

 Many species do not produce any young when 

 crossed; while many others do produce young, 

 which are themselves incapable of reproduction. 

 And in still other cases even this amount of sterility 

 is not found. 



Owing to the difficulty of experimenting upon 

 animals, observations upon this question have been 

 more largely confined to plants than is desirable. 



