the formation of vegetable mould-第33章
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thickness of the soil； if none be removed from the surface； will
steadily though slowly tend to 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。 ｛66｝ 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。
｛67｝ 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。 ｛68｝ 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； ｛69｝ 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。 ｛70｝ 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。 ｛71｝
When worms make their burrows through earth abounding with little
stones； no doubt many will be unavoidably swallowed； but it must
not be supposed that this fact accounts for the frequency with
which stones and sand are found in their gizzards。 For beads of
glass and fragments of brick and of hard tiles were scattered over
the surface of the earth； in pots in which worms were kept and had
already made their burrows； and very many of these beads and
fragments were picked up and swallowed by the worms； for they were
found in their castings； intestines； and gizzards。 They even
swallowed the coarse red dust； formed by the pounding of the tiles。
Nor can it be supposed that they mistook the beads and fragments
for food； for we have seen that their taste is delicate enough to
distinguish between different kinds of leaves。 It is therefore
manifest that they swallow hard objects； such as bits of stone；
beads of glass and angular fragments of bricks or tiles for some
special purpose； and it can hardly be doubted that this is to aid
their gizzards in crushing and grinding the earth； which they so
largely consume。 That such hard objects are not necessary for
crushing leaves； may be inferred from the fact that certain
species； which live in mud or water and feed on dead or living
vegetable matter； but which do not swallow earth； are not provided
with gizzards； ｛72｝ and therefore cannot have the power of
utilising stones。
During the grinding process； the particles of earth must be rubbed
against one another； and between the stones and the tough lining
membrane of the gizzard。 The softer particles will thus suffer
some attrition； and will perhaps even be crushed。 This conclusion
is supported by the appearance of freshly ejected castings； for
these often reminded me of the appearance of paint which has just
been ground by a workman between two flat stones。 Morren remarks
that the intestinal canal is 〃impleta tenuissima terra； veluti in
pulverem redacta。〃 ｛73｝ Perrier also speaks of 〃l'etat de pate
excessivement fine a laquelle est reduite la terre qu'ils
rejettent；〃 &c。 ｛74｝
As the amount of trituration which the particles of earth undergo
in the gizzards of worms possesses some interest （as we shall
hereafter see）； I endeavoured to obtain evidence on this head by
carefully examining many of the fragments which had passed through
their alimentary canals。 With worms living in a state of nature；
it is of course impossible to know how much the fragments may have
been worn before they were swallowed。 It is； however； clear that
worms do not habitually select already rounded particles； for
sharply angular bits of flint and of other hard rocks were often
found in their gizzards or intestines。 On three occasions sharp
spines from the stems of rose…bushes were thus found。 Worms kept
in confinement repeatedly swallowed angular fragments of hard tile；
coal； cinders； and even the sharpest fragments of glass。
Gallinaceous and struthious birds retain the same stones in their
gizzards for a long time； which thus become well rounded； but this
does not appear to be the case with worms； judging from the large
number of the fragments of tiles； glass beads； stones； &c。；
commonly found in their castings and intestines。 So that unless
the same fragments were to pass repeatedly through their gizzards；
visible signs of attrition in the fragments could hardly be
expected； except perhaps in the case of very soft stones。
I will now give such evidence of attrition as I have been able to
collect。 In the gizzards of some worms dug out of a thin bed of
mould over the chalk； there were many well…rounded small fragments
of chalk； and two fragments of the shells of a land…mollusc （as
ascertained by their microscopical structure）； which latter were
not only rounded but somewhat polished。 The calcareous concretions
formed in the calciferous glands； which are often found in their
gizzards； intestines； and occasionally in their castings； when of
large size； sometimes appeared to have been rounded； but with all
calcareous bodies the rounded appearance may be partly or wholly
due to their corrosion by carbonic acid and the humus…acids。 In
the gizzards of several worms collected in my kitchen garden near a
hothouse； eight little fragments of cinders were found； and of
these； six appeared more or less rounded； as were two bits of
brick； but some other bits were not at all rounded。 A farm…road
near Abinger Hall had been covered seven years before with brick…
rubbish to the depth of about 6 inches； turf had grown over this
rubbish on both sides of the road for a width of 18 inches； and on
this turf there were innumerable castings。 Some of them were
coloured of a uniform red owing to the presence of much brick…dust；
and they contained many particles of brick and of hard mortar from
1 to 3 mm。 in diameter； most of which were plainly rounded； but all
these particles may have been rounded before they were protected by
the turf and were swallowed； like those on the bare parts of the
road which were much worn。 A hole in a pasture…field had been
filled up with brick…rubbish at the same time； viz。； seven years
ago； and was no
