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.
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