ie 
—— a a — —= 
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 
