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.