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 remains barren. Facts of this kind destroy the value of the