The _Cambrian_ period was the time of the establishment of the chief
stocks of backboneless animals such as sponges, jellyfishes, worms,
sea-cucumbers, lamp-shells, trilobites, crustaceans, and molluscs. There
is something very eloquent in the broad fact that the peopling of the
seas had definitely begun some thirty million years ago, for Professor
H. F. Osborn points out that in the Cambrian period there was already a
colonisation of the shore of the sea, the open sea, and the deep waters.
The _Ordovician_ period was marked by abundant representation of the
once very successful class of Trilobites--jointed-footed,
antenna-bearing, segmented marine animals, with numerous appendages and
a covering of chitin. They died away entirely with the end of the
Palæozoic era. Also very notable was the abundance of predatory
cuttlefishes, the bullies of the ancient seas. But it was in this period
that the first backboned animals made their appearance--an epoch-making
step in evolution. In other words, true fishes were evolved--destined in
the course of ages to replace the cuttlefishes (which are mere molluscs)
in dominating the seas.
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_RECENT TIMES_ Human civilisation.
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{PLEISTOCENE OR GLACIAL TIME Last great Ice Age.
_CENOZOIC ERA_ {MIOCENE AND PLIOCENE TIMES Emergence of Man.
{EOCENE AND OLIGOCENE TIMES Rise of higher mammals.
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{CRETACEOUS PERIOD Rise of primitive mammals,
{ flowering plants,
{ and higher insects.
_MESOZOIC ERA_ {JURASSIC PERIOD Rise of birds and flying
{ reptiles.
{TRIASSIC PERIOD Rise of dinosaur reptiles.
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{PERMIAN PERIOD Rise of reptiles.
{CARBONIFEROUS PERIOD Rise of insects.
{DEVONIAN PERIOD First amphibians.
_PALÆOZOIC ERA_ {SILURIAN PERIOD Land animals began.
{ORDOVICIAN PERIOD First fishes.
{CAMBRIAN PERIOD Peopling of the sea.
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_PROTEROZOIC AGES_ Many of the Backboneless stocks began.
_ARCHÆOZOIC AGES_ Living creatures began to be upon the earth.
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{Making of continents and ocean-basins.
{Beginnings of atmosphere and hydrosphere.
_FORMATIVE TIMES_ {Cooling of the earth.
{Establishment of the solar system.
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In the _Silurian_ period in which the peopling of the seas went on
apace, there was the first known attempt at colonising the dry land. For
in Silurian rocks there are fossil scorpions, and that implies ability
to breathe dry air--by means of internal surfaces, in this case known as
lungbooks. It was also towards the end of the Silurian, when a period of
great aridity set in, that fishes appeared related to our mud-fishes or
double-breathers (Dipnoi), which have lungs as well as gills. This,
again, meant utilising dry air, just as the present-day mud-fishes do
when the water disappears from the pools in hot weather. The lung-fishes
or mud-fishes of to-day are but three in number, one in Queensland, one
in South America, and one in Africa, but they are extremely
interesting "living fossils," binding the class of fishes to that of
amphibians. It is highly probable that the first invasion of the dry
land should be put to the credit of some adventurous 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.