The Conditions of Existence as Affecting the Perpetuation of Living Beings | Page 5

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 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 to their primitive type. This is supposed by many to put an
absolute limit to the extent of selective and all other variations. People
say, "It is all very well to talk about producing these different races, but
you know very well that if you turned all these birds wild, these Pouters,
and Carriers, and so on, they would all