hibited by
physiological species would lead us into difficulties, and at this point
they begin to be obvious; for, if, as a result of spontaneous variation
and of selective breeding, the progeny of a common stock may become
separated into groups distinguished from one another by constant, not
sexual, morphological characters, it is clear that the physiological
definition of species is likely to clash with the morphological
definition. No one would hesitate to describe the pouter and the tumbler
as distinct species, if they were found fossil, or if their skins and
skeletons were imported, as those of exotic wild birds commonly
are--and, without doubt, if considered alone, they are good and distinct
morphological species. On the other hand, they are not physiological
species, for they are descended from a common stock, the rock-pigeon.

Under these circumstances, as it is admitted on all sides that races
occur in Nature, how are we to know whether any apparently distinct
animals are really of different physiological species, or not, seeing
that the amount of morphological difference is no safe guide? Is there
any test of a physiological species? The usual answer of physiologists
is in the affirmative. It is said that such a test is to be found in the
phaenomena of hybridization--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 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 m