describing, without the aid of a diagram, the various affinities which
they perceive between the many living and extinct members of the same
great natural class.
Extinction, as we have seen in the fourth chapter, has played an
important part in defining and widening the intervals between the
several groups in each class. We may thus account even for the
distinctness of whole classes from each other for instance, of birds
from all other vertebrate animals by the belief that many ancient forms
of life have been utterly lost, through which the early progenitors of
birds were formerly connected with the early progenitors of the other
vertebrate classes. There has been less entire extinction of the forms
of life which once connected fishes with batrachians. There has been
still less in some other classes, as in that of the Crustacea, for here
the most wonderfully diverse forms are still tied
together by a long, but broken, chain of affinities. Extinction has
only separated groups: it has by no means made them; for if every form
which has ever lived on this earth were suddenly to reappear, though it
would be quite impossible to give definitions by which each group could
be distinguished from other groups, as all would blend together by
steps as fine as those between the finest existing varieties,
nevertheless a natural classification, or at least a natural
arrangement, would be possible. We shall see this by turning to the
diagram: the letters, A to L, may represent eleven Silurian genera,
some of which have produced large groups of modified descendants. Every
intermediate link between these eleven genera and their primordial
parent, and every intermediate link in each branch and sub-branch of
their descendants, may be supposed to be still alive; and the links to
be as fine as those between the finest varieties. In this case it would
be quite impossible to give any definition by which the several members
of the several groups could be distinguished from their more immediate
parents; or these parents from their ancient and unknown progenitor.
Yet the natural arrangement in the diagram would still hold good; and,
on the principle of inheritance, all the forms descended from A, or
from I, would have something in common. In a tree we can specify this
or that branch, though at the actual fork the two unite and blend
together. We could not, as I have said, define the several groups; but
we could pick out types, or forms, representing most of the characters
of each group, whether large or small, and thus give a general idea of
the value of the differences between them. This is what we should be
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