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_. We may be certain, however, that others
of the Invertebrate sub-kingdoms besides the Protozoa were in
existence in the Laurentian period; and we may infer from known
analogies that they appeared successively, and not simultaneously.

When we come to smaller divisions than the sub-kingdoms--such
as classes, orders, and families--a similar succession of groups
is observable. The different classes of any given sub-kingdom,
or the different orders of any given class, do not make their
appearance together and all at once, but they are introduced
upon the earth in _succession_. More than this, the different
classes of a sub-kingdom, or the different orders of a class,
_in the main succeed one another in the relative order of their
zoological rank--the lower groups appearing first and the higher
groups last_. It is true that in the Cambrian formation--the
earliest series of sediments in which fossils are abundant--we
find numerous groups, some very low, others very high, in the
zoological scale, which _appear_ to have simultaneously flashed
into existence. For reasons stated above, however, we cannot
accept this appearance as real; and we must believe that many of
the Cambrian groups of animals really came into being long before
the commencement of the Cambrian period. At any rate, in the long
series of fossiliferous deposits of later date than the Cambrian
the above-stated