the formation of vegetable mould-第27部分
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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 now covered with turf； and here the castings contained
very many particles of brick； all more or less rounded； and this
brick…rubbish； after being shot into the hole； could not have
undergone any attrition。 Again； old bricks very little broken；
together with fragments of mortar； were laid down to form walks；
and were then covered with from 4 to 6 inches of gravel； six little
fragments of brick were extracted from castings collected on these
walks； three of which were plainly worn。 There were also very many
particles of hard mortar； about half of which were well rounded；
and it is not credible that these could have suffered so much
corrosion from the action of carbonic acid in the course of only
seven years。
Much better evidence of the attrition of hard objects in the
gizzards of worms； is afforded by the state of the small fragments
of tiles or bricks； and of concrete in the castings thrown up where
ancient buildings once stood。 As all the mould covering a field
passes every few years through the bodies of worms； the same small
fragments will probably be swallowed and brought to the surface
many times in the course of centuries。 It should be premised that
in the several following cases； the finer matter was first washed
away from the castings； and then all the particles of bricks； tiles
and concrete were collected without any selection； and were
afterwards examined。 Now in the castings ejected between the
tesserae on one of the buried floors of the Roman villa at Abinger；
there were many particles （from to 2 mm。 in diameter） of tiles and
concrete； which it was impossible to look at with the naked eye or
through a strong lens； and doubt for a moment that they had almost
all undergone much attrition。 I speak thus after having examined
small water…worn pebbles； formed from Roman bricks； which M。 Henri
de Saussure had the kindness to send me； and which he had extracted
from sand and gravel beds； deposited on the shores of the Lake of
Geneva； at a former period when the water stood at about two metres
above its present level。 The smallest of these water…worn pebbles
of brick from Geneva resembled closely many of those extracted from
the gizzards of worms； but the larger ones were somewhat smoother。
Four castings found on the recently uncovered； tesselated floor of
the great room in the Roman villa at Brading； contained many
particles of tile or brick； of mortar； and of hard white cement；
and the majority of these appeared plainly worn。 The particles of
mortar； however； seemed to have suffered more corrosion than
attrition； for grains of silex often projected from their surfaces。
Castings from within the nave of Beaulieu Abbey； which was
destroyed by Henry VIII。； were collected from a level expanse of
turf； overlying the buried tesselated pavement； through which worm…
burrows passed； and these castings contained innumerable particles
of tiles and bricks； of concrete and cement； the majority of which
had manifestly undergone some or much attrition。 There were also
many minute flakes of a micaceous slate； the points of which were
rounded。 If the above supposition； that in all these cases the
same minute fragments have passed several times through the
gizzards of worms； be rejected； notwithstanding its inherent
probability； we must then assume that in all the above cases the
many rounded fragments found in the castings had all accidentally
undergone much attrition before they were swallowed； and this is
highly improbable。
On the other hand it must be stated that fragments of ornamental
tiles； somewhat harder than common tiles or bricks； which had been
swallowed only once by worms kept in confinement； were with the
doubtful exception of one or two of the smallest grains； not at all
rounded。 Nevertheless some of them appeared a little worn； though
not rounded。 Notwithstanding these cases； if we consider the
evidence above given； there can be little doubt that the fragments；
which serve as millstones in the gizzards of worms； suffer； when of
a not very hard texture； some amount of attrition； and that the
smaller particles in the earth； which is habitually swallowed in
such astonishingly large quantities by worms； are ground together
and are thus levigated。 If this be the case； the 〃terra
tenuissima；〃the 〃pate excessivement fine；〃of which the castings
largely consist； is in part due to the mechanical action of the
gizzard； ｛75｝ and this fine matter； as we shall see in the next
chapter； is that which is chiefly washed away from the innumerable
castings on every field during each heavy shower of rain。 If the
softer stones yield at all； the harder ones will suffer some slight
amount of wear and tear。
The trituration of small particles of stone in the gizzards of
worms is of more importance under a geological point of view than
may at first appear to be the case； for Mr。 Sorby has clearly shown
that the ordinary means of disintegration； namely； running water
and the waves of the sea； act with less and less power on fragments
of rock the smaller they are。 〃Hence；〃 as he remarks； 〃even making
no allowance for the extra buoying up of very minute particles by a
current of water； depending on surface cohesion； the effects of
wearing on the form of the grains must vary directly as their
diameter or thereabouts。 If so； a grain of 1/10 an inch in
diameter would be worn ten times as much as one of an inch in
diameter； and at least a hundred times as much as one of 1/100 an
inch in diameter。 Perhaps； then； we may conclude that a grain 1/10
of an inch in diameter would be worn as much or more in drifting a
mile as a grain 1/1000 of an inch in being drifted 100 miles。 On
the same principle a pebble one inch in diameter would be worn
relatively more by being drifted only a few hundred yards。〃 ｛76｝
Nor should we forget； in considering the power which worms exert in
triturating particles of rock； that there is good evidence that on
each acre of land； whi
