s worms, but the
second great invasion was certainly due to air-breathing Arthropods,
like the pioneer scorpion we mentioned.

[Illustration: PICTORIAL REPRESENTATION OF THE SUCCESSIVE STRATA OF THE
EARTH'S CRUST, WITH SUGGESTIONS OF CHARACTERISTIC FOSSILS

E.g. Fish and Trilobite in the Devonian (red), a large Amphibian in the
Carboniferous (blue), Reptiles in Permian (light red), the first Mammal
in the Triassic (blue), the first Bird in the Jurassic (yellow), Giant
Reptiles in the Cretaceous (white), then follow the Tertiary strata with
progressive mammals, and Quaternary at the top with man and mammoth.]

The _Devonian_ period, including that of the Old Red Sandstone, was one
of the most significant periods in the earth's history. For it was the
time of the establishment of flowering plants upon the earth and of
terrestrial backboned animals. One would like to have been the
discoverer of the Devonian foot-print of _Thinopus_, the first known
Amphibian foot-print--an eloquent vestige of the third great invasion of
the dry land. It was probably from a stock of Devonian lung-fishes that
the first Amphibians sprang, but it was not till the next period that
they came to their own. While they were still feeling their way, there
was a remarkable exuberance of shark-like and heavily armoured fishes in
the Devonian seas.

EVOLUTION OF LAND ANIMALS

Sec. 1

Giant Amphibians and Coal-measures

The _Carboniferous_ period was marked by a mild moist climate and a
luxuriant vegetation in the swampy low grounds. It was a much less
strenuous time than the Devonian period; it was like a very long summer.
There were no trees of the type we see now, but there were forests of
club-mosses and horsetails which grew to a gigantic size compared with
their pigmy representatives of to-day. In these forests the
jointed-footed invaders of the dry land ran riot in the form of
centipedes, spiders, scorpions, and insects, and on these the primeval
Amphibians fed. The appearance of insects made possible a new linkage of
far-reaching importance, namely, the cross-fertilisation of flowering
plants by their insect visitors, and from this time onwards it may be
said that flowers and their visitors have evolved hand in hand.
Cross-fertilisation is much surer by insects than by the wind, and
cross-fertilisation is more advantageous than self-fertilisation because
it promotes both fertility and plasticity. It was probably in this
period that _c