only dating back to the Oolite; the
Hymenoptera, Diptera, and Homoptera go as far as the Lias; while the
Orthoptera and Neuroptera extend to the Trias. The recent discovery of
Coleoptera in the Carboniferous shows, however, that the preceding
limits are not absolute, and will probably soon be overpassed. Only the
more generalised ancestral forms of winged insects have been traced back
to Silurian time, and along with them the less highly organised
scorpions; facts which serve to show us the extreme imperfection of our
knowledge, and indicate possibilities of a world of terrestrial life in
the remotest Palaeozoic times.

_Geological Succession of Vertebrata._

The lowest forms of vertebrates are the fishes, and these appear first
in the geological record in the Upper Silurian formation. The most
ancient known fish is a Pteraspis, one of the bucklered ganoids or
plated fishes--by no means a very low type--allied to the sturgeon
(Accipenser) and alligator-gar (Lepidosteus), but, as a group, now
nearly extinct. Almost equally ancient are the sharks, which under
various forms still abound in our seas. We cannot suppose these to be
nearly the earliest fishes, especially as the two lowest orders, now
represented by the Amphioxus or lancelet and the lampreys, have not yet
been found fossil. The ganoids were greatly developed in the Devonian
era, and continued till the Cretaceous, when they gave way to the true
osseous fishes, which had first appeared in the Jurassic period, and
have continued to increase till the present day. This much later
appearance of the higher osseous fishes is quite in accordance with
evolution, although some of the very lowest forms, the lancelet and the
lampreys, together with the archaic ceratodus, have survived to our
time.

The Amphibia, represented by the extinct labyrinthodons, appear first in
the Carboniferous rocks, and these peculiar forms became extinct early
in the Secondary period. The labyrinthodons were, however, highly
specialised, and do not at all indicate the origin of the class, which
may be as ancient as the lower forms of fishes. Hardly any recognisable
remains of our existing groups--the frogs, toads, and salamanders--are
found before the Tertiary period, a fact which indicates the extreme
imperfection of the record as regards this class of animals.

True reptiles have not been found till we reach the Permian where
Prohatteria and Proterosaurus occur, the former closely allie