If we
may trust the observations of Philippi in Sicily, the successive
changes in the marine inhabitants of that island have been many and
most gradual. The secondary formations are more broken; but, as Bronn
has remarked, neither the appearance nor disappearance of their many
now extinct species has been simultaneous in each separate formation.
Species of different genera and classes have not changed at the same
rate, or in the same degree. In the oldest 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, in an
existing crocodile associated with many strange and 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 change at a quicker rate than
those of the sea, of which a striking instance has lately been
observed in Switzerland. There is some reason to believe that
organisms, considered high in the scale of nature, change more quickly
than those that are low: though there are exceptions to this rule. The
amount of organic change, as Pictet has remarked, does not strictly
correspond with the succession of our geological formations; so that
between each two consecutive formations, the forms of life have seldom
changed in exactly the same degree.
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