THE FORMATION OF VEGETABLE MOULD
THROUGH THE ACTION OF WORMS
WITH OBSERVATIONS ON THEIR HABITS.
by Charles Darwin
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 Elie de Beaumont, who
generally undervalues small agencies and their accumulated effects,
remarks: {1} "La couche tres-mince de la terre vegetale est un
monument d'une haute antiquite, et, par le fait de sa permanence,
un objet digne d'occuper le geologue, et capable de lui fournir des
remarques interessantes." 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, {2} "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 Elie de Beaumont, wrote
about my "singuliere theorie," and objected that it could apply
only to "les prairies basses et humides;" and that "les terres
labourees, les bois, les prairies elevees, n'apportent aucune
preuve a l'appui de cette maniere de voir." {3} 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. {4} With respect to "prairies elevees," 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 "Amenagement des Forets"
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 10th, 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; {7} 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. {8}
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. {9} 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.
{10} 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. {11} 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 larvae 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 {12} 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
oesophagus, 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 oesophagus 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 Claparede {13} 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.
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. {14}
Hoffmeister, however, states {15} 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 0.25 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. {16}
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. {17} 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 Perichaeta 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 {18} 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.
Leon Fredericq states {19} 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. {20} 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. {21}
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, {22} 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 Dionaea; 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 Urochaeta and some other genera. {23} The two posterior pairs
are formed by lamellae, which, according to Claparede, are
diverticula from the oesophagus. {24} These lamellae 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 microns. They contain in their centres a little excessively fine
granular matter; but they look so like oil globules that Claparede
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 lamellae; but I have seen only one case, and
Claparede 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 1.5
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.
Claparede 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, lamellae 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 lamellae. After the
formation and expulsion of a large concretion, new lamellae 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 lamellae.
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 lamellae 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 lamellae;
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. {25} 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." {26}
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." {27} 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. Claparede 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. {28} 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." {29} 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. {30}
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, {31} 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 i6 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 land 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 2.5 to 4.5
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 8.5 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. {32} 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 2.5 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:-
Drawn
into the Drawn in, Drawn in,
Nature of Object. burrows, by or by or
by or near near
near the the the
apex. middle. base.
Leaves of various kinds 80 11 9
- of the Lime, basal margin
of blade broad, apex
acuminated 79 17 4
- of a Laburnum, basal part of
blade as narrow as, or some-
times little narrower than
the apical part 63 10 27
- of the Rhododendron, basal
part of blade often narrower
than the apical part 34 ... 66
- of Pine-trees, consisting of
two needles arising from a
common base ... ... 100
Petioles of a Clematis,
somewhat pointed at the apex,
and blunt at the base 76 ... 24
- of the Ash, the thick basal
end often drawn in to serve
as food 48.5 ... 51.5
- of Robinia, extremely thin,
especially towards the apex,
so as to be ill-fitted for
plugging up the burrows 44 ... 56
Triangles of paper, of the
two sizes 62 15 23
- of the broad ones alone 59 25 16
- of the narrow ones alone 65 14 21
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 a
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 {33} 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 antennae. 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. {34} A cobra-snake was seen by Mr. Layard {35} 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, {36} 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 Claparede,
evidence in its favour must be given in some detail. There is no a
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. {37}
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 Claparede, 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
Perichaeta, which have been naturalized here from the East. {38}
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 2.5 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. {39} But we have seen that worms
eagerly devour raw meat, fat, and dead worms; and ordinary mould
can hardly fail to contain many ova, larvae, 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 1.5 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 1.5 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, 3.5 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 3.5 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 1.5 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. {40} 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. {41} They
inhabit the most isolated islands; they abound in Iceland, and are
known to exist in the West Indies, St. Helena, Madagascar, New
Caledonia and Tahiti. In the Antarctic regions, worms from
Kerguelen Land have been described by Ray Lankester; and I found
them in the Falkland Islands. How they reach such isolated islands
is at present quite unknown. They are easily killed by salt-water,
and it does not appear probable that young worms or their egg-
capsules could be carried in earth adhering to the feet or beaks of
land-birds. Moreover Kerguelen Land is not now inhabited by any
land-bird.
In this volume we are chiefly concerned with the earth cast up by
worms, and I have gleaned a few facts on this subject with respect
to distant lands. Worms throw up plenty of castings in the United
States. In Venezuela, castings, probably ejected by species of
Urochaeta, are common in the gardens and fields, but not in the
forests, as I hear from Dr. Ernst of Caracas. He collected 156
castings from the court-yard of his house, having an area of 200
square yards. They varied in bulk from half a cubic centimeter to
five cubic centimeters, and were on an average three cubic
centimeters. They were, therefore, of small size in comparison
with those often found in England; for six large castings from a
field near my house averaged 16 cubic centimeters. Several species
of earth-worms are common in St. Catharina in South Brazil, and
Fritz Muller informs me "that in most parts of the forests and
pasture-lands, the whole soil, to a depth of a quarter of a metre,
looks as if it had passed repeatedly through the intestines of
earth-worms, even where hardly any castings are to be seen on the
surface." A gigantic but very rare species is found there, the
burrows of which are sometimes even two centimeters or nearly 0.8
of an inch in diameter, and which apparently penetrate the ground
to a great depth.
In the dry climate of New South Wales, I hardly expected that worms
would be common; but Dr. G. Krefft of Sydney, to whom I applied,
after making inquiries from gardeners and others, and from his own
observations, informs me that their castings abound. He sent me
some collected after heavy rain, and they consisted of little
pellets, about 0.15 inch in diameter; and the blackened sandy earth
of which they were formed still cohered with considerable tenacity.
The late Mr. John Scott of the Botanic Gardens near Calcutta made
many observations for me on worms living under the hot and humid
climate of Bengal. The castings abound almost everywhere, in
jungles and in the open ground, to a greater degree, as he thinks,
than in England. After the water has subsided from the flooded
rice-fields, the whole surface very soon becomes studded with
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
Perichaeta. They stand up like towers, with an open passage in the
centre.
A figure of one of these castings from a photograph is here given
(Fig. 3). The largest received by me was 3.5 inches in height and
1.35 inch in diameter; another was only 0.75 inch in diameter and
2.75 in height. In the following year, Mr. Scott measured several
of the largest; one was 6 inches in height and nearly 1.5 in
diameter: two others were 5 inches in height and respectively 2
and rather more than 2.5 inches in diameter. The average weight of
the 22 castings sent to me was 35 grammes (1.25 oz.); and one of
them weighed 44.8 grammes (or 2 oz.). All these castings were
thrown up either in one night or in two. Where the ground in
Bengal is dry, as under large trees, castings of a different kind
are found in vast numbers: these consist of little oval or conical
bodies, from about the 1/20 to rather above 1/10 of an inch in
length. They are obviously voided by a distinct species of worms.
The period during which worms near Calcutta display such
extraordinary activity lasts for only a little over two months,
namely, during the cool season after the rains. At this time they
are generally found within about 10 inches beneath the surface.
During the hot season they burrow to a greater depth, and are then
found coiled up and apparently hybernating. Mr. Scott has never
seen them at a greater depth than 2.5 feet, but has heard of their
having been found at 4 feet. Within the forests, fresh castings
may be found even during the hot season. The worms in the Botanic
garden, during the cool and dry season, draw many leaves and little
sticks into the mouths of their burrows, like our English worms;
but they rarely act in this manner during the rainy season.
Mr. Scott saw worm-castings on the lofty mountains of Sikkim in
North India. In South India Dr. King found in one place, on the
plateau of the Nilgiris, at an elevation of 7000 feet, "a good many
castings," which are interesting for their great size. The worms
which eject them are seen only during the wet season, and are
reported to be from 12 to 15 inches in length, and as thick as a
man's little finger. These castings were collected by Dr. King
after a period of 110 days without any rain; and they must have
been ejected either during the north-east or more probably during
the previous south-west monsoon; for their surfaces had suffered
some disintegration and they were penetrated by many fine roots. A
drawing is here given (Fig. 4) of one which seems to have best
retained its original size and appearance. Notwithstanding some
loss from disintegration, five of the largest of these castings
(after having been well sun-dried) weighed each on an average 89.5
grammes, or above 3 oz.; and the largest weighed 123.14 grammes, or
4.33 oz.,--that is, above a quarter of a pound! The largest
convolutions were rather more than one inch in diameter; but it is
probable that they had subsided a little whilst soft, and that
their diameters had thus been increased. Some had flowed so much
that they now consisted of a pile of almost flat confluent cakes.
All were formed of fine, rather light-coloured earth, and were
surprisingly hard and compact, owing no doubt to the animal matter
by which the particles of earth had been cemented together. They
did not disintegrate, even when left for some hours in water.
Although they had been cast up on the surface of gravelly soil,
they contained extremely few bits of rock, the largest of which was
only 0.15 inch in diameter.
Dr. King saw in Ceylon a worm about 2 feet in length and 0.5 inch
in diameter; and he was told that it was a very common species
during the wet season. These worms must throw up castings at least
as large as those on the Nilgiri Mountains; but Dr. King saw none
during his short visit to Ceylon.
Sufficient facts have now been given, showing that worms do much
work in bringing up fine earth to the surface in most or all parts
of the world, and under the most different climates.
CHAPTER 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, 0.5 inch in
thickness, beneath which, at a depth of 2.5 inches (or 3 inches
from the surface), a layer of the lime in powder or in small lumps
could be distinctly seen running all round the vertical sides of
the holes. The soil beneath the layer of lime was either gravelly
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
4.75 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 0.75 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 3.5 inches, beneath which at a depth
of 9.5 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 3.5 inches beneath the surface; and below them at
a depth in parts of 9.5, and in other parts of 10.5 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.
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 1 inch thick, beneath which there was a
layer of vegetable mould 2.5 inches thick. This layer did not
contain fragments of any kind; but beneath it there was a layer of
mould, 1.5 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 4.5 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 2.5 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 6.5 years, about 1.5 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 21.5 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 2.5
years, the sand formed a layer at a depth of 0.75 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 1.5 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, {42}
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 32.5 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. {43} 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,
2.25 inches in thickness. In this latter case the mould was
altogether 9.25 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. {44}
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 5.5 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 4.125 to 4.375 inches in thickness. In another
and undisturbed place, the mould varied much in thickness, namely,
from 6.5 to 8.5 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 degrees to 15 degrees); 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, 2.5
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." {45} This parallelism and the length of the section
give interest to the case. Secondly, Mr. Dancer states {46} 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 {47} 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. {48}
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. {49} 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 0.5 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 1.5 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 2.5 inches, but in one place it
was as much as 5.5. 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 1.5 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 28.5 inches thick, a hole was dug; and here
the vegetable mould was at least 9.5 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 9.5
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 5.5 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 0.5 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 2.5
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. {50}
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, {51} 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, {52} 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 14.5 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.
(Weight in ounces given in parenthesis--DP.)
(1.) Down, Kent (sub-soil red clay, full of flints, 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 (3.98)
(2.) Down.--Largest casting which I could find (consisting chiefly
of calcareous matter), on extremely poor pasture land at the bottom
of the valley mentioned under (1.) (3.87)
(3.) Down.--A large casting, but not of unusual size, from a
nearly level field, poor pasture, laid down in a grass about 35
years before (1.22)
(4.) Down. Average weight of 11 not large castings 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 (0.7)
(5.) Near Nice in France.--Average weight of 12 castings of
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 (1.37)
(6.) The heaviest of the above twelve castings (1.76)
(7.) Lower Bengal.--Average weight of 22 castings, collected by
Mr. J. Scott, and stated by him to have been thrown up in the
course of one or two nights (1.24)
(8.) The heaviest of the above 22 castings (2.09)
(9.) Nilgiri Mountains, S. India; average weight of the 5 largest
castings collected by Dr. King. They had been exposed to the rain
of the last monsoon, and must have lost some weight (3.15)
(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 7.5 ounces; so that the weight
of those on a square yard would have been 4 lb. 3.5 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 9.5 oz., or 5
lb. 5.5 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 2.5 oz., or 1 lb. 6.5 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. 13.5 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 3.5 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 1.5 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 14.75 years on the surface of a
dry, sandy, grass-field near Maer Hall, amounted to 2.2 inches in
10 years.
The accumulation during 21.5 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 larvae 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 debris, 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 debris 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.
Archaeologists 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 Elie
de Beaumont, who, in discussing this subject, says, "pour une
voiture de materiaux qui en sort, on y en fait entrer cent." {53}
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 2.5 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 tesserae (small red tiles), and surrounded on two
sides by broken-down walls, was soon discovered. It is believed,
{54} 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 degrees; 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 degree, and in one place near the outside at as much as
8 degrees 30 minutes. 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 dug within an adjoining room (11 ft.
by 11 ft. 6 in. in size), the existence of which was not even
suspected whilst I was present.
On the side of the trench farthest from the buried wall (W), the
mould varied from 9 to 14 inches in thickness; it rested on a mass
(B) 23 inches thick of blackish earth, including many large stones.
Beneath this was a thin bed of very black mould (C), then a layer
of earth full of fragments of mortar (D), and then another thin bed
(about 3 inches thick) (E) of very black mould, which rested on the
undisturbed subsoil (F) of firm, yellowish, argillaceous sand. The
23-inch bed (B) was probably made ground, as this would have
brought up the floor of the room to a level with that of the
atrium. The two thin beds of black mould at the bottom of the
trench evidently marked two former land-surfaces. Outside the
walls of the northern room, many bones, ashes, oyster-shells,
broken pottery and an entire pot were subsequently found at a depth
of 16 inches beneath the surface.
The second trench was dug on the western or lower side of the
villa: the mould was here only 6.5 inches in thickness, and it
rested on a mass of fine earth full of stones, broken tiles and
fragments of mortar, 34 inches in thickness, beneath which was the
undisturbed sand. Most of this earth had probably been washed down
from the upper part of the field, and the fragments of stones,
tiles, &c., must have come from the immediately adjoining ruins.
It appears at first sight a surprising fact that this field of
light sandy soil should have been cultivated and ploughed during
many years, and that not a vestige of these buildings should have
been discovered. No one even suspected that the remains of a Roman
villa lay hidden close beneath the surface. But the fact is less
surprising when it is known that the field, as the bailiff
believed, had never been ploughed to a greater depth than 4 inches.
It is certain that when the land was first ploughed, the pavement
and the surrounding broken walls must have been covered by at least
4 inches of soil, for otherwise the rotten concrete floor would
have been scored by the ploughshare, the tesserae torn up, and the
tops of the old walls knocked down.
When the concrete and tesserae were first cleared over a space of
14 by 9 ft., the floor which was coated with trodden-down earth
exhibited no signs of having been penetrated by worms; and although
the overlying fine mould closely resembled that which in many
places has certainly been accumulated by worms, yet it seemed
hardly possible that this mould could have been brought up by worms
from beneath the apparently sound floor. It seemed also extremely
improbable that the thick walls, surrounding the room and still
united to the concrete, had been undermined by worms, and had thus
been caused to sink, being afterwards covered up by their castings.
I therefore at first concluded that all the fine mould above the
ruins had been washed down from the upper parts of the field; but
we shall soon see that this conclusion was certainly erroneous,
though much fine earth is known to be washed down from the upper
part of the field in its present ploughed state during heavy rains.
Although the concrete floor did not at first appear to have been
anywhere penetrated by worms, yet by the next morning little cakes
of the trodden-down earth had been lifted up by worms over the
mouths of seven burrows, which passed through the softer parts of
the naked concrete, or between the interstices of the tesserae. On
the third morning twenty-five burrows were counted; and by suddenly
lifting up the little cakes of earth, four worms were seen in the
act of quickly retreating. Two castings were thrown up during the
third night on the floor, and these were of large size. The season
was not favourable for the full activity of worms, and the weather
had lately been hot and dry, so that most of the worms now lived at
a considerable depth. In digging the two trenches many open
burrows and some worms were encountered at between 30 and 40 inches
beneath the surface; but at a greater depth they became rare. One
worm, however, was cut through at 48.5, and another at 51.5 inches
beneath the surface. A fresh humus-lined burrow was also met with
at a depth of 57 and another at 65.5 inches. At greater depths
than this, neither burrows nor worms were seen.
As I wished to learn how many worms lived beneath the floor of the
atrium--a space of about 14 by 9 feet--Mr. Farrer was so kind as to
make observations for me, during the next seven weeks, by which
time the worms in the surrounding country were in full activity,
and were working near the surface. It is very improbable that
worms should have migrated from the adjoining field into the small
space of the atrium, after the superficial mould in which they
prefer to live, had been removed. We may therefore conclude that
the burrows and the castings which were seen here during the
ensuing seven weeks were the work of the former inhabitants of the
space. I will now give a few extracts from Mr. Farrer's notes.
Aug. 26th, 1877; that is, five days after the floor had been
cleared. On the previous night there had been some heavy rain,
which washed the surface clean, and now the mouths of forty burrows
were counted. Parts of the concrete were seen to be solid, and had
never been penetrated by worms, and here the rain-water lodged.
Sept. 5th.--Tracks of worms, made during the previous night, could
be seen on the surface of the floor, and five or six vermiform
castings had been thrown up. These were defaced.
Sept. 12th.--During the last six days, the worms have not been
active, though many castings have been ejected in the neighbouring
fields; but on this day the earth was a little raised over the
mouths of the burrows, or castings were ejected, at ten fresh
points. These were defaced. It should be understood that when a
fresh burrow is spoken of, this generally means only that an old
burrow has been re-opened. Mr. Farrer was repeatedly struck with
the pertinacity with which the worms re-opened their old burrows,
even when no earth was ejected from them. I have often observed
the same fact, and generally the mouths of the burrows are
protected by an accumulation of pebbles, sticks or leaves. Mr.
Farrer likewise observed that the worms living beneath the floor of
the atrium often collected coarse grains of sand, and such little
stones as they could find, round the mouths of their burrows.
Sept. 13th; soft wet weather. The mouths of the burrows were re-
opened, or castings were ejected, at 31 points; these were all
defaced.
Sept. 14th; 34 fresh holes or castings; all defaced.
Sept. 15th; 44 fresh holes, only 5 castings; all defaced.
Sept. 18th; 43 fresh holes, 8 castings; all defaced.
The number of castings on the surrounding fields was now very
large.
Sept. 19th; 40 holes, 8 castings; all defaced.
Sept. 22nd; 43 holes, only a few fresh castings; all defaced.
Sept. 23rd; 44 holes, 8 castings.
Sept. 25th; 50 holes, no record of the number of castings.
Oct. 13th; 61 holes, no record of the number of castings.
After an interval of three years, Mr. Farrer, at my request, again
looked at the concrete floor, and found the worms still at work.
Knowing what great muscular power worms possess, and seeing how
soft the concrete was in many parts, I was not surprised at its
having been penetrated by their burrows; but it is a more
surprising fact that the mortar between the rough stones of the
thick walls, surrounding the rooms, was found by Mr. Farrer to have
been penetrated by worms. On August 26th, that is, five days after
the ruins had been exposed, he observed four open burrows on the
broken summit of the eastern wall (W in Fig. 8); and, on September
15th, other burrows similarly situated were seen. It should also
be noted that in the perpendicular side of the trench (which was
much deeper than is represented in Fig. 8) three recent burrows
were seen, which ran obliquely far down beneath the base of the old
wall.
We thus see that many worms lived beneath the floor and the walls
of the atrium at the time when the excavations were made; and that
they afterwards almost daily brought up earth to the surface from a
considerable depth. There is not the slightest reason to doubt
that worms have acted in this manner ever since the period when the
concrete was sufficiently decayed to allow them to penetrate it;
and even before that period they would have lived beneath the
floor, as soon as it became pervious to rain, so that the soil
beneath was kept damp. The floor and the walls must therefore have
been continually undermined; and fine earth must have been heaped
on them during many centuries, perhaps for a thousand years. If
the burrows beneath the floor and walls, which it is probable were
formerly as numerous as they now are, had not collapsed in the
course of time in the manner formerly explained, the underlying
earth would have been riddled with passages like a sponge; and as
this was not the case, we may feel sure that they have collapsed.
The inevitable result of such collapsing during successive
centuries, will have been the slow subsidence of the floor and of
the walls, and their burial beneath the accumulated worm-castings.
The subsidence of a floor, whilst it still remains nearly
horizontal, may at first appear improbable; but the case presents
no more real difficulty than that of loose objects strewed on the
surface of a field, which, as we have seen, become buried several
inches beneath the surface in the course of a few years, though
still forming a horizontal layer parallel to the surface. The
burial of the paved and level path on my lawn, which took place
under my own observation, is an analogous case. Even those parts
of the concrete floor which the worms could not penetrate would
almost certainly have been undermined, and would have sunk, like
the great stones at Leith Hill Place and Stonehenge, for the soil
would have been damp beneath them. But the rate of sinking of the
different parts would not have been quite equal, and the floor was
not quite level. The foundations of the boundary walls lie, as
shown in the section, at a very small depth beneath the surface;
they would therefore have tended to subside at nearly the same rate
as the floor. But this would not have occurred if the foundations
had been deep, as in the case of some other Roman ruins presently
to be described.
Finally, we may infer that a large part of the fine vegetable
mould, which covered the floor and the broken-down walls of this
villa, in some places to a thickness of 16 inches, was brought up
from below by worms. From facts hereafter to be given there can be
no doubt that some of the finest earth thus brought up will have
been washed down the sloping surface of the field during every
heavy shower of rain. If this had not occurred a greater amount of
mould would have accumulated over the ruins than that now present.
But beside the castings of worms and some earth brought up by
insects, and some accumulation of dust, much fine earth will have
been washed over the ruins from the upper parts of the field, since
it has been under cultivation; and from over the ruins to the lower
parts of the slope; the present thickness of the mould being the
resultant of these several agencies.
I may here append a modern instance of the sinking of a pavement,
communicated to me in 1871 by Mr. Ramsay, Director of the
Geological Survey of England. A passage without a roof, 7 feet in
length by 3 feet 2 inches in width, led from his house into the
garden, and was paved with slabs of Portland stone. Several of
these slabs were 16 inches square, others larger, and some a little
smaller. This pavement had subsided about 3 inches along the
middle of the passage, and two inches on each side, as could be
seen by the lines of cement by which the slabs had been originally
joined to the walls. The pavement had thus become slightly concave
along the middle; but there was no subsidence at the end close to
the house. Mr. Ramsay could not account for this sinking, until he
observed that castings of black mould were frequently ejected along
the lines of junction between the slabs; and these castings were
regularly swept away. The several lines of junction, including
those with the lateral walls, were altogether 39 feet 2 inches in
length. The pavement did not present the appearance of ever having
been renewed, and the house was believed to have been built about
eighty-seven years ago. Considering all these circumstances, Mr.
Ramsay does not doubt that the earth brought up by the worms since
the pavement was first laid down, or rather since the decay of the
mortar allowed the worms to burrow through it, and therefore within
a much shorter time than the eighty-seven years, has sufficed to
cause the sinking of the pavement to the above amount, except close
to the house, where the ground beneath would have been kept nearly
dry.
Beaulieu Abbey, Hampshire.--This abbey was destroyed by Henry
VIII., and there now remains only a portion of the southern aisle-
wall. It is believed that the king had most of the stones carried
away for building a castle; and it is certain that they have been
removed. The positions of the nave and transepts were ascertained
not long ago by the foundations having been found; and the place is
now marked by stones let into the ground. Where the abbey formerly
stood, there now extends a smooth grass-covered surface, which
resembles in all respects the rest of the field. The guardian, a
very old man, said the surface had never been levelled in his time.
In the year 1853, the Duke of Buccleuch had three holes dug in the
turf within a few yards of one another, at the western end of the
nave; and the old tesselated pavement of the abbey was thus
discovered. These holes were afterwards surrounded by brickwork,
and protected by trap-doors, so that the pavement might be readily
inspected and preserved. When my son William examined the place on
January 5, 1872, he found that the pavement in the three holes lay
at depths of 6.75, 10 and 11.5 inches beneath the surrounding turf-
covered surface. The old guardian asserted that he was often
forced to remove worm-castings from the pavement; and that he had
done so about six months before. My son collected all from one of
the holes, the area of which was 5.32 square feet, and they weighed
7.97 ounces. Assuming that this amount had accumulated in six
months, the accumulation during a year on a square yard would be
1.68 pounds, which, though a large amount, is very small compared
with what, as we have seen, is often ejected on fields and commons.
When I visited the abbey on June 22, 1877, the old man said that he
had cleared out the holes about a month before, but a good many
castings had since been ejected. I suspect that he imagined that
he swept the pavements oftener than he really did, for the
conditions were in several respects very unfavourable for the
accumulation of even a moderate amount of castings. The tiles are
rather large, viz., about 5.5 inches square, and the mortar between
them was in most places sound, so that the worms were able to bring
up earth from below only at certain points. The tiles rested on a
bed of concrete, and the castings in consequence consisted in large
part (viz., in the proportion of 19 to 33) of particles of mortar,
grains of sand, little fragments of rock, bricks or tile; and such
substances could hardly be agreeable, and certainly not nutritious,
to worms.
My son dug holes in several places within the former walls of the
abbey, at a distance of several yards from the above described
bricked squares. He did not find any tiles, though these are known
to occur in some other parts, but he came in one spot to concrete
on which tiles had once rested. The fine mould beneath the turf on
the sides of the several holes, varied in thickness from only 2 to
2.75 inches, and this rested on a layer from 8.75 to above 11
inches in thickness, consisting of fragments of mortar and stone-
rubbish with the interstices compactly filled up with black mould.
In the surrounding field, at a distance of 20 yards from the abbey,
the fine vegetable mould was 11 inches thick.
We may conclude from these facts that when the abbey was destroyed
and the stones removed, a layer of rubbish was left over the whole
surface, and that as soon as the worms were able to penetrate the
decayed concrete and the joints between the tiles, they slowly
filled up the interstices in the overlying rubbish with their
castings, which were afterwards accumulated to a thickness of
nearly three inches over the whole surface. If we add to this
latter amount the mould between the fragments of stones, some five
or six inches of mould must have been brought up from beneath the
concrete or tiles. The concrete or tiles will consequently have
subsided to nearly this amount. The bases of the columns of the
aisles are now buried beneath mould and turf. It is not probable
that they can have been undermined by worms, for their foundations
would no doubt have been laid at a considerable depth. If they
have not subsided, the stones of which the columns were constructed
must have been removed from beneath the former level of the floor.
Chedworth, Gloucestershire.--The remains of a large Roman villa
were discovered here in 1866, on ground which had been covered with
wood from time immemorial. No suspicion seems ever to have been
entertained that ancient buildings lay buried here, until a
gamekeeper, in digging for rabbits, encountered some remains. {55}
But subsequently the tops of some stone walls were detected in
parts of the wood, projecting a little above the surface of the
ground. Most of the coins found here belonged to Constans (who
died 350 A.D.) and the Constantine family. My sons Francis and
Horace visited the place in November 1877, for the sake of
ascertaining what part worms may have played in the burial of these
extensive remains. But the circumstances were not favourable for
this object, as the ruins are surrounded on three sides by rather
steep banks, down which earth is washed during rainy weather.
Moreover most of the old rooms have been covered with roofs, for
the protection of the elegant tesselated pavements.
A few facts may, however, be given on the thickness of the soil
over these ruins. Close outside the northern rooms there is a
broken wall, the summit of which was covered by 5 inches of black
mould; and in a hole dug on the outer side of this wall, where the
ground had never before been disturbed, black mould, full of
stones, 26 inches in thickness, was found, resting on the
undisturbed sub-soil of yellow clay. At a depth of 22 inches from
the surface a pig's jaw and a fragment of a tile were found. When
the excavations were first made, some large trees grew over the
ruins; and the stump of one has been left directly over a party-
wall near the bath-room, for the sake of showing the thickness of
the superincumbent soil, which was here 38 inches. In one small
room, which, after being cleared out, had not been roofed over, my
sons observed the hole of a worm passing through the rotten
concrete, and a living worm was found within the concrete. In
another open room worm-castings were seen on the floor, over which
some earth had by this means been deposited, and here grass now
grew.
Brading, Isle of Wight.--A fine Roman villa was discovered here in
1880; and by the end of October no less than 18 chambers had been
more or less cleared. A coin dated 337 A.D. was found. My son
William visited the place before the excavations were completed;
and he informs me that most of the floors were at first covered
with much rubbish and fallen stones, having their interstices
completely filled up with mould, abounding, as the workmen said,
with worms, above which there was mould without any stones. The
whole mass was in most places from 3 to above 4 ft. in thickness.
In one very large room the overlying earth was only 2 ft. 6 in.
thick; and after this had been removed, so many castings were
thrown up between the tiles that the surface had to be almost daily
swept. Most of the floors were fairly level. The tops of the
broken-down walls were covered in some places by only 4 or 5 inches
of soil, so that they were occasionally struck by the plough, but
in other places they were covered by from 13 to 18 inches of soil.
It is not probable that these walls could have been undermined by
worms and subsided, as they rested on a foundation of very hard red
sand, into which worms could hardly burrow. The mortar, however,
between the stones of the walls of a hypocaust was found by my son
to have been penetrated by many worm-burrows. The remains of this
villa stand on land which slopes at an angle of about 3 degrees;
and the land appears to have been long cultivated. Therefore no
doubt a considerable quantity of fine earth has been washed down
from the upper parts of the field, and has largely aided in the
burial of these remains.
Silchester, Hampshire.--The ruins of this small Roman town have
been better preserved than any other remains of the kind in
England. A broken wall, in most parts from 15 to 18 feet in height
and about 1.5 mile in compass, now surrounds a space of about 100
acres of cultivated land, on which a farm-house and a church stand.
{56} Formerly, when the weather was dry, the lines of the buried
walls could be traced by the appearance of the crops; and recently
very extensive excavations have been undertaken by the Duke of
Wellington, under the superintendence of the late Rev. J. G. Joyce,
by which means many large buildings have been discovered. Mr.
Joyce made careful coloured sections, and measured the thickness of
each bed of rubbish, whilst the excavations were in progress; and
he has had the kindness to send me copies of several of them. When
my sons Francis and Horace visited these ruins, he accompanied
them, and added his notes to theirs.
Mr. Joyce estimates that the town was inhabited by the Romans for
about three centuries; and no doubt much matter must have
accumulated within the walls during this long period. It appears
to have been destroyed by fire, and most of the stones used in the
buildings have since been carried away. These circumstances are
unfavourable for ascertaining the part which worms have played in
the burial of the ruins; but as careful sections of the rubbish
overlying an ancient town have seldom or never before been made in
England, I will give copies of the most characteristic portions of
some of those made by Mr. Joyce. They are of too great length to
be here introduced entire.
An east and west section, 30 ft. in length, was made across a room
in the Basilica, now called the Hall of the Merchants (Fig. 9).
The hard concrete floor, still covered here and there with
tesserae, was found at 3 ft. beneath the surface of the field,
which was here level. On the floor there were two large piles of
charred wood, one alone of which is shown in the part of the
section here given. This pile was covered by a thin white layer of
decayed stucco or plaster, above which was a mass, presenting a
singularly disturbed appearance, of broken tiles, mortar, rubbish
and fine gravel, together 27 inches in thickness. Mr. Joyce
believes that the gravel was used in making the mortar or concrete,
which has since decayed, some of the lime probably having been
dissolved. The disturbed state of the rubbish may have been due to
its having been searched for building stones. This bed was capped
by fine vegetable mould, 9 inches in thickness. From these facts
we may conclude that the Hall was burnt down, and that much rubbish
fell on the floor, through and from which the worms slowly brought
up the mould, now forming the surface of the level field.
A section across the middle of another hall in the Basilica, 32
feet 6 inches in length, called the AErarium, is shown in Fig. 10.
It appears that we have here evidence of two fires, separated by an
interval of time, during which the 6 inches of "mortar and concrete
with broken tiles" was accumulated. Beneath one of the layers of
charred wood, a valuable relic, a bronze eagle, was found; and this
shows that the soldiers must have deserted the place in a panic.
Owing to the death of Mr. Joyce, I have not been able to ascertain
beneath which of the two layers the eagle was found. The bed of
rubble overlying the undisturbed gravel originally formed, as I
suppose, the floor, for it stands on a level with that of a
corridor, outside the walls of the Hall; but the corridor is not
shown in the section as here given. The vegetable mould was 16
inches thick in the thickest part; and the depth from the surface
of the field, clothed with herbage, to the undisturbed gravel, was
40 inches.
The section shown in Fig. 11 represents an excavation made in the
middle of the town, and is here introduced because the bed of "rich
mould" attained, according to Mr. Joyce, the unusual thickness of
20 inches. Gravel lay at the depth of 48 inches from the surface;
but it was not ascertained whether this was in its natural state,
or had been brought here and had been rammed down, as occurs in
some other places.
The section shown in Fig. 12 was taken in the centre of the
Basilica, and though it was 5 feet in depth, the natural sub-soil
was not reached. The bed marked "concrete" was probably at one
time a floor; and the beds beneath seem to be the remnants of more
ancient buildings. The vegetable mould was here only 9 inches
thick. In some other sections, not copied, we likewise have
evidence of buildings having been erected over the ruins of older
ones. In one case there was a layer of yellow clay of very unequal
thickness between two beds of debris, the lower one of which rested
on a floor with tesserae. The ancient broken walls appear to have
been sometimes roughly cut down to a uniform level, so as to serve
as the foundations for a temporary building; and Mr. Joyce suspects
that some of these buildings were wattled sheds, plastered with
clay, which would account for the above-mentioned layer of clay.
Turning now to the points which more immediately concern us. Worm-
castings were observed on the floors of several of the rooms, in
one of which the tesselation was unusually perfect. The tesserae
here consisted of little cubes of hard sandstone of about 1 inch,
several of which were loose or projected slightly above the general
level. One or occasionally two open worm-burrows were found
beneath all the loose tesserae. Worms have also penetrated the old
walls of these ruins. A wall, which had just been exposed to view
during the excavations then in progress, was examined; it was built
of large flints, and was 18 inches in thickness. It appeared
sound, but when the soil was removed from beneath, the mortar in
the lower part was found to be so much decayed that the flints fell
apart from their own weight. Here, in the middle of the wall, at a
depth of 29 inches beneath the old floor and of 49.5 inches beneath
the surface of the field, a living worm was found, and the mortar
was penetrated by several burrows.
A second wall was exposed to view for the first time, and an open
burrow was seen on its broken summit. By separating the flints
this burrow was traced far down in the interior of the wall; but as
some of the flints cohered firmly, the whole mass was disturbed in
pulling down the wall, and the burrow could not be traced to the
bottom. The foundations of a third wall, which appeared quite
sound, lay at a depth of 4 feet beneath one of the floors, and of
course at a considerably greater depth beneath the level of the
ground. A large flint was wrenched out of the wall at about a foot
from the base, and this required much force, as the mortar was
sound; but behind the flint in the middle of the wall, the mortar
was friable, and here there were worm-burrows. Mr. Joyce and my
sons were surprised at the blackness of the mortar in this and in
several other cases, and at the presence of mould in the interior
of the walls. Some may have been placed there by the old builders
instead of mortar; but we should remember that worms line their
burrows with black humus. Moreover open spaces would almost
certainly have been occasionally left between the large irregular
flints; and these spaces, we may feel sure, would be filled up by
the worms with their castings, as soon as they were able to
penetrate the wall. Rain-water, oozing down the burrows would also
carry fine dark-coloured particles into every crevice. Mr. Joyce
was at first very sceptical about the amount of work which I
attributed to worms; but he ends his notes with reference to the
last-mentioned wall by saying, "This case caused me more surprise
and brought more conviction to me than any other. I should have
said, and did say, that it was quite impossible such a wall could
have been penetrated by earth-worms."
In almost all the rooms the pavement has sunk considerably,
especially towards the middle; and this is shown in the three
following sections. The measurements were made by stretching a
string tightly and horizontally over the floor. The section, Fig.
13, was taken from north to south across a room, 18 feet 4 inches
in length, with a nearly perfect pavement, next to the "Red Wooden
Hut." In the northern half, the subsidence amounted to 5.75 inches
beneath the level of the floor as it now stands close to the walls;
and it was greater in the northern than in the southern half; but,
according to Mr. Joyce, the entire pavement has obviously subsided.
In several places, the tesserae appeared as if drawn a little away
from the walls; whilst in other places they were still in close
contact with them.
In Fig. 14, we see a section across the paved floor of the southern
corridor or ambulatory of a quadrangle, in an excavation made near
"The Spring." The floor is 7 feet 9 inches wide, and the broken-
down walls now project only 0.75 of an inch above its level. The
field, which was in pasture, here sloped from north to south, at an
angle of 30 degrees, 40 seconds. The nature of the ground at some
little distance on each side of the corridor is shown in the
section. It consisted of earth full of stones and other debris,
capped with dark vegetable mould which was thicker on the lower or
southern than on the northern side. The pavement was nearly level
along lines parallel to the side-walls, but had sunk in the middle
as much as 7.75 inches.
A small room at no great distance from that represented in Fig. 13,
had been enlarged by the Roman occupier on the southern side, by an
addition of 5 feet 4 inches in breadth. For this purpose the
southern wall of the house had been pulled down, but the
foundations of the old wall had been left buried at a little depth
beneath the pavement of the enlarged room. Mr. Joyce believes that
this buried wall must have been built before the reign of Claudius
II., who died 270 A.D. We see in the accompanying section, Fig.
15, that the tesselated pavement has subsided to a less degree over
the buried wall than elsewhere; so that a slight convexity or
protuberance here stretched in a straight line across the room.
This led to a hole being dug, and the buried wall was thus
discovered.
We see in these three sections, and in several others not given,
that the old pavements have sunk or sagged considerably. Mr. Joyce
formerly attributed this sinking solely to the slow settling of the
ground. That there has been some settling is highly probable, and
it may be seen in Fig. 15 that the pavement for a width of 5 feet
over the southern enlargement of the room, which must have been
built on fresh ground, has sunk a little more than on the old
northern side. But this sinking may possibly have had no
connection with the enlargement of the room; for in Fig. 13 one
half of the pavement has subsided more than the other half without
any assignable cause. In a bricked passage to Mr. Joyce's own
house, laid down only about six years ago, the same kind of sinking
has occurred as in the ancient buildings. Nevertheless it does not
appear probable that the whole amount of sinking can be thus
accounted for. The Roman builders excavated the ground to an
unusual depth for the foundations of their walls, which were thick
and solid; it is therefore hardly credible that they should have
been careless about the solidity of the bed on which their
tesselated and often ornamented pavements were laid. The sinking
must, as it appears to me, be attributed in chief part to the
pavement having been undermined by worms, which we know are still
at work. Even Mr. Joyce at last admitted that this could not have
failed to have produced a considerable effect. Thus also the large
quantity of fine mould overlying the pavements can be accounted
for, the presence of which would otherwise be inexplicable. My
sons noticed that in one room in which the pavement had sagged very
little, there was an unusually small amount of overlying mould.
As the foundations of the walls generally lie at a considerable
depth, they will either have not subsided at all through the
undermining action of worms, or they will have subsided much less
than the floor. This latter result would follow from worms not
often working deep down beneath the foundations; but more
especially from the walls not yielding when penetrated by worms,
whereas the successively formed burrows in a mass of earth, equal
to one of the walls in depth and thickness, would have collapsed
many times since the desertion of the ruins, and would consequently
have shrunk or subsided. As the walls cannot have sunk much or at
all, the immediately adjoining pavement from adhering to them will
have been prevented from subsiding; and thus the present curvature
of the pavement is intelligible.
The circumstance which has surprised me most with respect to
Silchester is that during the many centuries which have elapsed
since the old buildings were deserted, the vegetable mould has not
accumulated over them to a greater thickness than that here
observed. In most places it is only about 9 inches in thickness,
but in some places 12 or even more inches. In Fig. 11, it is given
as 20 inches, but this section was drawn by Mr. Joyce before his
attention was particularly called to this subject. The land
enclosed within the old walls is described as sloping slightly to
the south; but there are parts which, according to Mr. Joyce, are
nearly level, and it appears that the mould is here generally
thicker than elsewhere. The surface slopes in other parts from
west to east, and Mr. Joyce describes one floor as covered at the
western end by rubbish and mould to a thickness of 28.5 inches, and
at the eastern end by a thickness of only 11.5 inches. A very
slight slope suffices to cause recent castings to flow downwards
during heavy rain, and thus much earth will ultimately reach the
neighbouring rills and streams and be carried away. By this means,
the absence of very thick beds of mould over these ancient ruins
may, as I believe, be explained. Moreover most of the land here
has long been ploughed, and this would greatly aid the washing away
of the finer earth during rainy weather.
The nature of the beds immediately beneath the vegetable mould in
some of the sections is rather perplexing. We see, for instance,
in the section of an excavation in a grass meadow (Fig. 14), which
sloped from north to south at an angle of 30 degrees 40 seconds,
that the mould on the upper side is only six inches and on the
lower side nine inches in thickness. But this mould lies on a mass
(25.5 inches in thickness on the upper side) "of dark brown mould,"
as described by Mr. Joyce, "thickly interspersed with small pebbles
and bits of tiles, which present a corroded or worn appearance.
The state of this dark-coloured earth is like that of a field which
has long been ploughed, for the earth thus becomes intermingled
with stones and fragments of all kinds which have been much exposed
to the weather. If during the course of many centuries this grass
meadow and the other now cultivated fields have been at times
ploughed, and at other times left as pasture, the nature of the
ground in the above section is rendered intelligible. For worms
will continually have brought up fine earth from below, which will
have been stirred up by the plough whenever the land was
cultivated. But after a time a greater thickness of fine earth
will thus have been accumulated than could be reached by the
plough; and a bed like the 25.5-inch mass, in Fig. 14, will have
been formed beneath the superficial mould, which latter will have
been brought to the surface within more recent times, and have been
well sifted by the worms.
Wroxeter, Shropshire. --The old Roman city of Uriconium was founded
in the early part of the second century, if not before this date;
and it was destroyed, according to Mr. Wright, probably between the
middle of the fourth and fifth century. The inhabitants were
massacred, and skeletons of women were found in the hypocausts.
Before the year 1859, the sole remnant of the city above ground,
was a portion of a massive wall about 20 ft. in height. The
surrounding land undulates slightly, and has long been under
cultivation. It had been noticed that the corn-crops ripened
prematurely in certain narrow lines, and that the snow remained
unmelted in certain places longer than in others. These
appearances led, as I was informed, to extensive excavations being
undertaken. The foundations of many large buildings and several
streets have thus been exposed to view. The space enclosed within
the old walls is an irregular oval, about 1 mile in length. Many
of the stones or bricks used in the buildings must have been
carried away; but the hypocausts, baths, and other underground
buildings were found tolerably perfect, being filled with stones,
broken tiles, rubbish and soil. The old floors of various rooms
were covered with rubble. As I was anxious to know how thick the
mantle of mould and rubbish was, which had so long concealed these
ruins, I applied to Dr. H. Johnson, who had superintended the
excavations; and he, with the greatest kindness, twice visited the
place to examine it in reference to my questions, and had many
trenches dug in four fields which had hitherto been undisturbed.
The results of his observations are given in the following Table.
He also sent me specimens of the mould, and answered, as far as he
could, all my questions.
MEASUREMENTS BY DR. H. JOHNSON OF THE THICKNESS OF THE VEGETABLE
MOULD OVER THE ROMAN RUINS AT WROXETER.
Trenches dug in a field called "Old Works."
(Thickness of mould in inches shown in parenthesis--DP.)
1. At a depth of 36 inches undisturbed sand was reached (20)
2. At a depth of 33 inches concrete was reached (21)
3. At a depth of 9 inches concrete was reached (9)
Trenches dug in a field called "Shop Leasows;" this is the highest
field within the old walls, and slopes down from a sub-central
point on all sides at about an angle of 2 degrees.
4. Summit of field, trench 45 inches deep (40)
5. Close to summit of field, trench 36 inches deep (26)
6. Close to summit of field, trench 28 inches deep (28)
7. Near summit of field, trench 36 inches deep (24)
8. Near summit of field, trench at one end 39 inches deep; the
mould here graduated into the underlying undisturbed sand, and its
thickness (24 inches) is somewhat arbitrary. At the other end of
the trench, a causeway was encountered at a depth of only 7 inches,
and the mould was here only 7 inches thick (24)
9. Trench close to the last, 28 inches in depth (24)
10. Lower part of same field, trench 30 inches deep (15)
11. Lower part of same field, trench 31 inches deep (17)
12. Lower part of same field, trench 36 inches deep, at which
depth undisturbed sand was reached (28)
13. In another part of same field, trench 9.5 inches deep stopped
by concrete (9.5)
14. In another part of same field, trench 9 inches deep, stopped
by concrete (9)
15. In another part of the same field, trench 24 inches deep, when
sand was reached (16)
16. In another part of same field, trench 30 inches deep, when
stones were reached; at one end of the trench mould 12 inches, at
the other end 14 inches thick (13)
Small field between "Old Works" and "Shop Leasows," I believe
nearly as high as the upper part of the latter field.
17. Trench 26 inches deep (24)
18. Trench 10 inches deep, and then came upon a causeway (10)
19. Trench 34 inches deep (30)
20. Trench 31 inches deep (31)
Field on the western side of the space enclosed within the old
walls.
21. Trench 28 inches deep, when undisturbed sand was reached (16)
22. Trench 29 inches deep, when undisturbed sand was reached (15)
23. Trench 14 inches deep, and then came upon a building (14)
Dr. Johnson distinguished as mould the earth which differed, more
or less abruptly, in its dark colour and in its texture from the
underlying sand or rubble. In the specimens sent to me, the mould
resembled that which lies immediately beneath the turf in old
pasture-land, excepting that it often contained small stones, too
large to have passed through the bodies of worms. But the trenches
above described were dug in fields, none of which were in pasture,
and all had been long cultivated. Bearing in mind the remarks made
in reference to Silchester on the effects of long-continued
culture, combined with the action of worms in bringing up the finer
particles to the surface, the mould, as so designated by Dr.
Johnson, seems fairly well to deserve its name. Its thickness,
where there was no causeway, floor or walls beneath, was greater
than has been elsewhere observed, namely, in many places above 2
ft., and in one spot above 3 ft. The mould was thickest on and
close to the nearly level summit of the field called "Shop
Leasows," and in a small adjoining field, which, as I believe, is
of nearly the same height. One side of the former field slopes at
an angle of rather above 2 degrees, and I should have expected that
the mould, from being washed down during heavy rain, would have
been thicker in the lower than in the upper part; but this was not
the case in two out of the three trenches here dug.
In many places, where streets ran beneath the surface, or where old
buildings stood, the mould was only 8 inches in thickness; and Dr.
Johnson was surprised that in ploughing the land, the ruins had
never been struck by the plough as far as he had heard. He thinks
that when the land was first cultivated the old walls were perhaps
intentionally pulled down, and that hollow places were filled up.
This may have been the case; but if after the desertion of the city
the land was left for many centuries uncultivated, worms would have
brought up enough fine earth to have covered the ruins completely;
that is if they had subsided from having been undermined. The
foundations of some of the walls, for instance those of the portion
still standing about 20 feet above the ground, and those of the
marketplace, lie at the extraordinary depth of 14 feet; but it is
highly improbable that the foundations were generally so deep. The
mortar employed in the buildings must have been excellent, for it
is still in parts extremely hard. Wherever walls of any height
have been exposed to view, they are, as Dr. Johnson believes,
still perpendicular. The walls with such deep foundations cannot
have been undermined by worms, and therefore cannot have subsided,
as appears to have occurred at Abinger and Silchester. Hence it is
very difficult to account for their being now completely covered
with earth; but how much of this covering consists of vegetable
mould and how much of rubble I do not know. The market-place, with
the foundations at a depth of 14 feet, was covered up, as Dr.
Johnson believes, by between 6 and 24 inches of earth. The tops of
the broken-down walls of a caldarium or bath, 9 feet in depth, were
likewise covered up with nearly 2 feet of earth. The summit of an
arch, leading into an ash-pit 7 feet in depth, was covered up with
not more than 8 inches of earth. Whenever a building which has not
subsided is covered with earth, we must suppose, either that the
upper layers of stone have been at some time carried away by man,
or that earth has since been washed down during heavy rain, or
blown down during storms, from the adjoining land; and this would
be especially apt to occur where the land has long been cultivated.
In the above cases the adjoining land is somewhat higher than the
three specified sites, as far as I can judge by maps and from
information given me by Dr. Johnson. If; however, a great pile of
broken stones, mortar, plaster, timber and ashes fell over the
remains of any building, their disintegration in the course of
time, and the sifting action of worms, would ultimately conceal the
whole beneath fine earth.
Conclusion. --The cases given in this chapter show that worms have
played a considerable part in the burial and concealment of several
Roman and other old buildings in England; but no doubt the washing
down of soil from the neighbouring higher lands, and the deposition
of dust, have together aided largely in the work of concealment.
Dust would be apt to accumulate wherever old broken-down walls
projected a little above the then existing surface and thus
afforded some shelter. The floors of the old rooms, halls and
passages have generally sunk, partly from the settling of the
ground, but chiefly from having been undermined by worms; and the
sinking has commonly been greater in the middle than near the
walls. The walls themselves, whenever their foundations do not lie
at a great depth, have been penetrated and undermined by worms, and
have consequently subsided. The unequal subsidence thus caused,
probably explains the great cracks which may be seen in many
ancient walls, as well as their inclination from the perpendicular.
CHAPTER V--THE ACTION OF WORMS IN THE DENUDATION OF THE LAND.
Evidence of the amount of denudation which the land has 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.
No one doubts that our world at one time consisted of crystalline
rocks, and that it is to their disintegration through the action of
air, water, changes of temperature, rivers, waves of the sea,
earthquakes and volcanic outbursts, that we owe our sedimentary
formations. These after being consolidated and sometimes
recrystallized, have often been again disintegrated. Denudation
means the removal of such disintegrated matter to a lower level.
Of the many striking results due to the modern progress of geology
there are hardly any more striking than those which relate to
denudation. It was long ago seen that there must have been an
immense amount of denudation; but until the successive formations
were carefully mapped and measured, no one fully realised how great
was the amount. One of the first and most remarkable memoirs ever
published on this subject was that by Ramsay, {57} who in 1846
showed that in Wales from 9000 to 11,000 feet in thickness of solid
rock had been stripped off large tracks of country. Perhaps the
plainest evidence of great denudation is afforded by faults or
cracks, which extend for many miles across certain districts, with
the strata on one side raised even ten thousand feet above the
corresponding strata on the opposite side; and yet there is not a
vestige of this gigantic displacement visible on the surface of the
land. A huge pile of rock has been planed away on one side and not
a remnant left.
Until the last twenty or thirty years, most geologists thought that
the waves of the sea were the chief agents in the work of
denudation; but we may now feel sure that air and rain, aided by
streams and rivers, are much more powerful agents,--that is if we
consider the whole area of the land. The long lines of escarpment
which stretch across several parts of England were formerly
considered to be undoubtedly ancient coast-lines; but we now know
that they stand up above the general surface merely from resisting
air, rain and frost better than the adjoining formations. It has
rarely been the good fortune of a geologist to bring conviction to
the minds of his fellow-workers on a disputed point by a single
memoir; but Mr. Whitaker, of the Geological Survey of England, was
so fortunate when, in 1867, he published his paper "On sub-aerial
Denudation, and on Cliffs and Escarpments of the Chalk." {58}
Before this paper appeared, Mr. A. Tylor had adduced important
evidence on sub-aerial denudation, by showing that the amount of
matter brought down by rivers must infallibly lower the level of
their drainage basins by many feet in no immense lapse of time.
This line of argument has since been followed up in the most
interesting manner by Archibald Geikie, Croll and others, in a
series of valuable memoirs. {59} For the sake of those who have
never attended to this subject, a single instance may be here
given, namely, that of the Mississippi, which is chosen because the
amount of sediment brought down by this great river has been
investigated with especial care by order of the United States
Government. The result is, as Mr. Croll shows, that the mean level
of its enormous area of drainage must be lowered 1/4566 of a foot
annually, or 1 foot in 4566 years. Consequently, taking the best
estimate of the mean height of the North American continent, viz.
748 feet, and looking to the future, the whole of the great
Mississippi basin will be washed away, and "brought down to the
sea-level in less than 4,500,000 years, if no elevation of the land
takes place." Some rivers carry down much more sediment relatively
to their size, and some much less than the Mississippi.
Disintegrated matter is carried away by the wind as well as by
running water. During volcanic outbursts much rock is triturated
and is thus widely dispersed; and in all arid countries the wind
plays an important part in the removal of such matter. Wind-driven
sand also wears down the hardest rocks. I have shown {60} that
during four months of the year a large quantity of dust is blown
from the north-western shores of Africa, and falls on the Atlantic
over a space of 1600 miles in latitude, and for a distance of from
300 to 600 miles from the coast. But dust has been seen to fall at
a distance of 1030 miles from the shores of Africa. During a stay
of three weeks at St. Jago in the Cape Verde Archipelago, the
atmosphere was almost always hazy, and extremely fine dust coming
from Africa was continually falling. In some of this dust which
fell in the open ocean at a distance of between 330 and 380 miles
from the African coast, there were many particles of stone, about
1/1000 of an inch square. Nearer to the coast the water has been
seen to be so much discoloured by the falling dust, that a sailing
vessel left a track behind her. In countries, like the Cape Verde
Archipelago, where it seldom rains and there are no frosts, the
solid rock nevertheless disintegrates; and in conformity with the
views lately advanced by a distinguished Belgian geologist, De
Koninck, such disintegration may be attributed in chief part to the
action of the carbonic and nitric acids, together with the nitrates
and nitrites of ammonia, dissolved in the dew.
In all humid, even moderately humid, countries, worms aid in the
work of denudation in several ways. The vegetable mould which
covers, as with a mantle, the surface of the land, has all passed
many times through their bodies. Mould differs in appearance from
the subsoil only in its dark colour, and in the absence of
fragments or particles of stone (when such are present in the
subsoil), larger than those which can pass through the alimentary
canal of a worm. This sifting of the soil is aided, as has already
been remarked, by burrowing animals of many kinds, especially by
ants. In countries where the summer is long and dry, the mould in
protected places must be largely increased by dust blown from other
and more exposed places. For instance, the quantity of dust
sometimes blown over the plains of La Plata, where there are no
solid rocks, is so great, that during the "gran seco," 1827 to
1830, the appearance of the land, which is here unenclosed, was so
completely changed that the inhabitants could not recognise the
limits of their own estates, and endless lawsuits arose. Immense
quantities of dust are likewise blown about in Egypt and in the
south of France. In China, as Richthofen maintains, beds appearing
like fine sediment, several hundred feet in thickness and extending
over an enormous area, owe their origin to dust blown from the high
lands of central Asia. {61} In humid countries like Great Britain,
as long as the land remains in its natural state clothed with
vegetation, the mould in any one place can hardly be much increased
by dust; but in its present condition, the fields near high roads,
where there is much traffic, must receive a considerable amount of
dust, and when fields are harrowed during dry and windy weather,
clouds of dust may be seen to be blown away. But in all these
cases the surface-soil is merely transported from one place to
another. The dust which falls so thickly within our houses
consists largely of organic matter, and if spread over the land
would in time decay and disappear almost entirely. It appears,
however, from recent observations on the snow-fields of the Arctic
regions, that some little meteoric dust of extra mundane origin is
continually falling.
The dark colour of ordinary mould is obviously due to the presence
of decaying organic matter, which, however, is present in but small
quantities. The loss of weight which mould suffers when heated to
redness seems to be in large part due to water in combination being
dispelled. In one sample of fertile mould the amount of organic
matter was ascertained to be only 1.76 per cent.; in some
artificially prepared soil it was as much as 5.5 per cent., and in
the famous black soil of Russia from 5 to even 12 per cent. {62}
In leaf-mould formed exclusively by the decay of leaves the amount
is much greater, and in peat the carbon alone sometimes amounts to
64 per cent.; but with these latter cases we are not here
concerned. The carbon in the soil tends gradually to oxidise and
to disappear, except where water accumulates and the climate is
cool; {63} so that in the oldest pasture-land there is no great
excess of organic matter, notwithstanding the continued decay of
the roots and the underground stems of plants, and the occasional
addition of manure. The disappearance of the organic matter from
mould is probably much aided by its being brought again and again
to the surface in the castings of worms.
Worms, on the other hand, add largely to the organic matter in the
soil by the astonishing number of half-decayed leaves which they
draw into their burrows to a depth of 2 or 3 inches. They do this
chiefly for obtaining food, but partly for closing the mouths of
their burrows and for lining the upper part. The leaves which they
consume are moistened, torn into small shreds, partially digested,
and intimately commingled with earth; and it is this process which
gives to vegetable mould its uniform dark tint. It is known that
various kinds of acids are generated by the decay of vegetable
matter; and from the contents of the intestines of worms and from
their castings being acid, it seems probable that the process of
digestion induces an analogous chemical change in the swallowed,
triturated, and half-decayed leaves. The large quantity of
carbonate of lime secreted by the calciferous glands apparently
serves to neutralise the acids thus generated; for the digestive
fluid of worms will not act unless it be alkaline. As the contents
of the upper part of their intestines are acid, the acidity can
hardly be due to the presence of uric acid. We may therefore
conclude that the acids in the alimentary canal of worms are formed
during the digestive process; and that probably they are nearly of
the same nature as those in ordinary mould or humus. The latter
are well known to have the power of de-oxidising or dissolving per-
oxide of iron, as may be seen wherever peat overlies red sand, or
where a rotten root penetrates such sand. Now I kept some worms in
a pot filled with very fine reddish sand, consisting of minute
particles of silex coated with the red oxide of iron; and the
burrows, which the worms made through this sand, were lined or
coated in the usual manner with their castings, formed of the sand
mingled with their intestinal secretions and the refuse of the
digested leaves; and this sand had almost wholly lost its red
colour. When small portions of it were placed under the
microscope, most of the grains were seen to be transparent and
colourless, owing to the dissolution of the oxide; whilst almost
all the grains taken from other parts of the pot were coated with
the oxide. Acetic acid produced hardly any effect on his sand; and
even hydrochloric, nitric and sulphuric acids, diluted as in the
Pharmacopoeia, produced less effect than did the acids in the
intestines of the worms.
Mr. A. A. Julien has lately collected all the extant information
about the acids generated in humus, which, according to some
chemists, amount to more than a dozen different kinds. These
acids, as well as their acid salts (i.e., in combination with
potash, soda, and ammonia), act energetically on carbonate of lime
and on the oxides of iron. It is also known that some of these
acids, which were called long ago by Thenard azohumic, are enabled
to dissolve colloid silica in proportion to the nitrogen which they
contain. {64} In the formation of these latter acids worms
probably afford some aid, for Dr. H. Johnson informs me that by
Nessler's test he found 0.018 per cent. of ammonia in their
castings.
It may be here added that I have recently been informed by Dr.
Gilbert "that several square yards on his lawn were swept clean,
and after two or three weeks all the worm-castings on the space
were collected and dried. These were found to contain 0.35 of
nitrogen. This is from two to three times as much as we find in
our ordinary arable surface-soil; more than in our ordinary pasture
surface-soil; but less than in rich kitchen-garden mould.
Supposing a quantity of castings equal to 10 tons in the dry state
were annually deposited on an acre, this would represent a manuring
of 78 lbs. of nitrogen per acre per annum; and this is very much
more than the amount of nitrogen in the annual yield of hay per
acre, if raised without any nitrogenous manure. Obviously, so far
as the nitrogen in the castings is derived from surface-growth or
from surface-soil, it is not a gain to the latter; but so far as it
is derived from below, it is a gain."
The several humus-acids, which appear, as we have just seen, to be
generated within the bodies of worms during the digestive process,
and their acid salts, play a highly important part, according to
the recent observations of Mr. Julien, in the disintegration of
various kinds of rocks. It has long been known that the carbonic
acid, and no doubt nitric and nitrous acids, which are present in
rain-water, act in like manner. There is, also, a great excess of
carbonic acid in all soils, especially in rich soils, and this is
dissolved by the water in the ground. The living roots of plants,
moreover, as Sachs and others have shown, quickly corrode and leave
their impressions on polished slabs of marble, dolomite and
phosphate of lime. They will attack even basalt and sandstone.
{65} But we are not here concerned with agencies which are wholly
independent of the action of worms.
The combination of any acid with a base is much facilitated by
agitation, as fresh surfaces are thus continually brought into
contact. This will be thoroughly effected with the particles of
stone and earth in the intestines of worms, during the digestive
process; and it should be remembered that the entire mass of the
mould over every field, passes, in the course of a few years,
through their alimentary canals. Moreover as the old burrows
slowly collapse, and as fresh castings are continually brought to
the surface, the whole superficial layer of mould slowly revolves
or circulates; and the friction of the particles one with another
will rub off the finest films of disintegrated matter as soon as
they are formed. Through these several means, minute fragments of
rocks of many kinds and mere particles in the soil will be
continually exposed to chemical decomposition; and thus the amount
of soil will tend to increase.
As worms line their burrows with their castings, and as the burrows
penetrate to a depth of 5 or 6, or even more feet, some small
amount of the humus-acids will be carried far down, and will there
act on the underlying rocks and fragments of rock. Thus the
thickness of the soil, if none be removed from the surface, will
steadily though slowly tend to increase; but the accumulation will
after a time delay the disintegration of the underlying rocks and
of the more deeply seated particles. For the humus-acids which are
generated chiefly in the upper layer of vegetable mould, are
extremely unstable compounds, and are liable to decomposition
before they reach any considerable depth. {66} A thick bed of
overlying soil will also check the downward extension of great
fluctuations of temperature, and in cold countries will check the
powerful action of frost. The free access of air will likewise be
excluded. From these several causes disintegration would be almost
arrested, if the overlying mould were to increase much in
thickness, owing to none or little being removed from the surface.
{67} In my own immediate neighbourhood we have a curious proof how
effectually a few feet of clay checks some change which goes on in
flints, lying freely exposed; for the large ones which have lain
for some time on the surface of ploughed fields cannot be used for
building; they will not cleave properly, and are said by the
workmen to be rotten. {68} It is therefore necessary to obtain
flints for building purposes from the bed of red clay overlying the
chalk (the residue of its dissolution by rain-water) or from the
chalk itself.
Not only do worms aid directly in the chemical disintegration of
rocks, but there is good reason to believe that they likewise act
in a direct and mechanical manner on the smaller particles. All
the species which swallow earth are furnished with gizzards; and
these are lined with so thick a chitinous membrane, that Perrier
speaks of it, {69} as "une veritable armature." The gizzard is
surrounded by powerful transverse muscles, which, according to
Claparede, are about ten times as thick as the longitudinal ones;
and Perrier saw them contracting energetically. Worms belonging to
one genus, Digaster, have two distinct but quite similar gizzards;
and in another genus, Moniligaster, the second gizzard consists of
four pouches, one succeeding the other, so that it may almost be
said to have five gizzards. {70} In the same manner as
gallinaceous and struthious birds swallow stones to aid in the
trituration of their food, so it appears to be with terricolous
worms. The gizzards of thirty-eight of our common worms were
opened, and in twenty-five of them small stones or grains of sand,
sometimes together with the hard calcareous concretions formed
within the anterior calciferous glands, were found, and in two
others concretions alone. In the gizzards of the remaining worms
there were no stones; but some of these were not real exceptions,
as the gizzards were opened late in the autumn, when the worms had
ceased to feed and their gizzards were quite empty. {71}
When worms make their burrows through earth abounding with little
stones, no doubt many will be unavoidably swallowed; but it must
not be supposed that this fact accounts for the frequency with
which stones and sand are found in their gizzards. For beads of
glass and fragments of brick and of hard tiles were scattered over
the surface of the earth, in pots in which worms were kept and had
already made their burrows; and very many of these beads and
fragments were picked up and swallowed by the worms, for they were
found in their castings, intestines, and gizzards. They even
swallowed the coarse red dust, formed by the pounding of the tiles.
Nor can it be supposed that they mistook the beads and fragments
for food; for we have seen that their taste is delicate enough to
distinguish between different kinds of leaves. It is therefore
manifest that they swallow hard objects, such as bits of stone,
beads of glass and angular fragments of bricks or tiles for some
special purpose; and it can hardly be doubted that this is to aid
their gizzards in crushing and grinding the earth, which they so
largely consume. That such hard objects are not necessary for
crushing leaves, may be inferred from the fact that certain
species, which live in mud or water and feed on dead or living
vegetable matter, but which do not swallow earth, are not provided
with gizzards, {72} and therefore cannot have the power of
utilising stones.
During the grinding process, the particles of earth must be rubbed
against one another, and between the stones and the tough lining
membrane of the gizzard. The softer particles will thus suffer
some attrition, and will perhaps even be crushed. This conclusion
is supported by the appearance of freshly ejected castings, for
these often reminded me of the appearance of paint which has just
been ground by a workman between two flat stones. Morren remarks
that the intestinal canal is "impleta tenuissima terra, veluti in
pulverem redacta." {73} Perrier also speaks of "l'etat de pate
excessivement fine a laquelle est reduite la terre qu'ils
rejettent," &c. {74}
As the amount of trituration which the particles of earth undergo
in the gizzards of worms possesses some interest (as we shall
hereafter see), I endeavoured to obtain evidence on this head by
carefully examining many of the fragments which had passed through
their alimentary canals. With worms living in a state of nature,
it is of course impossible to know how much the fragments may have
been worn before they were swallowed. It is, however, clear that
worms do not habitually select already rounded particles, for
sharply angular bits of flint and of other hard rocks were often
found in their gizzards or intestines. On three occasions sharp
spines from the stems of rose-bushes were thus found. Worms kept
in confinement repeatedly swallowed angular fragments of hard tile,
coal, cinders, and even the sharpest fragments of glass.
Gallinaceous and struthious birds retain the same stones in their
gizzards for a long time, which thus become well rounded; but this
does not appear to be the case with worms, judging from the large
number of the fragments of tiles, glass beads, stones, &c.,
commonly found in their castings and intestines. So that unless
the same fragments were to pass repeatedly through their gizzards,
visible signs of attrition in the fragments could hardly be
expected, except perhaps in the case of very soft stones.
I will now give such evidence of attrition as I have been able to
collect. In the gizzards of some worms dug out of a thin bed of
mould over the chalk, there were many well-rounded small fragments
of chalk, and two fragments of the shells of a land-mollusc (as
ascertained by their microscopical structure), which latter were
not only rounded but somewhat polished. The calcareous concretions
formed in the calciferous glands, which are often found in their
gizzards, intestines, and occasionally in their castings, when of
large size, sometimes appeared to have been rounded; but with all
calcareous bodies the rounded appearance may be partly or wholly
due to their corrosion by carbonic acid and the humus-acids. In
the gizzards of several worms collected in my kitchen garden near a
hothouse, eight little fragments of cinders were found, and of
these, six appeared more or less rounded, as were two bits of
brick; but some other bits were not at all rounded. A farm-road
near Abinger Hall had been covered seven years before with brick-
rubbish to the depth of about 6 inches; turf had grown over this
rubbish on both sides of the road for a width of 18 inches, and on
this turf there were innumerable castings. Some of them were
coloured of a uniform red owing to the presence of much brick-dust,
and they contained many particles of brick and of hard mortar from
1 to 3 mm. in diameter, most of which were plainly rounded; but all
these particles may have been rounded before they were protected by
the turf and were swallowed, like those on the bare parts of the
road which were much worn. A hole in a pasture-field had been
filled up with brick-rubbish at the same time, viz., seven years
ago, and was now covered with turf; and here the castings contained
very many particles of brick, all more or less rounded; and this
brick-rubbish, after being shot into the hole, could not have
undergone any attrition. Again, old bricks very little broken,
together with fragments of mortar, were laid down to form walks,
and were then covered with from 4 to 6 inches of gravel; six little
fragments of brick were extracted from castings collected on these
walks, three of which were plainly worn. There were also very many
particles of hard mortar, about half of which were well rounded;
and it is not credible that these could have suffered so much
corrosion from the action of carbonic acid in the course of only
seven years.
Much better evidence of the attrition of hard objects in the
gizzards of worms, is afforded by the state of the small fragments
of tiles or bricks, and of concrete in the castings thrown up where
ancient buildings once stood. As all the mould covering a field
passes every few years through the bodies of worms, the same small
fragments will probably be swallowed and brought to the surface
many times in the course of centuries. It should be premised that
in the several following cases, the finer matter was first washed
away from the castings, and then all the particles of bricks, tiles
and concrete were collected without any selection, and were
afterwards examined. Now in the castings ejected between the
tesserae on one of the buried floors of the Roman villa at Abinger,
there were many particles (from to 2 mm. in diameter) of tiles and
concrete, which it was impossible to look at with the naked eye or
through a strong lens, and doubt for a moment that they had almost
all undergone much attrition. I speak thus after having examined
small water-worn pebbles, formed from Roman bricks, which M. Henri
de Saussure had the kindness to send me, and which he had extracted
from sand and gravel beds, deposited on the shores of the Lake of
Geneva, at a former period when the water stood at about two metres
above its present level. The smallest of these water-worn pebbles
of brick from Geneva resembled closely many of those extracted from
the gizzards of worms, but the larger ones were somewhat smoother.
Four castings found on the recently uncovered, tesselated floor of
the great room in the Roman villa at Brading, contained many
particles of tile or brick, of mortar, and of hard white cement;
and the majority of these appeared plainly worn. The particles of
mortar, however, seemed to have suffered more corrosion than
attrition, for grains of silex often projected from their surfaces.
Castings from within the nave of Beaulieu Abbey, which was
destroyed by Henry VIII., were collected from a level expanse of
turf, overlying the buried tesselated pavement, through which worm-
burrows passed; and these castings contained innumerable particles
of tiles and bricks, of concrete and cement, the majority of which
had manifestly undergone some or much attrition. There were also
many minute flakes of a micaceous slate, the points of which were
rounded. If the above supposition, that in all these cases the
same minute fragments have passed several times through the
gizzards of worms, be rejected, notwithstanding its inherent
probability, we must then assume that in all the above cases the
many rounded fragments found in the castings had all accidentally
undergone much attrition before they were swallowed; and this is
highly improbable.
On the other hand it must be stated that fragments of ornamental
tiles, somewhat harder than common tiles or bricks, which had been
swallowed only once by worms kept in confinement, were with the
doubtful exception of one or two of the smallest grains, not at all
rounded. Nevertheless some of them appeared a little worn, though
not rounded. Notwithstanding these cases, if we consider the
evidence above given, there can be little doubt that the fragments,
which serve as millstones in the gizzards of worms, suffer, when of
a not very hard texture, some amount of attrition; and that the
smaller particles in the earth, which is habitually swallowed in
such astonishingly large quantities by worms, are ground together
and are thus levigated. If this be the case, the "terra
tenuissima,"--the "pate excessivement fine,"--of which the castings
largely consist, is in part due to the mechanical action of the
gizzard; {75} and this fine matter, as we shall see in the next
chapter, is that which is chiefly washed away from the innumerable
castings on every field during each heavy shower of rain. If the
softer stones yield at all, the harder ones will suffer some slight
amount of wear and tear.
The trituration of small particles of stone in the gizzards of
worms is of more importance under a geological point of view than
may at first appear to be the case; for Mr. Sorby has clearly shown
that the ordinary means of disintegration, namely, running water
and the waves of the sea, act with less and less power on fragments
of rock the smaller they are. "Hence," as he remarks, "even making
no allowance for the extra buoying up of very minute particles by a
current of water, depending on surface cohesion, the effects of
wearing on the form of the grains must vary directly as their
diameter or thereabouts. If so, a grain of 1/10 an inch in
diameter would be worn ten times as much as one of an inch in
diameter, and at least a hundred times as much as one of 1/100 an
inch in diameter. Perhaps, then, we may conclude that a grain 1/10
of an inch in diameter would be worn as much or more in drifting a
mile as a grain 1/1000 of an inch in being drifted 100 miles. On
the same principle a pebble one inch in diameter would be worn
relatively more by being drifted only a few hundred yards." {76}
Nor should we forget, in considering the power which worms exert in
triturating particles of rock, that there is good evidence that on
each acre of land, which is sufficiently damp and not too sandy,
gravelly or rocky for worms to inhabit, a weight of more than ten
tons of earth annually passes through their bodies and is brought
to the surface. The result for a country of the size of Great
Britain, within a period not very long in a geological sense, such
as a million years, cannot be insignificant; for the ten tons of
earth has to be multiplied first by the above number of years, and
then by the number of acres fully stocked with worms; and in
England, together with Scotland, the land which is cultivated and
is well fitted for these animals, has been estimated at above 32
million acres. The product is 320 million million tons of earth.
CHAPTER VI--THE DENUDATION OF THE LAND--continued.
Denudation aided by recently ejected castings flowing 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.
We are now prepared to consider the more direct part which worms
take in the denudation of the land. When reflecting on sub-aerial
denudation, it formerly appeared to me, as it has to others, that a
nearly level or very gently inclined surface, covered with turf,
could suffer no loss during even a long lapse of time. It may,
however, be urged that at long intervals, debacles of rain or
water-spouts would remove all the mould from a very gentle slope;
but when examining the steep, turf-covered slopes in Glen Roy, I
was struck with the fact how rarely any such event could have
happened since the Glacial period, as was plain from the well-
preserved state of the three successive "roads" or lake-margins.
But the difficulty in believing that earth in any appreciable
quantity can be removed from a gently inclined surface, covered
with vegetation and matted with roots, is removed through the
agency of worms. For the many castings which are thrown up during
rain, and those thrown up some little time before heavy rain, flow
for a short distance down an inclined surface. Moreover much of
the finest levigated earth is washed completely away from the
castings. During dry weather castings often disintegrate into
small rounded pellets, and these from their weight often roll down
any slope. This is more especially apt to occur when they are
started by the wind, and probably when started by the touch of an
animal, however small. We shall also see that a strong wind blows
all the castings, even on a level field, to leeward, whilst they
are soft; and in like manner the pellets when they are dry. If the
wind blows in nearly the direction of an inclined surface, the
flowing down of the castings is much aided.
The observations on which these several statements are founded must
now be given in some detail. Castings when first ejected are
viscid and soft; during rain, at which time worms apparently prefer
to eject them, they are still softer; so that I have sometimes
thought that worms must swallow much water at such times. However
this may be, rain, even when not very heavy, if long continued,
renders recently-ejected castings semi-fluid; and on level ground
they then spread out into thin, circular, flat discs, exactly as
would so much honey or very soft mortar, with all traces of their
vermiform structure lost. This latter fact was sometimes made
evident, when a worm had subsequently bored through a flat circular
disc of this kind, and heaped up a fresh vermiform mass in the
centre. These flat subsided discs have been repeatedly seen by me
after heavy rain, in many places on land of all kinds.
On the flowing of wet castings, and the rolling of dry
disintegrated castings down inclined surfaces.--When castings are
ejected on an inclined surface during or shortly before heavy rain,
they cannot fail to flow a little down the slope. Thus, on some
steep slopes in Knole Park, which were covered with coarse grass
and had apparently existed in this state from time immemorial, I
found (Oct. 22, 1872) after several wet days that almost all the
many castings were considerably elongated in the line of the slope;
and that they now consisted of smooth, only slightly conical
masses. Whenever the mouths of the burrows could be found from
which the earth had been ejected, there was more earth below than
above them. After some heavy storms of rain (Jan. 25, 1872) two
rather steeply inclined fields near Down, which had formerly been
ploughed and were now rather sparsely clothed with poor grass, were
visited, and many castings extended down the slopes for a length of
5 inches, which was twice or thrice the usual diameter of the
castings thrown up on the level parts of these same fields. On
some fine grassy slopes in Holwood Park, inclined at angles between
8 degrees and 11 degrees 30 seconds with the horizon, where the
surface apparently had never been disturbed by the hand of man,
castings abounded in extraordinary numbers: and a space 16 inches
in length transversely to the slope and 6 inches in the line of the
slope, was completely coated, between the blades of grass, with a
uniform sheet of confluent and subsided castings. Here also in
many places the castings had flowed down the slope, and now formed
smooth narrow patches of earth, 6, 7, and 7.5 inches in length.
Some of these consisted of two castings, one above the other, which
had become so completely confluent that they could hardly be
distinguished. On my lawn, clothed with very fine grass, most of
the castings are black, but some are yellowish from earth having
been brought up from a greater depth than usual, and the flowing-
down of these yellow castings after heavy rain, could be clearly
seen where the slope was 5 degrees; and where it was less than 1
degree some evidence of their flowing down could still be detected.
On another occasion, after rain which was never heavy, but which
lasted for 18 hours, all the castings on this same gently inclined
lawn had lost their vermiform structure; and they had flowed, so
that fully two-thirds of the ejected earth lay below the mouths of
the burrows.
These observations led me to make others with more care. Eight
castings were found on my lawn, where the grass-blades are fine and
close together, and three others on a field with coarse grass. The
inclination of the surface at the eleven places where these
castings were collected varied between 4 degrees 30 seconds and 17
degrees 30 seconds; the mean of the eleven inclinations being 9
degrees 26 seconds. The length of the castings in the direction of
the slope was first measured with as much accuracy as their
irregularities would permit. It was found possible to make these
measurements within about of an inch, but one of the castings was
too irregular to admit of measurement. The average length in the
direction of the slope of the remaining ten castings was 2.03
inches. The castings were then divided with a knife into two parts
along a horizontal line passing through the mouth of the burrow,
which was discovered by slicing off the turf; and all the ejected
earth was separately collected, namely, the part above the hole and
the part below. Afterwards these two parts were weighed. In every
case there was much more earth below than above; the mean weight of
that above being 103 grains, and of that below 205 grains; so that
the latter was very nearly double the former. As on level ground
castings are commonly thrown up almost equally round the mouths of
the burrows, this difference in weight indicates the amount of
ejected earth which had flowed down the slope. But very many more
observations would be requisite to arrive at any general result;
for the nature of the vegetation and other accidental
circumstances, such as the heaviness of the rain, the direction and
force of the wind, &c., appear to be more important in determining
the quantity of the earth which flows down a slope than its angle.
Thus with four castings on my lawn (included in the above eleven)
where the mean slope was 7 degrees 19 seconds, the difference in
the amount of earth above and below the burrows was greater than
with three other castings on the same lawn where the mean slope was
12 degrees 5 seconds.
We may, however, take the above eleven cases, which are accurate as
far as they go, and calculate the weight of the ejected earth which
annually flows down a slope having a mean inclination of 9 degrees
26 seconds. This was done by my son George. It has been shown
that almost exactly two-thirds of the ejected earth is found below
the mouth of the burrow and one-third above it. Now if the two-
thirds which is below the hole be divided into two equal parts, the
upper half of this two-thirds exactly counterbalances the one-third
which is above the hole, so that as far as regards the one-third
above and the upper half of the two-thirds below, there is no flow
of earth down the hill-side. The earth constituting the lower half
of the two-thirds is, however, displaced through distances which
are different for every part of it, but which may be represented by
the distance between the middle point of the lower half of the two-
thirds and the hole. So that the average distance of displacement
is a half of the whole length of the worm-casting. Now the average
length of ten out of the above eleven castings was 2.03 inches, and
half of this we may take as being 1 inch. It may therefore be
concluded that one-third of the whole earth brought to the surface
was in these cases carried down the slope through 1 inch. {77}
It was shown in the third chapter that on Leith Hill Common, dry
earth weighing at least 7.453 lbs. was brought up by worms to the
surface on a square yard in the course of a year. If a square yard
be drawn on a hillside with two of its sides horizontal, then it is
clear that only 1/36 part of the earth brought up on that square
yard would be near enough to its lower side to cross it, supposing
the displacement of the earth to be through one inch. But it
appears that only of the earth brought up can be considered to flow
downwards; hence 1/3 of 1/36 or 1/108 of 7.453 lbs. will cross the
lower side of our square yard in a year. Now 1/108 of 7.453 lbs.
is 1.1 oz. Therefore 1.1 oz. of dry earth will annually cross each
linear yard running horizontally along a slope having the above
inclination; or very nearly 7 lbs. will annually cross a horizontal
line, 100 yards in length, on a hill-side having this inclination.
A more accurate, though still very rough, calculation can be made
of the bulk of earth, which in its natural damp state annually
flows down the same slope over a yard-line drawn horizontally
across it. From the several cases given in the third chapter, it
is known that the castings annually brought to the surface on a
square yard, if uniformly spread out would form a layer 0.2 of an
inch in thickness: it therefore follows by a calculation similar
to the one already given, that 1/3 of 0.2x36, or 2.4 cubic inches
of damp earth will annually cross a horizontal line one yard in
length on a hillside with the above inclination. This bulk of damp
castings was found to weigh 1.85 oz. Therefore 11.56 lbs. of damp
earth, instead of 7 lbs. of dry earth as by the former calculation,
would annually cross a line 100 yards in length on our inclined
surface.
In these calculations it has been assumed that the castings flow a
short distance downwards during the whole year, but this occurs
only with those ejected during or shortly before rain; so that the
above results are thus far exaggerated. On the other hand, during
rain much of the finest earth is washed to a considerable distance
from the castings, even where the slope is an extremely gentle one,
and is thus wholly lost as far as the above calculations are
concerned. Castings ejected during dry weather and which have set
hard, lose in the same manner a considerable quantity of fine
earth. Dried castings, moreover, are apt to disintegrate into
little pellets, which often roll or are blown down any inclined
surface. Therefore the above result, namely, that 24 cubic inches
of earth (weighing 1.85 oz. whilst damp) annually crosses a yard-
line of the specified kind, is probably not much if at all
exaggerated.
This amount is small; but we should bear in mind how many branching
valleys intersect most countries, the whole length of which must be
very great; and that earth is steadily travelling down both turf-
covered sides of each valley. For every 100 yards in length in a
valley with sides sloping as in the foregoing cases, 480 cubic
inches of damp earth, weighing above 23 pounds, will annually reach
the bottom. Here a thick bed of alluvium will accumulate, ready to
be washed away in the course of centuries, as the stream in the
middle meanders from side to side.
If it could be shown that worms generally excavate their burrows at
right angles to an inclined surface, and this would be their
shortest course for bringing up earth from beneath, then as the old
burrows collapsed from the weight of the superincumbent soil, the
collapsing would inevitably cause the whole bed of vegetable mould
to sink or slide slowly down the inclined surface. But to
ascertain the direction of many burrows was found too difficult and
troublesome. A straight piece of wire was, however, pushed into
twenty-five burrows on several sloping fields, and in eight cases
the burrows were nearly at right angles to the slope; whilst in the
remaining cases they were indifferently directed at various angles,
either upwards or downwards with respect to the slope.
In countries where the rain is very heavy, as in the tropics, the
castings appear, as might have been expected, to be washed down in
a greater degree than in England. Mr. Scott informs me that near
Calcutta the tall columnar castings (previously described), the
diameter of which is usually between 1 and 1.5 inch, subside on a
level surface, after heavy rain, into almost circular, thin, flat
discs, between 3 and 4 and sometimes 5 inches in diameter. Three
fresh castings, which had been ejected in the Botanic Gardens "on a
slightly inclined, grass-covered, artificial bank of loamy clay,"
were carefully measured, and had a mean height of 2.17, and a mean
diameter of 1.43 inches; these after heavy rain, formed elongated
patches of earth, with a mean length in the direction of the slope
of 5.83 inches. As the earth had spread very little up the slope,
a large part, judging from the original diameter of these castings,
must have flowed bodily downwards about 4 inches. Moreover some of
the finest earth of which they were composed must have been washed
completely away to a still greater distance. In drier sites near
Calcutta, a species of worm ejects its castings, not in vermiform
masses, but in little pellets of varying sizes: these are very
numerous in some places, and Mr. Scott says that they "are washed
away by every shower."
I was led to believe that a considerable quantity of fine earth is
washed quite away from castings during rain, from the surfaces of
old ones being often studded with coarse particles. Accordingly a
little fine precipitated chalk, moistened with saliva or gum-water,
so as to be slightly viscid and of the same consistence as a fresh
casting, was placed on the summits of several castings and gently
mixed with them. These castings were then watered through a very
fine rose, the drops from which were closer together than those of
rain, but not nearly so large as those in a thunderstorm; nor did
they strike the ground with nearly so much force as drops during
heavy rain. A casting thus treated subsided with surprising
slowness, owing as I suppose to its viscidity. It did not flow
bodily down the grass-covered surface of the lawn, which was here
inclined at an angle of 16 degrees 20 seconds; nevertheless many
particles of the chalk were found three inches below the casting.
The experiment was repeated on three other castings on different
parts of the lawn, which sloped at 2 degrees 30 seconds, 3 degrees
and 6 degrees; and particles of chalk could be seen between 4 and 5
inches below the casting; and after the surface had become dry,
particles were found in two cases at a distance of 5 and 6 inches.
Several other castings with precipitated chalk placed on their
summits were left to the natural action of the rain. In one case,
after rain which was not heavy, the casting was longitudinally
streaked with white. In two other cases the surface of the ground
was rendered somewhat white for a distance of one inch from the
casting; and some soil collected at a distance of 2.5 inches, where
the slope was 7 degrees, effervesced slightly when placed in acid.
After one or two weeks, the chalk was wholly or almost wholly
washed away from all the castings on which it had been placed, and
these had recovered their natural colour.
It may be here remarked that after very heavy rain shallow pools
may be seen on level or nearly level fields, where the soil is not
very porous, and the water in them is often slightly muddy; when
such little pools have dried, the leaves and blades of grass at
their bottoms are generally coated with a thin layer of mud. This
mud I believe is derived in large part from recently ejected
castings.
Dr. King informs me that the majority of the before described
gigantic castings, which he found on a fully exposed, bare,
gravelly knoll on the Nilgiri Mountains in India, had been more or
less weathered by the previous north-east monsoon; and most of them
presented a subsided appearance. The worms here eject their
castings only during the rainy season; and at the time of Dr.
King's visit no rain had fallen for 110 days. He carefully
examined the ground between the place where these huge castings
lay, and a little watercourse at the base of the knoll, and nowhere
was there any accumulation of fine earth, such as would necessarily
have been left by the disintegration of the castings if they had
not been wholly removed. He therefore has no hesitation in
asserting that the whole of these huge castings are annually washed
during the two monsoons (when about 100 inches of rain fall) into
the little water-course, and thence into the plains lying below at
a depth of 3000 or 4000 feet.
Castings ejected before or during dry weather become hard,
sometimes surprisingly hard, from the particles of earth having
been cemented together by the intestinal secretions. Frost seems
to be less effective in their disintegration than might have been
expected. Nevertheless they readily disintegrate into small
pellets, after being alternately moistened with rain and again
dried. Those which have flowed during rain down a slope,
disintegrate in the same manner. Such pellets often roll a little
down any sloping surface; their descent being sometimes much aided
by the wind. The whole bottom of a broad dry ditch in my grounds,
where there were very few fresh castings, was completely covered
with these pellets or disintegrated castings, which had rolled down
the steep sides, inclined at an angle of 27 degrees.
Near Nice, in places where the great cylindrical castings,
previously described, abound, the soil consists of very fine
arenaceo-calcareous loam; and Dr. King informs me that these
castings are extremely liable to crumble during dry weather into
small fragments, which are soon acted on by rain, and then sink
down so as to be no longer distinguishable from the surrounding
soil. He sent me a mass of such disintegrated castings, collected
on the top of a bank, where none could have rolled down from above.
They must have been ejected within the previous five or six months,
but they now consisted of more or less rounded fragments of all
sizes, from 0.75 of an inch in diameter to minute grains and mere
dust. Dr. King witnessed the crumbling process whilst drying some
perfect castings, which he afterwards sent me. Mr. Scott also
remarks on the crumbling of the castings near Calcutta and on the
mountains of Sikkim during the hot and dry season.
When the castings near Nice had been ejected on an inclined
surface, the disintegrated fragments rolled downwards, without
losing their distinctive shape; and in some places could "be
collected in basketfuls." Dr. King observed a striking instance of
this fact on the Corniche road, where a drain, about 2.5 feet wide
and 9 inches deep, had been made to catch the surface drainage from
the adjoining hill-side. The bottom of this drain was covered for
a distance of several hundred yards, to a depth of from 1.5 to 3
inches, by a layer of broken castings, still retaining their
characteristic shape. Nearly all these innumerable fragments had
rolled down from above, for extremely few castings had been ejected
in the drain itself. The hill-side was steep, but varied much in
inclination, which Dr. King estimated at from 30 degrees to 60
degrees with the horizon. He climbed up the slope, and "found
every here and there little embankments, formed by fragments of the
castings that had been arrested in their downward progress by
irregularities of the surface, by stones, twigs, &c. One little
group of plants of Anemone hortensis had acted in this manner, and
quite a small bank of soil had collected round it. Much of this
soil had crumbled down, but a great deal of it still retained the
form of castings." Dr. King dug up this plant, and was struck with
the thickness of the soil which must have recently accumulated over
the crown of the rhizoma, as shown by the length of the bleached
petioles, in comparison with those of other plants of the same
kind, where there had been no such accumulation. The earth thus
accumulated had no doubt been secured (as I have everywhere seen)
by the smaller roots of the plants. After describing this and
other analogous cases, Dr. King concludes: "I can have no doubt
that worms help greatly in the process of denudation."
Ledges of earth on steep hill-sides.--Little horizontal ledges, one
above another, have been observed on steep grassy slopes in many
parts of the world. The formation has been attributed to animals
travelling repeatedly along the slope in the same horizontal lines
while grazing, and that they do thus move and use the ledges is
certain; but Professor Henslow (a most careful observer) told Sir
J. Hooker that he was convinced that this was not the sole cause of
their formation. Sir J. Hooker saw such ledges on the Himalayan
and Atlas ranges, where there were no domesticated animals and not
many wild ones; but these latter would, it is probable, use the
ledges at night while grazing like our domesticated animals. A
friend observed for me the ledges on the Alps of Switzerland, and
states that they ran at 3 or 4 ft. one above the other, and were
about a foot in breadth. They had been deeply pitted by the feet
of grazing cows. Similar ledges were observed by the same friend
on our Chalk downs, and on an old talus of chalk-fragments (thrown
out of a quarry) which had become clothed with turf.
My son Francis examined a Chalk escarpment near Lewes; and here on
a part which was very steep, sloping at 40 degrees with the
horizon, about 30 flat ledges extended horizontally for more than
100 yards, at an average distance of about 20 inches, one beneath
the other. They were from 9 to 10 inches in breadth. When viewed
from a distance they presented a striking appearance, owing to
their parallelism; but when examined closely, they were seen to be
somewhat sinuous, and one often ran into another, giving the
appearance of the ledge having forked into two. They are formed of
light-coloured earth, which on the outside, where thickest, was in
one case 9 inches, and in another case between 6 and 7 inches in
thickness. Above the ledges, the thickness of the earth over the
chalk was in the former case 4 and in the latter only 3 inches.
The grass grew more vigorously on the outer edges of the ledges
than on any other part of the slope, and here formed a tufted
fringe. Their middle part was bare, but whether this had been
caused by the trampling of sheep, which sometimes frequent the
ledges, my son could not ascertain. Nor could he feel sure how
much of the earth on the middle and bare parts, consisted of
disintegrated worm-castings which had rolled down from above; but
he felt convinced that some had thus originated; and it was
manifest that the ledges with their grass-fringed edges would
arrest any small object rolling down from above.
At one end or side of the bank bearing these ledges, the surface
consisted in parts of bare chalk, and here the ledges were very
irregular. At the other end of the bank, the slope suddenly became
less steep, and here the ledges ceased rather abruptly; but little
embankments only a foot or two in length were still present. The
slope became steeper lower down the hill, and the regular ledges
then reappeared. Another of my sons observed, on the inland side
of Beachy Head, where the surface sloped at about 25 degrees, many
short little embankments like those just mentioned. They extended
horizontally and were from a few inches to two or three feet in
length. They supported tufts of grass growing vigorously. The
average thickness of the mould of which they were formed, taken
from nine measurements, was 4.5 inches; while that of the mould
above and beneath them was on an average only 3.2 inches, and on
each side, on the same level, 3.1 inches. On the upper parts of
the slope, these embankments showed no signs of having been
trampled on by sheep, but in the lower parts such signs were fairly
plain. No long continuous ledges had here been formed.
If the little embankments above the Corniche road, which Dr. King
saw in the act of formation by the accumulation of disintegrated
and rolled worm-castings, were to become confluent along horizontal
lines, ledges would be formed. Each embankment would tend to
extend laterally by the lateral extension of the arrested castings;
and animals grazing on a steep slope would almost certainly make
use of every prominence at nearly the same level, and would indent
the turf between them; and such intermediate indentations would
again arrest the castings. An irregular ledge when once formed
would also tend to become more regular and horizontal by some of
the castings rolling laterally from the higher to the lower parts,
which would thus be raised. Any projection beneath a ledge would
not afterwards receive disintegrated matter from above, and would
tend to be obliterated by rain and other atmospheric agencies.
There is some analogy between the formation, as here supposed, of
these ledges, and that of the ripples of wind-drifted sand as
described by Lyell. {78}
The steep, grass-covered sides of a mountainous valley in
Westmoreland, called Grisedale, was marked in many places with
innumerable lines of miniature cliffs, with almost horizontal,
little ledges at their bases. Their formation was in no way
connected with the action of worms, for castings could not anywhere
be seen (and their absence is an inexplicable fact), although the
turf lay in many places over a considerable thickness of boulder-
clay and moraine rubbish. Nor, as far as I could judge, was the
formation of these little cliffs at all closely connected with the
trampling of cows or sheep. It appeared as if the whole
superficial, somewhat argillaceous earth, while partially held
together by the roots of the grasses, had slided a little way down
the mountain sides; and in thus sliding, had yielded and cracked in
horizontal lines, transversely to the slope.
Castings blown to leeward by the wind.--We have seen that moist
castings flow, and that disintegrated castings roll down any
inclined surface; and we shall now see that castings, recently
ejected on level grass-covered surfaces, are blown during gales of
wind accompanied by rain to leeward. This has been observed by me
many times on many fields during several successive years. After
such gales, the castings present a gently inclined and smooth, or
sometimes furrowed, surface to windward, while they are steeply
inclined or precipitous to leeward, so that they resemble on a
miniature scale glacier-ground hillocks of rock. They are often
cavernous on the leeward side, from the upper part having curled
over the lower part. During one unusually heavy south-west gale
with torrents of rain, many castings were wholly blown to leeward,
so that the mouths of the burrows were left naked and exposed on
the windward side. Recent castings naturally flow down an inclined
surface, but on a grassy field, which sloped between 10 degrees and
15 degrees, several were found after a heavy gale blown up the
slope. This likewise occurred on another occasion on a part of my
lawn where the slope was somewhat less. On a third occasion, the
castings on the steep, grass-covered sides of a valley, down which
a gale had blown, were directed obliquely instead of straight down
the slope; and this was obviously due to the combined action of the
wind and gravity. Four castings on my lawn, where the downward
inclination was 0 degrees 45 seconds, 1 degree, 3 degrees and 3
degrees 30 seconds (mean 2 degrees 45 seconds) towards the north-
east, after a heavy south-west gale with rain, were divided across
the mouths of the burrows and weighed in the manner formerly
described. The mean weight of the earth below the mouths of
burrows and to leeward, was to that above the mouths and on the
windward side as 2.75 to 1; whereas we have seen that with several
castings which had flowed down slopes having a mean inclination of
9 degrees 26 seconds, and with three castings where the inclination
was above 12 degrees; the proportional weight of the earth below to
that above the burrows was as only 2 to 1. These several cases
show how efficiently gales of wind accompanied by rain act in
displacing recently ejected castings. We may therefore conclude
that even a moderately strong wind will produce some slight effect
on them.
Dry and indurated castings, after their disintegration into small
fragments or pellets, are sometimes, probably often, blown by a
strong wind to leeward. This was observed on four occasions, but I
did not sufficiently attend to this point. One old casting on a
gently sloping bank was blown quite away by a strong south-west
wind. Dr. King believes that the wind removes the greater part of
the old crumbling castings near Nice. Several old castings on my
lawn were marked with pins and protected from any disturbance.
They were examined after an interval of 10 weeks, during which time
the weather had been alternately dry and rainy. Some, which were
of a yellowish colour had been washed almost completely away, as
could be seen by the colour of the surrounding ground. Others had
completely disappeared, and these no doubt had been blown away.
Lastly, others still remained and would long remain, as blades of
grass had grown through them. On poor pasture-land, which has
never been rolled and has not been much trampled on by animals, the
whole surface is sometimes dotted with little pimples, through and
on which grass grows; and these pimples consist of old worm-
castings.
In all the many observed cases of soft castings blown to leeward,
this had been effected by strong winds accompanied by rain. As
such winds in England generally blow from the south and south-west,
earth must on the whole tend to travel over our fields in a north
and north-east direction. This fact is interesting, because it
might be thought that none could be removed from a level, grass-
covered surface by any means. In thick and level woods, protected
from the wind, castings will never be removed as long as the wood
lasts; and mould will here tend to accumulate to the depth at which
worms can work. I tried to procure evidence as to how much mould
is blown, whilst in the state of castings, by our wet southern
gales to the north-east, over open and flat land, by looking to the
level of the surface on opposite sides of old trees and hedge-rows;
but I failed owing to the unequal growth of the roots of trees and
to most pasture-land having been formerly ploughed.
On an open plain near Stonehenge, there exist shallow circular
trenches, with a low embankment outside, surrounding level spaces
50 yards in diameter. These rings appear very ancient, and are
believed to be contemporaneous with the Druidical stones. Castings
ejected within these circular spaces, if blown to the north-east by
south-west winds would form a layer of mould within the trench,
thicker on the north-eastern than on any other side. But the site
was not favourable for the action of worms, for the mould over the
surrounding Chalk formation with flints, was only 3.37 inches in
thickness, from a mean of six observations made at a distance of 10
yards outside the embankment. The thickness of the mould within
two of the circular trenches was measured every 5 yards all round,
on the inner sides near the bottom. My son Horace protracted these
measurements on paper; and though the curved line representing the
thickness of the mould was extremely irregular, yet in both
diagrams it could be seen to be thicker on the north-eastern side
than elsewhere. When a mean of all the measurements in both the
trenches was laid down and the line smoothed, it was obvious that
the mould was thickest in the quarter of the circle between north-
west and north-east; and thinnest in the quarter between south-east
and south-west, especially at this latter point. Besides the
foregoing measurements, six others were taken near together in one
of the circular trenches, on the north-east side; and the mould
here had a mean thickness of 2.29 inches; while the mean of six
other measurements on the south-west side was only 1.46 inches.
These observations indicate that the castings had been blown by the
south-west winds from the circular enclosed space into the trench
on the north-east side; but many more measurements in other
analogous cases would be requisite for a trustworthy result.
The amount of fine earth brought to the surface under the form of
castings, and afterwards transported by the winds accompanied by
rain, or that which flows and rolls down an inclined surface, no
doubt is small in the course of a few scores of years; for
otherwise all the inequalities in our pasture fields would be
smoothed within a much shorter period than appears to be the case.
But the amount which is thus transported in the course of thousands
of years cannot fail to be considerable and deserves attention. E.
de Beaumont looks at the vegetable mould which everywhere covers
the land as a fixed line, from which the amount of denudation may
be measured. {79} He ignores the continued formation of fresh
mould by the disintegration of the underlying rocks and fragments
of rock; and it is curious to find how much more philosophical were
the views maintained long ago, by Playfair, who, in 1802, wrote,
"In the permanence of a coat of vegetable mould on the surface of
the earth, we have a demonstrative proof of the continued
destruction of the rocks." {80}
Ancient encampments and tumuli.--E. de Beaumont adduces the present
state of many ancient encampments and tumuli and of old ploughed
fields, as evidence that the surface of the land undergoes hardly
any degradation. But it does not appear that he ever examined the
thickness of the mould over different parts of such old remains.
He relies chiefly on indirect, but apparently trustworthy, evidence
that the slopes of the old embankments are the same as they
originally were; and it is obvious that he could know nothing about
their original heights. In Knole Park a mound had been thrown up
behind the rifle-targets, which appeared to have been formed of
earth originally supported by square blocks of turf. The sides
sloped, as nearly as I could estimate them, at an angle of 45
degrees or 50 degrees with the horizon, and they were covered,
especially on the northern side, with long coarse grass, beneath
which many worm-castings were found. These had flowed bodily
downwards, and others had rolled down as pellets. Hence it is
certain that as long as a mound of this kind is tenanted by worms,
its height will be continually lowered. The fine earth which flows
or rolls down the sides of such a mound accumulates at its base in
the form of a talus. A bed, even a very thin bed, of fine earth is
eminently favourable for worms; so that a greater number of
castings would tend to be ejected on a talus thus formed than
elsewhere; and these would be partially washed away by every heavy
shower and be spread over the adjoining level ground. The final
result would be the lowering of the whole mound, whilst the
inclination of the sides would not be greatly lessened. The same
result would assuredly follow with ancient embankments and tumuli;
except where they had been formed of gravel or of nearly pure sand,
as such matter is unfavourable for worms. Many old fortifications
and tumuli are believed to be at least 2000 years old; and we
should bear in mind that in many places about one inch of mould is
brought to the surface in 5 years or two inches in 10 years.
Therefore in so long a period as 2000 years, a large amount of
earth will have been repeatedly brought to the surface on most old
embankments and tumuli, especially on the talus round their bases,
and much of this earth will have been washed completely away. We
may therefore conclude that all ancient mounds, when not formed of
materials unfavourable to worms, will have been somewhat lowered in
the course of centuries, although their inclinations may not have
been greatly changed.
Fields formerly ploughed.--From a very remote period and in many
countries, land has been ploughed, so that convex beds, called
crowns or ridges, usually about 8 feet across and separated by
furrows, have been thrown up. The furrows are directed so as to
carry off the surface water. In my attempts to ascertain how long
a time these crowns and furrows last, when ploughed land has been
converted into pasture, obstacles of many kinds were encountered.
It is rarely known when a field was last ploughed; and some fields
which were thought to have been in pasture from time immemorial
were afterwards discovered to have been ploughed only 50 or 60
years before. During the early part of the present century, when
the price of corn was very high, land of all kinds seems to have
been ploughed in Britain. There is, however, no reason to doubt
that in many cases the old crowns and furrows have been preserved
from a very ancient period. {81} That they should have been
preserved for very unequal lengths of time would naturally follow
from the crowns, when first thrown up, having differed much in
height in different districts, as is now the case with recently
ploughed land.
In old pasture fields, the mould, wherever measurements were made,
was found to be from 0.5 to 2 inches thicker in the furrows than on
the crowns; but this would naturally follow from the finer earth
having been washed from the crowns into the furrows before the land
was well clothed with turf; and it is impossible to tell what part
worms may have played in the work. Nevertheless from what we have
seen, castings would certainly tend to flow and to be washed during
heavy rain from the crowns into the furrows. But as soon as a bed
of fine earth had by any means been accumulated in the furrows, it
would be more favourable for worms than the other parts, and a
greater number of castings would be thrown up here than elsewhere;
and as the furrows on sloping land are usually directed so as to
carry off the surface water, some of the finest earth would be
washed from the castings which had been here ejected and be carried
completely away. The result would be that the furrows would be
filled up very slowly, while the crowns would be lowered perhaps
still more slowly by the flowing and rolling of the castings down
their gentle inclinations into the furrows.
Nevertheless it might be expected that old furrows, especially
those on a sloping surface, would in the course of time be filled
up and disappear. Some careful observers, however, who examined
fields for me in Gloucestershire and Staffordshire could not detect
any difference in the state of the furrows in the upper and lower
parts of sloping fields, supposed to have been long in pasture; and
they came to the conclusion that the crowns and furrows would last
for an almost endless number of centuries. On the other hand the
process of obliteration seems to have commenced in some places.
Thus in a grass field in North Wales, known to have been ploughed
about 65 years ago, which sloped at an angle of 15 degrees to the
north-east, the depth of the furrows (only 7 feet apart) was
carefully measured, and was found to be about 4.5 inches in the
upper part of the slope, and only 1 inch near the base, where they
could be traced with difficulty. On another field sloping at about
the same angle to the south-west, the furrows were scarcely
perceptible in the lower part; although these same furrows when
followed on to some adjoining level ground were from 2.5 to 3.5
inches in depth. A third and closely similar case was observed.
In a fourth case, the mould in a furrow in the upper part of a
sloping field was 2.5 inches, and in the lower part 4.5 inches in
thickness.
On the Chalk Downs at about a mile distance from Stonehenge, my son
William examined a grass-covered, furrowed surface, sloping at from
8 degrees to 10 degrees, which an old shepherd said had not been
ploughed within the memory of man. The depth of one furrow was
measured at 16 points in a length of 68 paces, and was found to be
deeper where the slope was greatest and where less earth would
naturally tend to accumulate, and at the base it almost
disappeared. The thickness of the mould in this furrow in the
upper part was 2.5 inches, which increased to 5 inches, a little
above the steepest part of the slope; and at the base, in the
middle of the narrow valley, at a point which the furrow if
continued would have struck, it amounted to 7 inches. On the
opposite side of the valley, there were very faint, almost
obliterated, traces of furrows. Another analogous but not so
decided a case was observed at a few miles' distance from
Stonehenge. On the whole it appears that the crowns and furrows on
land formerly ploughed, but now covered with grass, tend slowly to
disappear when the surface is inclined; and this is probably in
large part due to the action of worms; but that the crowns and
furrows last for a very long time when the surface is nearly level.
Formation and amount of mould over the Chalk Formation.--Worm-
castings are often ejected in extraordinary numbers on steep,
grass-covered slopes, where the Chalk comes close to the surface,
as my son William observed near Winchester and elsewhere. If such
castings are largely washed away during heavy rains, it is
difficult to understand at first how any mould can still remain on
our Downs, as there does not appear any evident means for supplying
the loss. There is, moreover, another cause of loss, namely, in
the percolation of the finer particles of earth into the fissures
in the chalk and into the chalk itself. These considerations led
me to doubt for a time whether I had not exaggerated the amount of
fine earth which flows or rolls down grass-covered slopes under the
form of castings; and I sought for additional information. In some
places, the castings on Chalk Downs consist largely of calcareous
matter, and here the supply is of course unlimited. But in other
places, for instance on a part of Teg Down near Winchester, the
castings were all black and did not effervesce with acids. The
mould over the chalk was here only from 3 to 4 inches in thickness.
So again on the plain near Stonehenge, the mould, apparently free
from calcareous matter, averaged rather less than 3.5 inches in
thickness. Why worms should penetrate and bring up chalk in some
places and not in others I do not know.
In many districts where the land is nearly level, a bed several
feet in thickness of red clay full of unworn flints overlies the
Upper Chalk. This overlying matter, the surface of which has been
converted into mould, consists of the undissolved residue from the
chalk. It may be well here to recall the case of the fragments of
chalk buried beneath worm-castings on one of my fields, the angles
of which were so completely rounded in the course of 29 years that
the fragments now resembled water-worn pebbles. This must have
been effected by the carbonic acid in the rain and in the ground,
by the humus-acids, and by the corroding power of living roots.
Why a thick mass of residue has not been left on the Chalk,
wherever the land is nearly level, may perhaps be accounted for by
the percolation of the fine particles into the fissures, which are
often present in the chalk and are either open or are filled up
with impure chalk, or into the solid chalk itself. That such
percolation occurs can hardly be doubted. My son collected some
powdered and fragmentary chalk beneath the turf near Winchester;
the former was found by Colonel Parsons, R. E., to contain 10 per
cent., and the fragments 8 per cent. of earthy matter. On the
flanks of the escarpment near Abinger in Surrey, some chalk close
beneath a layer of flints, 2 inches in thickness and covered by 8
inches of mould, yielded a residue of 3.7 per cent. of earthy
matter. On the other hand the Upper Chalk properly contains, as I
was informed by the late David Forbes who had made many analyses,
only from 1 to 2 per cent. of earthy matter; and two samples from
pits near my house contained 1.3 and 0.6 per cent. I mention these
latter cases because, from the thickness of the overlying bed of
red clay with flints, I had imagined that the underlying chalk
might here be less pure than elsewhere. The cause of the residue
accumulating more in some places than in others, may be attributed
to a layer of argillaceous matter having been left at an early
period on the chalk, and this would check the subsequent
percolation of earthy matter into it.
From the facts now given we may conclude that castings ejected on
our Chalk Downs suffer some loss by the percolation of their finer
matter into the chalk. But such impure superficial chalk, when
dissolved, would leave a larger supply of earthy matter to be added
to the mould than in the case of pure chalk. Besides the loss
caused by percolation, some fine earth is certainly washed down the
sloping grass-covered surfaces of our Downs. The washing-down
process, however, will be checked in the course of time; for
although I do not know how thin a layer of mould suffices to
support worms, yet a limit must at last be reached; and then their
castings would cease to be ejected or would become scanty.
The following cases show that a considerable quantity of fine earth
is washed down. The thickness of the mould was measured at points
12 yards apart across a small valley in the Chalk near Winchester.
The sides sloped gently at first; then became inclined at about 20
degrees; then more gently to near the bottom, which transversely
was almost level and about 50 yards across. In the bottom, the
mean thickness of the mould from five measurements was 8.3 inches;
whilst on the sides of the valley, where the inclination varied
between 14 degrees and 20 degrees, its mean thickness was rather
less than 3.5 inches. As the turf-covered bottom of the valley
sloped at an angle of only between 2 degrees and 3 degrees, it is
probable that most of the 8.3-inch layer of mould had been washed
down from the flanks of the valley, and not from the upper part.
But as a shepherd said that he had seen water flowing in this
valley after the sudden thawing of snow, it is possible that some
earth may have been brought down from the upper part; or, on the
other hand, that some may have been carried further down the
valley. Closely similar results, with respect to the thickness of
the mould, were obtained in a neighbouring valley.
St. Catherine's Hill, near Winchester, is 327 feet in height, and
consists of a steep cone of chalk about 0.25 of a mile in diameter.
The upper part was converted by the Romans, or, as some think, by
the ancient Britons, into an encampment, by the excavation of a
deep and broad ditch all round it. Most of the chalk removed
during the work was thrown upwards, by which a projecting bank was
formed; and this effectually prevents worm-castings (which are
numerous in parts), stones, and other objects from being washed or
rolled into the ditch. The mould on the upper and fortified part
of the hill was found to be in most places only from 2.5 to 3.5
inches in thickness; whereas it had accumulated at the foot of the
embankment above the ditch to a thickness in most places of from 8
to 9.5 inches. On the embankment itself the mould was only 1 to
1.5 inch in thickness; and within the ditch at the bottom it varied
from 2.5 to 3.5, but was in one spot 6 inches in thickness. On the
north-west side of the hill, either no embankment had ever been
thrown up above the ditch, or it had subsequently been removed; so
that here there was nothing to prevent worm-castings, earth and
stones being washed into the ditch, at the bottom of which the
mould formed a layer from 11 to 22 inches in thickness. It should
however be stated that here and on other parts of the slope, the
bed of mould often contained fragments of chalk and flint which had
obviously rolled down at different times from above. The
interstices in the underlying fragmentary chalk were also filled up
with mould.
My son examined the surface of this hill to its base in a south-
west direction. Beneath the great ditch, where the slope was about
24 degrees, the mould was very thin, namely, from 1.5 to 2.5
inches; whilst near the base, where the slope was only 3 degrees to
4 degrees, it increased to between 8 and 9 inches in thickness. We
may therefore conclude that on this artificially modified hill, as
well as in the natural valleys of the neighbouring Chalk Downs,
some fine earth, probably derived in large part from worm-castings,
is washed down, and accumulates in the lower parts, notwithstanding
the percolation of an unknown quantity into the underlying chalk; a
supply of fresh earthy matter being afforded by the dissolution of
the chalk through atmospheric and other agencies.
CHAPTER VII--CONCLUSION.
Summary of the part which worms have played in the 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.
Worms have played a more important part in the history of the world
than most persons would at first suppose. In almost all humid
countries they are extraordinarily numerous, and for their size
possess great muscular power. In many parts of England a weight of
more than ten tons (10,516 kilogrammes) of dry earth annually
passes through their bodies and is brought to the surface on each
acre of land; so that the whole superficial bed of vegetable mould
passes through their bodies in the course of every few years. From
the collapsing of the old burrows the mould is in constant though
slow movement, and the particles composing it are thus rubbed
together. By these means fresh surfaces are continually exposed to
the action of the carbonic acid in the soil, and of the humus-acids
which appear to be still more efficient in the decomposition of
rocks. The generation of the humus-acids is probably hastened
during the digestion of the many half-decayed leaves which worms
consume. Thus the particles of earth, forming the superficial
mould, are subjected to conditions eminently favourable for their
decomposition and disintegration. Moreover, the particles of the
softer rocks suffer some amount of mechanical trituration in the
muscular gizzards of worms, in which small stones serve as mill-
stones.
The finely levigated castings, when brought to the surface in a
moist condition, flow during rainy weather down any moderate slope;
and the smaller particles are washed far down even a gently
inclined surface. Castings when dry often crumble into small
pellets and these are apt to roll down any sloping surface. Where
the land is quite level and is covered with herbage, and where the
climate is humid so that much dust cannot be blown away, it appears
at first sight impossible that there should be any appreciable
amount of sub-aerial denudation; but worm-castings are blown,
especially whilst moist and viscid, in one uniform direction by the
prevalent winds which are accompanied by rain. By these several
means the superficial mould is prevented from accumulating to a
great thickness; and a thick bed of mould checks in many ways the
disintegration of the underlying rocks and fragments of rock.
The removal of worm-castings by the above means leads to results
which are far from insignificant. It has been shown that a layer
of earth, 0.2 of an inch in thickness, is in many places annually
brought to the surface; and if a small part of this amount flows,
or rolls, or is washed, even for a short distance, down every
inclined surface, or is repeatedly blown in one direction, a great
effect will be produced in the course of ages. It was found by
measurements and calculations that on a surface with a mean
inclination of 9 degrees 26 seconds, 2.4 cubic inches of earth
which had been ejected by worms crossed, in the course of a year, a
horizontal line one yard in length; so that 240 cubic inches would
cross a line 100 yards in length. This latter amount in a damp
state would weigh 11.5 pounds. Thus a considerable weight of earth
is continually moving down each side of every valley, and will in
time reach its bed. Finally this earth will be transported by the
streams flowing in the valleys into the ocean, the great receptacle
for all matter denuded from the land. It is known from the amount
of sediment annually delivered into the sea by the Mississippi,
that its enormous drainage-area must on an average be lowered
.00263 of an inch each year; and this would suffice in four and
half million years to lower the whole drainage-area to the level of
the sea-shore. So that, if a small fraction of the layer of fine
earth, 0.2 of an inch in thickness, which is annually brought to
the surface by worms, is carried away, a great result cannot fail
to be produced within a period which no geologist considers
extremely long.
Archaeologists ought to be grateful to worms, as they protect and
preserve for an indefinitely long period every object, not liable
to decay, which is dropped on the surface of the land, by burying
it beneath their castings. Thus, also, many elegant and curious
tesselated pavements and other ancient remains have been preserved;
though no doubt the worms have in these cases been largely aided by
earth washed and blown from the adjoining land, especially when
cultivated. The old tesselated pavements have, however, often
suffered by having subsided unequally from being unequally
undermined by the worms. Even old massive walls may be undermined
and subside; and no building is in this respect safe, unless the
foundations lie 6 or 7 feet beneath the surface, at a depth at
which worms cannot work. It is probable that many monoliths and
some old walls have fallen down from having been undermined by
worms.
Worms prepare the ground {82} in an excellent manner for the growth
of fibrous-rooted plants and for seedlings of all kinds. They
periodically expose the mould to the air, and sift it so that no
stones larger than the particles which they can swallow are left in
it. They mingle the whole intimately together, like a gardener who
prepares fine soil for his choicest plants. In this state it is
well fitted to retain moisture and to absorb all soluble
substances, as well as for the process of nitrification. The bones
of dead animals, the harder parts of insects, the shells of land-
molluscs, leaves, twigs, &c., are before long all buried beneath
the accumulated castings of worms, and are thus brought in a more
or less decayed state within reach of the roots of plants. Worms
likewise drag an infinite number of dead leaves and other parts of
plants into their burrows, partly for the sake of plugging them up
and partly as food.
The leaves which are dragged into the burrows as food, after being
torn into the finest shreds, partially digested, and saturated with
the intestinal and urinary secretions, are commingled with much
earth. This earth forms the dark coloured, rich humus which almost
everywhere covers the surface of the land with a fairly well-
defined layer or mantle. Hensen {83} placed two worms in a vessel
18 inches in diameter, which was filled with sand, on which fallen
leaves were strewed; and these were soon dragged into their burrows
to a depth of 3 inches. After about 6 weeks an almost uniform
layer of sand, a centimeter (0.4 inch) in thickness, was converted
into humus by having passed through the alimentary canals of these
two worms. It is believed by some persons that worm-burrows, which
often penetrate the ground almost perpendicularly to a depth of 5
or 6 feet, materially aid in its drainage; notwithstanding that the
viscid castings piled over the mouths of the burrows prevent or
check the rain-water directly entering them. They allow the air to
penetrate deeply into the ground. They also greatly facilitate the
downward passage of roots of moderate size; and these will be
nourished by the humus with which the burrows are lined. Many
seeds owe their germination to having been covered by castings; and
others buried to a considerable depth beneath accumulated castings
lie dormant, until at some future time they are accidentally
uncovered and germinate.
Worms are poorly provided with sense-organs, for they cannot be
said to see, although they can just distinguish between light and
darkness; they are completely deaf, and have only a feeble power of
smell; the sense of touch alone is well developed. They can
therefore learn but little about the outside world, and it is
surprising that they should exhibit some skill in lining their
burrows with their castings and with leaves, and in the case of
some species in piling up their castings into tower-like
constructions. But it is far more surprising that they should
apparently exhibit some degrees of intelligence instead of a mere
blind instinctive impulse, in their manner of plugging up the
mouths of their burrows. They act in nearly the same manner as
would a man, who had to close a cylindrical tube with different
kinds of leaves, petioles, triangles of paper, &c., for they
commonly seize such objects by their pointed ends. But with thin
objects a certain number are drawn in by their broader ends. They
do not act in the same unvarying manner in all cases, as do most of
the lower animals; for instance, they do not drag in leaves by
their foot-stalks, unless the basal part of the blade is as narrow
as the apex, or narrower than it.
When we behold a wide, turf-covered expanse, we should remember
that its smoothness, on which so much of its beauty depends, is
mainly due to all the inequalities having been slowly levelled by
worms. It is a marvellous reflection that the whole of the
superficial mould over any such expanse has passed, and will again
pass, every few years through the bodies of worms. The plough is
one of the most ancient and most valuable of man's inventions; but
long before he existed the land was in fact regularly ploughed, and
still continues to be thus ploughed by earth-worms. It may be
doubted whether there are many other animals which have played so
important a part in the history of the world, as have these lowly
organized creatures. Some other animals, however, still more lowly
organized, namely corals, have done far more conspicuous work in
having constructed innumerable reefs and islands in the great
oceans; but these are almost confined to the tropical zones.
Footnotes:
{1} 'Lecons de Geologie Pratique,' tom. i. 1845, p. 140.
{2} 'Transactions Geolog. Soc.' vol. v. p. 505. Read November 1,
1837.
{3} 'Histoire des progres de la Geologie,' tom. i. 1847, p. 224.
{4} 'Zeitschrift fur wissenschaft. Zoologie,' B. xxviii. 1877, p.
361.
{5} 'Gardeners' Chronicle,' April 17, 1869, p. 418.
{6} Mr. Darwin's attention was called by Professor Hensen to P. E.
Muller's work on Humus in 'Tidsskrift for Skovbrug,' Band iii. Heft
1 and 2, Copenhagen, 1878. He had, however, no opportunity of
consulting Muller's work. Dr. Muller published a second paper in
1884 in the same periodical--a Danish journal of forestry. His
results have also been published in German, in a volume entitled
'Studien uber die naturlichen Humusformen, unter deren Einwirkung
auf Vegetation und Boden,' 8vo., Berlin, 1887.
{7} 'Bidrag till Skandinaviens Oligochaetfauna,' 1871.
{8} 'Die bis jetzt bekannten Arten aus der Familie der
Regenwurmer,' 1845.
{9} There is even some reason to believe that pressure is actually
favourable to the growth of grasses, for Professor Buckman, who
made many observations on their growth in the experimental gardens
of the Royal Agricultural College, remarks ('Gardeners' Chronicle,'
1854, p. 619): "Another circumstance in the cultivation of grasses
in the separate form or small patches, is the impossibility of
rolling or treading them firmly, without which no pasture can
continue good."
{10} I shall have occasion often to refer to M. Perrier's
admirable memoir, 'Organisation des Lombriciens terrestres' in
'Archives de Zoolog. exper.' tom. iii. 1874, p. 372. C. F. Morren
('De Lumbrici terrestris Hist. Nat.' 1829, p. 14) found that worms
endured immersion for fifteen to twenty days in summer, but that in
winter they died when thus treated.
{11} Morren, 'De Lumbrici terrestris Hist. Nat.' &c., 1829, p. 67.
{12} 'De Lumbrici terrestris Hist. Nat.' &c., p. 14.
{13} Histolog. Untersuchungen uber die Regenwurmer. 'Zeitschrift
fur wissenschaft. Zoologie,' B. xix., 1869, p. 611.
{14} For instance, Mr. Bridgman and Mr. Newman ('The Zoologist,'
vol. vii. 1849, p. 2576), and some friends who observed worms for
me.
{15} 'Familie der Regenwurmer,' 1845, p. 18.
{16} 'The Zoologist,' vol. vii. 1849, p. 2576.
{17} 'Familie der Regenwurmer,' p. 13. Dr. Sturtevant states in
the 'New York Weekly Tribune' (May 19, 1880) that he kept three
worms in a pot, which was allowed to become extremely dry; and
these worms were found "all entwined together, forming a round mass
and in good condition."
{18} 'De Lumbrici terrestris Hist. Nat.' p. 19.
{19} 'Archives de Zoologie experimentale,' tom. vii. 1878, p. 394.
When I wrote the above passage, I was not aware that Krukenberg
('Untersuchungen a. d. physiol. Inst. d. Univ. Heidelberg,' Bd.
ii. p. 37, 1877) had previously investigated the digestive juice of
Lumbricus. He states that it contains a peptic, and diastatic, as
well as a tryptic ferment.
{20} On the action of the pancreatic ferment, see 'A Text-Book of
Physiology,' by Michael Foster, 2nd edit. pp. 198-203. 1878.
{21} Schmulewitsch, 'Action des Sucs digestifs sur la Cellulose.'
Bull. de l'Acad. Imp. de St. Petersbourg, tom. xxv. p. 549. 1879.
{22} Claparede doubts whether saliva is secreted by worms: see
'Zeitschrift fur wissenschaft. Zoologie,' B. xix. 1869, p. 601.
{23} Perrier, 'Archives de Zoolog. exper.' July, 1874, pp. 416,
419.
{24} 'Zeitschrift fur wissenschaft. Zoologie,' B. xix, 1869, pp.
603-606.
{25} De Vries, 'Landwirth. Jahrbucher,' 1881, p. 77.
{26} M. Foster, 'A Text-Book of Physiology,' 2nd edit. 1878, p.
243.
{27} M. Foster, ut sup. p. 200.
{28} Claparede remarks ('Zeitschrift fur wisseuschaft. Zoolog.'
B. 19, 1869, p. 602) that the pharynx appears from its structure to
be adapted for suction.
{29} An account of her observations is given in the 'Gardeners'
Chronicle,' March 28th, 1868, p. 324.
{30} London's 'Gard. Mag.' xvii. p. 216, as quoted in the
'Catalogue of the British Museum Worms,' 1865, p. 327.
{31} 'Familie der Regenwurmer,' p. 19.
{32} In these narrow triangles the apical angle is 9 degrees 34
seconds, and the basal angles 85 degrees 13 seconds. In the
broader triangles the apical angle is 19 degrees 10 seconds and the
basal angles 80 degrees 25 seconds.
{33} See his interesting work, 'Souvenirs entomologiques,' 1879,
pp. 168-177.
{34} Mobius, 'Die Bewegungen der Thiere,' &c., 1873, p. 111.
{35} 'Annals and Mag. of N. History,' series ii. vol. ix. 1852, p.
333.
{36} 'Archives de Zoolog. exper.' tom. iii. 1874, p. 405.
{37} I state this on the authority of Semper, 'Reisen im Archipel
der Philippinen,' Th. ii. 1877, p. 30.
{38} Dr. King gave me some worms collected near Nice, which, as he
believes, had constructed these castings. They were sent to M.
Perrier, who with great kindness examined and named them for me:
they consisted of Perichaeta affinis, a native of Cochin China and
of the Philippines; P. Luzonica, a native of Luzon in the
Philippines; and P. Houlleti, which lives near Calcutta. M.
Perrier informs me that species of Perichaeta have been naturalized
in the gardens near Montpellier and in Algiers. Before I had any
reason to suspect that the tower-like castings from Nice had been
formed by worms not endemic in the country, I was greatly surprised
to see how closely they resembled castings sent to me from near
Calcutta, where it is known that species of Perichaeta abound.
{39} 'Zeitschrift fur wissenschaft. Zoolog.' B. xxviii. 1877, p.
364.
{40} 'Zeitschrift fur wissenschaft. Zoolog.' B. xxviii. 1877, p.
356.
{41} Perrier, 'Archives de Zoolog. exper.' tom. 3, p. 378, 1874.
{42} This case is given in a postscript to my paper in the
'Transact. Geolog. Soc.' (Vol. v. p. 505), and contains a serious
error, as in the account received I mistook the figure 30 for 80.
The tenant, moreover, formerly said that he had marled the field
thirty years before, but was now positive that this was done in
1809, that is twenty-eight years before the first examination of
the field by my friend. The error, as far as the figure 80 is
concerned, was corrected in an article by me, in the 'Gardeners'
Chronicle,' 1844, p. 218.
{43} These pits or pipes are still in process of formation.
During the last forty years I have seen or heard of five cases, in
which a circular space, several feet in diameter, suddenly fell in,
leaving on the field an open hole with perpendicular sides, some
feet in depth. This occurred in one of my own fields, whilst it
was being rolled, and the hinder quarters of the shaft horse fell
in; two or three cart-loads of rubbish were required to fill up the
hole. The subsidence occurred where there was a broad depression,
as if the surface had fallen in at several former periods. I heard
of a hole which must have been suddenly formed at the bottom of a
small shallow pool, where sheep had been washed during many years,
and into which a man thus occupied fell to his great terror. The
rain-water over this whole district sinks perpendicularly into the
ground, but the chalk is more porous in certain places than in
others. Thus the drainage from the overlying clay is directed to
certain points, where a greater amount of calcareous matter is
dissolved than elsewhere. Even narrow open channels are sometimes
formed in the solid chalk. As the chalk is slowly dissolved over
the whole country, but more in some parts than in others, the
undissolved residue--that is the overlying mass of red clay with
flints,--likewise sinks slowly down, and tends to fill up the pipes
or cavities. But the upper part of the red clay holds together,
aided probably by the roots of plants, for a longer time than the
lower parts, and thus forms a roof, which sooner or later falls in,
as in the above mentioned five cases. The downward movement of the
clay may be compared with that of a glacier, but is incomparably
slower; and this movement accounts for a singular fact, namely,
that the much elongated flints which are embedded in the chalk in a
nearly horizontal position, are commonly found standing nearly or
quite upright in the red clay. This fact is so common that the
workmen assured me that this was their natural position. I roughly
measured one which stood vertically, and it was of the same length
and of the same relative thickness as one of my arms. These
elongated flints must get placed in their upright position, on the
same principle that a trunk of a tree left on a glacier assumes a
position parallel to the line of motion. The flints in the clay
which form almost half its bulk, are very often broken, though not
rolled or abraded; and this may he accounted for by their mutual
pressure, whilst the whole mass is subsiding. I may add that the
chalk here appears to have been originally covered in parts by a
thin bed of fine sand with some perfectly rounded flint pebbles,
probably of Tertiary age; for such sand often partly fills up the
deeper pits or cavities in the chalk.
{44} S. W. Johnson, 'How Crops Feed,' 1870, p. 139.
{45} 'Nature,' November 1877, p. 28.
{46} 'Proc. Phil. Soc.' of Manchester, 1877, p. 247.
{47} 'Trans. of the New Zealand Institute,' vol. xii., 1880, p.
152.
{48} Mr. Lindsay Carnagie, in a letter (June 1838) to Sir C.
Lyell, remarks that Scotch farmers are afraid of putting lime on
ploughed land until just before it is laid down for pasture, from a
belief that it has some tendency to sink. He adds: "Some years
since, in autumn, I laid lime on an oat-stubble and ploughed it
down; thus bringing it into immediate contact with the dead
vegetable matter, and securing its thorough mixture through the
means of all the subsequent operations of fallow. In consequence
of the above prejudice, I was considered to have committed a great
fault; but the result was eminently successful, and the practice
was partially followed. By means of Mr. Darwin's observations, I
think the prejudice will be removed."
{49} This conclusion, which, as we shall immediately see, is fully
justified, is of some little importance, as the so-called bench-
stones, which surveyors fix in the ground as a record of their
levels, may in time become false standards. My son Horace intends
at some future period to ascertain how far this has occurred.
{50} Mr. R. Mallet remarks ('Quarterly Journal of Geolog. Soc.'
vol. xxxiii., 1877, p. 745) that "the extent to which the ground
beneath the foundations of ponderous architectural structures, such
as cathedral towers, has been known to become compressed, is as
remarkable as it is instructive and curious. The amount of
depression in some cases may be measured by feet." He instances
the Tower of Pisa, but adds that it was founded on "dense clay."
{51} 'Zeitschrift fur wissensch. Zoolog.' Bd. xxviii., 1877, p.
360.
{52} See Mr. Dancer's paper in 'Proc. Phil. Soc. of Manchester,'
1877, p. 248.
{53} 'Lecons de Geologie pratique,' 1845, p. 142.
{54} A short account of this discovery was published in 'The
Times' of January 2, 1878; and a fuller account in 'The Builder,'
January 5, 1878.
{55} Several accounts of these ruins have been published; the best
is by Mr. James Farrer in 'Proc. Soc. of Antiquaries of Scotland,'
vol. vi., Part II., 1867, p. 278. Also J. W. Grover, 'Journal of
the British Arch. Assoc.' June 1866. Professor Buckman has
likewise published a pamphlet, 'Notes on the Roman Villa at
Chedworth,' 2nd edit. 1873 Cirencester.
{56} These details are taken from the 'Penny Cyclopaedia,' article
Hampshire.
{57} "On the denudation of South Wales," &c., 'Memoirs of the
Geological Survey of Great Britain,' vol. 1., p. 297, 1846.
{58} 'Geological Magazine,' October and November, 1867, vol. iv.
pp. 447 and 483. Copious references on the subject are given in
this remarkable memoir.
{59} A. Tylor "On changes of the sea-level," &c., ' Philosophical
Mag.' (Ser. 4th) vol. v., 1853, p. 258. Archibald Geikie,
Transactions Geolog. Soc. of Glasgow, vol. iii., p. 153 (read
March, 1868). Croll "On Geological Time," 'Philosophical Mag.,'
May, August, and November, 1868. See also Croll, 'Climate and
Time,' 1875, Chap. XX. For some recent information on the amount
of sediment brought down by rivers, see 'Nature,' Sept. 23rd,
1880. Mr. T. Mellard Reade has published some interesting articles
on the astonishing amount of matter brought down in solution by
rivers. See Address, Geolog. Soc., Liverpool, 1876-77.
{60} "An account of the fine dust which often falls on Vessels in
the Atlantic Ocean," Proc. Geolog. Soc. of London, June 4th, 1845.
{61} For La Plata, see my 'Journal of Researches,' during the
voyage of the Beagle, 1845, p. 133. Elie de Beaumont has given
('Lecons de Geolog. pratique,' tom. I. 1845, p. 183) an excellent
account of the enormous quantity of dust which is transported in
some countries. I cannot but think that Mr. Proctor has somewhat
exaggerated ('Pleasant Ways in Science,' 1879, p. 379) the agency
of dust in a humid country like Great Britain. James Geikie has
given ('Prehistoric Europe,' 1880, p. 165) a full abstract of
Richthofen's views, which, however, he disputes.
{62} These statements are taken from Hensen in 'Zeitschrift fur
wissenschaft. Zoologie.' Bd. xxviii., 1877, p. 360. Those with
respect to peat are taken from Mr. A. A. Julien in 'Proc. American
Assoc. Science,' 1879, p. 354.
{63} I have given some facts on the climate necessary or
favourable for the formation of peat, in my 'Journal of
Researches,' 1845, p. 287.
{64} A. A. Julien "On the Geological action of the Humus-acids,"
'Proc. American Assoc. Science,' vol. xxviii., 1879, p. 311. Also
on "Chemical erosion on Mountain Summits;" 'New York Academy of
Sciences,' Oct. 14, 1878, as quoted in the 'American Naturalist.'
See also, on this subject, S. W. Johnson, 'How Crops Feed,' 1870,
p. 138.
{65} See, for references on this subject, S. W. Johnson, 'How
Crops Feed,' 1870, p. 326.
{66} This statement is taken from Mr. Julien, 'Proc. American
Assoc. Science,' vol. xxviii., 1879, p. 330.
{67} The preservative power of a layer of mould and turf is often
shown by the perfect state of the glacial scratches on rocks when
first uncovered. Mr. J. Geikie maintains, in his last very
interesting work ('Prehistoric Europe,' 1881), that the more
perfect scratches are probably due to the last access of cold and
increase of ice, during the long-continued, intermittent glacial
period.
{68} Many geologists have felt much surprise at the complete
disappearance of flints over wide and nearly level areas, from
which the chalk has been removed by subaerial denudation. But the
surface of every flint is coated by an opaque modified layer, which
will just yield to a steel point, whilst the freshly fractured,
translucent surface will not thus yield. The removal by
atmospheric agencies of the outer modified surfaces of freely
exposed flints, though no doubt excessively slow, together with the
modification travelling inwards, will, as may be suspected,
ultimately lead to their complete disintegration, notwithstanding
that they appear to be so extremely durable.
{69} 'Archives de Zoolog. exper.' tom. iii. 1874, p. 409.
{70} 'Nouvelles Archives du Museum,' tom. viii. 1872, pp. 95,
131.
{71} Morren, in speaking of the earth in the alimentary canals of
worms, says, "praesepe cum lapillis commixtam vidi:" 'De Lumbrici
terrestris Hist. Nat.' &c., 1829, p. 16.
{72} Perrier, 'Archives de Zoolog. exper.' tom. iii. 1874, p. 419.
{73} Morren, 'De Lumbrici terrestris Hist. Nat.' &c., p. 16.
{74} 'Archives de Zoolog. exper.' tom. iii. 1874, p. 418.
{75} This conclusion reminds me of the vast amount of extremely
fine chalky mud which is found within the lagoons of many atolls,
where the sea is tranquil and waves cannot triturate the blocks of
coral. This mud must, as I believe ('The Structure and
Distribution of Coral-Reefs,' 2nd edit. 1874, p. 19), be attributed
to the innumerable annelids and other animals which burrow into the
dead coral, and to the fishes, Holothurians, &c., which browse on
the living corals.
{76} Anniversary Address: 'The Quarterly Journal of the
Geological Soc.' May 1880, p. 59.
{77} Mr. James Wallace has pointed out that it is necessary to
take into consideration the possibility of burrows being made at
right angles to the surface instead of vertically down, in which
case the lateral displacement of the soil would be increased.
{78} 'Elements of Geology,' 1865, p. 20.
{79} 'Lecons de Geologie pratique, 1845; cinquieme Lecon. All
Elie de Beaumont's arguments are admirably controverted by Prof. A.
Geikie in his essay in Transact. Geolog. Soc. of Glasgow, vol. iii.
p. 153, 1868.
{80} 'Illustrations of the Huttonian Theory of the Earth,' p. 107.
{81} Mr. E. Tylor in his Presidential address ('Journal of the
Anthropological Institute,' May 1880, p. 451) remarks: "It appears
from several papers of the Berlin Society as to the German 'high-
fields' or 'heathen-fields' (Hochacker, and Heidenacker) that they
correspond much in their situation on hills and wastes with the
'elf-furrows' of Scotland, which popular mythology accounts for by
the story of the fields having been put under a Papal interdict, so
that people took to cultivating the hills. There seems reason to
suppose that, like the tilled plots in the Swedish forest which
tradition ascribes to the old 'hackers,' the German heathen-fields
represent tillage by an ancient and barbaric population."
{82} White of Selborne has some good remarks on the service
performed by worms in loosening, &c., the soil. Edit, by L.
Jenyns, 1843, p. 281.
{83} 'Zeitschrift fur wissenschaft. Zoolog.' B. xxviii. 1877, p.
360.