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.
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