This general replacement of the tertiary species of a country by others
so much like them is a noteworthy fact. The hypothesis of the
independent creation of all species, irrespective of their antecedents,
leaves this fact just as mysterious as is creation itself; that of
derivation undertakes to account for it. Whether it satisfactorily does
so or not, it must be allowed that the facts well accord with that
hypothesis. The same may be said of another conclusion, namely, that
the geological succession of animals and plants appears to correspond
in a general way with their relative standing or rank in a natural
system of classification. It seems clear that, though no one of the
grand types of the animal kingdom can be traced back farther than the
rest, yet the lower classes long preceded the higher; that there has
been on the whole a steady progression within each class and order; and
that the highest plants and animals have appeared only in relatively
modern times. It is only, however, in a broad sense that this
generalization is now thought to hold good. It encounters many apparent
exceptions, and sundry real ones. So far as the rule holds, all is as
it should be upon an hypothesis of derivation.
The rule has its exceptions. But, curiously enough, the most striking
class of exceptions, if such they be, seems to us even more favorable
to the doctrine of derivation than is the general rule of a pure and
simple ascending gradation.
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