SUCCESSION OF LIFE UPON THE GLOBE. 371 



Tetrabranchiates, also, are of a much simpler type than the 

 highly complex Ammonitida 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 tf 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-organised groups 

 of the Cryptogamous or Flowerless plants. The Mesozoic 

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

 naked-seeded Flowering plants the Conifers and the Cycads; 

 whilst the higher groups of the Angiospermous Exogens and 

 Monocotyledons characterise the Upper Cretaceous and Ter- 

 tiary rocks. 



Facts of the above nature and they could be greatly multi- 

 plied seem to point clearly to the existence of some law of 

 progression, though we certainly are not yet in a position to 

 formulate this law, or to indicate the precise manner in which 

 it has operated. Two considerations, also, must not be over- 

 looked. In the first place, there are various groups, some of 

 them highly organised, which make their appearance at an ex- 

 tremely ancient date, but which continue throughout geological 

 time almost unchanged, and certainly unprogressive. Many of 

 these " persistent types " are known such as various of the 



