at present, individuals, of what are
certainly known to be mere races produced by selection, however distinct
they may appear to be, not only breed freely together, but the offspring
of such crossed races are only perfectly fertile with one another. Thus,
the spaniel and the greyhound, the dray-horse and the Arab, the
pouter and the tumbler, breed together with perfect freedom, and their
mongrels, if matched with other mongrels of the same kind, are equally
fertile.

On the other hand, there can be no doubt that the individuals of
many natural species are either absolutely infertile if crossed with
individuals of other species, or, if they give rise to hybrid offspring,
the hybrids so produced are infertile when paired together. The horse
and the ass, for instance, if so crossed, give rise to the mule, and
there is no certain evidence of offspring ever having been produced by a
male and female mule. The unions of the rock-pigeon and the
ring-pigeon appear to be equally barren of result. Here, then, says the
physiologist, we have a means of distinguishing any two true species
from any two varieties. If a male and a female, selected from each
group, produce offspring, and that offspring is fertile with others
produced in the same way, the groups are races and not species. If, on
the other hand, no result ensues, or if the offspring are infertile with
others produced in the same way, they are true physiological species.
The test would be an admirable one, if, in the first place, it were
always practicable to apply it, and if, in the second, it always yielded
results susceptible of a definite interpretation. Unfortunately, in
the great majority of cases, this touchstone for species is wholly
inapplicable.

The constitution of many wild animals is so altered by confinement that
they will not breed even with their own females, so that the negative
results obtained from crosses are of no value; and the antipathy of wild
animals of the same species for one another, or even of wild and tame
members of the same species, is ordinarily so great, that it is hopeless
to look for such unions in Nature. The hermaphrodism of most plants,
the difficulty in the way of insuring the absence of their own, or the
proper working of other pollen, are obstacles of no less magnitude
in applying the test to them. And, in both animals and plants, is
superadded the further difficulty, that experiments must be continued
over a long time for the pur