the formation of vegetable mould-第41章
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to 9。5 inches。 On the embankment itself the mould was only 1 to
1。5 inch in thickness； and within the ditch at the bottom it varied
from 2。5 to 3。5； but was in one spot 6 inches in thickness。 On the
north…west side of the hill； either no embankment had ever been
thrown up above the ditch； or it had subsequently been removed； so
that here there was nothing to prevent worm…castings； earth and
stones being washed into the ditch； at the bottom of which the
mould formed a layer from 11 to 22 inches in thickness。 It should
however be stated that here and on other parts of the slope； the
bed of mould often contained fragments of chalk and flint which had
obviously rolled down at different times from above。 The
interstices in the underlying fragmentary chalk were also filled up
with mould。
My son examined the surface of this hill to its base in a south…
west direction。 Beneath the great ditch； where the slope was about
24 degrees； the mould was very thin； namely； from 1。5 to 2。5
inches； whilst near the base； where the slope was only 3 degrees to
4 degrees； it increased to between 8 and 9 inches in thickness。 We
may therefore conclude that on this artificially modified hill； as
well as in the natural valleys of the neighbouring Chalk Downs；
some fine earth； probably derived in large part from worm…castings；
is washed down； and accumulates in the lower parts； notwithstanding
the percolation of an unknown quantity into the underlying chalk； a
supply of fresh earthy matter being afforded by the dissolution of
the chalk through atmospheric and other agencies。
CHAPTER VIICONCLUSION。
Summary of the part which worms have played in the history of the
worldTheir aid in the disintegration of rocksIn the denudation
of the landIn the preservation of ancient remainsIn the
preparation of the soil for the growth of plantsMental powers of
wormsConclusion。
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 favourable 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。
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 whilst 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。
The removal of worm…castings by the above means leads to results
which are far from insignificant。 It has been shown that a layer
of earth； 0。2 of an inch in thickness； is in many places annually
brought to the surface； and if a small part of this amount flows；
or rolls； or is washed； even for a short distance； down every
inclined surface； or is repeatedly blown in one direction； a great
effect will be produced in the course of ages。 It was found by
measurements and calculations that on a surface with a mean
inclination of 9 degrees 26 seconds； 2。4 cubic inches of earth
which had been ejected by worms crossed； in the course of a year； a
horizontal line one yard in length； so that 240 cubic inches would
cross a line 100 yards in length。 This latter amount in a damp
state would weigh 11。5 pounds。 Thus a considerable weight of earth
is continually moving down each side of every valley； and will in
time reach its bed。 Finally this earth will be transported by the
streams flowing in the valleys into the ocean； the great receptacle
for all matter denuded from the land。 It is known from the amount
of sediment annually delivered into the sea by the Mississippi；
that its enormous drainage…area must on an average be lowered
。00263 of an inch each year； and this would suffice in four and
half million years to lower the whole drainage…area to the level of
the sea…shore。 So that； if a small fraction of the layer of fine
earth； 0。2 of an inch in thickness； which is annually brought to
the surface by worms； is carried away； a great result cannot fail
to be produced within a period which no geologist considers
extremely long。
Archaeologists ought to be grateful to worms； as they protect and
preserve for an indefinitely long period every object； not liable
to decay； which is dropped on the surface of the land； by burying
it beneath their castings。 Thus； also； many elegant and curious
tesselated pavements and other ancient remains have been preserved；
though no doubt the worms have in these cases been largely aided by
earth washed and blown from the adjoining land； especially when
cultivated。 The old tesselated pavements have； however； often
suffered by having subsided unequally from being unequally
undermined by the worms。 Even old massive walls may be undermined
and subside； and no building is in this respect safe； unless the
foundations lie 6 or 7 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 ｛82｝ 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…
molluscs； leaves； twigs； &c。； 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 coloured； rich humus which almost
everywhere covers the surface of the land with a fairly well…
defined layer or mantle。 Hensen ｛83｝ placed two worms in a vessel
18 inches in diameter； which was f
