tigrada, Felidae, Lion. Most of the
other groups in each grade are also subdivisible, though some of them
contain far fewer sub-classes than others.

To see the true character of this classification, we must consider that
it is based chiefly upon knowledge of existing animals. Some extinct
animals, known by their fossils, find places in it; for others new
places have been made. But it represents, on the whole, a cross-section,
or cross-sections of Nature as developing in time; and, in order to give
a just view of the relations of animals, it must be seen in the light of
other considerations. The older systems of classification, and the rules
for making them, seem to have assumed that an actual system of classes,
or of what Mill calls 'Kinds,' exists in nature, and that the relations
of Kinds in this system are determined by quantity of resemblance in
co-inherent qualities, as the ground of their affinity.

Sec. 8. Darwin's doctrine of the origin of species affects the conception
of natural classification in several ways, (1) If all living things are
blood-relations, modified in the course of ages according to their
various conditions of life, 'affinity' must mean 'nearness of common
descent'; and it seems irrational to propose a classification upon any
other basis. We have to consider the Animal (or the Vegetable) Kingdom
as a family tree, exhibiting a long line of ancestors, and (descended
from them) all sorts of cousins, first, second, third, etc., perhaps
once, twice, or oftener 'removed.' Animals in the relation of first
cousins must be classed as nearer than second cousins, and so on.

But, if we accept this principle, and are able to trace relationship, it
may not lead to the same results as would be reached by simply relying
upon the present 'quantity of resemblance,' unless we understand this
in a very particular way. For the most obvious features of an animal may
have been recently acquired; which often happens with those characters
that adapt an animal to its habits of life, as the wings of a bat, or
the fish-like shape of a dolphin; or as in cases of 'mimicry.' Some
butterflies, snakes, etc., have grown to resemble closely, in a
superficial way, other butterflies and snakes, from which a stricter
investigation widely separates them; and this superficial resemblance is
probably a recent acquisition, for the sake of protection; the imitated
butterflies being nauseous, and the imitated snakes poisonous. On the