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 rule holds good as a broad generalisation--that
the lower groups, namely, precede the higher in point of time;
and though there are apparent exceptions to the rule, there are
none of such a nature as not to admit of explanation.
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