ecies--such as genera, families, orders, and the
rest--naturalists have at all times recognised the fact that the one
shades off into the other by such imperceptible gradations, that it is
impossible to regard such divisions as other than conventional. It is
important to remember that this fact was fully recognised before the
days of Darwin. In those days the scientifically orthodox doctrine was,
that although species were to be regarded as fixed units, bearing the
stamp of a special creation, all the higher taxonomic divisions were
to be considered as what may be termed the artificial creation of
naturalists themselves. In other words, it was believed, and in many
cases known, that if we could go far enough back in the history of the
earth, we should everywhere find a tendency to mutual approximation
between allied _groups of species_; so that, for instance, birds and
reptiles would be found to be drawing nearer and nearer together, until
eventually they would seem to become fused in a single type; that the
existing distinctions between herbivorous and carnivorous mammals
would be found to do likewise; and so on with all the larger
group-distinctions, at any rate within the limits of the same
sub-kingdoms. But although naturalists recognised this even in the
pre-Darwinian days, they stoutly believed that a great exception was to
be made in the case of species. These, the lowest or initial members of
their taxonomic series, they supposed to be permanent--the miraculously
created units of organic nature. Now, all that I have at present to
remark is, that this pre-Darwinian exception which was made in favour of
species to the otherwise recognised principle of gradual change, was an
exception which can at no time have been recommended by any antecedent
considerations. At all times it stood out of analogy with the principle
of continuity; and, as we shall fully find in subsequent chapters, it is
now directly contradicted by all the facts of biological science.

There remains one other fact of high generality to which prominent
attention should be drawn from the present, or merely antecedent, point
of view. On the theory of special creation no reason can be assigned why
distinct specific types should present any correlation, either in time
or in space, with their nearest allies; for there is evidently no
conceivable reason why any given species, A, should have been specially
created on the same area and at about the same time