re 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. On the contrary, it is just as perfectly
adapted by its organisation to fill its own sphere and to meet
the exigencies of its own existence as is the Cuttle-fish; but
the latter lives a life which is, physiologically, higher than
the former, and its organisation is correspondingly increased
in complexity.

This being understood, it may be repeated that, in the main,
the succession of life upon the globe in point of _time_ has
corresponded with the relative order of succession of the great
groups of animals in _zoological rank_; and some of the more
striking examples of this may be here alluded to. Amongst the
_Echinoderms_, for instance, the two orders generally admitted to
be the "lowest" in the zoological scale--namely, the _Crinoids_
and the _Cystoids_--are likewise the oldest, both, appearing in
the Cambrian, the former slowly dying out as we approach the
Recent period, and the latter disappearing wholly before the
close of the Palaeozoic period. Amongst the _Crustaceans_, the
ancient groups of the Trilobites, Ostracodes, Phyllopods,
Eurypterids, and Limuloids, some of which exist at the present
day, are all "low" types; whereas the highly-organised Decapods
do not make their appearance till near the close of the Palaeozoic
epoch, and they do not become abundant till we reach Mesozoic
times. Amongst the _Mollusca_, those Bivalves which possess
breathing-tubes (the "siphonate" Bivalves) are generally admitted
to be higher than those which are destitute of these organs (the
"asiphonate" Bivalves); and the latter are especially characteristic
of the Palaeozoic period, whilst the former abound in Mesozoic
and Kainozoic formations. Similarly, the Univalves with
breathing-tubes and a corresponding notch in the mouth of the
shell ("siphonostomatous" Univalves) are regarded as higher in