SUCCESSION OF LIFE UPON THE GLOBE. 371 
Tetrabranchiates, also, are of a much simpler type than the 
highly complex Ammonitide of the Mesozoic. 
Similar facts are observable amongst the Vertebrate animals. 
The /ishes 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 /epiiles, 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 AJammals are mainly, if not exclusively, 
referable to the JZarsupzals, 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 Palzo- 
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 
