oups as they now are.

Some writers have objected to any extinct species or group of species being
considered as intermediate between living species or groups. If by this
term it is meant that an extinct form is directly intermediate in all its
characters between two living forms, the objection is probably valid. But I
apprehend that in a perfectly 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 {331} 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, a^{14}, q^{14}, p^{14},
will form a small family; b^{14} and f^{14} a closely allied family or
sub-family; and o^{14}, e^{14}, m^{14}, a third family. Th