Passing from these difficulties, the other great leading facts in
palaeontology agree admirably with the theory of descent with modification
through variation and 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 belonging to large and dominant groups
tend to leave many modified descendants, which form new sub-groups and
groups. 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 has sometimes
been a 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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