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 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
phaenomena of hybridization which are exhibited by many species when
crossed with other species. On the other hand, not only is it not
proved that all species give rise to hybrids infertile _inter se_, but
there is much reason to believe that, in crossing, species exhibit every
gradation from perfect sterility to perfect fertility.

Such are the most essential characteristics of species. Even were man
not one of them--a member of the same system and subject to the same
laws--the question of their origin, their causal connexion, that is,
with the other phaenomena of the universe, must have attracted his
attention, as soon as his intelligence had raised itself above the level
of his daily wants.

Indeed history relates that such was the case, and has embalmed for us
the speculations upon the origin of living beings, which were among the
earliest produ