Species belonging to different genera and classes have not changed at the same rate, or in the same degree.

In the older tertiary beds a few living shells may still be found in the midst of a multitude of extinct forms.

Falconer has given a striking instance of a similar fact, for an existing crocodile is associated with many lost mammals and reptiles in the sub-Himalayan deposits.

The Silurian Lingula differs but little from the living species of this genus; whereas most of the other Silurian Molluscs and all the Crustaceans have changed greatly.