RWIN.

[Illustration: W]

Worms have played a more important part in the history of the world
than most persons would at first suppose. In almost all humid
countries they are extraordinarily numerous, and for their size
possess great muscular power. In many parts of England a weight of
more than ten tons (10,516 kilogrammes) of dry earth annually passes
through their bodies and is brought to the surface on each acre of
land; so that the whole superficial bed of vegetable mould passes
through their bodies in the course of every few years. From the
collapsing of the old burrows the mould is in constant though slow
movement, and the particles composing it are thus rubbed together. By
these means fresh surfaces are continually exposed to the action of
the carbonic acid in the soil, and of the humus-acids which appear to
be still more efficient in the decomposition of rocks. The generation
of the humus-acids is probably hastened during the digestion of the
many half-decayed leaves which worms consume. Thus the particles of
earth, forming the superficial mould, are subjected to conditions
eminently favorable for their decomposition and disintegration.
Moreover, the particles of the softer rocks suffer some amount of
mechanical trituration in the muscular gizzards of worms, in which
small stones serve as mill-stones.

[Illustration: DIAGRAM OF THE ALIMENTARY CANAL OF AN EARTH-WORM.]

The finely levigated castings, when brought to the surface in a moist
condition, flow during rainy weather down any moderate slope; and the
smaller particles are washed far down even a gently inclined surface.
Castings when dry often crumble into small pellets and these are apt
to roll down any sloping surface. Where the land is quite level and is
covered with herbage, and where the climate is humid so that much dust
cannot be blown away, it appears at first sight impossible that there
should be any appreciable amount of sub-aerial denudation; but worm
castings are blown, especially while moist and viscid, in one uniform
direction by the prevalent winds which are accompanied by rain. By
these several means the superficial mould is prevented from
accumulating to a great thickness; and a thick bed of mould checks in
many ways the disintegration of the underlying rocks and fragments of
rock.

[Illustration: A WORM CASTING, FROM NICE. (Natural Size.)]

The removal of worm-castings by the above means leads to results which
are far from insign