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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.
_PALAEOZOIC 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.
_ARCHAEOZOIC 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 adventurou