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.