We cannot leave the last period of the Palæozoic era and its prolonged
ice age without noticing that it meant the entire cessation of a large
number of ancient types, especially among plants and backboneless
animals, which now disappear for ever. It is necessary to understand
that the animals of ancient days stand in three different relations to
those of to-day. (_a_) There are ancient types that have living
representatives, sometimes few and sometimes many, sometimes much
changed and sometimes but slightly changed. The lamp-shell,
_Lingulella_, of the Cambrian and Ordovician period has a very near
relative in the _Lingula_ of to-day. There are a few extremely
conservative animals. (_b_) There are ancient types which have no living
representatives, except in the guise of transformed descendants, as the
King-crab (_Limulus_) may be said to be a transformed descendant of the
otherwise quite extinct race to which Eurypterids or Sea-scorpions
belonged. (_c_) There are altogether extinct types--_lost races_--which
have left not a wrack behind. For there is not any representation to-day
of such races as Graptolites and Trilobites.
Looking backwards over the many millions of years comprised in the
Palæozoic era, what may we emphasise as the most salient features? There
was in the _Cambrian_ the establishment of the chief classes of
backboneless animals; in the _Ordovician_ the first fishes and perhaps
the first terrestrial plants; in the _Silurian_ the emergence of
air-breathing Invertebrates and mud-fishes; in the _Devonian_ the
appearance of the first Amphibians, from which all higher land animals
are descended, and the establishment of a land flora; in the
_Carboniferous_ the great Club-moss forests and an exuberance of
air-breathing insects and their allies; in the _Permian_ the first
reptiles and a new flora.