surpasses imagination. At least 100,000 such kinds of insects alone have
been described and may be identified in collections, and the number of
separable kinds of living things is under-estimated at half a million.
Seeing that most of these obvious kinds have their accidental varieties,
and that they often shade into others by imperceptible degrees, it may well
be imagined that the task of distinguishing between what is permanent and
what fleeting, what is a species and what a mere variety, is sufficiently
formidable.

But is it not possible to apply a test whereby a true species may be known
from a mere variety? Is there no criterion of species? Great authorities
affirm that there is--that the unions of members of the same species are
always fertile, while those of distinct species are either sterile, or
their offspring, called hybrids, are so. It is affirmed not only that this
is an experimental fact, but that it is a provision for the preservation of
the purity of species. Such a criterion as this would be invaluable; but,
unfortunately, not only is it not obvious how to apply it in the great
majority of cases in which its aid is needed, but its general validity is
stoutly denied. The Hon. and Rev. Mr. Herbert, a most trustworthy
authority, not only asserts as the result of his own observations and
experiments that many hybrids are quite as fertile as the parent species,
but he goes so far as to assert that the particular plant _Crinum
capense_ is much more fertile when crossed by a distinct species than
when fertilised by its proper pollen! On the other hand, the famous
Gaertner, though he took the greatest pains to cross the Primrose and the
Cowslip, succeeded only once or twice in several years; and yet it is a
well-established fact that the Primrose and the Cowslip are only varieties
of the same kind of plant. Again, such cases as the following are well
established. The female of species A, if crossed with the male of species
B, is fertile; but, if the female of B is crossed with the male of A, she