XI PHENOMENA OF ORGANIC NATURE 423 



elements necessary to generation. It is said to 

 be invariably the case with the male mule, the 

 cross between the Ass and the Mare ; and hence 

 it is, that, although crossing the Horse with the 

 Ass is easy enough, and is constantly done, as far 

 as I am aware, if you take two mules, a male and 

 a female, and endeavour to breed from them, you 

 get no offspring whatever ; no generation will take 

 place. This is what is called the sterility of the 

 hybrids between two distinct species. 



You see that this is a very extraordinary cir- 

 cumstance ; one does not see why it should be. 

 The common teleological explanation is, that it is 

 to prevent the impurity of the blood resulting 

 from the crossing of one species with another, but 

 you see it does not in reality do anything of the 

 kind. There is nothing in this fact that hybrids 

 cannot breed with each other, to establish such a 

 theory ; there is nothing to prevent the Horse 

 breeding with the Ass, or the Ass with the Horse. 

 So that this explanation breaks down, as a great 

 many explanations of this kind do, that are only 

 founded on mere assumptions. 



Thus you see that there is a great difference 

 between " mongrels," which are crosses between 

 distinct races, and "hybrids," which are crosses 

 between distinct species. The mongrels are, so 

 far as we know, fertile with one another. But 

 between species, in many cases, you cannot suc- 

 ceed in obtaining even the first cross ; at any rate 



