) of animals and vegetables. These great groups
did not all come into existence at once, but they made their
appearance successively. It is true that we cannot be said to
be certainly acquainted with the first _absolute_ appearance of
any great group of animals. No one dare assert positively that
the apparent first appearance of Fishes in the Upper Silurian
is really their first introduction upon the earth: indeed, there
is a strong probability against any such supposition. To whatever
extent, however, future discoveries may push back the first advent
of any or of all of the great groups of life, there is no likelihood
that anything will be found out which will materially alter the
_relative_ succession of these groups as at present known to us.
It is not likely, for example, that the future has in store for
us any discovery by which it would be shown that Fishes were in
existence before Molluscs, or that Mammals made their appearance
before Fishes. The sub-kingdoms of Invertebrate animals were
all represented in Cambrian times--and it might therefore be
inferred that _these_ had all come simultaneously into existence;
but it is clear that this inference, though incapable of actual
disproof, is in the last degree improbable. Anterior to the Cambrian
is the great series of the Laurentian, which, owing to the
metamorphism to which it has been subjected, has so far yielded
but the singular _Eozooen_.
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