belonging to the same orders, families, or genera with those living at
the present day, were not at this early epoch limited in such distinct
groups 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
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
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