rphological conformity is explained by
community of descent; and it has not been explained in any other way.

Naturalists are constantly speaking of "related species," of
the "affinity" of a genus or other group, and of "family
resemblance,"--vaguely conscious that these terms of kinship are
something more than mere metaphors, but unaware of the grounds of their
aptness. Mr. Darwin assures them that they have been talking derivative
doctrine all their lives without knowing it.

If it is difficult and in some cases practically impossible to fix the
limits of species, it is still more so to fix those of genera; and those
of tribes and families are still less susceptible of exact natural
circumscription. Intermediate forms occur, connecting one group with
another in a manner sadly perplexing to systematists, except to those
who have ceased to expect absolute limitations in Nature. All this
blending could hardly fail to suggest a former material connection among
allied forms, such as that which an hypothesis of derivation demands.

Here it would not be amiss to consider the general principle of
gradation throughout organic Nature,--a principle which answers in a
general way to the law of continuity in the inorganic world, or
rather is so analogous to it that both may fairly be expressed by
the Leibnitzian axiom, _Natura non agit saltatim_. As an axiom or
philosophical principle, used to test modal laws or hypotheses, this in
strictness belongs only to physics. In the investigation of Nature at
large, at least in the organic world, nobody would undertake to apply
this principle as a test of the validity of any theory or supposed law.
But naturalists of enlarged views will not fail to infer the principle
from the phenomena they investigate,--to perceive that the rule holds,
under due qualifications and altered forms, throughout the realm of
Nature; although we do not suppose that Nature in the organic world
makes no distinct steps, but only short and serial steps,--not
infinitely fine gradations, but no long leaps, or few of them.

To glance at a few illustrations out of many that present themselves. It
would be thought that the distinction between the two organic kingdoms
was broad and absolute. Plants and animals belong to two very different
categories, fulfil opposite offices, and, as to the mass of them, are
so unlike that the difficulty of the ordinary observer would be to find
points of comparison. Without entering i