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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.

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