The Formation of Vegetable Mould Through the Action of Worms
by Charles Darwin · English · Gutenberg #2355 · 374,513 chars · public domain (US)
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Title: The Formation of Vegetable Mould Through the Action of Worms
Author: Charles Darwin
Release date: October 1, 2000 [eBook #2355]
Most recently updated: November 5, 2021
Language: English
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*** START OF THE PROJECT GUTENBERG EBOOK THE FORMATION OF VEGETABLE MOULD THROUGH THE ACTION OF WORMS ***
[Picture: Book cover]
THE FORMATION OF
VEGETABLE MOULD
THROUGH THE ACTION OF WORMS
WITH OBSERVATIONS ON THEIR HABITS.
BY CHARLES DARWIN, LL.D., F.R.S.
THIRTEENTH THOUSAND
WITH ILLUSTRATIONS
* * * * *
LONDON
JOHN MURRAY, ALBEMARLE STREET
1904
* * * * *
PRINTED BY
WILLIAM CLOWES AND SONS, LIMITED,
LONDON AND BECCLES.
* * * * *
CONTENTS.
INTRODUCTION Page 1–6
CHAPTER I.
HABITS OF WORMS.
Nature of the sites inhabited—Can live long under 7–15
water—Nocturnal—Wander about at night—Often lie close
to the mouths of their burrows, and are thus destroyed
in large numbers by birds—Structure—Do not possess
eyes, but can distinguish between light and
darkness—Retreat rapidly when brightly illuminated, not
by a reflex action—Power of attention—Sensitive to heat
and cold—Completely deaf—Sensitive to vibrations and to
touch—Feeble power of smell—Taste—Mental
qualities—Nature of food—Omnivorous—Digestion—Leaves
before being swallowed, moistened with a fluid of the
nature of the pancreatic secretion—Extra-stomachal
digestion—Calciferous glands, structure of—Calcareous
concretions formed in the anterior pair of glands—The
calcareous matter primarily an excretion, but
secondarily serves to neutralise the acids generated
during the digestive process.
CHAPTER II.
HABITS OF WORMS—_continued_.
Manner in which worms seize objects—Their power of 52–120
suction—The instinct of plugging up the mouths of their
burrows—Stones piled over the burrows—The advantages
thus gained—Intelligence shown by worms in their manner
of plugging up their burrows—Various kinds of leaves
and other objects thus used—Triangles of paper—Summary
of reasons for believing that worms exhibit some
intelligence—Means by which they excavate their
burrows, by pushing away the earth and swallowing
it—Earth also swallowed for the nutritious matter which
it contains—Depth to which worms burrow, and the
construction of their burrows—Burrows lined with
castings, and in the upper part with leaves—The lowest
part paved with little stones or seeds—Manner in which
the castings are ejected—The collapse of old
burrows—Distribution of worms—Tower-like castings in
Bengal—Gigantic castings on the Nilgiri
Mountains—Castings ejected in all countries.
CHAPTER III.
THE AMOUNT OF FINE EARTH BROUGHT UP BY WORMS TO THE SURFACE.
Rate at which various objects strewed on the surface of 121–163
grass-fields are covered up by the castings of
worms—The burial of a paved path—The slow subsidence of
great stones left on the surface—The number of worms
which live within a given space—The weight of earth
ejected from a burrow, and from all the burrows within
a given space—The thickness of the layer of mould which
the castings on a given space would form within a given
time if uniformly spread out—The slow rate at which
mould can increase to a great thickness—Conclusion.
CHAPTER IV.
THE PART WHICH WORMS HAVE PLAYED IN THE BURIAL OF ANCIENT BUILDINGS.
The accumulation of rubbish on the sites of great 164–208
cities independent of the action of worms—The burial of
a Roman villa at Abinger—The floors and walls
penetrated by worms—Subsidence of a modern pavement—The
buried pavement at Beaulieu Abbey—Roman villas at
Chedworth and Brading—The remains of the Roman town at
Silchester—The nature of the débris by which the
remains are covered—The penetration of the tesselated
floors and walls by worms—Subsidence of the
floors—Thickness of the mould—The old Roman city of
Wroxeter—Thickness of the mould—Depth of the
foundations of some of the Buildings—Conclusion.
CHAPTER V.
THE ACTION OF WORMS IN THE DENUDATION OF THE LAND.
Evidence of the amount of denudation which the land has 209–236
undergone—Sub-aerial denudation—The deposition of
dust—Vegetable mould, its dark colour and fine texture
largely due to the action of worms—The disintegration
of rocks by the humus-acids—Similar acids apparently
generated within the bodies of worms—The action of
these acids facilitated by the continued movement of
the particles of earth—A thick bed of mould checks the
disintegration of the underlying soil and rocks.
Particles of stone worn or triturated in the gizzards
of worms—Swallowed stones serve as mill-stones—The
levigated state of the castings—Fragments of brick in
the castings over ancient buildings well rounded. The
triturating power of worms not quite insignificant
under a geological point of view.
CHAPTER VI.
THE DENUDATION OF THE LAND—_continued_.
Denudation aided by recently ejected castings flowing 237–279
down inclined grass-covered surfaces—The amount of
earth which annually flows downwards—The effect of
tropical rain on worm castings—The finest particles of
earth washed completely away from castings—The
disintegration of dried castings into pellets, and
their rolling down inclined surfaces—The formation of
little ledges on hill-sides, in part due to the
accumulation of disintegrated castings—Castings blown
to leeward over level land—An attempt to estimate the
amount thus blown—The degradation of ancient
encampments and tumuli—The preservation of the crowns
and furrows on land anciently ploughed—The formation
and amount of mould over the Chalk formation.
CHAPTER VII.
CONCLUSION.
Summary of the part which worms have played in the 280–288
history of the world—Their aid in the disintegration of
rocks—In the denudation of the land—In the preservation
of ancient remains—In the preparation of the soil for
the growth of plants—Mental powers of worms—Conclusion.
INTRODUCTION.
THE share which worms have taken in the formation of the layer of
vegetable mould, which covers the whole surface of the land in every
moderately humid country, is the subject of the present volume. This
mould is generally of a blackish colour and a few inches in thickness.
In different districts it differs but little in appearance, although it
may rest on various subsoils. The uniform fineness of the particles of
which it is composed is one of its chief characteristic features; and
this may be well observed in any gravelly country, where a
recently-ploughed field immediately adjoins one which has long remained
undisturbed for pasture, and where the vegetable mould is exposed on the
sides of a ditch or hole. The subject may appear an insignificant one,
but we shall see that it possesses some interest; and the maxim “de
minimis non curat lex,” does not apply to science. Even Élie de
Beaumont, who generally undervalues small agencies and their accumulated
effects, remarks: {2} “La couche très-mince de la terre végétale est un
monument d’une haute antiquité, et, par le fait de sa permanence, un
objet digne d’occuper le géologue, et capable de lui fournir des
remarques intéressantes.” Although the superficial layer of vegetable
mould as a whole no doubt is of the highest antiquity, yet in regard to
its permanence, we shall hereafter see reason to believe that its
component particles are in most cases removed at not a very slow rate,
and are replaced by others due to the disintegration of the underlying
materials.
As I was led to keep in my study during many months worms in pots filled
with earth, I became interested in them, and wished to learn how far they
acted consciously, and how much mental power they displayed. I was the
more desirous to learn something on this head, as few observations of
this kind have been made, as far as I know, on animals so low in the
scale of organization and so poorly provided with sense-organs, as are
earth-worms.
In the year 1837, a short paper was read by me before the Geological
Society of London, {3} “On the Formation of Mould,” in which it was shown
that small fragments of burnt marl, cinders, &c., which had been thickly
strewed over the surface of several meadows, were found after a few years
lying at the depth of some inches beneath the turf, but still forming a
layer. This apparent sinking of superficial bodies is due, as was first
suggested to me by Mr. Wedgwood of Maer Hall in Staffordshire, to the
large quantity of fine earth continually brought up to the surface by
worms in the form of castings. These castings are sooner or later spread
out and cover up any object left on the surface. I was thus led to
conclude that all the vegetable mould over the whole country has passed
many times through, and will again pass many times through, the
intestinal canals of worms. Hence the term “animal mould” would be in
some respects more appropriate than that commonly used of “vegetable
mould.”
Ten years after the publication of my paper, M. D’Archiac, evidently
influenced by the doctrines of Élie de Beaumont, wrote about my
“singulière théorie,” and objected that it could apply only to “les
prairies basses et humides;” and that “les terres labourées, les bois,
les prairies élevées, n’apportent aucune preuve à l’appui de cette
manière de voir.” {4a} But M. D’Archiac must have thus argued from inner
consciousness and not from observation, for worms abound to an
extraordinary degree in kitchen gardens where the soil is continually
worked, though in such loose soil they generally deposit their castings
in any open cavities or within their old burrows instead of on the
surface. Hensen estimates that there are about twice as many worms in
gardens as in corn-fields. {4b} With respect to “prairies élevées,” I do
not know how it may be in France, but nowhere in England have I seen the
ground so thickly covered with castings as on commons, at a height of
several hundred feet above the sea. In woods again, if the loose leaves
in autumn are removed, the whole surface will be found strewed with
castings. Dr. King, the superintendent of the Botanic Garden in
Calcutta, to whose kindness I am indebted for many observations on
earth-worms, informs me that he found, near Nancy in France, the bottom
of the State forests covered over many acres with a spongy layer,
composed of dead leaves and innumerable worm-castings. He there heard
the Professor of “Aménagement des Forêts” lecturing to his pupils, and
pointing out this case as a “beautiful example of the natural cultivation
of the soil; for year after year the thrown-up castings cover the dead
leaves; the result being a rich humus of great thickness.”
In the year 1869, Mr. Fish {5} rejected my conclusions with respect to
the part which worms have played in the formation of vegetable mould,
merely on account of their assumed incapacity to do so much work. He
remarks that “considering their weakness and their size, the work they
are represented to have accomplished is stupendous.” Here we have an
instance of that inability to sum up the effects of a continually
recurrent cause, which has often retarded the progress of science, as
formerly in the case of geology, and more recently in that of the
principle of evolution.
Although these several objections seemed to me to have no weight, yet I
resolved to make more observations of the same kind as those published,
and to attack the problem on another side; namely, to weigh all the
castings thrown up within a given time in a measured space, instead of
ascertaining the rate at which objects left on the surface were buried by
worms. But some of my observations have been rendered almost superfluous
by an admirable paper by Hensen, already alluded to, which appeared in
1877. {6} Before entering on details with respect to the castings, it
will be advisable to give some account of the habits of worms from my own
observations and from those of other naturalists.
[FIRST EDITION,
_October_ 10_th_, 1881.]
CHAPTER I.
HABITS OF WORMS.
Nature of the sites inhabited—Can live long under water—Nocturnal—Wander
about at night—Often lie close to the mouths of their burrows, and are
thus destroyed in large numbers by birds—Structure—Do not possess eyes,
but can distinguish between light and darkness—Retreat rapidly when
brightly illuminated, not by a reflex action—Power of attention—Sensitive
to heat and cold—Completely deaf—Sensitive to vibrations and to
touch—Feeble power of smell—Taste—Mental qualities—Nature of
food—Omnivorous—Digestion—Leaves before being swallowed, moistened with a
fluid of the nature of the pancreatic secretion—Extra-stomachal
digestion—Calciferous glands, structure of—Calcareous concretions formed
in the anterior pair of glands—The calcareous matter primarily an
excretion, but secondarily serves to neutralise the acids generated
during the digestive process.
EARTH-WORMS are distributed throughout the world under the form of a few
genera, which externally are closely similar to one another. The British
species of Lumbricus have never been carefully monographed; but we may
judge of their probable number from those inhabiting neighbouring
countries. In Scandinavia there are eight species, according to Eisen;
{8a} but two of these rarely burrow in the ground, and one inhabits very
wet places or even lives under the water. We are here concerned only
with the kinds which bring up earth to the surface in the form of
castings. Hoffmeister says that the species in Germany are not well
known, but gives the same number as Eisen, together with some strongly
marked varieties. {8b}
Earth-worms abound in England in many different stations. Their castings
may be seen in extraordinary numbers on commons and chalk-downs, so as
almost to cover the whole surface, where the soil is poor and the grass
short and thin. But they are almost or quite as numerous in some of the
London parks, where the grass grows well and the soil appears rich. Even
on the same field worms are much more frequent in some places than in
others, without any visible difference in the nature of the soil. They
abound in paved court-yards close to houses; and an instance will be
given in which they had burrowed through the floor of a very damp cellar.
I have seen worms in black peat in a boggy field; but they are extremely
rare, or quite absent in the drier, brown, fibrous peat, which is so much
valued by gardeners. On dry, sandy or gravelly tracks, where heath with
some gorse, ferns, coarse grass, moss and lichens alone grow, hardly any
worms can be found. But in many parts of England, wherever a path
crosses a heath, its surface becomes covered with a fine short sward.
Whether this change of vegetation is due to the taller plants being
killed by the occasional trampling of man and animals, or to the soil
being occasionally manured by the droppings from animals, I do not know.
{9b} On such grassy paths worm-castings may often be seen. On a heath
in Surrey, which was carefully examined, there were only a few castings
on these paths, where they were much inclined; but on the more level
parts, where a bed of fine earth had been washed down from the steeper
parts and had accumulated to a thickness of a few inches, worm-castings
abounded. These spots seemed to be overstocked with worms, so that they
had been compelled to spread to a distance of a few feet from the grassy
paths, and here their castings had been thrown up among the heath; but
beyond this limit, not a single casting could be found. A layer, though
a thin one, of fine earth, which probably long retains some moisture, is
in all cases, as I believe, necessary for their existence; and the mere
compression of the soil appears to be in some degree favourable to them,
for they often abound in old gravel walks, and in foot-paths across
fields.
Beneath large trees few castings can be found during certain seasons of
the year, and this is apparently due to the moisture having been sucked
out of the ground by the innumerable roots of the trees; for such places
may be seen covered with castings after the heavy autumnal rains.
Although most coppices and woods support many worms, yet in a forest of
tall and ancient beech-trees in Knole Park, where the ground beneath was
bare of all vegetation, not a single casting could be found over wide
spaces, even during the autumn. Nevertheless, castings were abundant on
some grass-covered glades and indentations which penetrated this forest.
On the mountains of North Wales and on the Alps, worms, as I have been
informed, are in most places rare; and this may perhaps be due to the
close proximity of the subjacent rocks, into which worms cannot burrow
during the winter so as to escape being frozen. Dr. McIntosh, however,
found worm-castings at a height of 1500 feet on Schiehallion in Scotland.
They are numerous on some hills near Turin at from 2000 to 3000 feet
above the sea, and at a great altitude on the Nilgiri Mountains in South
India and on the Himalaya.
Earth-worms must be considered as terrestrial animals, though they are
still in one sense semi-aquatic, like the other members of the great
class of annelids to which they belong. M. Perrier found that their
exposure to the dry air of a room for only a single night was fatal to
them. On the other hand he kept several large worms alive for nearly
four months, completely submerged in water. {11} During the summer when
the ground is dry, they penetrate to a considerable depth and cease to
work, as they do during the winter when the ground is frozen. Worms are
nocturnal in their habits, and at night may be seen crawling about in
large numbers, but usually with their tails still inserted in their
burrows. By the expansion of this part of their bodies, and with the
help of the short, slightly reflexed bristles, with which their bodies
are armed, they hold so fast that they can seldom be dragged out of the
ground without being torn into pieces. {12} During the day they remain
in their burrows, except at the pairing season, when those which inhabit
adjoining burrows expose the greater part of their bodies for an hour or
two in the early morning. Sick individuals, which are generally affected
by the parasitic larvæ of a fly, must also be excepted, as they wander
about during the day and die on the surface. After heavy rain succeeding
dry weather, an astonishing number of dead worms may sometimes be seen
lying on the ground. Mr. Galton informs me that on one such occasion
(March, 1881), the dead worms averaged one for every two and a half paces
in length on a walk in Hyde Park, four paces in width. He counted no
less than 45 dead worms in one place in a length of sixteen paces. From
the facts above given, it is not probable that these worms could have
been drowned, and if they had been drowned they would have perished in
their burrows. I believe that they were already sick, and that their
deaths were merely hastened by the ground being flooded.
It has often been said that under ordinary circumstances healthy worms
never, or very rarely, completely leave their burrows at night; but this
is an error, as White of Selborne long ago knew. In the morning, after
there has been heavy rain, the film of mud or of very fine sand over
gravel-walks is often plainly marked with their tracks. I have noticed
this from August to May, both months included, and it probably occurs
during the two remaining months of the year when they are wet. On these
occasions, very few dead worms could anywhere be seen. On January 31,
1881, after a long-continued and unusually severe frost with much snow,
as soon as a thaw set in, the walks were marked with innumerable tracks.
On one occasion, five tracks were counted crossing a space of only an
inch square. They could sometimes be traced either to or from the mouths
of the burrows in the gravel-walks, for distances between 2 or 3 up to 15
yards. I have never seen two tracks leading to the same burrow; nor is
it likely, from what we shall presently see of their sense-organs, that a
worm could find its way back to its burrow after having once left it.
They apparently leave their burrows on a voyage of discovery, and thus
they find new sites to inhabit.
Morren states {14} that worms often lie for hours almost motionless close
beneath the mouths of their burrows. I have occasionally noticed the
same fact with worms kept in pots in the house; so that by looking down
into their burrows, their heads could just be seen. If the ejected earth
or rubbish over the burrows be suddenly removed, the end of the worm’s
body may very often be seen rapidly retreating. This habit of lying near
the surface leads to their destruction to an immense extent. Every
morning during certain seasons of the year, the thrushes and blackbirds
on all the lawns throughout the country draw out of their holes an
astonishing number of worms, and this they could not do, unless they lay
close to the surface. It is not probable that worms behave in this
manner for the sake of breathing fresh air, for we have seen that they
can live for a long time under water. I believe that they lie near the
surface for the sake of warmth, especially in the morning; and we shall
hereafter find that they often coat the mouths of their burrows with
leaves, apparently to prevent their bodies from coming into close contact
with the cold damp earth. It is said that they completely close their
burrows during the winter.
_Structure_.—A few remarks must be made on this subject. The body of a
large worm consists of from 100 to 200 almost cylindrical rings or
segments, each furnished with minute bristles. The muscular system is
well developed. Worms can crawl backwards as well as forwards, and by
the aid of their affixed tails can retreat with extraordinary rapidity
into their burrows. The mouth is situated at the anterior end of the
body, and is provided with a little projection (lobe or lip, as it has
been variously called) which is used for prehension. Internally, behind
the mouth, there is a strong pharynx, shown in the accompanying diagram
(Fig. 1) which is pushed forwards when the animal eats, and this part
corresponds, according to Perrier, with the protrudable trunk or
proboscis of other annelids. The pharynx leads into the œsophagus, on
each side of which in the lower part there are three pairs of large
glands, which secrete a surprising amount of carbonate of lime. These
calciferous glands are highly remarkable, for nothing like them is known
in any other animal. Their use will be discussed when we treat of the
digestive process. In most of the species, the œsophagus is enlarged
into a crop in front of the gizzard. This latter organ is lined with a
smooth thick chitinous membrane, and is surrounded by weak longitudinal,
but powerful transverse muscles. Perrier saw these muscles in energetic
action; and, as he remarks, the trituration of the food must be chiefly
effected by this organ, for worms possess no jaws or teeth of any kind.
Grains of sand and small stones, from the 1/20 to a little more than the
1/10 inch in diameter, may generally be found in their gizzards and
intestines. As it is certain that worms swallow many little stones,
independently of those swallowed while excavating their burrows, it is
probable that they serve, like mill-stones, to triturate their food. The
gizzard opens into the intestine, which runs in a straight course to the
vent at the posterior end of the body. The intestine presents a
remarkable structure, the typhlosolis, or, as the old anatomists called
it, an intestine within an intestine; and Claparède {17} has shown that
this consists of a deep longitudinal involution of the walls of the
intestine, by which means an extensive absorbent surface is gained.
[Picture: Fig. 1: Diagram of the alimentary canal of an earth-worm. Fig.
2: Tower-like casting from near Nice]
The circulatory system is well developed. Worms breathe by their skin,
as they do not possess any special respiratory organs. The two sexes are
united in the same individual, but two individuals pair together. The
nervous system is fairly well developed; and the two almost confluent
cerebral ganglia are situated very near to the anterior end of the body.
_Senses_.—Worms are destitute of eyes, and at first I thought that they
were quite insensible to light; for those kept in confinement were
repeatedly observed by the aid of a candle, and others out of doors by
the aid of a lantern, yet they were rarely alarmed, although extremely
timid animals. Other persons have found no difficulty in observing worms
at night by the same means. {18a}
Hoffmeister, however, states {18b} that worms, with the exception of a
few individuals, are extremely sensitive to light; but he admits that in
most cases a certain time is requisite for its action. These statements
led me to watch on many successive nights worms kept in pots, which were
protected from currents of air by means of glass plates. The pots were
approached very gently, in order that no vibration of the floor should be
caused. When under these circumstances worms were illuminated by a
bull’s-eye lantern having slides of dark red and blue glass, which
intercepted so much light that they could be seen only with some
difficulty, they were not at all affected by this amount of light,
however long they were exposed to it. The light, as far as I could
judge, was brighter than that from the full moon. Its colour apparently
made no difference in the result. When they were illuminated by a
candle, or even by a bright paraffin lamp, they were not usually affected
at first. Nor were they when the light was alternately admitted and shut
off. Sometimes, however, they behaved very differently, for as soon as
the light fell on them, they withdrew into their burrows with almost
instantaneous rapidity. This occurred perhaps once out of a dozen times.
When they did not withdraw instantly, they often raised the anterior
tapering ends of their bodies from the ground, as if their attention was
aroused or as if surprise was felt; or they moved their bodies from side
to side as if feeling for some object. They appeared distressed by the
light; but I doubt whether this was really the case, for on two occasions
after withdrawing slowly, they remained for a long time with their
anterior extremities protruding a little from the mouths of their
burrows, in which position they were ready for instant and complete
withdrawal.
When the light from a candle was concentrated by means of a large lens on
the anterior extremity, they generally withdrew instantly; but this
concentrated light failed to act perhaps once out of half a dozen trials.
The light was on one occasion concentrated on a worm lying beneath water
in a saucer, and it instantly withdrew into its burrow. In all cases the
duration of the light, unless extremely feeble, made a great difference
in the result; for worms left exposed before a paraffin lamp or a candle
invariably retreated into their burrows within from five to fifteen
minutes; and if in the evening the pots were illuminated before the worms
had come out of their burrows, they failed to appear.
From the foregoing facts it is evident that light affects worms by its
intensity and by its duration. It is only the anterior extremity of the
body, where the cerebral ganglia lie, which is affected by light, as
Hoffmeister asserts, and as I observed on many occasions. If this part
is shaded, other parts of the body may be fully illuminated, and no
effect will be produced. As these animals have no eyes, we must suppose
that the light passes through their skins, and in some manner excites
their cerebral ganglia. It appeared at first probable that the different
manner in which they were affected on different occasions might be
explained, either by the degree of extension of their skin and its
consequent transparency, or by some particular incident of the light; but
I could discover no such relation. One thing was manifest, namely, that
when worms were employed in dragging leaves into their burrows or in
eating them, and even during the short intervals whilst they rested from
their work, they either did not perceive the light or were regardless of
it; and this occurred even when the light was concentrated on them
through a large lens. So, again, whilst they are paired, they will
remain for an hour or two out of their burrows, fully exposed to the
morning light; but it appears from what Hoffmeister says that a light
will occasionally cause paired individuals to separate.
When a worm is suddenly illuminated and dashes like a rabbit into its
burrow—to use the expression employed by a friend—we are at first led to
look at the action as a reflex one. The irritation of the cerebral
ganglia appears to cause certain muscles to contract in an inevitable
manner, independently of the will or consciousness of the animal, as if
it were an automaton. But the different effect which a light produced on
different occasions, and especially the fact that a worm when in any way
employed and in the intervals of such employment, whatever set of muscles
and ganglia may then have been brought into play, is often regardless of
light, are opposed to the view of the sudden withdrawal being a simple
reflex action. With the higher animals, when close attention to some
object leads to the disregard of the impressions which other objects must
be producing on them, we attribute this to their attention being then
absorbed; and attention implies the presence of a mind. Every sportsman
knows that he can approach animals whilst they are grazing, fighting or
courting, much more easily than at other times. The state, also, of the
nervous system of the higher animals differs much at different times, for
instance, a horse is much more readily startled at one time than at
another. The comparison here implied between the actions of one of the
higher animals and of one so low in the scale as an earth-worm, may
appear far-fetched; for we thus attribute to the worm attention and some
mental power, nevertheless I can see no reason to doubt the justice of
the comparison.
Although worms cannot be said to possess the power of vision, their
sensitiveness to light enables them to distinguish between day and night;
and they thus escape extreme danger from the many diurnal animals which
prey on them. Their withdrawal into their burrows during the day
appears, however, to have become an habitual action; for worms kept in
pots covered by glass plates, over which sheets of black paper were
spread, and placed before a north-east window, remained during the
day-time in their burrows and came out every night; and they continued
thus to act for a week. No doubt a little light may have entered between
the sheets of glass and the blackened paper; but we know from the trials
with coloured glass, that worms are indifferent to a small amount of
light.
Worms appear to be less sensitive to moderate radiant heat than to a
bright light. I judge of this from having held at different times a
poker heated to dull redness near some worms, at a distance which caused
a very sensible degree of warmth in my hand. One of them took no notice;
a second withdrew into its burrow, but not quickly; the third and fourth
much more quickly, and the fifth as quickly as possible. The light from
a candle, concentrated by a lens and passing through a sheet of glass
which would intercept most of the heat-rays, generally caused a much more
rapid retreat than did the heated poker. Worms are sensitive to a low
temperature, as may be inferred from their not coming out of their
burrows during a frost.
Worms do not possess any sense of hearing. They took not the least
notice of the shrill notes from a metal whistle, which was repeatedly
sounded near them; nor did they of the deepest and loudest tones of a
bassoon. They were indifferent to shouts, if care was taken that the
breath did not strike them. When placed on a table close to the keys of
a piano, which was played as loudly as possible, they remained perfectly
quiet.
Although they are indifferent to undulations in the air audible by us,
they are extremely sensitive to vibrations in any solid object. When the
pots containing two worms which had remained quite indifferent to the
sound of the piano, were placed on this instrument, and the note C in the
bass clef was struck, both instantly retreated into their burrows. After
a time they emerged, and when G above the line in the treble clef was
struck they again retreated. Under similar circumstances on another
night one worm dashed into its burrow on a very high note being struck
only once, and the other worm when C in the treble clef was struck. On
these occasions the worms were not touching the sides of the pots, which
stood in saucers; so that the vibrations, before reaching their bodies,
had to pass from the sounding board of the piano, through the saucer, the
bottom of the pot and the damp, not very compact earth on which they lay
with their tails in their burrows. They often showed their sensitiveness
when the pot in which they lived, or the table on which the pot stood,
was accidentally and lightly struck; but they appeared less sensitive to
such jars than to the vibrations of the piano; and their sensitiveness to
jars varied much at different times.
It has often been said that if the ground is beaten or otherwise made to
tremble, worms believe that they are pursued by a mole and leave their
burrows. From one account that I have received, I have no doubt that
this is often the case; but a gentleman informs me that he lately saw
eight or ten worms leave their burrows and crawl about the grass on some
boggy land on which two men had just trampled while setting a trap; and
this occurred in a part of Ireland where there were no moles. I have
been assured by a Volunteer that he has often seen many large earth-worms
crawling quickly about the grass, a few minutes after his company had
fired a volley with blank cartridges. The Peewit (_Tringa vanellus_,
Linn.) seems to know instinctively that worms will emerge if the ground
is made to tremble; for Bishop Stanley states (as I hear from Mr.
Moorhouse) that a young peewit kept in confinement used to stand on one
leg and beat the turf with the other leg until the worms crawled out of
their burrows, when they were instantly devoured. Nevertheless, worms do
not invariably leave their burrows when the ground is made to tremble, as
I know by having beaten it with a spade, but perhaps it was beaten too
violently.
The whole body of a worm is sensitive to contact. A slight puff of air
from the mouth causes an instant retreat. The glass plates placed over
the pots did not fit closely, and blowing through the very narrow chinks
thus left, often sufficed to cause a rapid retreat. They sometimes
perceived the eddies in the air caused by quickly removing the glass
plates. When a worm first comes out of its burrow, it generally moves
the much extended anterior extremity of its body from side to side in all
directions, apparently as an organ of touch; and there is some reason to
believe, as we shall see in the next chapter, that they are thus enabled
to gain a general notion of the form of an object. Of all their senses
that of touch, including in this term the perception of a vibration,
seems much the most highly developed.
In worms the sense of smell apparently is confined to the perception of
certain odours, and is feeble. They were quite indifferent to my breath,
as long as I breathed on them very gently. This was tried, because it
appeared possible that they might thus be warned of the approach of an
enemy. They exhibited the same indifference to my breath whilst I chewed
some tobacco, and while a pellet of cotton-wool with a few drops of
millefleurs perfume or of acetic acid was kept in my mouth. Pellets of
cotton-wool soaked in tobacco juice, in millefleurs perfume, and in
paraffin, were held with pincers and were waved about within two or three
inches of several worms, but they took no notice. On one or two
occasions, however, when acetic acid had been placed on the pellets, the
worms appeared a little uneasy, and this was probably due to the
irritation of their skins. The perception of such unnatural odours would
be of no service to worms; and as such timid creatures would almost
certainly exhibit some signs of any new impression, we may conclude that
they did not perceive these odours.
The result was different when cabbage-leaves and pieces of onion were
employed, both of which are devoured with much relish by worms. Small
square pieces of fresh and half-decayed cabbage-leaves and of onion bulbs
were on nine occasions buried in my pots, beneath about ¼ of an inch of
common garden soil; and they were always discovered by the worms. One
bit of cabbage was discovered and removed in the course of two hours;
three were removed by the next morning, that is, after a single night;
two others after two nights; and the seventh bit after three nights. Two
pieces of onion were discovered and removed after three nights. Bits of
fresh raw meat, of which worms are very fond, were buried, and were not
discovered within forty-eight hours, during which time they had not
become putrid. The earth above the various buried objects was generally
pressed down only slightly, so as not to prevent the emission of any
odour. On two occasions, however, the surface was well watered, and was
thus rendered somewhat compact. After the bits of cabbage and onion had
been removed, I looked beneath them to see whether the worms had
accidentally come up from below, but there was no sign of a burrow; and
twice the buried objects were laid on pieces of tin-foil which were not
in the least displaced. It is of course possible that the worms whilst
moving about on the surface of the ground, with their tails affixed
within their burrows, may have poked their heads into the places where
the above objects were buried; but I have never seen worms acting in this
manner. Some pieces of cabbage-leaf and of onion were twice buried
beneath very fine ferruginous sand, which was slightly pressed down and
well watered, so as to be rendered very compact, and these pieces were
never discovered. On a third occasion the same kind of sand was neither
pressed down nor watered, and the pieces of cabbage were discovered and
removed after the second night. These several facts indicate that worms
possess some power of smell; and that they discover by this means
odoriferous and much-coveted kinds of food.
It may be presumed that all animals which feed on various substances
possess the sense of taste, and this is certainly the case with worms.
Cabbage-leaves are much liked by worms; and it appears that they can
distinguish between different varieties; but this may perhaps be owing to
differences in their texture. On eleven occasions pieces of the fresh
leaves of a common green variety and of the red variety used for pickling
were given them, and they preferred the green, the red being either
wholly neglected or much less gnawed. On two other occasions, however,
they seemed to prefer the red. Half-decayed leaves of the red variety
and fresh leaves of the green were attacked about equally. When leaves
of the cabbage, horse-radish (a favourite food) and of the onion were
given together, the latter were always, and manifestly preferred. Leaves
of the cabbage, lime-tree, Ampelopsis, parsnip (Pastinaca), and celery
(Apium) were likewise given together; and those of the celery were first
eaten. But when leaves of cabbage, turnip, beet, celery, wild cherry and
carrots were given together, the two latter kinds, especially those of
the carrot, were preferred to all the others, including those of celery.
It was also manifest after many trials that wild cherry leaves were
greatly preferred to those of the lime-tree and hazel (Corylus).
According to Mr. Bridgman the half-decayed leaves of _Phlox verna_ are
particularly liked by worms. {31}
Pieces of the leaves of cabbage, turnip, horse-radish and onion were left
on the pots during 22 days, and were all attacked and had to be renewed;
but during the whole of this time leaves of an Artemisia and of the
culinary sage, thyme and mint, mingled with the above leaves, were quite
neglected excepting those of the mint, which were occasionally and very
slightly nibbled. These latter four kinds of leaves do not differ in
texture in a manner which could make them disagreeable to worms; they all
have a strong taste, but so have the four first mentioned kinds of
leaves; and the wide difference in the result must be attributed to a
preference by the worms for one taste over another.
_Mental Qualities_.—There is little to be said on this head. We have
seen that worms are timid. It may be doubted whether they suffer as much
pain when injured, as they seem to express by their contortions. Judging
by their eagerness for certain kinds of food, they must enjoy the
pleasure of eating. Their sexual passion is strong enough to overcome
for a time their dread of light. They perhaps have a trace of social
feeling, for they are not disturbed by crawling over each other’s bodies,
and they sometimes lie in contact. According to Hoffmeister they pass
the winter either singly or rolled up with others into a ball at the
bottom of their burrows. {32} Although worms are so remarkably deficient
in the several sense-organs, this does not necessarily preclude
intelligence, as we know from such cases as those of Laura Bridgman; and
we have seen that when their attention is engaged, they neglect
impressions to which they would otherwise have attended; and attention
indicates the presence of a mind of some kind. They are also much more
easily excited at certain times than at others. They perform a few
actions instinctively, that is, all the individuals, including the young,
perform such actions in nearly the same fashion. This is shown by the
manner in which the species of Perichæta eject their castings, so as to
construct towers; also by the manner in which the burrows of the common
earth-worm are smoothly lined with fine earth and often with little
stones, and the mouths of their burrows with leaves. One of their
strongest instincts is the plugging up the mouths of their burrows with
various objects; and very young worms act in this manner. But some
degree of intelligence appears, as we shall see in the next chapter, to
be exhibited in this work,—a result which has surprised me more than
anything else in regard to worms.
_Food and Digestion_.—Worms are omnivorous. They swallow an enormous
quantity of earth, out of which they extract any digestible matter which
it may contain; but to this subject I must recur. They also consume a
large number of half-decayed leaves of all kinds, excepting a few which
have an unpleasant taste or are too tough for them; likewise petioles,
peduncles, and decayed flowers. But they will also consume fresh leaves,
as I have found by repeated trials. According to Morren {33} they will
eat particles of sugar and liquorice; and the worms which I kept drew
many bits of dry starch into their burrows, and a large bit had its
angles well rounded by the fluid poured out of their mouths. But as they
often drag particles of soft stone, such as of chalk, into their burrows,
I feel some doubt whether the starch was used as food. Pieces of raw and
roasted meat were fixed several times by long pins to the surface of the
soil in my pots, and night after night the worms could be seen tugging at
them, with the edges of the pieces engulfed in their mouths, so that much
was consumed. Raw fat seems to be preferred even to raw meat or to any
other substance which was given them, and much was consumed. They are
cannibals, for the two halves of a dead worm placed in two of the pots
were dragged into the burrows and gnawed; but as far as I could judge,
they prefer fresh to putrid meat, and in so far I differ from
Hoffmeister.
Léon Fredericq states {34} that the digestive fluid of worms is of the
same nature as the pancreatic secretion of the higher animals; and this
conclusion agrees perfectly with the kinds of food which worms consume.
Pancreatic juice emulsifies fat, and we have just seen how greedily worms
devour fat; it dissolves fibrin, and worms eat raw meat; it converts
starch into grape-sugar with wonderful rapidity, and we shall presently
show that the digestive fluid of worms acts on starch. {35a} But they
live chiefly on half-decayed leaves; and these would be useless to them
unless they could digest the cellulose forming the cell-walls; for it is
well known that all other nutritious substances are almost completely
withdrawn from leaves, shortly before they fall off. It has, however,
now been ascertained that some forms of cellulose, though very little or
not at all attacked by the gastric secretion of the higher animals, are
acted on by that from the pancreas. {35b}
The half-decayed or fresh leaves which worms intend to devour, are
dragged into the mouths of their burrows to a depth of from one to three
inches, and are then moistened with a secreted fluid. It has been
assumed that this fluid serves to hasten their decay; but a large number
of leaves were twice pulled out of the burrows of worms and kept for many
weeks in a very moist atmosphere under a bell-glass in my study; and the
parts which had been moistened by the worms did not decay more quickly in
any plain manner than the other parts. When fresh leaves were given in
the evening to worms kept in confinement and examined early on the next
morning, therefore not many hours after they had been dragged into the
burrows, the fluid with which they were moistened, when tested with
neutral litmus paper, showed an alkaline reaction. This was repeatedly
found to be the case with celery, cabbage and turnip leaves. Parts of
the same leaves which had not been moistened by the worms, were pounded
with a few drops of distilled water, and the juice thus extracted was not
alkaline. Some leaves, however, which had been drawn into burrows out of
doors, at an unknown antecedent period, were tried, and though still
moist, they rarely exhibited even a trace of alkaline reaction.
The fluid, with which the leaves are bathed, acts on them whilst they are
fresh or nearly fresh, in a remarkable manner; for it quickly kills and
discolours them. Thus the ends of a fresh carrot-leaf, which had been
dragged into a burrow, were found after twelve hours of a dark brown
tint. Leaves of celery, turnip, maple, elm, lime, thin leaves of ivy,
and, occasionally those of the cabbage were similarly acted on. The end
of a leaf of _Triticum repens_, still attached to a growing plant, had
been drawn into a burrow, and this part was dark brown and dead, whilst
the rest of the leaf was fresh and green. Several leaves of lime and elm
removed from burrows out of doors were found affected in different
degrees. The first change appears to be that the veins become of a dull
reddish-orange. The cells with chlorophyll next lose more or less
completely their green colour, and their contents finally become brown.
The parts thus affected often appeared almost black by reflected light;
but when viewed as a transparent object under the microscope, minute
specks of light were transmitted, and this was not the case with the
unaffected parts of the same leaves. These effects, however, merely show
that the secreted fluid is highly injurious or poisonous to leaves; for
nearly the same effects were produced in from one to two days on various
kinds of young leaves, not only by artificial pancreatic fluid, prepared
with or without thymol, but quickly by a solution of thymol by itself.
On one occasion leaves of Corylus were much discoloured by being kept for
eighteen hours in pancreatic fluid, without any thymol. With young and
tender leaves immersion in human saliva during rather warm weather, acted
in the same manner as the pancreatic fluid, but not so quickly. The
leaves in all these cases often became infiltrated with the fluid.
Large leaves from an ivy plant growing on a wall were so tough that they
could not be gnawed by worms, but after four days they were affected in a
peculiar manner by the secretion poured out of their mouths. The upper
surfaces of the leaves, over which the worms had crawled, as was shown by
the dirt left on them, were marked in sinuous lines, by either a
continuous or broken chain of whitish and often star-shaped dots, about 2
mm. in diameter. The appearance thus presented was curiously like that
of a leaf, into which the larva of some minute insect had burrowed. But
my son Francis, after making and examining sections, could nowhere find
that the cell-walls had been broken down or that the epidermis had been
penetrated. When the section passed through the whitish dots, the grains
of chlorophyll were seen to be more or less discoloured, and some of the
palisade and mesophyll cells contained nothing but broken down granular
matter. These effects must be attributed to the transudation of the
secretion through the epidermis into the cells.
The secretion with which worms moisten leaves likewise acts on the
starch-granules within the cells. My son examined some leaves of the ash
and many of the lime, which had fallen off the trees and had been partly
dragged into worm-burrows. It is known that with fallen leaves the
starch-grains are preserved in the guard-cells of the stomata. Now in
several cases the starch had partially or wholly disappeared from these
cells, in the parts which had been moistened by the secretion; while it
was still well preserved in the other parts of the same leaves.
Sometimes the starch was dissolved out of only one of the two
guard-cells. The nucleus in one case had disappeared, together with the
starch-granules. The mere burying of lime-leaves in damp earth for nine
days did not cause the destruction of the starch-granules. On the other
hand, the immersion of fresh lime and cherry leaves for eighteen hours in
artificial pancreatic fluid, led to the dissolution of the
starch-granules in the guard-cells as well as in the other cells.
From the secretion with which the leaves are moistened being alkaline,
and from its acting both on the starch-granules and on the protoplasmic
contents of the cells, we may infer that it resembles in nature not
saliva, {40} but pancreatic secretion; and we know from Fredericq that a
secretion of this kind is found in the intestines of worms. As the
leaves which are dragged into the burrows are often dry and shrivelled,
it is indispensable for their disintegration by the unarmed mouths of
worms that they should first be moistened and softened; and fresh leaves,
however soft and tender they may be, are similarly treated, probably from
habit. The result is that they are partially digested before they are
taken into the alimentary canal. I am not aware of any other case of
extra-stomachal digestion having been recorded. The boa-constrictor is
said to bathe its prey with saliva, but this is doubtful; and it is done
solely for the sake of lubricating its prey. Perhaps the nearest analogy
may be found in such plants as Drosera and Dionæa; for here animal matter
is digested and converted into peptone not within a stomach, but on the
surfaces of the leaves.
_Calciferous Glands_.—These glands (see Fig. 1), judging from their size
and from their rich supply of blood-vessels, must be of much importance
to the animal. But almost as many theories have been advanced on their
use as there have been observers. They consist of three pairs, which in
the common earth-worm debouch into the alimentary canal in advance of the
gizzard, but posteriorly to it in Urochæta and some other genera. {41a}
The two posterior pairs are formed by lamellæ, which, according to
Claparède, are diverticula from the œsophagus. {41b} These lamellæ are
coated with a pulpy cellular layer, with the outer cells lying free in
infinite numbers. If one of these glands is punctured and squeezed, a
quantity of white pulpy matter exudes, consisting of these free cells.
They are minute, and vary in diameter from 2 to 6 _μ_. They contain in
their centres a little excessively fine granular matter; but they look so
like oil globules that Claparède and others at first treated them with
ether. This produces no effect; but they are quickly dissolved with
effervescence in acetic acid, and when oxalate of ammonia is added to the
solution a white precipitate is thrown down. We may therefore conclude
that they contain carbonate of lime. If the cells are immersed in a very
little acid, they become more transparent, look like ghosts, and are soon
lost to view; but if much acid is added, they disappear instantly. After
a very large number have been dissolved, a flocculent residue is left,
which apparently consists of the delicate ruptured cell-walls. In the
two posterior pairs of glands the carbonate of lime contained in the
cells occasionally aggregates into small rhombic crystals or into
concretions, which lie between the lamellæ; but I have seen only one
case, and Claparède only a very few such cases.
The two anterior glands differ a little in shape from the four posterior
ones, by being more oval. They differ also conspicuously in generally
containing several small, or two or three larger, or a single very large
concretion of carbonate of lime, as much as 1½ mm. in diameter. When a
gland includes only a few very small concretions, or, as sometimes
happens, none at all, it is easily overlooked. The large concretions are
round or oval, and exteriorly almost smooth. One was found which filled
up not only the whole gland, as is often the case, but its neck; so that
it resembled an olive-oil flask in shape. These concretions when broken
are seen to be more or less crystalline in structure. How they escape
from the gland is a marvel; but that they do escape is certain, for they
are often found in the gizzard, intestines, and in the castings of worms,
both with those kept in confinement and those in a state of nature.
Claparède says very little about the structure of the two anterior
glands, and he supposes that the calcareous matter of which the
concretions are formed is derived from the four posterior glands. But if
an anterior gland which contains only small concretions is placed in
acetic acid and afterwards dissected, or if sections are made of such a
gland without being treated with acid, lamellæ like those in the
posterior glands and coated with cellular matter could be plainly seen,
together with a multitude of free calciferous cells readily soluble in
acetic acid. When a gland is completely filled with a single large
concretion, there are no free cells, as these have been all consumed in
forming the concretion. But if such a concretion, or one of only
moderately large size, is dissolved in acid, much membranous matter is
left, which appears to consist of the remains of the formerly active
lamellæ. After the formation and expulsion of a large concretion, new
lamellæ must be developed in some manner. In one section made by my son,
the process had apparently commenced, although the gland contained two
rather large concretions, for near the walls several cylindrical and oval
pipes were intersected, which were lined with cellular matter and were
quite filled with free calciferous cells. A great enlargement in one
direction of several oval pipes would give rise to the lamellæ.
Besides the free calciferous cells in which no nucleus was visible, other
and rather larger free cells were seen on three occasions; and these
contained a distinct nucleus and nucleolus. They were only so far acted
on by acetic acid that the nucleus was thus rendered more distinct. A
very small concretion was removed from between two of the lamellæ within
an anterior gland. It was imbedded in pulpy cellular matter, with many
free calciferous cells, together with a multitude of the larger, free,
nucleated cells, and these latter cells were not acted on by acetic acid,
while the former were dissolved. From this and other such cases I am led
to suspect that the calciferous cells are developed from the larger
nucleated ones; but how this was effected was not ascertained.
When an anterior gland contains several minute concretions, some of these
are generally angular or crystalline in outline, while the greater number
are rounded with an irregular mulberry-like surface. Calciferous cells
adhered to many parts of these mulberry-like masses, and their gradual
disappearance could be traced while they still remained attached. It was
thus evident that the concretions are formed from the lime contained
within the free calciferous cells. As the smaller concretions increase
in size, they come into contact and unite, thus enclosing the now
functionless lamellæ; and by such steps the formation of the largest
concretions could be followed. Why the process regularly takes place in
the two anterior glands, and only rarely in the four posterior glands, is
quite unknown. Morren says that these glands disappear during the
winter; and I have seen some instances of this fact, and others in which
either the anterior or posterior glands were at this season so shrunk and
empty, that they could be distinguished only with much difficulty.
With respect to the function of the calciferous glands, it is probable
that they primarily serve as organs of excretion, and secondarily as an
aid to digestion. Worms consume many fallen leaves; and it is known that
lime goes on accumulating in leaves until they drop off the parent-plant,
instead of being re-absorbed into the stem or roots, like various other
organic and inorganic substances. {46} The ashes of a leaf of an acacia
have been known to contain as much as 72 per cent. of lime. Worms
therefore would be liable to become charged with this earth, unless there
were some special means for its excretion; and the calciferous glands are
well adapted for this purpose. The worms which live in mould close over
the chalk, often have their intestines filled with this substance, and
their castings are almost white. Here it is evident that the supply of
calcareous matter must be super-abundant. Nevertheless with several
worms collected on such a site, the calciferous glands contained as many
free calciferous cells, and fully as many and large concretions, as did
the glands of worms which lived where there was little or no lime; and
this indicates that the lime is an excretion, and not a secretion poured
into the alimentary canal for some special purpose.
On the other hand, the following considerations render it highly probable
that the carbonate of lime, which is excreted by the glands, aids the
digestive process under ordinary circumstances. Leaves during their
decay generate an abundance of various kinds of acids, which have been
grouped together under the term of humus acids. We shall have to recur
to this subject in our fifth chapter, and I need here only say that these
acids act strongly on carbonate of lime. The half-decayed leaves which
are swallowed in such large quantities by worms would, therefore, after
they have been moistened and triturated in the alimentary canal, be apt
to produce such acids. And in the case of several worms, the contents of
the alimentary canal were found to be plainly acid, as shown by litmus
paper. This acidity cannot be attributed to the nature of the digestive
fluid, for pancreatic fluid is alkaline; and we have seen that the
secretion which is poured out of the mouths of worms for the sake of
preparing the leaves for consumption, is likewise alkaline. The acidity
can hardly be due to uric acid, as the contents of the upper part of the
intestine were often acid. In one case the contents of the gizzard were
slightly acid, those of the upper intestines being more plainly acid. In
another case the contents of the pharynx were not acid, those of the
gizzard doubtfully so, while those of the intestine were distinctly acid
at a distance of 5 cm. below the gizzard. Even with the higher
herbivorous and omnivorous animals, the contents of the large intestine
are acid. “This, however, is not caused by any acid secretion from the
mucous membrane; the reaction of the intestinal walls in the larger as in
the small intestine is alkaline. It must therefore arise from acid
fermentations going on in the contents themselves . . . In Carnivora the
contents of the coecum are said to be alkaline, and naturally the amount
of fermentation will depend largely on the nature of the food.” {49}
With worms not only the contents of the intestines, but their ejected
matter or the castings, are generally acid. Thirty castings from
different places were tested, and with three or four exceptions were
found to be acid; and the exceptions may have been due to such castings
not having been recently ejected; for some which were at first acid, were
on the following morning, after being dried and again moistened, no
longer acid; and this probably resulted from the humus acids being, as is
known to be the case, easily decomposed. Five fresh castings from worms
which lived in mould close over the chalk, were of a whitish colour and
abounded with calcareous matter; and these were not in the least acid.
This shows how effectually carbonate of lime neutralises the intestinal
acids. When worms were kept in pots filled with fine ferruginous sand,
it was manifest that the oxide of iron, with which the grains of silex
were coated, had been dissolved and removed from them in the castings.
The digestive fluid of worms resembles in its action, as already stated,
the pancreatic secretion of the higher animals; and in these latter,
“pancreatic digestion is essentially alkaline; the action will not take
place unless some alkali be present; and the activity of an alkaline
juice is arrested by acidification, and hindered by neutralization.” {50}
Therefore it seems highly probable that the innumerable calciferous
cells, which are poured from the four posterior glands into the
alimentary canal of worms, serve to neutralise more or less completely
the acids there generated by the half-decayed leaves. We have seen that
these cells are instantly dissolved by a small quantity of acetic acid,
and as they do not always suffice to neutralise the contents of even the
upper part of the alimentary canal, the lime is perhaps aggregated into
concretions in the anterior pair of glands, in order that some may be
carried down to the posterior parts of the intestine, where these
concretions would be rolled about amongst the acid contents. The
concretions found in the intestines and in the castings often have a worn
appearance, but whether this is due to some amount of attrition or of
chemical corrosion could not be told. Claparède believes that they are
formed for the sake of acting as mill-stones, and of thus aiding in the
trituration of the food. They may give some aid in this way; but I fully
agree with Perrier that this must be of quite subordinate importance,
seeing that the object is already attained by stones being generally
present in the gizzards and intestines of worms.
CHAPTER II.
HABITS OF WORMS—_continued_.
Manner in which worms seize objects—Their power of suction—The instinct
of plugging up the mouths of their burrows—Stones piled over the
burrows—The advantages thus gained—Intelligence shown by worms in their
manner of plugging up their burrows—Various kinds of leaves and other
objects thus used—Triangles of paper—Summary of reasons for believing
that worms exhibit some intelligence—Means by which they excavate their
burrows, by pushing away the earth and swallowing it—Earth also swallowed
for the nutritious matter which it contains—Depth to which worms burrow,
and the construction of their burrows—Burrows lined with castings, and in
the upper part with leaves—The lowest part paved with little stones or
seeds—Manner in which the castings are ejected—The collapse of old
burrows—Distribution of worms—Tower-like castings in Bengal—Gigantic
castings on the Nilgiri Mountains—Castings ejected in all countries.
IN the pots in which worms were kept, leaves were pinned down to the
soil, and at night the manner in which they were seized could be
observed. The worms always endeavoured to drag the leaves towards their
burrows; and they tore or sucked off small fragments, whenever the leaves
were sufficiently tender. They generally seized the thin edge of a leaf
with their mouths, between the projecting upper and lower lip; the thick
and strong pharynx being at the same time, as Perrier remarks, pushed
forward within their bodies, so as to afford a point of resistance for
the upper lip. In the case of broad flat objects they acted in a wholly
different manner. The pointed anterior extremity of the body, after
being brought into contact with an object of this kind, was drawn within
the adjoining rings, so that it appeared truncated and became as thick as
the rest of the body. This part could then be seen to swell a little;
and this, I believe, is due to the pharynx being pushed a little
forwards. Then by a slight withdrawal of the pharynx or by its
expansion, a vacuum was produced beneath the truncated slimy end of the
body whilst in contact with the object; and by this means the two adhered
firmly together. {53} That under these circumstances a vacuum was
produced was plainly seen on one occasion, when a large worm lying
beneath a flaccid cabbage leaf tried to drag it away; for the surface of
the leaf directly over the end of the worm’s body became deeply pitted.
On another occasion a worm suddenly lost its hold on a flat leaf; and the
anterior end of the body was momentarily seen to be cup-formed. Worms
can attach themselves to an object beneath water in the same manner; and
I saw one thus dragging away a submerged slice of an onion-bulb.
The edges of fresh or nearly fresh leaves affixed to the ground were
often nibbled by the worms; and sometimes the epidermis and all the
parenchyma on one side was gnawed completely away over a considerable
space; the epidermis alone on the opposite side being left quite clean.
The veins were never touched, and leaves were thus sometimes partly
converted into skeletons. As worms have no teeth and as their mouths
consist of very soft tissue, it may be presumed that they consume by
means of suction the edges and the parenchyma of fresh leaves, after they
have been softened by the digestive fluid. They cannot attack such
strong leaves as those of sea-kale or large and thick leaves of ivy;
though one of the latter after it had become rotten was reduced in parts
to the state of a skeleton.
Worms seize leaves and other objects, not only to serve as food, but for
plugging up the mouths of their burrows; and this is one of their
strongest instincts. They sometimes work so energetically that Mr. D. F.
Simpson, who has a small walled garden where worms abound in Bayswater,
informs me that on a calm damp evening he there heard so extraordinary a
rustling noise from under a tree from which many leaves had fallen, that
he went out with a light and discovered that the noise was caused by many
worms dragging the dry leaves and squeezing them into the burrows. Not
only leaves, but petioles of many kinds, some flower-peduncles, often
decayed twigs of trees, bits of paper, feathers, tufts of wool and
horse-hairs are dragged into their burrows for this purpose. I have seen
as many as seventeen petioles of a Clematis projecting from the mouth of
one burrow, and ten from the mouth of another. Some of these objects,
such as the petioles just named, feathers, &c., are never gnawed by
worms. In a gravel-walk in my garden I found many hundred leaves of a
pine-tree (_P. austriaca_ or _nigricans_) drawn by their bases into
burrows. The surfaces by which these leaves are articulated to the
branches are shaped in as peculiar a manner as is the joint between the
leg-bones of a quadruped; and if these surfaces had been in the least
gnawed, the fact would have been immediately visible, but there was no
trace of gnawing. Of ordinary dicotyledonous leaves, all those which are
dragged into burrows are not gnawed. I have seen as many as nine leaves
of the lime-tree drawn into the same burrow, and not nearly all of them
had been gnawed; but such leaves may serve as a store for future
consumption. Where fallen leaves are abundant, many more are sometimes
collected over the mouth of a burrow than can be used, so that a small
pile of unused leaves is left like a roof over those which have been
partly dragged in.
A leaf in being dragged a little way into a cylindrical burrow is
necessarily much folded or crumpled. When another leaf is drawn in, this
is done exteriorly to the first one, and so on with the succeeding
leaves; and finally all become closely folded and pressed together.
Sometimes the worm enlarges the mouth of its burrow, or makes a fresh one
close by, so as to draw in a still larger number of leaves. They often
or generally fill up the interstices between the drawn-in leaves with
moist viscid earth ejected from their bodies; and thus the mouths of the
burrows are securely plugged. Hundreds of such plugged burrows may be
seen in many places, especially during the autumnal and early winter
months. But, as will hereafter be shown, leaves are dragged into the
burrows not only for plugging them up and for food, but for the sake of
lining the upper part or mouth.
When worms cannot obtain leaves, petioles, sticks, &c., with which to
plug up the mouths of their burrows, they often protect them by little
heaps of stones; and such heaps of smooth rounded pebbles may frequently
be seen on gravel-walks. Here there can be no question about food. A
lady, who was interested in the habits of worms, removed the little heaps
of stones from the mouths of several burrows and cleared the surface of
the ground for some inches all round. She went out on the following
night with a lantern, and saw the worms with their tails fixed in their
burrows, dragging the stones inwards by the aid of their mouths, no doubt
by suction. “After two nights some of the holes had 8 or 9 small stones
over them; after four nights one had about 30, and another 34 stones.”
{58} One stone—which had been dragged over the gravel-walk to the mouth
of a burrow weighed two ounces; and this proves how strong worms are.
But they show greater strength in sometimes displacing stones in a
well-trodden gravel-walk; that they do so, may be inferred from the
cavities left by the displaced stones being exactly filled by those lying
over the mouths of adjoining burrows, as I have myself observed.
Work of this kind is usually performed during the night; but I have
occasionally known objects to be drawn into the burrows during the day.
What advantage the worms derive from plugging up the mouths of their
burrows with leaves, &c., or from piling stones over them, is doubtful.
They do not act in this manner at the times when they eject much earth
from their burrows; for their castings then serve to cover the mouths.
When gardeners wish to kill worms on a lawn, it is necessary first to
brush or rake away the castings from the surface, in order that the
lime-water may enter the burrows. {59a} It might be inferred from this
fact that the mouths are plugged up with leaves, &c., to prevent the
entrance of water during heavy rain; but it may be urged against this
view that a few, loose, well-rounded stones are ill-adapted to keep out
water. I have moreover seen many burrows in the perpendicularly cut
turf-edgings to gravel-walks, into which water could hardly flow, as well
plugged as burrows on a level surface. It is not probable that the plugs
or piles of stones serve to conceal the burrows from scolopendras, which,
according to Hoffmeister, {59b} are the bitterest enemies of worms, or
from the larger species of Carabus and Staphylinus which attack them
ferociously, for these animals are nocturnal, and the burrows are opened
at night. May not worms when the mouth of the burrow is protected be
able to remain with safety with their heads close to it, which we know
that they like to do, but which costs so many of them their lives? Or
may not the plugs check the free ingress of the lowest stratum of air,
when chilled by radiation at night, from the surrounding ground and
herbage? I am inclined to believe in this latter view: firstly, because
when worms were kept in pots in a room with a fire, in which case cold
air could not enter the burrows, they plugged them up in a slovenly
manner; and secondarily, because they often coat the upper part of their
burrows with leaves, apparently to prevent their bodies from coming into
close contact with the cold damp earth. Mr. E. Parfitt has suggested to
me that the mouths of the burrows are closed in order that the air within
them may be kept thoroughly damp, and this seems the most probable
explanation of the habit. But the plugging-up process may serve for all
the above purposes.
Whatever the motive may be, it appears that worms much dislike leaving
the mouths of their burrows open. Nevertheless they will reopen them at
night, whether or not they can afterwards close them. Numerous open
burrows may be seen on recently-dug ground, for in this case the worms
eject their castings in cavities left in the ground, or in the old
burrows instead of piling them over the mouths of their burrows, and they
cannot collect objects on the surface by which the mouths might be
protected. So again on a recently disinterred pavement of a Roman villa
at Abinger (hereafter to be described) the worms pertinaciously opened
their burrows almost every night, when these had been closed by being
trampled on, although they were rarely able to find a few minute stones
wherewith to protect them.
_Intelligence shown by worms in their manner of plugging up their
burrows_.—If a man had to plug up a small cylindrical hole, with such
objects as leaves, petioles or twigs, he would drag or push them in by
their pointed ends; but if these objects were very thin relatively to the
size of the hole, he would probably insert some by their thicker or
broader ends. The guide in his case would be intelligence. It seemed
therefore worth while to observe carefully how worms dragged leaves into
their burrows; whether by their tips or bases or middle parts. It seemed
more especially desirable to do this in the case of plants not natives to
our country; for although the habit of dragging leaves into their burrows
is undoubtedly instinctive with worms, yet instinct could not tell them
how to act in the case of leaves about which their progenitors knew
nothing. If, moreover, worms acted solely through instinct or an
unvarying inherited impulse, they would draw all kinds of leaves into
their burrows in the same manner. If they have no such definite
instinct, we might expect that chance would determine whether the tip,
base or middle was seized. If both these alternatives are excluded,
intelligence alone is left; unless the worm in each case first tries many
different methods, and follows that alone which proves possible or the
most easy; but to act in this manner and to try different methods makes a
near approach to intelligence.
In the first place 227 withered leaves of various kinds, mostly of
English plants, were pulled out of worm-burrows in several places. Of
these, 181 had been drawn into the burrows by or near their tips, so that
the foot-stalk projected nearly upright from the mouth of the burrow; 20
had been drawn in by their bases, and in this case the tips projected
from the burrows; and 26 had been seized near the middle, so that these
had been drawn in transversely and were much crumpled. Therefore 80 per
cent. (always using the nearest whole number) had been drawn in by the
tip, 9 per cent. by the base or foot-stalk, and 11 per cent. transversely
or by the middle. This alone is almost sufficient to show that chance
does not determine the manner in which leaves are dragged into the
burrows.
Of the above 227 leaves, 70 consisted of the fallen leaves of the common
lime-tree, which is almost certainly not a native of England. These
leaves are much acuminated towards the tip, and are very broad at the
base with a well-developed foot-stalk. They are thin and quite flexible
when half-withered. Of the 70, 79 per cent. had been drawn in by or near
the tip; 4 per cent. by or near the base; and 17 per cent. transversely
or by the middle. These proportions agree very closely, as far as the
tip is concerned, with those before given. But the percentage drawn in
by the base is smaller, which may be attributed to the breadth of the
basal part of the blade. We here, also, see that the presence of a
foot-stalk, which it might have been expected would have tempted the
worms as a convenient handle, has little or no influence in determining
the manner in which lime leaves are dragged into the burrows. The
considerable proportion, viz., 17 per cent., drawn in more or less
transversely depends no doubt on the flexibility of these half-decayed
leaves. The fact of so many having been drawn in by the middle, and of
some few having been drawn in by the base, renders it improbable that the
worms first tried to draw in most of the leaves by one or both of these
methods, and that they afterwards drew in 79 per cent. by their tips; for
it is clear that they would not have failed in drawing them in by the
base or middle.
The leaves of a foreign plant were next searched for, the blades of which
were not more pointed towards the apex than towards the base. This
proved to be the case with those of a laburnum (a hybrid between _Cytisus
alpinus_ and _laburnum_) for on doubling the terminal over the basal
half, they generally fitted exactly; and when there was any difference,
the basal half was a little the narrower. It might, therefore, have been
expected that an almost equal number of these leaves would have been
drawn in by the tip and base, or a slight excess in favour of the latter.
But of 73 leaves (not included in the first lot of 227) pulled out of
worm-burrows, 63 per cent. had been drawn in by the tip; 27 per cent. by
the base, and 10 per cent. transversely. We here see that a far larger
proportion, viz., 27 per cent. were drawn in by the base than in the case
of lime leaves, the blades of which are very broad at the base, and of
which only 4 per cent. had thus been drawn in. We may perhaps account
for the fact of a still larger proportion of the laburnum leaves not
having been drawn in by the base, by worms having acquired the habit of
generally drawing in leaves by their tips and thus avoiding the
foot-stalk. For the basal margin of the blade in many kinds of leaves
forms a large angle with the foot-stalk; and if such a leaf were drawn in
by the foot-stalk, the basal margin would come abruptly into contact with
the ground on each side of the burrow, and would render the drawing in of
the leaf very difficult.
Nevertheless worms break through their habit of avoiding the foot-stalk,
if this part offers them the most convenient means for drawing leaves
into their burrows. The leaves of the endless hybridised varieties of
the Rhododendron vary much in shape; some are narrowest towards the base
and others towards the apex. After they have fallen off, the blade on
each side of the midrib often becomes curled up while drying, sometimes
along the whole length, sometimes chiefly at the base, sometimes towards
the apex. Out of 28 fallen leaves on one bed of peat in my garden, no
less than 23 were narrower in the basal quarter than in the terminal
quarter of their length; and this narrowness was chiefly due to the
curling in of the margins. Out of 36 fallen leaves on another bed, in
which different varieties of the Rhododendron grew, only 17 were narrower
towards the base than towards the apex. My son William, who first called
my attention to this case, picked up 237 fallen leaves in his garden
(where the Rhododendron grows in the natural soil) and of these 65 per
cent. could have been drawn by worms into their burrows more easily by
the base or foot-stalk than by the tip; and this was partly due to the
shape of the leaf and in a less degree to the curling in of the margins:
27 per cent. could have been drawn in more easily by the tip than by the
base: and 8 per cent. with about equal ease by either end. The shape of
a fallen leaf ought to be judged of before one end has been drawn into a
burrow, for after this has happened, the free end, whether it be the base
or apex, will dry more quickly than the end imbedded in the damp ground;
and the exposed margins of the free end will consequently tend to become
more curled inwards than they were when the leaf was first seized by the
worm. My son found 91 leaves which had been dragged by worms into their
burrows, though not to a great depth; of these 66 per cent. had been
drawn in by the base or foot-stalk; and 34 per cent. by the tip. In this
case, therefore, the worms judged with a considerable degree of
correctness how best to draw the withered leaves of this foreign plant
into their burrows; notwithstanding that they had to depart from their
usual habit of avoiding the foot-stalk.
On the gravel-walks in my garden a very large number of leaves of three
species of Pinus (_P. austriaca_, _nigricans_ and _sylvestris_) are
regularly drawn into the mouths of worm burrows. These leaves consist of
two so-called needles, which are of considerable length in the two first
and short in the last named species, and are united to a common base; and
it is by this part that they are almost invariably drawn into the
burrows. I have seen only two or at most three exceptions to this rule
with worms in a state of nature. As the sharply pointed needles diverge
a little, and as several leaves are drawn into the same burrow, each tuft
forms a perfect _chevaux de frise_. On two occasions many of these tufts
were pulled up in the evening, but by the following morning fresh leaves
had been pulled in, and the burrows were again well protected. These
leaves could not be dragged into the burrows to any depth, except by
their bases, as a worm cannot seize hold of the two needles at the same
time, and if one alone were seized by the apex, the other would be
pressed against the ground and would resist the entry of the seized one.
This was manifest in the above mentioned two or three exceptional cases.
In order, therefore, that worms should do their work well, they must drag
pine-leaves into their burrows by their bases, where the two needles are
conjoined. But how they are guided in this work is a perplexing
question.
This difficulty led my son Francis and myself to observe worms in
confinement during several nights by the aid of a dim light, while they
dragged the leaves of the above named pines into their burrows. They
moved the anterior extremities of their bodies about the leaves, and on
several occasions when they touched the sharp end of a needle they
withdrew suddenly as if pricked. But I doubt whether they were hurt, for
they are indifferent to very sharp objects, and will swallow even
rose-thorns and small splinters of glass. It may also be doubted,
whether the sharp ends of the needles serve to tell them that this is the
wrong end to seize; for the points were cut off many leaves for a length
of about one inch, and fifty-seven of them thus treated were drawn into
the burrows by their bases, and not one by the cut-off ends. The worms
in confinement often seized the needles near the middle and drew them
towards the mouths of their burrows; and one worm tried in a senseless
manner to drag them into the burrow by bending them. They sometimes
collected many more leaves over the mouths of their burrows (as in the
case formerly mentioned of lime-leaves) than could enter them. On other
occasions, however, they behaved very differently; for as soon as they
touched the base of a pine-leaf, this was seized, being sometimes
completely engulfed in their mouths, or a point very near the base was
seized, and the leaf was then quickly dragged or rather jerked into their
burrows. It appeared both to my son and myself as if the worms instantly
perceived as soon as they had seized a leaf in the proper manner. Nine
such cases were observed, but in one of them the worm failed to drag the
leaf into its burrow, as it was entangled by other leaves lying near. In
another case a leaf stood nearly upright with the points of the needles
partly inserted into a burrow, but how placed there was not seen; and
then the worm reared itself up and seized the base, which was dragged
into the mouth of the burrow by bowing the whole leaf. On the other
hand, after a worm had seized the base of a leaf, this was on two
occasions relinquished from some unknown motive.
As already remarked, the habit of plugging up the mouths of the burrows
with various objects, is no doubt instinctive in worms; and a very young
one, born in one of my pots, dragged for some little distance a
Scotch-fir leaf, one needle of which was as long and almost as thick as
its own body. No species of pine is endemic in this part of England, it
is therefore incredible that the proper manner of dragging pine-leaves
into the burrows can be instinctive with our worms. But as the worms on
which the above observations were made, were dug up beneath or near some
pines, which had been planted there about forty years, it was desirable
to prove that their actions were not instinctive. Accordingly,
pine-leaves were scattered on the ground in places far removed from any
pine-tree, and 90 of them were drawn into the burrows by their bases.
Only two were drawn in by the tips of the needles, and these were not
real exceptions, as one was drawn in for a very short distance, and the
two needles of the other cohered. Other pine-leaves were given to worms
kept in pots in a warm room, and here the result was different; for out
of 42 leaves drawn into the burrows, no less than 16 were drawn in by the
tips of the needles. These worms, however, worked in a careless or
slovenly manner; for the leaves were often drawn in to only a small
depth; sometimes they were merely heaped over the mouths of the burrows,
and sometimes none were drawn in. I believe that this carelessness may
be accounted for either by the warmth of the air, or by its dampness, as
the pots were covered by glass plates; the worms consequently did not
care about plugging up their holes effectually. Pots tenanted by worms
and covered with a net which allowed the free entrance of air, were left
out of doors for several nights, and now 72 leaves were all properly
drawn in by their bases.
It might perhaps be inferred from the facts as yet given, that worms
somehow gain a general notion of the shape or structure of pine-leaves,
and perceive that it is necessary for them to seize the base where the
two needles are conjoined. But the following cases make this more than
doubtful. The tips of a large number of needles of _P. austriaca_ were
cemented together with shell-lac dissolved in alcohol, and were kept for
some days, until, as I believe, all odour or taste had been lost; and
they were then scattered on the ground where no pine-trees grew, near
burrows from which the plugging had been removed. Such leaves could have
been drawn into the burrows with equal ease by either end; and judging
from analogy and more especially from the case presently to be given of
the petioles of _Clematis montana_, I expected that the apex would have
been preferred. But the result was that out of 121 leaves with the tips
cemented, which were drawn into burrows, 108 were drawn in by their
bases, and only 13 by their tips. Thinking that the worms might possibly
perceive and dislike the smell or taste of the shell-lac, though this was
very improbable, especially after the leaves had been left out during
several nights, the tips of the needles of many leaves were tied together
with fine thread. Of leaves thus treated 150 were drawn into burrows—123
by the base and 27 by the tied tips; so that between four and five times
as many were drawn in by the base as by the tip. It is possible that the
short cut-off ends of the thread with which they were tied, may have
tempted the worms to drag in a larger proportional number by the tips
than when cement was used. Of the leaves with tied and cemented tips
taken together (271 in number) 85 per cent. were drawn in by the base and
15 per cent. by the tips. We may therefore infer that it is not the
divergence of the two needles which leads worms in a state of nature
almost invariably to drag pine-leaves into their burrows by the base.
Nor can it be the sharpness of the points of the needles which determines
them; for, as we have seen, many leaves with the points cut off were
drawn in by their bases. We are thus led to conclude, that with
pine-leaves there must be something attractive to worms in the base,
notwithstanding that few ordinary leaves are drawn in by the base or
foot-stalk.
_Petioles_.—We will now turn to the petioles or foot-stalks of compound
leaves, after the leaflets have fallen off. Those from _Clematis
montana_, which grew over a verandah, were dragged early in January in
large numbers into the burrows on an adjoining gravel-walk, lawn, and
flower-bed. These petioles vary from 2½ to 4½ inches in length, are
rigid and of nearly uniform thickness, except close to the base where
they thicken rather abruptly, being here about twice as thick as in any
other part. The apex is somewhat pointed, but soon withers and is then
easily broken off. Of these petioles, 314 were pulled out of burrows in
the above specified sites; and it was found that 76 per cent. had been
drawn in by their tips, and 24 per cent by their bases; so that those
drawn in by the tip were a little more than thrice as many as those drawn
in by the base. Some of those extracted from the well-beaten gravel-walk
were kept separate from the others; and of these (59 in number) nearly
five times as many had been drawn in by the tip as by the base; whereas
of those extracted from the lawn and flower-bed, where from the soil
yielding more easily, less care would be necessary in plugging up the
burrows, the proportion of those drawn in by the tip (130) to those drawn
in by the base (48) was rather less than three to one. That these
petioles had been dragged into the burrows for plugging them up, and not
for food, was manifest, as neither end, as far as I could see, had been
gnawed. As several petioles are used to plug up the same burrow, in one
case as many as 10, and in another case as many as 15, the worms may
perhaps at first draw in a few by the thicker end so as to save labour;
but afterwards a large majority are drawn in by the pointed end, in order
to plug up the hole securely.
The fallen petioles of our native ash-tree were next observed, and the
rule with most objects, viz., that a large majority are dragged into the
burrows by the more pointed end, had not here been followed; and this
fact much surprised me at first. These petioles vary in length from 5 to
8½ inches; they are thick and fleshy towards the base, whence they taper
gently towards the apex, which is a little enlarged and truncated where
the terminal leaflet had been originally attached. Under some ash-trees
growing in a grass-field, 229 petioles were pulled out of worm burrows
early in January, and of these 51.5 per cent. had been drawn in by the
base, and 48.5 per cent. by the apex. This anomaly was however readily
explained as soon as the thick basal part was examined; for in 78 out of
103 petioles, this part had been gnawed by worms, just above the
horse-shoe shaped articulation. In most cases there could be no mistake
about the gnawing; for ungnawed petioles which were examined after being
exposed to the weather for eight additional weeks had not become more
disintegrated or decayed near the base than elsewhere. It is thus
evident that the thick basal end of the petiole is drawn in not solely
for the sake of plugging up the mouths of the burrows, but as food. Even
the narrow truncated tips of some few petioles had been gnawed; and this
was the case in 6 out of 37 which were examined for this purpose. Worms,
after having drawn in and gnawed the basal end, often push the petioles
out of their burrows; and then drag in fresh ones, either by the base for
food, or by the apex for plugging up the mouth more effectually. Thus,
out of 37 petioles inserted by their tips, 5 had been previously drawn in
by the base, for this part had been gnawed. Again, I collected a handful
of petioles lying loose on the ground close to some plugged-up burrows,
where the surface was thickly strewed with other petioles which
apparently had never been touched by worms; and 14 out of 47 (_i.e._
nearly one-third), after having had their bases gnawed had been pushed
out of the burrows and were now lying on the ground. From these several
facts we may conclude that worms draw in some petioles of the ash by the
base to serve as food, and others by the tip to plug up the mouths of
their burrows in the most efficient manner.
The petioles of _Robinia pseudo-acacia_ vary from 4 or 5 to nearly 12
inches in length; they are thick close to the base before the softer
parts have rotted off, and taper much towards the upper end. They are so
flexible that I have seen some few doubled up and thus drawn into the
burrows of worms. Unfortunately these petioles were not examined until
February, by which time the softer parts had completely rotted off, so
that it was impossible to ascertain whether worms had gnawed the bases,
though this is in itself probable. Out of 121 petioles extracted from
burrows early in February, 68 were imbedded by the base, and 53 by the
apex. On February 5 all the petioles which had been drawn into the
burrows beneath a Robinia, were pulled up; and after an interval of
eleven days, 35 petioles had been again dragged in, 19 by the base, and
16 by the apex. Taking these two lots together, 56 per cent. were drawn
in by the base, and 44 per cent. by the apex. As all the softer parts
had long ago rotted off, we may feel sure, especially in the latter case,
that none had been drawn in as food. At this season, therefore, worms
drag these petioles into their burrows indifferently by either end, a
slight preference being given to the base. This latter fact may be
accounted for by the difficulty of plugging up a burrow with objects so
extremely thin as are the upper ends. In support of this view, it may be
stated that out of the 16 petioles which had been drawn in by their upper
ends, the more attenuated terminal portion of 7 had been previously
broken off by some accident.
_Triangles of paper_.—Elongated triangles were cut out of moderately
stiff writing-paper, which was rubbed with raw fat on both sides, so as
to prevent their becoming excessively limp when exposed at night to rain
and dew. The sides of all the triangles were three inches in length,
with the bases of 120 one inch, and of the other 183 half an inch in
length. These latter triangles were very narrow or much acuminated. {79}
As a check on the observations presently to be given, similar triangles
in a damp state were seized by a very narrow pair of pincers at different
points and at all inclinations with reference to the margins, and were
then drawn into a short tube of the diameter of a worm-burrow. If seized
by the apex, the triangle was drawn straight into the tube, with its
margins infolded; if seized at some little distance from the apex, for
instance at half an inch, this much was doubled back within the tube. So
it was with the base and basal angles, though in this case the triangles
offered, as might have been expected, much more resistance to being drawn
in. If seized near the middle the triangle was doubled up, with the apex
and base left sticking out of the tube. As the sides of the triangles
were three inches in length, the result of their being drawn into a tube
or into a burrow in different ways, may be conveniently divided into
three groups: those drawn in by the apex or within an inch of it; those
drawn in by the base or within an inch of it; and those drawn in by any
point in the middle inch.
In order to see how the triangles would be seized by worms, some in a
damp state were given to worms kept in confinement. They were seized in
three different manners in the case of both the narrow and broad
triangles: viz., by the margin; by one of the three angles, which was
often completely engulfed in their mouths; and lastly, by suction applied
to any part of the flat surface. If lines parallel to the base and an
inch apart, are drawn across a triangle with the sides three inches in
length, it will be divided into three parts of equal length. Now if
worms seized indifferently by chance any part, they would assuredly seize
on the basal part or division far oftener than on either of the two other
divisions. For the area of the basal to the apical part is as 5 to 1, so
that the chance of the former being drawn into a burrow by suction, will
be as 5 to 1, compared with the apical part. The base offers two angles
and the apex only one, so that the former would have twice as good a
chance (independently of the size of the angles) of being engulfed in a
worm’s mouth, as would the apex. It should, however, be stated that the
apical angle is not often seized by worms; the margin at a little
distance on either side being preferred. I judge of this from having
found in 40 out of 46 cases in which triangles had been drawn into
burrows by their apical ends, that the tip had been doubled back within
the burrow for a length of between 1/20 of an inch and 1 inch. Lastly,
the proportion between the margins of the basal and apical parts is as 3
to 2 for the broad, and 2½ to 2 for the narrow triangles. From these
several considerations it might certainly have been expected, supposing
that worms seized hold of the triangles by chance, that a considerably
larger proportion would have been dragged into the burrows by the basal
than by the apical part; but we shall immediately see how different was
the result.
Triangles of the above specified sizes were scattered on the ground in
many places and on many successive nights near worm-burrows, from which
the leaves, petioles, twigs, &c., with which they had been plugged, were
removed. Altogether 303 triangles were drawn by worms into their
burrows: 12 others were drawn in by both ends, but as it was impossible
to judge by which end they had been first seized, these are excluded. Of
the 303, 62 per cent. had been drawn in by the apex (using this term for
all drawn in by the apical part, one inch in length); 15 per cent. by the
middle; and 23 per cent. by the basal part. If they had been drawn
indifferently by any point, the proportion for the apical, middle and
basal parts would have been 33.3 per cent. for each; but, as we have just
seen, it might have been expected that a much larger proportion would
have been drawn in by the basal than by any other part. As the case
stands, nearly three times as many were drawn in by the apex as by the
base. If we consider the broad triangles by themselves, 59 per cent.
were drawn in by the apex, 25 per cent. by the middle, and 16 per cent.
by the base. Of the narrow triangles, 65 per cent. were drawn in by the
apex, 14 per cent, by the middle, and 21 per cent. by the base; so that
here those drawn in by the apex were more than 3 times as many as those
drawn in by the base. We may therefore conclude that the manner in which
the triangles are drawn into the burrows is not a matter of chance.
In eight cases, two triangles had been drawn into the same burrow, and in
seven of these cases, one had been drawn in by the apex and the other by
the base. This again indicates that the result is not determined by
chance. Worms appear sometimes to revolve in the act of drawing in the
triangles, for five out of the whole lot had been wound into an irregular
spire round the inside of the burrow. Worms kept in a warm room drew 63
triangles into their burrows; but, as in the case of the pine-leaves,
they worked in a rather careless manner, for only 44 per cent. were drawn
in by the apex, 22 per cent. by the middle, and 33 per cent. by the base.
In five cases, two triangles were drawn into the same burrow.
It may be suggested with much apparent probability that so large a
proportion of the triangles were drawn in by the apex, not from the worms
having selected this end as the most convenient for the purpose, but from
having first tried in other ways and failed. This notion was
countenanced by the manner in which worms in confinement were seen to
drag about and drop the triangles; but then they were working carelessly.
I did not at first perceive the importance of this subject, but merely
noticed that the bases of those triangles which had been drawn in by the
apex, were generally clean and not crumpled. The subject was afterwards
attended to carefully. In the first place several triangles which had
been drawn in by the basal angles, or by the base, or a little above the
base, and which were thus much crumpled and dirtied, were left for some
hours in water and were then well shaken while immersed; but neither the
dirt nor the creases were thus removed. Only slight creases could be
obliterated, even by pulling the wet triangles several times through my
fingers. Owing to the slime from the worms’ bodies, the dirt was not
easily washed off. We may therefore conclude that if a triangle, before
being dragged in by the apex, had been dragged into a burrow by its base
with even a slight degree of force, the basal part would long retain its
creases and remain dirty. The condition of 89 triangles (65 narrow and
24 broad ones), which had been drawn in by the apex, was observed; and
the bases of only 7 of them were at all creased, being at the same time
generally dirty. Of the 82 uncreased triangles, 14 were dirty at the
base; but it does not follow from this fact that these had first been
dragged towards the burrows by their bases; for the worms sometimes
covered large portions of the triangles with slime, and these when
dragged by the apex over the ground would be dirtied; and during rainy
weather, the triangles were often dirtied over one whole side or over
both sides. If the worms had dragged the triangles to the mouths of
their burrows by their bases, as often as by their apices, and had then
perceived, without actually trying to draw them into the burrow, that the
broader end was not well adapted for this purpose—even in this case a
large proportion would probably have had their basal ends dirtied. We
may therefore infer—improbable as is the inference—that worms are able by
some means to judge which is the best end by which to draw triangles of
paper into their burrows.
The percentage results of the foregoing observations on the manner in
which worms draw various kinds of objects into the mouths of their
burrows may be abridged as follows:—
Nature of Drawn into the Drawn in, by or Drawn in, by or
Object. burrows, by or near the near the base.
near the apex. middle.
Leaves of 80 11 9
various kinds
—of the Lime, 79 17 4
basal margin of
blade broad,
apex acuminated
—of a Laburnum, 63 10 27
basal part of
blade as narrow
as, or
sometimes
little narrower
than the apical
part
—of the 34 ... 66
Rhododendron,
basal part of
blade often
narrower than
the apical part
—of Pine-trees, ... ... 100
consisting of
two needles
arising from a
common base
Petioles of a 76 ... 24
Clematis,
somewhat
pointed at the
apex, and blunt
at the base
—of the Ash, 48.5 ... 51.5
the thick basal
end often drawn
in to serve as
food
—of Robinia, 44 ... 56
extremely thin,
especially
towards the
apex, so as to
be ill-fitted
for plugging up
the burrows
Triangles of 62 15 23
paper, of the
two sizes
—of the broad 59 25 16
ones alone
—of the narrow 65 14 21
ones alone
If we consider these several cases, we can hardly escape from the
conclusion that worms show some degree of intelligence in their manner of
plugging up their burrows. Each particular object is seized in too
uniform a manner, and from causes which we can generally understand, for
the result to be attributed to mere chance. That every object has not
been drawn in by its pointed end, may be accounted for by labour having
been saved through some being inserted by their broader or thicker ends.
No doubt worms are led by instinct to plug up their burrows; and it might
have been expected that they would have been led by instinct how best to
act in each particular case, independently of intelligence. We see how
difficult it is to judge whether intelligence comes into play, for even
plants might sometimes be thought to be thus directed; for instance when
displaced leaves re-direct their upper surfaces towards the light by
extremely complicated movements and by the shortest course. With
animals, actions appearing due to intelligence may be performed through
inherited habit without any intelligence, although aboriginally thus
acquired. Or the habit may have been acquired through the preservation
and inheritance of beneficial variations of some other habit; and in this
case the new habit will have been acquired independently of intelligence
throughout the whole course of its development. There is no _à priori_
improbability in worms having acquired special instincts through either
of these two latter means. Nevertheless it is incredible that instincts
should have been developed in reference to objects, such as the leaves of
petioles of foreign plants, wholly unknown to the progenitors of the
worms which act in the described manner. Nor are their actions so
unvarying or inevitable as are most true instincts.
As worms are not guided by special instincts in each particular case,
though possessing a general instinct to plug up their burrows, and as
chance is excluded, the next most probable conclusion seems to be that
they try in many different ways to draw in objects, and at last succeed
in some one way. But it is surprising that an animal so low in the scale
as a worm should have the capacity for acting in this manner, as many
higher animals have no such capacity. For instance, ants may be seen
vainly trying to drag an object transversely to their course, which could
be easily drawn longitudinally; though after a time they generally act in
a wiser manner, M. Fabre states {89a} that a Sphex—an insect belonging to
the same highly-endowed order with ants—stocks its nest with paralysed
grass-hoppers, which are invariably dragged into the burrow by their
antennæ. When these were cut off close to the head, the Sphex seized the
palpi; but when these were likewise cut off, the attempt to drag its prey
into the burrow was given up in despair. The Sphex had not intelligence
enough to seize one of the six legs or the ovipositor of the grasshopper,
which, as M. Fabre remarks, would have served equally well. So again, if
the paralysed prey with an egg attached to it be taken out of the cell,
the Sphex after entering and finding the cell empty, nevertheless closes
it up in the usual elaborate manner. Bees will try to escape and go on
buzzing for hours on a window, one half of which has been left open.
Even a pike continued during three months to dash and bruise itself
against the glass sides of an aquarium, in the vain attempt to seize
minnows on the opposite side. {89b} A cobra-snake was seen by Mr. Layard
{90} to act much more wisely than either the pike or the Sphex; it had
swallowed a toad lying within a hole, and could not withdraw its head;
the toad was disgorged, and began to crawl away; it was again swallowed
and again disgorged; and now the snake had learnt by experience, for it
seized the toad by one of its legs and drew it out of the hole. The
instincts of even the higher animals are often followed in a senseless or
purposeless manner: the weaver-bird will perseveringly wind threads
through the bars of its cage, as if building a nest: a squirrel will pat
nuts on a wooden floor, as if he had just buried them in the ground: a
beaver will cut up logs of wood and drag them about, though there is no
water to dam up; and so in many other cases.
Mr. Romanes, who has specially studied the minds of animals, believes
that we can safely infer intelligence, only when we see an individual
profiting by its own experience. By this test the cobra showed some
intelligence; but this would have been much plainer if on a second
occasion he had drawn a toad out of a hole by its leg. The Sphex failed
signally in this respect. Now if worms try to drag objects into their
burrows first in one way and then in another, until they at last succeed,
they profit, at least in each particular instance, by experience.
But evidence has been advanced showing that worms do not habitually try
to draw objects into their burrows in many different ways. Thus
half-decayed lime-leaves from their flexibility could have been drawn in
by their middle or basal parts, and were thus drawn into the burrows in
considerable numbers; yet a large majority were drawn in by or near the
apex. The petioles of the Clematis could certainly have been drawn in
with equal ease by the base and apex; yet three times and in certain
cases five times as many were drawn in by the apex as by the base. It
might have been thought that the foot-stalks of leaves would have tempted
the worms as a convenient handle; yet they are not largely used, except
when the base of the blade is narrower than the apex. A large number of
the petioles of the ash are drawn in by the base; but this part serves
the worms as food. In the case of pine-leaves worms plainly show that
they at least do not seize the leaf by chance; but their choice does not
appear to be determined by the divergence of the two needles, and the
consequent advantage or necessity of drawing them into their burrows by
the base. With respect to the triangles of paper, those which had been
drawn in by the apex rarely had their bases creased or dirty; and this
shows that the worms had not often first tried to drag them in by this
end.
If worms are able to judge, either before drawing or after having drawn
an object close to the mouths of their burrows, how best to drag it in,
they must acquire some notion of its general shape. This they probably
acquire by touching it in many places with the anterior extremity of
their bodies, which serves as a tactile organ. It may be well to
remember how perfect the sense of touch becomes in a man when born blind
and deaf, as are worms. If worms have the power of acquiring some
notion, however rude, of the shape of an object and of their burrows, as
seems to be the case, they deserve to be called intelligent; for they
then act in nearly the same manner as would a man under similar
circumstances.
To sum up, as chance does not determine the manner in which objects are
drawn into the burrows, and as the existence of specialized instincts for
each particular case cannot be admitted, the first and most natural
supposition is that worms try all methods until they at last succeed; but
many appearances are opposed to such a supposition. One alternative
alone is left, namely, that worms, although standing low in the scale of
organization, possess some degree of intelligence. This will strike
every one as very improbable; but it may be doubted whether we know
enough about the nervous system of the lower animals to justify our
natural distrust of such a conclusion. With respect to the small size of
the cerebral ganglia, we should remember what a mass of inherited
knowledge, with some power of adapting means to an end, is crowded into
the minute brain of a worker-ant.
_Means by which worms excavate their burrows_.—This is effected in two
ways; by pushing away the earth on all sides, and by swallowing it. In
the former case, the worm inserts the stretched out and attenuated
anterior extremity of its body into any little crevice, or hole; and
then, as Perrier remarks, {93} the pharynx is pushed forwards into this
part, which consequently swells and pushes away the earth on all sides.
The anterior extremity thus serves as a wedge. It also serves, as we
have before seen, for prehension and suction, and as a tactile organ. A
worm was placed on loose mould, and it buried itself in between two and
three minutes. On another occasion four worms disappeared in 15 minutes
between the sides of the pot and the earth, which had been moderately
pressed down. On a third occasion three large worms and a small one were
placed on loose mould well mixed with fine sand and firmly pressed down,
and they all disappeared, except the tail of one, in 35 minutes. On a
fourth occasion six large worms were placed on argillaceous mud mixed
with sand firmly pressed down, and they disappeared, except the extreme
tips of the tails of two of them, in 40 minutes. In none of these cases,
did the worms swallow, as far as could be seen, any earth. They
generally entered the ground close to the sides of the pot.
A pot was next filled with very fine ferruginous sand, which was pressed
down, well watered, and thus rendered extremely compact. A large worm
left on the surface did not succeed in penetrating it for some hours, and
did not bury itself completely until 25 hrs. 40 min. had elapsed. This
was effected by the sand being swallowed, as was evident by the large
quantity ejected from the vent, long before the whole body had
disappeared. Castings of a similar nature continued to be ejected from
the burrow during the whole of the following day.
As doubts have been expressed by some writers whether worms ever swallow
earth solely for the sake of making their burrows, some additional cases
may be given. A mass of fine reddish sand, 23 inches in thickness, left
on the ground for nearly two years, had been penetrated in many places by
worms; and their castings consisted partly of the reddish sand and partly
of black earth brought up from beneath the mass. This sand had been dug
up from a considerable depth, and was of so poor a nature that weeds
could not grow on it. It is therefore highly improbable that it should
have been swallowed by the worms as food. Again in a field near my house
the castings frequently consist of almost pure chalk, which lies at only
a little depth beneath the surface; and here again it is very improbable
that the chalk should have been swallowed for the sake of the very little
organic matter which could have percolated into it from the poor
overlying pasture. Lastly, a casting thrown up through the concrete and
decayed mortar between the tiles, with which the now ruined aisle of
Beaulieu Abbey had formerly been paved, was washed, so that the coarser
matter alone was left. This consisted of grains of quartz, micaceous
slate, other rocks, and bricks or tiles, many of them from 1/20 to 1/10
inch in diameter. No one will suppose that these grains were swallowed
as food, yet they formed more than half of the casting, for they weighed
19 grains, the whole casting having weighed 33 grains. Whenever a worm
burrows to a depth of some feet in undisturbed compact ground, it must
form its passage by swallowing the earth; for it is incredible that the
ground could yield on all sides to the pressure of the pharynx when
pushed forwards within the worm’s body.
That worms swallow a larger quantity of earth for the sake of extracting
any nutritious matter which it may contain than for making their burrows,
appears to me certain. But as this old belief has been doubted by so
high an authority as Claparède, evidence in its favour must be given in
some detail. There is no _à priori_ improbability in such a belief, for
besides other annelids, especially the _Arenicola marina_, which throws
up such a profusion of castings on our tidal sands, and which it is
believed thus subsists, there are animals belonging to the most distinct
classes, which do not burrow, but habitually swallow large quantities of
sand; namely, the molluscan Onchidium and many Echinoderms. {97}
If earth were swallowed only when worms deepened their burrows or made
new ones, castings would be thrown up only occasionally; but in many
places fresh castings may be seen every morning, and the amount of earth
ejected from the same burrow on successive days is large. Yet worms do
not burrow to a great depth, except when the weather is very dry or
intensely cold. On my lawn the black vegetable mould or humus is only
about 5 inches in thickness, and overlies light-coloured or reddish
clayey soil: now when castings are thrown up in the greatest profusion,
only a small proportion are light coloured, and it is incredible that the
worms should daily make fresh burrows in every direction in the thin
superficial layer of dark-coloured mould, unless they obtained nutriment
of some kind from it. I have observed a strictly analogous case in a
field near my house where bright red clay lay close beneath the surface.
Again on one part of the Downs near Winchester the vegetable mould
overlying the chalk was found to be only from 3 to 4 inches in thickness;
and the many castings here ejected were as black as ink and did not
effervesce with acids; so that the worms must have confined themselves to
this thin superficial layer of mould, of which large quantities were
daily swallowed. In another place at no great distance the castings were
white; and why the worms should have burrowed into the chalk in some
places and not in others, I am unable to conjecture.
Two great piles of leaves had been left to decay in my grounds, and
months after their removal, the bare surface, several yards in diameter,
was so thickly covered during several months with castings that they
formed an almost continuous layer; and the large number of worms which
lived here must have subsisted during these months on nutritious matter
contained in the black earth.
The lowest layer from another pile of decayed leaves mixed with some
earth was examined under a high power, and the number of spores of
various shapes and sizes which it contained was astonishingly great; and
these crushed in the gizzards of worms may largely aid in supporting
them. Whenever castings are thrown up in the greatest number, few or no
leaves are drawn into the burrows; for instance the turf along a
hedgerow, about 200 yards in length, was daily observed in the autumn
during several weeks, and every morning many fresh castings were seen;
but not a single leaf was drawn into these burrows. These castings from
their blackness and from the nature of the subsoil could not have been
brought up from a greater depth than 6 or 8 inches. On what could these
worms have subsisted during this whole time, if not on matter contained
in the black earth? On the other hand, whenever a large number of leaves
are drawn into the burrows, the worms seem to subsist chiefly on them,
for few earth-castings are then ejected on the surface. This difference
in the behaviour of worms at different times, perhaps explains a
statement by Claparède, namely, that triturated leaves and earth are
always found in distinct parts of their intestines.
Worms sometimes abound in places where they can rarely or never obtain
dead or living leaves; for instance, beneath the pavement in well-swept
courtyards, into which leaves are only occasionally blown. My son Horace
examined a house, one corner of which had subsided; and he found here in
the cellar, which was extremely damp, many small worm-castings thrown up
between the stones with which the cellar was paved; and in this case it
is improbable that the worms could ever have obtained leaves. Mr. A. C.
Horner confirms this account, as he has seen castings in the cellars of
his house, which is an old one at Tonbridge.
But the best evidence, known to me, of worms subsisting for at least
considerable periods of time solely on the organic matter contained in
earth, is afforded by some facts communicated to me by Dr. King. Near
Nice large castings abound in extraordinary numbers, so that 5 or 6 were
often found within the space of a square foot. They consist of fine,
pale-coloured earth, containing calcareous matter, which after having
passed through the bodies of worms and being dried, coheres with
considerable force. I have reason to believe that these castings had
been formed by species of Perichæta, which have been naturalized here
from the East. {101} They rise like towers, with their summits often a
little broader than their bases, sometimes to a height of above 3 and
often to a height of 2½ inches. The tallest of those which were measured
was 3.3 inches in height and 1 inch in diameter. A small cylindrical
passage runs up the centre of each tower, through which the worm ascends
to eject the earth which it has swallowed, and thus to add to its height.
A structure of this kind would not allow leaves being easily dragged from
the surrounding ground into the burrows; and Dr. King, who looked
carefully, never saw even a fragment of a leaf thus drawn in. Nor could
any trace be discovered of the worms having crawled down the exterior
surfaces of the towers in search of leaves; and had they done so, tracks
would almost certainly have been left on the upper part whilst it
remained soft. It does not, however, follow that these worms do not draw
leaves into their burrows during some other season of the year, at which
time they would not build up their towers.
From the several foregoing cases, it can hardly be doubted that worms
swallow earth, not only for the sake of making their burrows, but for
obtaining food. Hensen, however, concludes from his analyses of mould
that worms probably could not live on ordinary vegetable mould, though he
admits that they might be nourished to some extent by leaf-mould. {102}
But we have seen that worms eagerly devour raw meat, fat, and dead worms;
and ordinary mould can hardly fail to contain many ova, larvæ, and small
living or dead creatures, spores of cryptogamic plants, and micrococci,
such as those which give rise to saltpetre. These various organisms,
together with some cellulose from any leaves and roots not utterly
decayed, might well account for such large quantities of mould being
swallowed by worms. It may be worth while here to recall the fact that
certain species of Utricularia, which grow in damp places in the tropics,
possess bladders beautifully constructed for catching minute subterranean
animals; and these traps would not have been developed unless many small
animals inhabited such soil.
_The depth to which worms penetrate_, _and the construction of their
burrows_.—Although worms usually live near the surface, yet they burrow
to a considerable depth during long-continued dry weather and severe
cold. In Scandinavia, according to Eisen, and in Scotland, according to
Mr. Lindsay Carnagie, the burrows run down to a depth of from 7 to 8
feet; in North Germany, according to Hoffmeister, from 6 to 8 feet, but
Hensen says, from 3 to 6 feet. This latter observer has seen worms
frozen at a depth of 1½ feet beneath the surface. I have not myself had
many opportunities for observation, but I have often met with worms at
depths of 3 to 4 feet. In a bed of fine sand overlying the chalk, which
had never been disturbed, a worm was cut into two at 55 inches, and
another was found here at Down in December at the bottom of its burrow,
at 61 inches beneath the surface. Lastly, in earth near an old Roman
Villa, which had not been disturbed for many centuries, a worm was met
with at a depth of 66 inches; and this was in the middle of August.
The burrows run down perpendicularly, or more commonly a little
obliquely. They are said sometimes to branch, but as far as I have seen
this does not occur, except in recently dug ground and near the surface.
They are generally, or as I believe invariably, lined with a thin layer
of fine, dark-coloured earth voided by the worms; so that they must at
first be made a little wider than their ultimate diameter. I have seen
several burrows in undisturbed sand thus lined at a depth of 4 ft. 6 in.;
and others close to the surface thus lined in recently dug ground. The
walls of fresh burrows are often dotted with little globular pellets of
voided earth, still soft and viscid; and these, as it appears, are spread
out on all sides by the worm as it travels up or down its burrow. The
lining thus formed becomes very compact and smooth when nearly dry, and
closely fits the worm’s body. The minute reflexed bristles which project
in rows on all sides from the body, thus have excellent points of
support; and the burrow is rendered well adapted for the rapid movement
of the animal. The lining appears also to strengthen the walls, and
perhaps saves the worm’s body from being scratched. I think so because
several burrows which passed through a layer of sifted coal-cinders,
spread over turf to a thickness of 1½ inch, had been thus lined to an
unusual thickness. In this case the worms, judging from the castings,
had pushed the cinders away on all sides and had not swallowed any of
them. In another place, burrows similarly lined, passed through a layer
of coarse coal-cinders, 3½ inches in thickness. We thus see that the
burrows are not mere excavations, but may rather be compared with tunnels
lined with cement.
The mouths of the burrow are in addition often lined with leaves; and
this is an instinct distinct from that of plugging them up, and does not
appear to have been hitherto noticed. Many leaves of the Scotch-fir or
pine (_Pinus sylvestris_) were given to worms kept in confinement in two
pots; and when after several weeks the earth was carefully broken up, the
upper parts of three oblique burrows were found surrounded for lengths of
7, 4, and 3½ inches with pine-leaves, together with fragments of other
leaves which had been given the worms as food. Glass beads and bits of
tile, which had been strewed on the surface of the soil, were stuck into
the interstices between the pine-leaves; and these interstices were
likewise plastered with the viscid castings voided by the worms. The
structures thus formed cohered so well, that I succeeded in removing one
with only a little earth adhering to it. It consisted of a slightly
curved cylindrical case, the interior of which could be seen through
holes in the sides and at either end. The pine-leaves had all been drawn
in by their bases; and the sharp points of the needles had been pressed
into the lining of voided earth. Had this not been effectually done, the
sharp points would have prevented the retreat of the worms into their
burrows; and these structures would have resembled traps armed with
converging points of wire, rendering the ingress of an animal easy and
its egress difficult or impossible. The skill shown by these worms is
noteworthy and is the more remarkable, as the Scotch pine is not a native
of this district.
After having examined these burrows made by worms in confinement, I
looked at those in a flower-bed near some Scotch pines. These had all
been plugged up in the ordinary manner with the leaves of this tree,
drawn in for a length of from 1 to 1½ inch; but the mouths of many of
them were likewise lined with them, mingled with fragments of other kinds
of leaves, drawn in to a depth of 4 or 5 inches. Worms often remain, as
formerly stated, for a long time close to the mouths of their burrows,
apparently for warmth; and the basket-like structures formed of leaves
would keep their bodies from coming into close contact with the cold damp
earth. That they habitually rested on the pine-leaves, was rendered
probable by their clean and almost polished surfaces.
The burrows which run far down into the ground, generally, or at least
often, terminate in a little enlargement or chamber. Here, according to
Hoffmeister, one or several worms pass the winter rolled up into a ball.
Mr. Lindsay Carnagie informed me (1838) that he had examined many burrows
over a stone-quarry in Scotland, where the overlying boulder-clay and
mould had recently been cleared away, and a little vertical cliff thus
left. In several cases the same burrow was a little enlarged at two or
three points one beneath the other; and all the burrows terminated in a
rather large chamber, at a depth of 7 or 8 feet from the surface. These
chambers contained many small sharp bits of stone and husks of
flax-seeds. They must also have contained living seeds, for on the
following spring Mr. Carnagie saw grass-plants sprouting out of some of
the intersected chambers. I found at Abinger in Surrey two burrows
terminating in similar chambers at a depth of 36 and 41 inches, and these
were lined or paved with little pebbles, about as large as mustard seeds;
and in one of the chambers there was a decayed oat-grain, with its husk.
Hensen likewise states that the bottoms of the burrows are lined with
little stones; and where these could not be procured, seeds, apparently
of the pear, had been used, as many as fifteen having been carried down
into a single burrow, one of which had germinated. {108} We thus see how
easily a botanist might be deceived who wished to learn how long deeply
buried seeds remained alive, if he were to collect earth from a
considerable depth, on the supposition that it could contain only seeds
which had long lain buried. It is probable that the little stones, as
well as the seeds, are carried down from the surface by being swallowed;
for a surprising number of glass beads, bits of tile and of glass were
certainly thus carried down by worms kept in pots; but some may have been
carried down within their mouths. The sole conjecture which I can form
why worms line their winter-quarters with little stones and seeds, is to
prevent their closely coiled-up bodies from coming into close contact
with the surrounding cold soil; and such contact would perhaps interfere
with their respiration which is effected by the skin alone.
A worm after swallowing earth, whether for making its burrow or for food,
soon comes to the surface to empty its body. The ejected earth is
thoroughly mingled with the intestinal secretions, and is thus rendered
viscid. After being dried it sets hard. I have watched worms during the
act of ejection, and when the earth was in a very liquid state it was
ejected in little spurts, and by a slow peristaltic movement when not so
liquid. It is not cast indifferently on any side, but with some care,
first on one and then on another side; the tail being used almost like a
trowel. When a worm comes to the surface to eject earth, the tail
protrudes, but when it collects leaves its head must protrude. Worms
therefore must have the power of turning round in their closely-fitting
burrows; and this, as it appears to us, would be a difficult feat. As
soon as a little heap has been formed, the worm apparently avoids, for
the sake of safety, protruding its tail; and the earthy matter is forced
up through the previously deposited soft mass. The mouth of the same
burrow is used for this purpose for a considerable time. In the case of
the tower-like castings (see Fig. 2) near Nice, and of the similar but
still taller towers from Bengal (hereafter to be described and figured),
a considerable degree of skill is exhibited in their construction. Dr.
King also observed that the passage up these towers hardly ever ran in
the same exact line with the underlying burrow, so that a thin
cylindrical object such as a haulm of grass, could not be passed down the
tower into the burrow; and this change of direction probably serves in
some manner as a protection.
Worms do not always eject their castings on the surface of the ground.
When they can find any cavity, as when burrowing in newly turned-up
earth, or between the stems of banked-up plants, they deposit their
castings in such places. So again any hollow beneath a large stone lying
on the surface of the ground, is soon filled up with their castings.
According to Hensen, old burrows are habitually used for this purpose;
but as far as my experience serves, this is not the case, excepting with
those near the surface in recently dug ground. I think that Hensen may
have been deceived by the walls of old burrows, lined with black earth,
having sunk in or collapsed; for black streaks are thus left, and these
are conspicuous when passing through light-coloured soil, and might be
mistaken for completely filled-up burrows.
It is certain that old burrows collapse in the course of time; for as we
shall see in the next chapter, the fine earth voided by worms, if spread
out uniformly, would form in many places in the course of a year a layer
0.2 of an inch in thickness; so that at any rate this large amount is not
deposited within the old unused burrows. If the burrows did not
collapse, the whole ground would be first thickly riddled with holes to a
depth of about ten inches, and in fifty years a hollow unsupported space,
ten inches in depth, would be left. The holes left by the decay of
successively formed roots of trees and plants must likewise collapse in
the course of time.
The burrows of worms run down perpendicularly or a little obliquely, and
where the soil is at all argillaceous, there is no difficulty in
believing that the walls would slowly flow or slide inwards during very
wet weather. When, however, the soil is sandy or mingled with many small
stones, it can hardly be viscous enough to flow inwards during even the
wettest weather; but another agency may here come into play. After much
rain the ground swells, and as it cannot expand laterally, the surface
rises; during dry weather it sinks again. For instance, a large flat
stone laid on the surface of a field sank 3.33 mm. whilst the weather was
dry between May 9th and June 13th, and rose 1.91 mm, between September
7th and 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. {113} 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 Urochæta, 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 Müller
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 castings—a 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 Perichæta. They
stand up like towers, with an open passage in the centre.
[Picture: Fig. 3: A tower-like casting. Fig. 4: A casting from the
Nilgiri Mountains]
A figure of one of these castings from a photograph is here given (Fig.
3). The largest received by me was 3½ inches in height and 1.35 inch in
diameter; another was only ¾ inch in diameter and 2¾ in height. In the
following year, Mr. Scott measured several of the largest; one was 6
inches in height and nearly 1½ in diameter: two others were 5 inches in
height and respectively 2 and rather more than 2½ inches in diameter.
The average weight of the 22 castings sent to me was 35 grammes (1¼ 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 2½
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 4⅓ 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 ½ 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 III.
THE 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 worms—The burial of a paved path—The slow
subsidence of great stones left on the surface—The number of worms which
live within a given space—The weight of earth ejected from a burrow, and
from all the burrows within a given space—The thickness of the layer of
mould which the castings on a given space would form within a given time
if uniformly spread out—The slow rate at which mould can increase to a
great thickness—Conclusion.
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, ½ inch in thickness, beneath which, at a
depth of 2½ 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 or of a coarse sandy nature, and differed considerably in
appearance from the overlying dark-coloured fine mould. Coal-cinders had
been spread over a part of this same field either in the year 1833 or
1834; and when the above holes were dug, that is after an interval of 3
or 4 years, the cinders formed a line of black spots round the holes, at
a depth of 1 inch beneath the surface, parallel to and above the white
layer of lime. Over another part of this field cinders had been strewed,
only about half-a-year before, and these either still lay on the surface
or were entangled among the roots of the grasses; and I here saw the
commencement of the burying process, for worm-castings had been heaped on
several of the smaller fragments. After an interval of 4¾ years this
field was re-examined, and now the two layers of lime and cinders were
found almost everywhere at a greater depth than before by nearly 1 inch,
we will say by ¾ of an inch. Therefore mould to an average thickness of
0.22 of an inch had been annually brought up by the worms, and had been
spread over the surface of this field.
Coal-cinders had been strewed over another field, at a date which could
not be positively ascertained, so thickly that they formed (October,
1837) a layer, 1 inch in thickness at a depth of about 3 inches from the
surface. The layer was so continuous that the over-lying dark vegetable
mould was connected with the sub-soil of red clay only by the roots of
the grasses; and when these were broken, the mould and the red clay fell
apart. In a third field, on which coal-cinders and burnt marl had been
strewed several times at unknown dates, holes were dug in 1842; and a
layer of cinders could be traced at a depth of 3½ inches, beneath which
at a depth of 9½ inches from the surface there was a line of cinders
together with burnt marl. On the sides of one hole there were two layers
of cinders, at 2 and 3½ inches beneath the surface; and below them at a
depth in parts of 9½, and in other parts of 10½ inches there were
fragments of burnt marl. In a fourth field two layers of lime, one above
the other, could be distinctly traced, and beneath them a layer of
cinders and burnt marl at a depth of from 10 to 12 inches below the
surface.
[Picture: Fig. 5: Section of the vegetable mould in a field. Fig. 6:
Traverse section across a large stone]
A piece of waste, swampy land was enclosed, drained, ploughed, harrowed
and thickly covered in the year 1822 with burnt marl and cinders. It was
sowed with grass seeds, and now supports a tolerably good but coarse
pasture. Holes were dug in this field in 1837, or 15 years after its
reclamation, and we see in the accompanying diagram (Fig. 5), reduced to
half of the natural scale, that the turf was ½ inch thick, beneath which
there was a layer of vegetable mould 2½ inches thick. This layer did not
contain fragments of any kind; but beneath it there was a layer of mould,
1½ inch in thickness, full of fragments of burnt marl, conspicuous from
their red colour, one of which near the bottom was an inch in length; and
other fragments of coal-cinders together with a few white quartz pebbles.
Beneath this layer and at a depth of 4½ inches from the surface, the
original black, peaty, sandy soil with a few quartz pebbles was
encountered. Here therefore the fragments of burnt marl and cinders had
been covered in the course of 15 years by a layer of fine vegetable
mould, only 2½ inches in thickness, excluding the turf. Six and a half
years subsequently this field was re-examined, and the fragments were now
found at from 4 to 5 inches beneath the surface. So that in this
interval of 6½ years, about 1½ inch of mould had been added to the
superficial layer. I am surprised that a greater quantity had not been
brought up during the whole 21½ years, for in the closely underlying
black, peaty soil there were many worms. It is, however, probable that
formerly, whilst the land remained poor, worms were scanty; and the mould
would then have accumulated slowly. The average annual increase of
thickness for the whole period is 0.19 of an inch.
Two other cases are worth recording. In the spring of 1835, a field,
which had long existed as poor pasture and was so swampy that it trembled
slightly when stamped on, was thickly covered with red sand so that the
whole surface appeared at first bright red. When holes were dug in this
field after an interval of about 2½ years, the sand formed a layer at a
depth of ¾ in. beneath the surface. In 1842 (i.e., 7 years after the
sand had been laid on) fresh holes were dug, and now the red sand formed
a distinct layer, 2 inches beneath the surface, or 1½ inch beneath the
turf; so that on an average, 0.21 inch of mould had been annually brought
to the surface. Immediately beneath the layer of red sand, the original
substratum of black sandy peat extended.
A grass field, likewise not far from Maer Hall, had formerly been thickly
covered with marl, and was then left for several years as pasture; it was
afterwards ploughed. A friend had three trenches dug in this field 28
years after the application of the marl, {126} and a layer of the marl
fragments could be traced at a depth, carefully measured, of 12 inches in
some parts, and of 14 inches in other parts. This difference in depth
depended on the layer being horizontal, whilst the surface consisted of
ridges and furrows from the field having been ploughed. The tenant
assured me that it had never been turned up to a greater depth than from
6 to 8 inches; and as the fragments formed an unbroken horizontal layer
from 12 to 14 inches beneath the surface, these must have been buried by
the worms whilst the land was in pasture before it was ploughed, for
otherwise they would have been indiscriminately scattered by the plough
throughout the whole thickness of the soil. Four-and-a-half years
afterwards I had three holes dug in this field, in which potatoes had
been lately planted, and the layer of marl-fragments was now found 13
inches beneath the bottoms of the furrows, and therefore probably 15
inches beneath the general level of the field. It should, however, be
observed that the thickness of the blackish sandy soil, which had been
thrown up by the worms above the marl-fragments in the course of 32½
years, would have measured less than 15 inches, if the field had always
remained as pasture, for the soil would in this case have been much more
compact. The fragments of marl almost rested on an undisturbed
substratum of white sand with quartz pebbles; and as this would be little
attractive to worms, the mould would hereafter be very slowly increased
by their action.
We will now give some cases of the action of worms, on land differing
widely from the dry sandy or the swampy pastures just described. The
chalk formation extends all round my house in Kent; and its surface, from
having been exposed during an immense period to the dissolving action of
rain-water, is extremely irregular, being abruptly festooned and
penetrated by many deep well-like cavities. {128} During the dissolution
of the chalk, the insoluble matter, including a vast number of unrolled
flints of all sizes, has been left on the surface and forms a bed of
stiff red clay, full of flints, and generally from 6 to 14 feet in
thickness. Over the red clay, wherever the land has long remained as
pasture, there is a layer a few inches in thickness, of dark-coloured
vegetable mould.
A quantity of broken chalk was spread, on December 20, 1842, over a part
of a field near my house, which had existed as pasture certainly for 30,
probably for twice or thrice as many years. The chalk was laid on the
land for the sake of observing at some future period to what depth it
would become buried. At the end of November, 1871, that is after an
interval of 29 years, a trench was dug across this part of the field; and
a line of white nodules could be traced on both sides of the trench, at a
depth of 7 inches from the surface. The mould, therefore, (excluding the
turf) had here been thrown up at an average rate of 0.22 inch per year.
Beneath the line of chalk nodules there was in parts hardly any fine
earth free of flints, while in other parts there was a layer, 2¼ inches
in thickness. In this latter case the mould was altogether 9¼ inches
thick; and in one such spot a nodule of chalk and a smooth flint pebble,
both of which must have been left at some former time on the surface,
were found at this depth. At from 11 to 12 inches beneath the surface,
the undisturbed reddish clay, full of flints, extended. The appearance
of the above nodules of chalk surprised me, much at first, as they
closely resembled water-worn pebbles, whereas the freshly-broken
fragments had been angular. But on examining the nodules with a lens,
they no longer appeared water-worn, for their surfaces were pitted
through unequal corrosion, and minute, sharp points, formed of broken
fossil shells, projected from them. It was evident that the corners of
the original fragments of chalk had been wholly dissolved, from
presenting a large surface to the carbonic acid dissolved in the
rain-water and to that generated in soil containing vegetable matter, as
well as to the humus-acids. {131} The projecting corners would also,
relatively to the other parts, have been embraced by a larger number of
living rootlets; and these have the power of even attacking marble, as
Sachs has shown. Thus, in the course of 29 years, buried angular
fragments of chalk had been converted into well-rounded nodules.
Another part of this same field was mossy, and as it was thought that
sifted coal-cinders would improve the pasture, a thick layer was spread
over this part either in 1842 or 1843, and another layer some years
afterwards. In 1871 a trench was here dug, and many cinders lay in a
line at a depth of 7 inches beneath the surface, with another line at a
depth of 5½ inches parallel to the one beneath. In another part of this
field, which had formerly existed as a separate one, and which it was
believed had been pasture-land for more than a century, trenches were dug
to see how thick the vegetable mould was. By chance the first trench was
made at a spot where at some former period, certainly more than forty
years before, a large hole had been filled up with coarse red clay,
flints, fragments of chalk, and gravel; and here the fine vegetable mould
was only from 4⅛ to 4⅜ inches in thickness. In another and undisturbed
place, the mould varied much in thickness, namely, from 6½ to 8½ inches;
beneath which a few small fragments of brick were found in one place.
From these several cases, it would appear that during the last 29 years
mould has been heaped on the surface at an average annual rate of from
0.2 to 0.22 of an inch. But in this district when a ploughed field is
first laid down in grass, the mould accumulates at a much slower rate.
The rate, also, must become very much slower after a bed of mould,
several inches in thickness, has been formed; for the worms then live
chiefly near the surface, and burrow down to a greater depth so as to
bring up fresh earth from below, only during the winter when the weather
is very cold (at which time worms were found in this field at a depth of
26 inches) and during summer, when the weather is very dry.
A field, which adjoins the one just described, slopes in one part rather
steeply (viz., at from 10° to 15°); this part was last ploughed in 1841,
was then harrowed and left to become pasture-land. For several years it
was clothed with an extremely scant vegetation, and was so thickly
covered with small and large flints (some of them half as large as a
child’s head) that the field was always called by my sons “the stony
field.” When they ran down the slope the stones clattered together, I
remember doubting whether I should live to see these larger flints
covered with vegetable mould and turf. But the smaller stones
disappeared before many years had elapsed, as did every one of the larger
ones after a time; so that after thirty years (1871) a horse could gallop
over the compact turf from one end of the field to the other, and not
strike a single stone with his shoes. To anyone who remembered the
appearance of the field in 1842, the transformation was wonderful. This
was certainly the work of the worms, for though castings were not
frequent for several years, yet some were thrown up month after month,
and these gradually increased in numbers as the pasture improved. In the
year 1871 a trench was dug on the above slope, and the blades of grass
were cut off close to the roots, so that the thickness of the turf and of
the vegetable mould could be measured accurately. The turf was rather
less than half an inch, and the mould, which did not contain any stones,
2½ inches in thickness. Beneath this lay coarse clayey earth full of
flints, like that in any of the neighbouring ploughed fields. This
coarse earth easily fell apart from the overlying mould when a spit was
lifted up. The average rate of accumulation of the mould during the
whole thirty years was only .083 inch per year (i.e., nearly one inch in
twelve years); but the rate must have been much slower at first, and
afterwards considerably quicker.
The transformation in the appearance of this field, which had been
effected beneath my eyes, was afterwards rendered the more striking, when
I examined in Knole Park a dense forest of lofty beech-trees, beneath
which nothing grew. Here the ground was thickly strewed with large naked
stones, and worm-castings were almost wholly absent. Obscure lines and
irregularities on the surface indicated that the land had been cultivated
some centuries ago. It is probable that a thick wood of young
beech-trees sprung up so quickly, that time enough was not allowed for
worms to cover up the stones with their castings, before the site became
unfitted for their existence. Anyhow the contrast between the state of
the now miscalled “stony field,” well stocked with worms, and the present
state of the ground beneath the old beech-trees in Knole Park, where
worms appeared to be absent, was striking.
A narrow path running across part of my lawn was paved in 1843 with small
flagstones, set edgeways; but worms threw up many castings and weeds grew
thickly between them. During several years the path was weeded and
swept; but ultimately the weeds and worms prevailed, and the gardener
ceased to sweep, merely mowing off the weeds, as often as the lawn was
mowed. The path soon became almost covered up, and after several years
no trace of it was left. On removing, in 1877, the thin overlying layer
of turf, the small flag-stones, all in their proper places, were found
covered by an inch of fine mould.
Two recently published accounts of substances strewed on the surface of
pasture-land, having become buried through the action of worms, may be
here noticed. The Rev. H. C. Key had a ditch cut in a field, over which
coal-ashes had been spread, as it was believed, eighteen years before;
and on the clean-cut perpendicular sides of the ditch, at a depth of at
least seven inches, there could be seen, for a length of 60 yards, “a
distinct, very even, narrow line of coal-ashes, mixed with small coal,
perfectly parallel with the top-sward.” {136a} This parallelism and the
length of the section give interest to the case. Secondly, Mr. Dancer
states {136b} that crushed bones had been thickly strewed over a field;
and “some years afterwards” these were found “several inches below the
surface, at a uniform depth.”
The Rev. Mr. Zincke informs me that he has lately had an orchard dug to
the unusual depth of 4 feet. The upper 18 inches consisted of
dark-coloured vegetable mould, and the next 18 inches of sandy loam,
containing in the lower part many rolled pieces of sandstone, with some
bits of brick and tile, probably of Roman origin, as remains of this
period have been found close by. The sandy loam rested on an indurated
ferruginous pan of yellow clay, on the surface of which two perfect celts
were found. If, as seems probable, the celts were originally left on the
surface of the land, they have since been covered up with earth 3 feet in
thickness, all of which has probably passed through the bodies of worms,
excepting the stones which may have been scattered on the surface at
different times, together with manure or by other means. It is difficult
otherwise to understand the source of the 18 inches of sandy loam, which
differed from the overlying dark vegetable mould, after both had been
burnt, only in being of a brighter red colour, and in not being quite so
fine-grained. But on this view we must suppose that the carbon in
vegetable mould, when it lies at some little depth beneath the surface
and does not continually receive decaying vegetable matter from above,
loses its dark colour in the course of centuries; but whether this is
probable I do not know.
Worms appear to act in the same manner in New Zealand as in Europe; for
Professor J. von Haast has described {138a} a section near the coast,
consisting of mica-schist, “covered by 5 or 6 feet of loess, above which
about 12 inches of vegetable soil had accumulated.” Between the loess
and the mould there was a layer from 3 to 6 inches in thickness,
consisting of “cores, implements, flakes, and chips, all manufactured
from hard basaltic rock.” It is therefore probable that the aborigines,
at some former period, had left these objects on the surface, and that
they had afterwards been slowly covered up by the castings of worms.
Farmers in England are well aware that objects of all kinds, left on the
surface of pasture-land, after a time disappear, or, as they say, work
themselves downwards. How powdered lime, cinders, and heavy stones, can
work down, and at the same rate, through the matted roots of a
grass-covered surface, is a question which has probably never occurred to
them. {138b}
_The Sinking of great Stones through the Action of Worms_.—When a stone
of large size and of irregular shape is left on the surface of the
ground, it rests, of course, on the more protuberant parts; but worms
soon fill up with their castings all the hollow spaces on the lower side;
for, as Hensen remarks, they like the shelter of stones. As soon as the
hollows are filled up, the worms eject the earth which they have
swallowed beyond the circumference of the stones; and thus the surface of
the ground is raised all round the stone. As the burrows excavated
directly beneath the stone after a time collapse, the stone sinks a
little. {139} Hence it is, that boulders which at some ancient period
have rolled down from a rocky mountain or cliff on to a meadow at its
base, are always somewhat imbedded in the soil; and, when removed, leave
an exact impression of their lower surfaces in the underlying fine mould.
If, however, a boulder is of such huge dimensions, that the earth beneath
is kept dry, such earth will not be inhabited by worms, and the boulder
will not sink into the ground.
A lime-kiln formerly stood in a grass-field near Leith Hill Place in
Surrey, and was pulled down 35 years before my visit; all the loose
rubbish had been carted away, excepting three large stones of quartzose
sandstone, which it was thought might hereafter be of some use. An old
workman remembered that they had been left on a bare surface of broken
bricks and mortar, close to the foundations of the kiln; but the whole
surrounding surface is now covered with turf and mould. The two largest
of these stones had never since been moved; nor could this easily have
been done, as, when I had them removed, it was the work of two men with
levers. One of these stones, and not the largest, was 64 inches long, 17
inches broad, and from 9 to 10 inches in thickness. Its lower surface
was somewhat protuberant in the middle; and this part still rested on
broken bricks and mortar, showing the truth of the old workman’s account.
Beneath the brick rubbish the natural sandy soil, full of fragments of
sandstone was found; and this could have yielded very little, if at all,
to the weight of the stone, as might have been expected if the sub-soil
had been clay. The surface of the field, for a distance of about 9
inches round the stone, gradually sloped up to it, and close to the stone
stood in most places about 4 inches above the surrounding ground. The
base of the stone was buried from 1 to 2 inches beneath the general
level, and the upper surface projected about 8 inches above this level,
or about 4 inches above the sloping border of turf. After the removal of
the stone it became evident that one of its pointed ends must at first
have stood clear above the ground by some inches, but its upper surface
was now on a level with the surrounding turf. When the stone was
removed, an exact cast of its lower side, forming a shallow crateriform
hollow, was left, the inner surface of which consisted of fine black
mould, excepting where the more protuberant parts rested on the
brick-rubbish. A transverse section of this stone, together with its
bed, drawn from measurements made after it had been displaced, is here
given on a scale of ½ inch to a foot (Fig. 6). The turf-covered border
which sloped up to the stone, consisted of fine vegetable mould, in one
part 7 inches in thickness. This evidently consisted of worm-castings,
several of which had been recently ejected. The whole stone had sunk in
the thirty-five years, as far as I could judge, about 1½ inch; and this
must have been due to the brick-rubbish beneath the more protuberant
parts having been undermined by worms. At this rate the upper surface of
the stone, if it had been left undisturbed, would have sunk to the
general level of the field in 247 years; but before this could have
occurred, some earth would have been washed down by heavy rain from the
castings on the raised border of turf over the upper surface of the
stone.
The second stone was larger that the one just described, viz., 67 inches
in length, 39 in breadth, and 15 in thickness. The lower surface was
nearly flat, so that the worms must soon have been compelled to eject
their castings beyond its circumference. The stone as a whole had sunk
about 2 inches into the ground. At this rate it would have required 262
years for its upper surface to have sunk to the general level of the
field. The upwardly sloping, turf-covered border round the stone was
broader than in the last case, viz., from 14 to 16 inches; and why this
should be so, I could see no reason. In most parts this border was not
so high as in the last case, viz., from 2 to 2½ inches, but in one place
it was as much as 5½. Its average height close to the stone was probably
about 3 inches, and it thinned out to nothing. If so, a layer of fine
earth, 15 inches in breadth and 1½ inch in average thickness, of
sufficient length to surround the whole of the much elongated slab, must
have been brought up by the worms in chief part from beneath the stone in
the course of 35 years. This amount would be amply sufficient to account
for its having sunk about 2 inches into the ground; more especially if we
bear in mind that a good deal of the finest earth would have been washed
by heavy rain from the castings ejected on the sloping border down to the
level of the field. Some fresh castings were seen close to the stone.
Nevertheless, on digging a large hole to a depth of 18 inches where the
stone had lain, only two worms and a few burrows were seen, although the
soil was damp and seemed favourable for worms. There were some large
colonies of ants beneath the stone, and possibly since their
establishment the worms had decreased in number.
The third stone was only about half as large as the others; and two
strong boys could together have rolled it over. I have no doubt that it
had been rolled over at a moderately recent time, for it now lay at some
distance from the two other stones at the bottom of a little adjoining
slope. It rested also on fine earth, instead of partly on brick-rubbish.
In agreement with this conclusion, the raised surrounding border of turf
was only 1 inch high in some parts, and 2 inches in other parts. There
were no colonies of ants beneath this stone, and on digging a hole where
it had lain, several burrows and worms were found.
At Stonehenge, some of the outer Druidical stones are now prostrate,
having fallen at a remote but unknown period; and these have become
buried to a moderate depth in the ground. They are surrounded by sloping
borders of turf, on which recent castings were seen. Close to one of
these fallen stones, which was 17 ft long, 6 ft. broad, and 28½ inches
thick, a hole was dug; and here the vegetable mould was at least 9½
inches in thickness. At this depth a flint was found, and a little
higher up on one side of the hole a fragment of glass. The base of the
stone lay about 9½ inches beneath the level of the surrounding ground,
and its upper surface 19 inches above the ground.
A hole was also dug close to a second huge stone, which in falling had
broken into two pieces; and this must have happened long ago, judging
from the weathered aspect of the fractured ends. The base was buried to
a depth of 10 inches, as was ascertained by driving an iron skewer
horizontally into the ground beneath it. The vegetable mould forming the
turf-covered sloping border round the stone, on which many castings had
recently been ejected, was 10 inches in thickness; and most of this mould
must have been brought up by worms from beneath its base. At a distance
of 8 yards from the stone, the mould was only 5½ inches in thickness
(with a piece of tobacco pipe at a depth of 4 inches), and this rested on
broken flint and chalk which could not have easily yielded to the
pressure or weight of the stone.
A straight rod was fixed horizontally (by the aid of a spirit-level)
across a third fallen stone, which was 7 feet 9 inches long; and the
contour of the projecting parts and of the adjoining ground, which was
not quite level, was thus ascertained, as shown in the accompanying
diagram (Fig. 7) on a scale of ½ inch to a foot. The turf-covered border
sloped up to the stone on one side to a height of 4 inches, and on the
opposite side to only 2½ inches above the general level. A hole was dug
on the eastern side, and the base of the stone was here found to lie at a
depth of 4 inches beneath the general level of the ground, and of 8
inches beneath the top of the sloping turf-covered border.
* * * * *
Sufficient evidence has now been given showing that small objects left on
the surface of the land where worms abound soon get buried, and that
large stones sink slowly downwards through the same means. Every step of
the process could be followed, from the accidental deposition of a single
casting on a small object lying loose on the surface, to its being
entangled amidst the matted roots of the turf, and lastly to its being
embedded in the mould at various depths beneath the surface. When the
same field was re-examined after the interval of a few years, such
objects were found at a greater depth than before. The straightness and
regularity of the lines formed by the imbedded objects, and their
parallelism with the surface of the land, are the most striking features
of the case; for this parallelism shows how equably the worms must have
worked; the result being, however, partly the effect of the washing down
of the fresh castings by rain. The specific gravity of the objects does
not affect their rate of sinking, as could be seen by porous cinders,
burnt marl, chalk and quartz pebbles, having all sunk to the same depth
within the same time. Considering the nature of the substratum, which at
Leith Hill Place was sandy soil including many bits of rock, and at
Stonehenge, chalk-rubble with broken flints; considering, also, the
presence of the turf-covered sloping border of mould round the great
fragments of stone at both these places, their sinking does not appear to
have been sensibly aided by their weight, though this was considerable.
{147}
_On the number of worms which live within a given space_.—We will now
show, firstly, what a vast number of worms live unseen by us beneath our
feet, and, secondly, the actual weight of the earth which they bring up
to the surface within a given space and within a given time. Hensen, who
has published so full and interesting an account of the habits of worms,
{148} calculates, from the number which he found in a measured space,
that there must exist 133,000 living worms in a hectare of land, or
53,767 in an acre. This latter number of worms would weigh 356 pounds,
taking Hensen’s standard of the weight of a single worm, namely, three
grams. It should, however, be noted that this calculation is founded on
the numbers found in a garden, and Hensen believes that worms are here
twice as numerous as in corn-fields. The above result, astonishing
though it be, seems to me credible, judging from the number of worms
which I have sometimes seen, and from the number daily destroyed by birds
without the species being exterminated. Some barrels of bad ale were
left on Mr. Miller’s land, {149} in the hope of making vinegar, but the
vinegar proved bad, and the barrels were upset. It should be premised
that acetic acid is so deadly a poison to worms that Perrier found that a
glass rod dipped into this acid and then into a considerable body of
water in which worms were immersed, invariably killed them quickly. On
the morning after the barrels had been upset, “the heaps of worms which
lay dead on the ground were so amazing, that if Mr. Miller had not seen
them, he could not have thought it possible for such numbers to have
existed in the space.” As further evidence of the large number of worms
which live in the ground, Hensen states that he found in a garden
sixty-four open burrows in a space of 14½ square feet, that is, nine in 2
square feet. But the burrows are sometimes much more numerous, for when
digging in a grass-field near Maer Hall, I found a cake of dry earth, as
large as my two open hands, which was penetrated by seven burrows, as
large as goose-quills.
_Weight of the earth ejected from a single burrow_, _and from all the
burrows within a given space_.—With respect to the weight of the earth
daily ejected by worms, Hensen found that it amounted, in the case of
some worms which he kept in confinement, and which he appears to have fed
with leaves, to only 0.5 gram, or less than 8 grains per diem. But a
very much larger amount must be ejected by worms in their natural state,
at the periods when they consume earth as food instead of leaves, and
when they are making deep burrows. This is rendered almost certain by
the following weights of the castings thrown up at the mouths of single
burrows; the whole of which appeared to have been ejected within no long
time, as was certainly the case in several instances. The castings were
dried (excepting in one specified instance) by exposure during many days
to the sun or before a hot fire.
WEIGHT OF THE CASTINGS ACCUMULATED AT THE MOUTH OF A SINGLE BURROW.
(1.) Down, Kent (sub-soil red clay, full of flints, 3.98
over-lying the chalk). The largest casting which I could
find on the flanks of a steep valley, the sub-soil being
here shallow. In this one case, the casting was not well
dried
(2.) Down.—Largest casting which I could find (consisting 3.87
chiefly of calcareous matter), on extremely poor pasture
land at the bottom of the valley mentioned under (1.)
(3.) Down.—A large casting, but not of unusual size, from 1.22
a nearly level field, poor pasture, laid down in a grass
about 35 years before
(4.) Down. Average weight of 11 not large castings 0.7
ejected on a sloping surface on my lawn, after they had
suffered some loss of weight from being exposed during a
considerable length of time to rain
(5.) Near Nice in France.—Average weight of 12 castings of 1.37
ordinary dimensions, collected by Dr. King on land which
had not been mown for a long time and where worms abounded,
viz., a lawn protected by shrubberies near the sea; soil
sandy and calcareous; these castings had been exposed for
some time to rain, before being collected, and must have
lost some weight by disintegration, but they still retained
their form
(6.) The heaviest of the above twelve castings 1.76
(7.) Lower Bengal.—Average weight of 22 castings, 1.24
collected by Mr. J. Scott, and stated by him to have been
thrown up in the course of one or two nights
(8.) The heaviest of the above 22 castings 2.09
(9.) Nilgiri Mountains, S. India; average weight of the 5 3.15
largest castings collected by Dr. King. They had been
exposed to the rain of the last monsoon, and must have lost
some weight
(10.) The heaviest of the above 5 castings 4.34
In this table we see that castings which had been ejected at the mouth of
the same burrow, and which in most cases appeared fresh and always
retained their vermiform configuration, generally exceeded an ounce in
weight after being dried, and sometimes nearly equalled a quarter of a
pound. On the Nilgiri mountains one casting even exceeded this latter
weight. The largest castings in England were found on extremely poor
pasture-land; and these, as far as I have seen, are generally larger than
those on land producing a rich vegetation. It would appear that worms
have to swallow a greater amount of earth on poor than on rich land, in
order to obtain sufficient nutriment.
With respect to the tower-like castings near Nice (Nos. 5 and 6 in the
above table), Dr. King often found five or six of them on a square foot
of surface; and these, judging from their average weight, would have
weighed together 7½ ounces; so that the weight of those on a square yard
would have been 4 lb. 3½ oz. Dr. King collected, near the close of the
year 1872, all the castings which still retained their vermiform shape,
whether broken down or not, from a square foot, in a place abounding with
worms, on the summit of a bank, where no castings could have rolled down
from above. These castings must have been ejected, as he judged from
their appearance in reference to the rainy and dry periods near Nice,
within the previous five or six months; they weighed 9½ oz., or 5 lb. 5½
oz. per square yard. After an interval of four months, Dr. King
collected all the castings subsequently ejected on the same square foot
of surface, and they weighed 2½ oz., or 1 lb. 6½ oz. per square yard.
Therefore within about ten months, or we will say for safety’s sake
within a year, 12 oz. of castings were thrown up on this one square foot,
or 6.75 pounds on the square yard; and this would give 14.58 tons per
acre.
In a field at the bottom of a valley in the chalk (see No. 2 in the
foregoing table), a square yard was measured at a spot where very large
castings abounded; they appeared, however, almost equally numerous in a
few other places. These castings, which retained perfectly their
vermiform shape, were collected; and they weighed when partially dried, 1
lb. 13½ oz. This field had been rolled with a heavy agricultural roller
fifty-two days before, and this would certainly have flattened every
single casting on the land. The weather had been very dry for two or
three weeks before the day of collection, so that not one casting
appeared fresh or had been recently ejected. We may therefore assume
that those which were weighed had been ejected within, we will say, forty
days from the time when the field was rolled,—that is, twelve days short
of the whole intervening period. I had examined the same part of the
field shortly before it was rolled, and it then abounded with fresh
castings. Worms do not work in dry weather during the summer, or in
winter during severe frosts. If we assume that they work for only half
the year—though this is too low an estimate—then the worms in this field
would eject during the year, 8.387 pounds per square yard; or 18.12 tons
per acre, assuming the whole surface to be equally productive in
castings.
In the foregoing cases some of the necessary data had to be estimated,
but in the two following cases the results are much more trustworthy. A
lady, on whose accuracy I can implicitly rely, offered to collect during
a year all the castings thrown up on two separate square yards, near
Leith Hill Place, in Surrey. The amount collected was, however, somewhat
less than that originally ejected by the worms; for, as I have repeatedly
observed, a good deal of the finest earth is washed away, whenever
castings are thrown up during or shortly before heavy rain. Small
portions also adhered to the surrounding blades of grass, and it required
too much time to detach every one of them.
On sandy soil, as in the present instance, castings are liable to crumble
after dry weather, and particles were thus often lost. The lady also
occasionally left home for a week or two, and at such times the castings
must have suffered still greater loss from exposure to the weather.
These losses were, however, compensated to some extent by the collections
having been made on one of the squares for four days, and on the other
square for two days more than the year.
A space was selected (October 9th, 1870) for one of the squares on a
broad, grass-covered terrace, which had been mowed and swept during many
years. It faced the south, but was shaded during part of the day by
trees. It had been formed at least a century ago by a great accumulation
of small and large fragments of sandstone, together with some sandy
earth, rammed down level. It is probable that it was at first protected
by being covered with turf. This terrace, judging from the number of
castings on it, was rather unfavourable for the existence of worms, in
comparison with the neighbouring fields and an upper terrace. It was
indeed surprising that as many worms could live here as were seen; for on
digging a hole in this terrace, the black vegetable mould together with
the turf was only four inches in thickness, beneath which lay the level
surface of light-coloured sandy soil, with many fragments of sandstone.
Before any castings were collected all the previously existing ones were
carefully removed. The last day’s collection was on October 14th, 1871.
The castings were then well dried before a fire; and they weighed exactly
3½ lbs. This would give for an acre of similar land 7.56 tons of dry
earth annually ejected by worms.
The second square was marked on unenclosed common land, at a height of
about 700 ft. above the sea, at some little distance from Leith Hill
Tower. The surface was clothed with short, fine turf, and had never been
disturbed by the hand of man. The spot selected appeared neither
particularly favourable nor the reverse for worms; but I have often
noticed that castings are especially abundant on common land, and this
may, perhaps, be attributed to the poorness of the soil. The vegetable
mould was here between three and four inches in thickness. As this spot
was at some distance from the house where the lady lived, the castings
were not collected at such short intervals of time as those on the
terrace; consequently the loss of fine earth during rainy weather must
have been greater in this than in the last case. The castings moreover
were more sandy, and in collecting them during dry weather they sometimes
crumbled into dust, and much was thus lost. Therefore it is certain that
the worms brought up to the surface considerably more earth than that
which was collected. The last collection was made on October 27th, 1871;
i.e., 367 days after the square had been marked out and the surface
cleared of all pre-existing castings. The collected castings, after
being well dried, weighed 7.453 pounds; and this would give, for an acre
of the same kind of land, 16.1 tons of annually ejected dry earth.
SUMMARY OF THE FOUR FOREGOING CASES.
(1.) Castings ejected near Nice within about a year, collected by Dr.
King on a square foot of surface, calculated to yield per acre 14.58
tons.
(2.) Castings ejected during about 40 days on a square yard, in a
field of poor pasture at the bottom of a large valley in the Chalk,
calculated to yield annually per acre 18.12 tons.
(3.) Castings collected from a square yard on an old terrace at Leith
Hill Place, during 369 days, calculated to yield annually per acre
7.56 tons.
(4.) Castings collected from a square yard on Leith Hill Common
during 367 days, calculated to yield annually per acre 16.1 tons.
_The thickness of the layer of mould_, _which castings ejected during a
year would form if uniformly spread out_.—As we know, from the two last
cases in the above summary, the weight of the dried castings ejected by
worms during a year on a square yard of surface, I wished to learn how
thick a layer of ordinary mould this amount would form if spread
uniformly over a square yard. The dry castings were therefore broken
into small particles, and whilst being placed in a measure were well
shaken and pressed down. Those collected on the Terrace amounted to
124.77 cubic inches; and this amount, if spread out over a square yard,
would make a layer 0.9627 inch in thickness. Those collected on the
Common amounted to 197.56 cubic inches, and would make a similar layer
0.1524 inch in thickness.
These thicknesses must, however, be corrected, for the triturated
castings, after being well shaken down and pressed, did not make nearly
so compact a mass as vegetable mould, though each separate particle was
very compact. Yet mould is far from being compact, as is shown by the
number of air-bubbles which rise up when the surface is flooded with
water. It is moreover penetrated by many fine roots. To ascertain
approximately by how much ordinary vegetable mould would be increased in
bulk by being broken up into small particles and then dried, a thin
oblong block of somewhat argillaceous mould (with the turf pared off) was
measured before being broken up, was well dried and again measured. The
drying caused it to shrink by 1/7 of its original bulk, judging from
exterior measurements alone. It was then triturated and partly reduced
to powder, in the same manner as the castings had been treated, and its
bulk now exceeded (notwithstanding shrinkage from drying) by 1/16 that of
the original block of damp mould. Therefore the above calculated
thickness of the layer, formed by the castings from the Terrace, after
being damped and spread over a square yard, would have to be reduced by
1/16; and this will reduce the layer to 0.09 of an inch, so that a layer
0.9 inch in thickness would be formed in the course of ten years. On the
same principle the castings from the Common would make in the course of a
single year a layer 0.1429 inch, or in the course of 10 years 1.429 inch,
in thickness. We may say in round numbers that the thickness in the
former case would amount to nearly 1 inch, and in the second case to
nearly 1½ inch in 10 years.
In order to compare these results with those deduced from the rates at
which small objects left on the surfaces of grass-fields become buried
(as described in the early part of this chapter), we will give the
following summary:—
SUMMARY OF THE THICKNESS OF THE MOULD ACCUMULATED OVER OBJECTS LEFT
STREWED ON THE SURFACE, IN THE COURSE OF TEN YEARS.
The accumulation of mould during 14¾ years on the surface of a dry,
sandy, grass-field near Maer Hall, amounted to 2.2 inches in 10 years.
The accumulation during 21½ years on a swampy field near Maer Hall,
amounted to nearly 1.9 inch in 10 years.
The accumulation during 7 years on a very swampy field near Maer Hall
amounted to 2.1 inches in 10 years.
The accumulation during 29 years, on good, argillaceous pasture-land
over the Chalk at Down, amounted to 2.2 inches in 10 years.
The accumulation during 30 years on the side of a valley over the
Chalk at Down, the soil being argillaceous, very poor, and only just
converted into pasture (so that it was for some years unfavourable for
worms), amounted to 0.83 inch in 10 years.
In these cases (excepting the last) it may be seen that the amount of
earth brought to the surface during 10 years is somewhat greater than
that calculated from the castings which were actually weighed. This
excess may be partly accounted for by the loss which the weighed castings
had previously undergone through being washed by rain, by the adhesion of
particles to the blades of the surrounding grass, and by their crumbling
when dry. Nor must we overlook other agencies which in all ordinary
cases add to the amount of mould, and which would not be included in the
castings that were collected, namely, the fine earth brought up to the
surface by burrowing larvæ and insects, especially by ants. The earth
brought up by moles generally has a somewhat different appearance from
vegetable mould; but after a time would not be distinguishable from it.
In dry countries, moreover, the wind plays an important part in carrying
dust from one place to another, and even in England it must add to the
mould on fields near great roads. But in our country these latter
several agencies appear to be of quite subordinate importance in
comparison with the action of worms.
We have no means of judging how great a weight of earth a single
full-sized worm ejects during a year. Hensen estimates that 53,767 worms
exist in an acre of land; but this is founded on the number found in
gardens, and he believes that only about half as many live in
corn-fields. How many live in old pasture land is unknown; but if we
assume that half the above number, or 26,886 worms live on such land,
then taking from the previous summary 15 tons as the weight of the
castings annually thrown up on an acre of land, each worm must annually
eject 20 ounces. A full-sized casting at the mouth of a single burrow
often exceeds, as we have seen, an ounce in weight; and it is probable
that worms eject more than 20 full-sized castings during a year. If they
eject annually more than 20 ounces, we may infer that the worms which
live in an acre of pasture land must be less than 26,886 in number.
Worms live chiefly in the superficial mould, which is usually from 4 or 5
to 10 and even 12 inches in thickness; and it is this mould which passes
over and over again through their bodies and is brought to the surface.
But worms occasionally burrow into the subsoil to a much greater depth,
and on such occasions they bring up earth from this greater depth; and
this process has gone on for countless ages. Therefore the superficial
layer of mould would ultimately attain, though at a slower and slower
rate, a thickness equal to the depth to which worms ever burrow, were
there not other opposing agencies at work which carry away to a lower
level some of the finest earth which is continually being brought to the
surface by worms. How great a thickness vegetable mould ever attains, I
have not had good opportunities for observing; but in the next chapter,
when we consider the burial of ancient buildings, some facts will be
given on this head. In the two last chapters we shall see that the soil
is actually increased, though only to a small degree, through the agency
of worms; but their chief work is to sift the finer from the coarser
particles, to mingle the whole with vegetable débris, and to saturate it
with their intestinal secretions.
Finally, no one who considers the facts given in this chapter—on the
burying of small objects and on the sinking of great stones left on the
surface—on the vast number of worms which live within a moderate extent
of ground on the weight of the castings ejected from the mouth of the
same burrow—on the weight of all the castings ejected within a known time
on a measured space—will hereafter, as I believe, doubt that worms play
an important part in nature.
CHAPTER IV.
THE PART WHICH WORMS HAVE PLAYED IN THE BURIAL OF ANCIENT BUILDINGS.
The accumulation of rubbish on the sites of great cities independent of
the action of worms—The burial of a Roman villa at Abinger—The floors and
walls penetrated by worms—Subsidence of a modern pavement—The buried
pavement at Beaulieu Abbey—Roman villas at Chedworth and Brading—The
remains of the Roman town at Silchester—The nature of the débris by which
the remains are covered—The penetration of the tesselated floors and
walls by worms—Subsidence of the floors—Thickness of the mould—The old
Roman city of Wroxeter—Thickness of the mould—Depth of the foundations of
some of the Buildings—Conclusion.
ARCHÆOLOGISTS are probably not aware how much they owe to worms for the
preservation of many ancient objects. Coins, gold ornaments, stone
implements, &c., if dropped on the surface of the ground, will infallibly
be buried by the castings of worms in a few years, and will thus be
safely preserved, until the land at some future time is turned up. For
instance, many years ago a grass-field was ploughed on the northern side
of the Severn, not far from Shrewsbury; and a surprising number of iron
arrow-heads were found at the bottom of the furrows, which, as Mr.
Blakeway, a local antiquary, believed, were relics of the battle of
Shrewsbury in the year 1403, and no doubt had been originally left
strewed on the battle-field. In the present chapter I shall show that
not only implements, &c., are thus preserved, but that the floors and the
remains of many ancient buildings in England have been buried so
effectually, in large part through the action of worms, that they have
been discovered in recent times solely through various accidents. The
enormous beds of rubbish, several yards in thickness, which underlie many
cities, such as Rome, Paris, and London, the lower ones being of great
antiquity, are not here referred to, as they have not been in any way
acted on by worms. When we consider how much matter is daily brought
into a great city for building, fuel, clothing and food, and that in old
times when the roads were bad and the work of the scavenger was
neglected, a comparatively small amount was carried away, we may agree
with Élie de Beaumont, who, in discussing this subject, says, “pour une
voiture de matériaux qui en sort, on y en fait entrer cent.” {166a} Nor
should we overlook the effects of fires, the demolition of old buildings,
and the removal of rubbish to the nearest vacant space.
_Abinger_, _Surrey_.—Late in the autumn of 1876, the ground in an old
farm-yard at this place was dug to a depth of 2 to 2½ feet, and the
workmen found various ancient remains. This led Mr. T. H. Farrer of
Abinger Hall to have an adjoining ploughed field searched. On a trench
being dug, a layer of concrete, still partly covered with tesseræ (small
red tiles), and surrounded on two sides by broken-down walls, was soon
discovered. It is believed, {166b} that this room formed part of the
atrium or reception-room of a Roman villa. The walls of two or three
other small rooms were afterwards discovered. Many fragments of pottery,
other objects, and coins of several Roman emperors, dating from 133 to
361, and perhaps to 375 A.D., were likewise found. Also a half-penny of
George I., 1715. The presence of this latter coin seems an anomaly; but
no doubt it was dropped on the ground during the last century, and since
then there has been ample time for its burial under a considerable depth
of the castings of worms. From the different dates of the Roman coins we
may infer that the building was long inhabited. It was probably ruined
and deserted 1400 or 1500 years ago.
I was present during the commencement of the excavations (August 20,
1877) and Mr. Farrer had two deep trenches dug at opposite ends of the
atrium, so that I might examine the nature of the soil near the remains.
The field sloped from east to west at an angle of about 7°; and one of
the two trenches, shown in the accompanying section (Fig. 8) was at the
upper or eastern end. The diagram is on a scale of 1/20 of an inch to an
inch; but the trench, which was between 4 and 5 feet broad, and in parts
above 5 feet deep, has necessarily been reduced out of all proportion.
The fine mould over the floor of the atrium varied in thickness from 11
to 16 inches; and on the side of the trench in the section was a little
over 13 inches. After the mould had been removed, the floor appeared as
a whole moderately level; but it sloped in parts at an angle of 1°, and
in one place near the outside at as much as 8° 30′. The wall surrounding
the pavement was built of rough stones, and was 23 inches in thickness
where the trench was dug. Its broken summit was here 13 inches, but in
another part 15 inches, beneath the surface of the field, being covered
by this thickness of mould. In one spot, however, it rose to within 6
inches of the surface. On two sides of the room, where the junction of
the concrete floor with the bounding walls could be carefully examined,
there was no crack or separation. This trench afterwards proved to have
been
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