results of
physiological differences and peculiarities, which have been developed
from a common stock, at least there is every reason to believe so.
But it is a most singular circumstance, that while you may run through
almost the whole series of physiological processes, without finding a
check to your argument, you come at last to a point where you do find
a check, and that is in the reproductive processes. For there is a most
singular circumstance in respect to natural species--at least about some
of them--and it would be sufficient 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
reproductive process. If you breed from the male and female of the same
race, you of course have offspring of the like kind, and 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, 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 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 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 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