the formation of vegetable mould-第14部分
按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
19th of the same year； much rain having fallen during the latter
part of this time。 During frosts and thaws the movements were
twice as great。 These observations were made by my son Horace； who
will hereafter publish an account of the movements of this stone
during successive wet and dry seasons； and of the effects of its
being undermined by worms。 Now when the ground swells； if it be
penetrated by cylindrical holes； such as worm…burrows； their walls
will tend to yield and be pressed inwards； and the yielding will be
greater in the deeper parts （supposing the whole to be equally
moistened） from the greater weight of the superincumbent soil which
has to be raised； than in the parts near the surface。 When the
ground dries； the walls will shrink a little and the burrows will
be a little enlarged。 Their enlargement； however； through the
lateral contraction of the ground； will not be favoured； but rather
opposed； by the weight of the superincumbent soil。
Distribution of Worms。Earth…worms are found in all parts of the
world； and some of the genera have an enormous range。 ｛41｝ They
inhabit the most isolated islands； they abound in Iceland； and are
known to exist in the West Indies； St。 Helena； Madagascar； New
Caledonia and Tahiti。 In the Antarctic regions； worms from
Kerguelen Land have been described by Ray Lankester； and I found
them in the Falkland Islands。 How they reach such isolated islands
is at present quite unknown。 They are easily killed by salt…water；
and it does not appear probable that young worms or their egg…
capsules could be carried in earth adhering to the feet or beaks of
land…birds。 Moreover Kerguelen Land is not now inhabited by any
land…bird。
In this volume we are chiefly concerned with the earth cast up by
worms； and I have gleaned a few facts on this subject with respect
to distant lands。 Worms throw up plenty of castings in the United
States。 In Venezuela； castings； probably ejected by species of
Urochaeta； are common in the gardens and fields； but not in the
forests； as I hear from Dr。 Ernst of Caracas。 He collected 156
castings from the court…yard of his house； having an area of 200
square yards。 They varied in bulk from half a cubic centimeter to
five cubic centimeters； and were on an average three cubic
centimeters。 They were； therefore； of small size in comparison
with those often found in England； for six large castings from a
field near my house averaged 16 cubic centimeters。 Several species
of earth…worms are common in St。 Catharina in South Brazil； and
Fritz Muller informs me 〃that in most parts of the forests and
pasture…lands； the whole soil； to a depth of a quarter of a metre；
looks as if it had passed repeatedly through the intestines of
earth…worms； even where hardly any castings are to be seen on the
surface。〃 A gigantic but very rare species is found there； the
burrows of which are sometimes even two centimeters or nearly 0。8
of an inch in diameter； and which apparently penetrate the ground
to a great depth。
In the dry climate of New South Wales； I hardly expected that worms
would be common； but Dr。 G。 Krefft of Sydney； to whom I applied；
after making inquiries from gardeners and others； and from his own
observations； informs me that their castings abound。 He sent me
some collected after heavy rain； and they consisted of little
pellets； about 0。15 inch in diameter； and the blackened sandy earth
of which they were formed still cohered with considerable tenacity。
The late Mr。 John Scott of the Botanic Gardens near Calcutta made
many observations for me on worms living under the hot and humid
climate of Bengal。 The castings abound almost everywhere； in
jungles and in the open ground； to a greater degree； as he thinks；
than in England。 After the water has subsided from the flooded
rice…fields； the whole surface very soon becomes studded with
castingsa fact which much surprised Mr。 Scott； as he did not know
how long worms could survive beneath water。 They cause much
trouble in the Botanic garden； 〃for some of the finest of our lawns
can be kept in anything like order only by being almost daily
rolled； if left undisturbed for a few days they become studded with
large castings。〃 These closely resemble those described as
abounding near Nice； and they are probably the work of a species of
Perichaeta。 They stand up like towers； with an open passage in the
centre。
A figure of one of these castings from a photograph is here given
（Fig。 3）。 The largest received by me was 3。5 inches in height and
1。35 inch in diameter； another was only 0。75 inch in diameter and
2。75 in height。 In the following year； Mr。 Scott measured several
of the largest； one was 6 inches in height and nearly 1。5 in
diameter： two others were 5 inches in height and respectively 2
and rather more than 2。5 inches in diameter。 The average weight of
the 22 castings sent to me was 35 grammes （1。25 oz。）； and one of
them weighed 44。8 grammes （or 2 oz。）。 All these castings were
thrown up either in one night or in two。 Where the ground in
Bengal is dry； as under large trees； castings of a different kind
are found in vast numbers： these consist of little oval or conical
bodies； from about the 1/20 to rather above 1/10 of an inch in
length。 They are obviously voided by a distinct species of worms。
The period during which worms near Calcutta display such
extraordinary activity lasts for only a little over two months；
namely； during the cool season after the rains。 At this time they
are generally found within about 10 inches beneath the surface。
During the hot season they burrow to a greater depth； and are then
found coiled up and apparently hybernating。 Mr。 Scott has never
seen them at a greater depth than 2。5 feet； but has heard of their
having been found at 4 feet。 Within the forests； fresh castings
may be found even during the hot season。 The worms in the Botanic
garden； during the cool and dry season； draw many leaves and little
sticks into the mouths of their burrows； like our English worms；
but they rarely act in this manner during the rainy season。
Mr。 Scott saw worm…castings on the lofty mountains of Sikkim in
North India。 In South India Dr。 King found in one place； on the
plateau of the Nilgiris； at an elevation of 7000 feet； 〃a good many
castings；〃 which are interesting for their great size。 The worms
which eject them are seen only during the wet season； and are
reported to be from 12 to 15 inches in length； and as thick as a
man's little finger。 These castings were collected by Dr。 King
after a period of 110 days without any rain； and they must have
been ejected either during the north…east or more probably during
the previous south…west monsoon； for their surfaces had suffered
some disintegration and they were penetrated by many fine roots。 A
drawing is here given （Fig。 4） of one which seems to have best
retained its original size and appearance。 Notwithstanding some
loss from disintegration； five of the largest of these castings
（after having been well sun…dried） weighed each on an average 89。5
grammes； or above 3 oz。； and the largest weighed 123。14 grammes； or
4。33 oz。；that is； above a quarter of a pound！ The largest
convolutions were rather more than one inch in diameter； but it is
probable that they had subsided a little whilst soft； and that
their diameters had thus been increased。 Some had flowed so much
that they now consisted of a pile of almost flat confluent cakes。
All were formed of fine； rather light…coloured earth； and were
surprisingly hard and compact； owing no doubt to the animal matter
by which the particles of earth had been cemented together。 They
did not disintegrate； even when left for some hours in water。
Although they had been cast up on the surface of gravelly soil；
they contained extremely few bits of rock； the largest of which was
only 0。15 inch in diameter。
Dr。 King saw in Ceylon a worm about 2 feet in length and 0。5 inch
in diameter； and he was told that it was a very common species
during the wet season。 These worms must throw up castings at least
as large as those on the Nilgiri Mountains； but Dr。 King saw none
during his short visit to Ceylon。
Sufficient facts have now been given； showing that worms do much
work in bringing up fine earth to the surface in most or all parts
of the world； and under the most different climates。
CHAPTER IIITHE AMOUNT OF FINE EARTH BROUGHT UP BY WORMS TO THE
SURFACE。
Rate at which various objects strewed on the surface of grass…
fields are covered up by the castings of wormsThe burial of a
paved pathThe slow subsidence of great stones left on the
surfaceThe number of worms which live within a given spaceThe
weight of earth ejected from a burrow； and from all the burrows
within a given spaceThe thickness of the layer of mould which the
castings on a given space would form within a given time if
uniformly spread outThe slow rate at which mould can increase to
a great thicknessConclusion。
We now come to the more immediate subject of this volume； namely；
the amount of earth which is brought up by worms from beneath the
surface； and is afterwards spread out more or less completely by
the rain and wind。 The amount can be judged of by two methods；by
the rate at which objects left on the surface are buried； and more
accurately by weighing the quantity brought up within a given time。
We will begin with the first method； as it was first followed。
Near Mael Hall in Staffordshire； quick…lime had been spread about
the year 1827 thickly over a field of good pasture…land； which had
not since been ploughed。 Some square holes were dug in this field
in the beginning of October 1837； and the sections showed a layer
of turf； formed by the matted roots of the grasses； 0。5 inch in
thickness； beneath which； at a depth of 2。5 inches （or 3 inches
from the surface）； a layer of the lime in powder or in small lumps
could be distinctly seen running all round the vertical sides of
the holes。 The soil beneath the layer of lime was either gravelly
