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.