PERPETUATION OF LIVING BEINGS. 107 



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 be- 

 tween two distinct species. 



You see that this is a very extraordinary circum- 

 stance ; one does not see why it should be. The com- 

 mon 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 be- 

 tween " mongrels," which are crosses between distinct 

 races, and " hybrids," which are crosses between dis- 

 tinct species. The mongrels are, so far as we know, 

 fertile with one another. But between species, in many 

 cases, you cannot succeed in obtaining even the first 

 cross : at any rate it is quite certain that the hybrids 

 are often absolutely infertile one with another. 



Here is a feature, then, great or small as it may be, 

 which distinguishes natural species of animals. Can 

 we find any approximation to this in the different races 

 known to be produced by selective breeding from a 

 common stock? Up to the present time the answer 

 to that question is absolutely a negative one. As far 

 as we know at present, there is nothing approximating 



