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 formations; for the forms are more closely
linked together by generation: we can clearly see wh