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.
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 extinct forms
often tend to fill up gaps between existing forms, sometimes blending
two groups previously classed as distinct into one; but more commonly
only bringing them a little closer together. The more ancient a form
is, the more often, apparently, it displays characters in some degree
intermediate between groups now distinct; for the more ancient a form
is, the more nearly it will be related to, and consequently resemble,
the common progenitor of groups, since become
widely divergent. Extinct forms are seldom directly intermediate
between existing forms; but are intermediate only by a long and
circuitous course through many extinct and very different forms. We can
clearly see why the organic remains of closely consecutive formations
are more closely allied to each other, than are those of remote
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