very extraordinary circumstance; 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 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 original cross, or than Carrier with Carrier.
Here, you see, is a physiological contrast between the races produced
by selective modification and natural species. I shall inquire into the
value of this fact, and of some modifying circumstances by and by; for
the present I merely put it broadly before you.

But while considering this question of the limitations of species, a
word must be said about what is called RECURRENCE--the tendency of races
which have been developed by selective breeding from varieties to return