66 THE FIXITY OF SPECIES; 
if you make the offspring breed together, you obtain the 
same result, and if you breed from these again, you will 
still have the same kind of offspring; there is no check. 
But if you take members of two distinct species, how- 
ever similar they may be to each other, and make them 
breed together, you will And a check. If you cross two 
such species with each other, then—although you may 
get offspring in the case of the first cross, yet, if you at- 
tempt to breed from the products of that crossing, which 
are what are called hybrids—that is, if you couple a 
male and a female hybrid—then the result is that in 
ninety-nine cases out of a hundred you will get no off- 
spring at all; there will be no result whatsoever. 
“The reason of this is quite obvious in some cases; 
the female hybrids, although possessing all the external 
appearances and characteristics of perfect animals, are 
physiologically imperfect and deficient in the structural 
parts of the reproductive 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 en- 
deavor to breed from them, you get no offspring what- 
ever; no generation will take place. This is what is 
called the sterility of the hybrids between two distinct 
species.” (Huxley, “On the Origin of Species.” p. 
212.) He continues: 
“Thus you see that there is a great difference be- 
tween ‘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 succeed in obtaining even the first cross; at 
