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.

We can understand how it is that dominant forms which spread widely and
yield the greatest number of varieties tend to people the world with
allied, but modified, descendants; and these will generally succeed
in displacing the groups which are their inferiors in the struggle for
existence. Hence, after long intervals of time, the productions of the
world appear to have changed simultaneously.

We can understand how it is that all the forms of life, ancient and
recent, make together a few grand classes. 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 bringing them only a little closer together.
The more ancient a form is, the more often it stands in some degree
intermediate between groups now distinct; for the more ancient a form
is, the more nearly it will be related to