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.
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