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.

The productions of the land seem to have changed at a quicker rate than those of the sea, of which a striking instance has been observed in Switzerland.
There is some reason to believe that organisms high in the scale, change more quickly than those that are low: though there are exceptions to this rule.

The amount of organic change, as Pictet has remarked, is not the same in each successive so-called formation.