Passing from these difficulties, all the other great leading facts in
palaeontology seem to me simply to follow on the theory of descent
with modification through natural selection. We can thus understand
how it is that new species come in slowly and successively; how
species of different classes do not necessarily change together, or at
the same rate, or in the same degree; yet in the long run that all
undergo modification to some extent. The extinction of old forms is
the almost inevitable consequence of the production of new forms. We
can understand why when a species has once disappeared it never
reappears. Groups of species increase in numbers slowly, and endure
for unequal periods of time; for the process of modification is
necessarily slow, and depends on many complex contingencies. The
dominant species of the larger dominant groups tend to leave many
modified descendants, and thus new sub-groups and groups are formed.
As these are formed, the species of the less vigorous groups, from
their inferiority inherited from a common progenitor, tend to become
extinct together, and to leave no modified offspring on the face of
the earth. But the utter extinction of a whole group of species may
often be a very slow process, from the survival of a few descendants,
lingering in protected and isolated situations. When a group has once
wholly disappeared, it does not reappear; for the link of generation
has been broken.
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