difficulty which naturalists have experienced in 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 {432} 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 g