sil species certainly 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 by a score of characters,
the ancient members are separated by a somewhat lesser number of
characters, so that the two groups formerly made a somewhat nearer
approach to each other than they now do.

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
recent members of the same classes, we must admit that there is 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 in italics 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 subfamily; and o14, i14, 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
progenitor. On the principle of the continued tendency to divergence
of character, which was formerly illustrated by this diagram, the more
recent any form is the more it will generally differ from its ancient
progenitor. Hence, we can understand the rule that t