384 HISTORICAL PALAEONTOLOGY. 



plan of structure ; but they nevertheless admit of being ar- 

 ranged 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 arrange- 

 ment 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 consistitute 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 organization 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 organization 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 gen- 

 erally 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- 

 organized Decapods do not make their appearance till the near the 

 close of the Palaeozoic epoch, and they do not become abun- 

 dant 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 forma- 

 tions. Similarly, the Univalves with breathing-tubes and a 

 corresponding notch in the mouth of the shell (" siphonosto- 



