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 phenomena 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