of flight brings obvious advantages. A bird feeding on the
ground is able to evade the stalking carnivore by suddenly rising into
the air; food and water can be followed rapidly and to great distances;
the eggs or the young can be placed in safe situations; and birds in
their migrations have made a brilliant conquest both of time and space.
Many of them know no winter in their year, and the migratory flight of
the Pacific Golden Plover from Hawaii to Alaska and back again does not
stand alone.

THE PROCESSION OF LIFE THROUGH THE AGES

Sec. 1

The Rock Record

How do we know when the various classes of animals and plants were
established on the earth? How do we know the order of their appearance
and the succession of their advances? The answer is: by reading the Rock
Record. In the course of time the crust of the earth has been elevated
into continents and depressed into ocean-troughs, and the surface of the
land has been buckled up into mountain ranges and folded in gentler
hills and valleys. The high places of the land have been weathered by
air and water in many forms, and the results of the weathering have been
borne away by rivers and seas, to be laid down again elsewhere as
deposits which eventually formed sandstones, mudstones, and similar
sedimentary rocks. Much of the material of the original crust has thus
been broken down and worked up again many times over, and if the total
thickness of the sedimentary rocks is added up it amounts, according to
some geologists, to a total of 67 miles. In most cases, however, only a
small part of this thickness is to be seen in one place, for the
deposits were usually formed in limited areas at any one time.

The Use of Fossils

When the sediments were accumulating age after age, it naturally came
about that remains of the plants and animals living at the time were
buried, and these formed the fossils by the aid of which it is possible
to read the story of the past. By careful piecing together of evidence
the geologist is able to determine the order in which the different
sedimentary rocks were laid down, and thus to say, for instance, that
the Devonian period was the time of the origin of Amphibians. In other
cases the geologist utilises the fossils in his attempt to work out the
order of the strata when these have been much disarranged. For the
simpler fossil forms of any type must be older than those that are more
complex. There is no vicious circle here, for the