ons lie six or
seven feet beneath the surface, at a depth at which worms cannot work.
It is probable that many monoliths and some old walls have fallen
down from having been undermined by worms.

Worms prepare the ground in an excellent manner for the growth of
fibrous-rooted plants and for seedlings of all kinds. They
periodically expose the mould to the air, and sift it so that no
stones larger than the particles which they can swallow are left in
it. They mingle the whole intimately together, like a gardener who
prepares fine soil for his choicest plants. In this state it is well
fitted to retain moisture and to absorb all soluble substances, as
well as for the process of nitrification. The bones of dead animals,
the harder parts of insects, the shells of land mollusks, leaves,
twigs, etc., are before long all buried beneath the accumulated
castings of worms, and are thus brought in a more or less decayed
state within reach of the roots of plants. Worms likewise drag an
infinite number of dead leaves and other parts of plants into their
burrows, partly for the sake of plugging them up and partly as food.

The leaves which are dragged into the burrows as food, after being
torn into the finest shreds, partially digested and saturated with the
intestinal and urinary secretions, are commingled with much earth.
This earth forms the dark-colored, rich humus which almost everywhere
covers the surface of the land with a fairly well-defined layer or
mantle. Von Hensen placed two worms in a vessel eighteen inches in
diameter, which was filled with sand, on which fallen leaves were
strewed; and these were soon dragged into their burrows to a depth of
three inches. After about six weeks an almost uniform layer of sand, a
centimetre (.4 inch) in thickness, was converted into humus by having
passed through the alimentary canals of these two worms. It is
believed by some persons that worm-burrows, which often penetrate the
ground almost perpendicularly to a depth of five or six feet,
materially aid in its drainage; notwithstanding that the viscid
castings piled over the mouths of the burrows prevent or check the
rain-water directly entering them. They allow the air to penetrate
deeply into the ground. They also greatly facilitate the downward
passage of roots of moderate size; and these will be nourished by the
humus with which the burrows are lined. Many seeds owe their
germination to having been covered by castings; and others