; but that long before that period, the world may have
presented a wholly different aspect; and that the older continents, formed
of formations older than any known to us, may now all be in a metamorphosed
condition, or may lie buried under the ocean.

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 {344} 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.

We can understand how the spreading of the dominant forms of life, which
are those that oftenest vary, will in the long run tend to people the world
with allied, but modified, descendants; and these will generally succeed in
taking the places of those groups of species which are their inferiors in
the struggle for existence. Hence, after long intervals of time, the
productions of the world will appear to have changed simultaneously.

We can understand how it is that all the forms of life, ancient and recent,
make together one grand system; for all are connected by generation. We can
understand, from the continued tendency to divergence of character, why the
more ancient a form is, the more it generally differs from those now
living. Why ancient and extin