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 Mesoz