We must, however, allow for the entire
extinction of some preceding forms, and in any one region for the
immigration of new forms from other regions, and for a large amount of
modification during the long and blank intervals between the successive
formations. Subject to these allowances, the fauna of each geological
period undoubtedly is intermediate in character, between the preceding and
succeeding faunas. I need give only one instance, namely, the manner in
which the fossils of the Devonian system, when this system was first
discovered, were at once recognised by palaeontologists as intermediate in
character between those of the overlying carboniferous and underlying
Silurian systems. But each fauna is not necessarily exactly intermediate,
as unequal intervals of time have elapsed between consecutive formations.
It is no real objection to the truth of the statement that the fauna of
each period as a whole is nearly intermediate in character between the
preceding and succeeding faunas, that certain genera offer exceptions to
the rule. For instance, the species of mastodons and elephants, when
arranged by Dr. Falconer in two series--in the first place according to
their mutual affinities, and in the second place according to their periods
of existence--do not accord in arrangement. The species extreme in
character are not the oldest or the most recent; nor are those which are
intermediate in character, intermediate in age.
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