hybridisation--in the results of crossing races, as compared with the
results of crossing species.

So far as the evidence goes 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 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.