th fossil-bearing rocks to a thickness of about
twenty-one miles--in itself an astounding fact. Perhaps thirty million
years must be allotted to the Pre-Cambrian eras, eighteen to the
Palaeozoic, nine to the Mesozoic, three to the Cenozoic, making a grand
total of sixty millions.

The Establishment of Invertebrate Stocks

It is an astounding fact that at least half of geological time (the
Archaeozoic and Proterozoic eras) passed before there were living
creatures with parts sufficiently hard to form fossils. In the latter
part of the Proterozoic era there are traces of one-celled marine
animals (Radiolarians) with shells of flint, and of worms that wallowed
in the primal mud. It is plain that as regards the most primitive
creatures the rock record tells us little.

[Illustration: _From Knipe's "Nebula to Man."_

ANIMALS OF THE CAMBRIAN PERIOD e.g. Sponges, Jellyfish, Starfish,
Sea-lilies, Water-fleas, and Trilobites]

[Illustration: _Photo: J. J. Ward, F.E.S._

A TRILOBITE

Trilobites were ancient seashore animals, abundant from the Upper
Cambrian to the Carboniferous eras. They have no direct descendants
to-day. They were jointed-footed animals, allied to Crustaceans and
perhaps also to King-crabs. They were able to roll themselves up in
their ring-armour.]

[Illustration: _Photo: British Museum (Natural History)._

THE GAMBIAN MUD-FISH, PROTOPTERUS

It can breathe oxygen dissolved in water by its gills; it can also
breathe dry air by means of its swim-bladder, which has become a lung.
It is a _double-breather_, showing evolution in process. For seven
months of the year, the dry season, it can remain inert in the mud,
getting air through an open pipe to the surface. When water fills the
pools it can use its gills again. Mud-nests or mud encasements with the
lung-fish inside have often been brought to Britain and the fish when
liberated were quite lively.]

[Illustration: THE ARCHAEOPTERYX

(_After William Leche of Stockholm._)

A good restoration of the oldest known bird, Archaeopteryx (Jurassic
Era). It was about the size of a crow; it had teeth on both jaws; it had
claws on the thumb and two fingers; and it had a long lizard-like tail.
But it had feathers, proving itself a true bird.]

[Illustration: WING OF A BIRD, SHOWING THE ARRANGEMENT OF THE FEATHERS

The longest feathers or primaries (PR) are borne by the two fingers (2
and 3), and their palm-bones (CMC); the second longest or secondaries