y? 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 supposed criterion.

If, weary of the endless difficulties involved in the determination of
species, the investigator, contenting himself with the rough practical
distinction of separable kinds, endeavours to study them as they occur
in nature--to ascertain their relations to the conditions which surround
them, their mutual harmonies and discordances of structure, the bond of
union of their parts and their past history, he finds himself, according
to the received notions, in a mighty maze, and with, at most, the
dimmest adumbration of a plan. If he starts with any one clear
conviction, it is that every part of a living creature is cunningly
adapted to some special use in its life. Has not his Paley told him that
that seemingly useless organ, the spleen, is beautifully adjusted as
so much packing between the other organs? And yet, at the outset of his
studies, he finds that no adaptive reason whatsoever can be given for
one-half of the peculiarities of vegetable struc