When a species has once disappeared from the face of the earth, we have no reason to believe that the same identical form ever reappears.

The strongest apparent exception to this latter rule is that of the so-called "colonies" of M.

Barrande, which intrude for a period in the midst of an older formation, and then allow the pre-existing fauna to reappear; but Lyell's explanation, namely, that it is a case of temporary migration from a distinct geographical province, seems satisfactory.