natural classification many
fossil species would have to stand between living species, and some
extinct genera between living genera, even between genera belonging to
distinct families. The most common case, especially with respect to very
distinct groups, such as fish and reptiles, seems to be, that supposing
them to be distinguished at the present day from each other by a
dozen characters, the ancient members of the same two groups would be
distinguished by a somewhat lesser number of characters, so that the two
groups, though formerly quite distinct, at that period made some small
approach to each other.

It is a common belief that the more ancient a form is, by so much the
more it tends to connect by some of its characters groups now widely
separated from each other. This remark no doubt must be restricted
to those groups which have undergone much change in the course of
geological ages; and it would be difficult to prove the truth of
the proposition, for every now and then even a living animal, as the
Lepidosiren, is discovered having affinities directed towards very
distinct groups. Yet if we compare the older Reptiles and Batrachians,
the older Fish, the older Cephalopods, and the eocene Mammals, with the
more recent members of the same classes, we must admit that there is
some truth in the remark.

Let us see how far these several facts and inferences accord with the
theory of descent with modification. As the subject is somewhat complex,
I must request the reader to turn to the diagram in the fourth chapter.
We may suppose that the numbered letters represent genera, and the
dotted lines diverging from them the species in each genus. The diagram
is much too simple, too few genera and too few species being given,
but this is unimportant for us. The horizontal lines may represent
successive geological formations, and all the forms beneath the
uppermost line may be considered as extinct. The three existing genera,
a14, q14, p14, will form a small family; b14 and f14 a closely allied
family or sub-family; and o14, e14, m14, a third family. These three
families, together with the many extinct genera on the several lines of
descent diverging from the parent-form A, will form an order; for all
will have inherited something in common from their ancient and common
progenitor. On the principle of the continued tendency to divergence
of character, which was formerly illustrated by this diagram, the more
recent any form