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.
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