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
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 to this check. In
crossing the breeds between the Fantail and the Pouter, the Carrier and
the Tumbler, or any other variety or race you may name--so far as we
know at present--there is no difficulty in breeding together the
mongrels. Take the Carrier and the Fantail, for instance, and let them
represent the Horse and the Ass in the case of distinct species; then
you have, as the result of their breeding, the Carrier-Fantail
mongrel,--we will say the male and female mongrel,--and, as far as we
know, these two when crossed would not be less fertile than the