species goes, is, that there are such things in Nature as groups of
animals and of plants, whose members are incapable of fertile union
with those of other groups; and that there are such things as hybrids,
which are absolutely sterile when crossed with other hybrids. For, if
such phenomena as these were exhibited by only two of those assemblages
of living objects, to which the name of species (whether it be used in
its physiological or in its morphological sense) is given, it would
have to be accounted for by any theory of the origin of species, and
every theory which could not account for it would be, so far,
imperfect.

Up to this point, we have been dealing with matters of fact, and the
statements which we have laid before the reader would, to the best of
our knowledge, be admitted to contain a fair exposition of what is at
present known respecting the essential properties of species, by all
who have studied the question. And whatever may be his theoretical
views, no naturalist will probably be disposed to demur to the
following summary of that exposition:--

Living beings, whether animals or plants, are divisible into multitudes
of distinctly definable kinds, which are morphological species. They
are also divisible into groups of individuals, which breed freely
together, tending to reproduce their like, and are physiological
species. Normally resembling their parents, the offspring of members of
these species are still liable to vary; and the variation may be
perpetuated by selection, as a race, which race, in many cases,
presents all the characteristics of a morphological species. But it is
not as yet proved that a race ever exhibits, when crossed with another
race of the same species, those phenomena of hybridization which are
exhibited by many species when crossed with other species. On the