Each is equally perfect in its own way; but the structural
pattern of the Coral is the highest, and therefore it occupies a
higher place in the zoological scale. It is upon this principle,
then, that the primary subdivisions of the animal kingdom (the
so-called "sub-kingdoms") are arranged in a certain order. Coming,
again, to the minor subdivisions (classes, orders, &c.) of each
sub-kingdom, we find a different but entirely analogous principle
employed as a means of classification. The numerous animals belonging
to any given sub-kingdom are formed upon the same fundamental
plan of structure; but they nevertheless admit of being arranged
in a regular series of groups. All the Shell-fish, for example,
are built upon a common plan, this plan representing the ideal
Mollusc; but there are at the same time various groups of the
_Mollusca_, and these groups admit of an arrangement in a given
sequence. The principle adopted in this case is simply of _the
relative elaboration of the common type_. The Oyster is built
upon the same ground-plan as the Cuttle-fish; but this plan is
carried out with much greater elaboration, and with many more
complexities, in the latter than in the former: and in accordance
with this, the _Cephalopoda_ constitute a higher group than the
Bivalve Shell-fish. As in the case of superiority of structural
type, so in this case also, it is not in the least that the Oyster
is an _imperfect_ animal.
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