422 THE CAUSES OF THE XI 
for the purposes of this argument if it were true 
of only one of them, but there is, in fact, a great 
number of such cases—and that is, that, similar 
as they may appear to be to mere races or breeds, 
they present a marked peculiarity in the repro- 
ductive process. If you breed from the male and 
female of the same race, you of course have off- 
spring of the like kind, and if you make the off- 
spring 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, however similar they may be to each other, 
and make them breed together, you will find a 
check, with some modifications and exceptions, 
however, which I shall speak of presently. 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 attempt to breed from 
the products of that crossing, which are what are 
called Hysrips—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 offspring at all; there will be no result 
whatsoever. 
The reason of this is quite obvious in some 
cases ; the male hybrids, although possessing all 
the external appearances and characteristics of 
perfect animals, are physiologically imperfect and 
deficient in the structural parts of the reproductive 
