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. Yet if we compare any but the most closely related
formations, all the species will be found to have undergone some change.
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.
These several facts accord well with our theory, which includes no fixed
law of development, causing all the inhabitants of an area to change
abruptly, or simultaneously, or to an equal degree.
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