increase; but the accumulation will after a time delay the disintegration
of the underlying rocks and of the more deeply seated particles. For the
humus-acids which are generated chiefly in the upper layer of vegetable
mould, are extremely unstable compounds, and are liable to decomposition
before they reach any considerable depth. {223} A thick bed of overlying
soil will also check the downward extension of great fluctuations of
temperature, and in cold countries will check the powerful action of
frost. The free access of air will likewise be excluded. From these
several causes disintegration would be almost arrested, if the overlying
mould were to increase much in thickness, owing to none or little being
removed from the surface. {224a} In my own immediate neighbourhood we
have a curious proof how effectually a few feet of clay checks some
change which goes on in flints, lying freely exposed; for the large ones
which have lain for some time on the surface of ploughed fields cannot be
used for building; they will not cleave properly, and are said by the
workmen to be rotten. {224b} It is therefore necessary to obtain flints
for building purposes from the bed of red clay overlying the chalk (the
residue of its dissolution by rain-water) or from the chalk itself.
Not only do worms aid directly in the chemical disintegration of rocks,
but there is good reason to believe that they likewise act in a direct
and mechanical manner on the smaller particles. All the species which
swallow earth are furnished with gizzards; and these are lined with so
thick a chitinous membrane, that Perrier speaks of it, {225a} as une
veritable armature. The gizzard is surrounded by powerful transverse
muscles, which, according to Claparede, are about ten times as thick as
the longitudinal ones; and Perrier saw them contracting energetically.
Worms belonging to one genus, Digaster, have two distinct but quite
similar gizzards; and in another genus, Moniligaster, the second gizzard
consists of four pouches, one succeeding the other, so that it may almost
be said to have five gizzards. {225b} In the same manner as gallinaceous
and struthious birds swallow stones to aid in the trituration of their
food, so it appears to be with terricolous worms. The gizzards of
thirty-eight of our common worms were opened, and in twenty-five of them
small stones or grains of sand, sometimes together with the hard
calcareous concretions formed within the anterior calciferous glands,
were found, and in two others concretions alone. In the gizzards of the
remaining worms there were no stones; but some of these were not real
exceptions, as the gizzards were opened late in the autumn, when the
worms had ceased to feed and their gizzards were quite empty. {226}
106