affinities of distinct orders
of plants.

On the principle of the multiplication and gradual divergence in
character of the species descended from a common parent, together with
their retention by inheritance of some characters in common, we can
understand the excessively complex and radiating affinities by which all
the members of the same family or higher group are connected together.
For the common parent of a whole family of species, now broken up by
extinction into distinct groups and sub-groups, will have transmitted
some of its characters, modified in various ways and degrees, to all;
and the several species 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 this 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 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 le