It is an astounding fact that at least half of geological time (the
Archæozoic 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._