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.  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