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, and consequently resemble, the common progenitor of groups, since become widely divergent.