and a corresponding notch in the mouth of the shell ("siphonostomatous” 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 specialised 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 Ammonitidoe 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 Invertebrates, 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 preceded 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 commencement 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 Palaeozoic 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 Tertiary rocks.