differences in the reproductive system. In all other respects, excluding
fertility, there is a close general resemblance between hybrids and
mongrels."--Pp. 276-8.

We fully agree with the general tenor of this weighty passage; but forcible
as are these arguments, and little as the value of fertility or infertility
as a test of species may be, it must not be forgotten that the really
important fact, so far as the inquiry into the origin of species goes, is,
that there are such things in Nature as groups of animals and of plants,
the members of which 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 phaenomena 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