substance to fertilize it. Leaves and stems falling on the surface
are turned under by several agents. Earthworms and other animals
whose home is in the waste drag them into their burrows either for
food or to line their nests. Trees overthrown by the wind, roots
and all, turn over the soil and subsoil and mingle them together.
Bacteria also work in the waste and contribute to its enrichment.
The animals living in the mantle do much in other ways toward the
making of soil. They bring the coarser fragments from beneath to
the surface, where the waste weathers more rapidly. Their burrows
allow air and water to penetrate the waste more freely and to
affect it to greater depths.

ANTS. In the tropics the mantle of waste is worked over chiefly by
ants. They excavate underground galleries and chambers, extending
sometimes as much as fourteen feet below the surface, and build
mounds which may reach as high above it. In some parts of Paraguay
and southern Brazil these mounds, like gigantic potato hills,
cover tracts of considerable area.

In search for its food--the dead wood of trees--the so-called
white ant constructs runways of earth about the size of gas pipes,
reaching from the base of the tree to the topmost branches. On the
plateaus of central Africa explorers have walked for miles through
forests every tree of which was plastered with these galleries of
mud. Each grain of earth used in their construction is moistened
and cemented by slime as it is laid in place by the ant, and is
thus acted on by organic chemical agents. Sooner or later these
galleries are beaten down by heavy rains, and their fertilizing
substances are scattered widely by the winds.