repared. In this way a great deal of animal matter is
introduced into the earth, and contributes to its fertility.

Many of our small mammals have the habit of making their dwelling
places in the soil. Some of them, such as the moles, normally abide in
the subterranean realm for all their lives. Others use the excavations
as places of retreat. In any case, these excavations serve to move the
particles of the soil about, and the materials which the animals drag
into the earth, as well as the excrement of the creatures, act to
enrich it. This habit of taking food underground is not limited to the
mammals; it is common with the ants, and even the earthworms, as noted
by Charles Darwin in his wonderful essay on these creatures, are
accustomed to drag into their burrows bits of grass and the slender
leaves of pines. It is not known what purpose they attain by these
actions, but it is sufficiently common somewhat to affect the
conditions of the soil.

The result of these complicated works done by animals and plants on
the soil is that the material to a considerable depth are constantly
being supplied with organic matter, which, along with the mineral
material, constitutes that part of the earth which can support
vegetation. Experiment will readily show that neither crushed rock nor
pure vegetable mould will of itself serve to maintain any but the
lowliest vegetation. It requires that the two materials be mixed in
order that the earth may yield food for ordinary plants, particularly
for those which are of use to man, as crops. On this account all the
processes above noted whereby the waste of plant and animal life is
carried below the surface are of the utmost importance in the creation
and preservation of the soil. It has been found, indeed, in almost all
cases, necessary for the farmer to maintain the fertility of his
fields to plough-in quantities of such organic waste. By so doing he
imitates the work which is effected in virgin soil by natural action.
As the process is costly in time and material, it is often neglected
or imperfectly done, with the result that the fields rapidly diminish
in fertility.

The way in which the buried organic matter acts upon the soil is not
yet thoroughly understood. In part it accomplishes the results by the
materials which on its decay it contributes to the soil in a state in
which they may readily be dissolved and taken up by the roots into
their sap; in part, however, it is believed th