SUCCESSION OF LIFE UPON THE GLOBE. 385 



matous " Univalves) are regarded as higher in the scale than 

 the round-mouthed vegetable-eating Sea-snails, in which no 

 respiratory siphons exist (" holostomatous " Univalves) ; but 

 the latter abound in the Palaeozoic rocks whereas the former 

 do not make their appearance till the Jurassic period, and 

 their higher groups do not seem to have existed till the close 

 of the Cretaceous. The Cephalopods, again the highest of all 

 the groups of Mollusca are represented in the Palaeozoic 

 rocks exclusively by Tetrabranchiate forms, which constitute 

 the lowest of the two orders of this class; whereas the more 

 highly specialized Dibranchiates do not make their appearance 

 till the commencement of the Mesozoic. The Palaeozoic 

 Tetrabranchiates, also are of a much simpler type than the 

 highly complex Ammonitidce of the Mesozoic. 



Similar facts are observable amongst the Vertebrate animals. 

 The Fishes are the lowest class of Vertebrates, and they are 

 the first to appear, their first certain occurrence being in the 

 Upper Silurian ; whilst, even if the Lower Silurian and Upper 

 Cambrian " Conodonts " were shown to be the teeth of Fishes, 

 there would still remain the enormously long periods of the 

 Laurentian and Lower Cambrian, during which there were In- 

 vertebrates, but no Vertebrates. The Amphibians, the next 

 class in zoological order, appears later than the Fishes, and 

 is not represented till the Carboniferous ; whilst its highest 

 group (that of the Frogs and Toads) does not make its entrance 

 upon the scene till Tertiary times are reached. The class of 

 the Reptiles, again, the next in order, does not appear till 

 the Permian, and therefore not till after Amphibians of very 

 varied forms had been in existence for a protracted period. 

 The Birds seem to be undoubtedly later than the Reptiles; 

 but, owing to the uncertainty as to the exact point of their first 

 appearance, it cannot be positively asserted that they pre- 

 ceded Mammals, as they should have done. Finally, the 

 Mesozoic types of Mammals are mainly, if not exclusively, 

 referable to the Marsupials, one of the lowest orders of the 

 class ; whilst the higher orders of the " Placental " Quadrupeds 

 are not with certainty known to have existed prior to the com- 

 mencement of the Tertiary period. 



Facts of a very similar nature are offered by the succession 

 of Plants upon the globe. Thus the vegetation of the Palaeo- 

 zoic period consisted principally of the lowly-organized groups 

 of the Cryptogamous or Flowerless plants. The Mesozoic 

 formations, up to the Chalk, are especially characterized by the 

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