ther.
For the common progenitor of a whole family, now broken up by extinction
into distinct groups and subgroups, will have transmitted some of its
characters, modified in various ways and degrees, to all the species;
and they will consequently be related to each other by circuitous lines
of affinity of various lengths (as may be seen in the diagram so often
referred to), mounting up through many predecessors. As it is difficult
to show the blood-relationship between the numerous kindred of any
ancient and noble family, even by the aid of a genealogical tree, and
almost impossible to do so without this aid, we can understand
the extraordinary 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 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 and at that
time less differentiated vertebrate classes. There has been much
less extinction of the forms of life which once connected fishes with
Batrachians. There has been still less within some whole classes, for
instance the Crustacea, for here the most wonderfully diverse forms
are still linked together by a long and only partially broken chain of
affinities. Extinction has only defined the 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, still 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, with every link in each branch and
sub-branch still alive; and the links not greater than those between
existing varieties. In this case it would be quite impossible to give
definitions by which the several members of the several groups could be
disti