are borne by the ulna bone (U) of the fore-arm; there is a separate tuft
(AS) on the thumb (TH).]

The rarity of direct traces of life in the oldest rocks is partly due to
the fact that the primitive animals would be of delicate build, but it
must also be remembered that the ancient rocks have been profoundly and
repeatedly changed by pressure and heat, so that the traces which did
exist would be very liable to obliteration. And if it be asked what
right we have to suppose the presence of living creatures in the absence
or extreme rarity of fossils, we must point to great accumulations of
limestone which indicate the existence of calcareous algae, and to
deposits of iron which probably indicate the activity of iron-forming
Bacteria. Ancient beds of graphite similarly suggest that green plants
flourished in these ancient days.

Sec. 3

The Era of Ancient Life (Palaeozoic)

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
Palaeozoic 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.