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.
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