ng to the depth of the soil and the frequency with
which they are visited by fertilizing inundations, they yield rich
harvests without fertilizing for thousands of years. It is therefore
not surprising that we find the peoples who depended upon tillage for
subsistence first developed on the great river plains. There, indeed,
were laid the foundations of our higher civilization; there alone
could the state which demands of its citizens fixed abodes and
continuous labour take rise. In the conditions which these fields of
abundance afforded, dense populations were possible, and all the arts
which lead toward culture were greatly favoured. Thus it is that the
civilization of China, India, Persia, and Egypt, the beginnings of
man's higher development, began near the mouths of the great river
valleys. These fields were, moreover, most favourably placed for the
institution of commerce, in that the arts of navigation, originating
in the sheltered reaches of the streams, readily found its way through
the estuaries to the open sea.

Passing down the reaches of a great river as it approaches the sea, we
find that the alluvial plains usually widen and become lower. At
length we attain a point where the flood waters cover the surface for
so large a part of the year that the ground is swampy and untillable
unless it is artificially and at great expense of labour won to
agriculture in the manner in which this task has been effected in the
lower portion of the Rhine Valley. Still farther toward the sea, the
plain gradually dips downward until it passes below the level of the
waters. Through this mud-flat section the stream continues to cut
channels, but with the ever-progressive slowing of its motion the
burden of fine mud which it carries drops to the bottom, and
constantly closes the paths through which the water escapes. Every few
years they tend to break a new way on one side or the other of their
former path. Some of the greatest engineering work done in modern
times has been accomplished by the engineers engaged in controlling
the exits of large rivers to the sea. The outbreak of the Yellow River
in 1887, in which the stream, hindered by its own accumulations,
forced a new path across its alluvial plains, destroyed a vast deal of
life and property, and made the new exit seventy miles from the path
which it abandoned.

Below the surface of the open water the alluvial deposits spread out
into a broad fan, which slopes gradually t