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Title: The Formation of Vegetable Mould
       through the action of worms with observations on their habits

Author: Charles Darwin

Release Date: December 1, 1999 [eBook #2355]
[Most recently updated: February 20, 2021]

Language: English

Character set encoding: UTF-8

Produced by: David Price

*** START OF THE PROJECT GUTENBERG EBOOK THE FORMATION OF VEGETABLE MOULD ***

                          [Picture: Book cover]





                             THE FORMATION OF
                             VEGETABLE MOULD
                       THROUGH THE ACTION OF WORMS
                    WITH OBSERVATIONS ON THEIR HABITS.


                     BY CHARLES DARWIN, LL.D., F.R.S.

                           THIRTEENTH THOUSAND
                            WITH ILLUSTRATIONS

                                * * * * *

                                  LONDON
                      JOHN MURRAY, ALBEMARLE STREET
                                   1904

                                * * * * *

                                PRINTED BY
                    WILLIAM CLOWES AND SONS, LIMITED,
                           LONDON AND BECCLES.

                                * * * * *




CONTENTS.

INTRODUCTION                                                  Page 1–6
                              CHAPTER I.
                           HABITS OF WORMS.
Nature of the sites inhabited—Can live long under                 7–15
water—Nocturnal—Wander about at night—Often lie close
to the mouths of their burrows, and are thus destroyed
in large numbers by birds—Structure—Do not possess
eyes, but can distinguish between light and
darkness—Retreat rapidly when brightly illuminated, not
by a reflex action—Power of attention—Sensitive to heat
and cold—Completely deaf—Sensitive to vibrations and to
touch—Feeble power of smell—Taste—Mental
qualities—Nature of food—Omnivorous—Digestion—Leaves
before being swallowed, moistened with a fluid of the
nature of the pancreatic secretion—Extra-stomachal
digestion—Calciferous glands, structure of—Calcareous
concretions formed in the anterior pair of glands—The
calcareous matter primarily an excretion, but
secondarily serves to neutralise the acids generated
during the digestive process.
                             CHAPTER II.
                     HABITS OF WORMS—_continued_.
Manner in which worms seize objects—Their power of              52–120
suction—The instinct of plugging up the mouths of their
burrows—Stones piled over the burrows—The advantages
thus gained—Intelligence shown by worms in their manner
of plugging up their burrows—Various kinds of leaves
and other objects thus used—Triangles of paper—Summary
of reasons for believing that worms exhibit some
intelligence—Means by which they excavate their
burrows, by pushing away the earth and swallowing
it—Earth also swallowed for the nutritious matter which
it contains—Depth to which worms burrow, and the
construction of their burrows—Burrows lined with
castings, and in the upper part with leaves—The lowest
part paved with little stones or seeds—Manner in which
the castings are ejected—The collapse of old
burrows—Distribution of worms—Tower-like castings in
Bengal—Gigantic castings on the Nilgiri
Mountains—Castings ejected in all countries.
                             CHAPTER III.
     THE AMOUNT OF FINE EARTH BROUGHT UP BY WORMS TO THE SURFACE.
Rate at which various objects strewed on the surface of        121–163
grass-fields are covered up by the castings of
worms—The burial of a paved path—The slow subsidence of
great stones left on the surface—The number of worms
which live within a given space—The weight of earth
ejected from a burrow, and from all the burrows within
a given space—The thickness of the layer of mould which
the castings on a given space would form within a given
time if uniformly spread out—The slow rate at which
mould can increase to a great thickness—Conclusion.
                             CHAPTER IV.
 THE PART WHICH WORMS HAVE PLAYED IN THE BURIAL OF ANCIENT BUILDINGS.
The accumulation of rubbish on the sites of great              164–208
cities independent of the action of worms—The burial of
a Roman villa at Abinger—The floors and walls
penetrated by worms—Subsidence of a modern pavement—The
buried pavement at Beaulieu Abbey—Roman villas at
Chedworth and Brading—The remains of the Roman town at
Silchester—The nature of the débris by which the
remains are covered—The penetration of the tesselated
floors and walls by worms—Subsidence of the
floors—Thickness of the mould—The old Roman city of
Wroxeter—Thickness of the mould—Depth of the
foundations of some of the Buildings—Conclusion.
                              CHAPTER V.
          THE ACTION OF WORMS IN THE DENUDATION OF THE LAND.
Evidence of the amount of denudation which the land has        209–236
undergone—Sub-aerial denudation—The deposition of
dust—Vegetable mould, its dark colour and fine texture
largely due to the action of worms—The disintegration
of rocks by the humus-acids—Similar acids apparently
generated within the bodies of worms—The action of
these acids facilitated by the continued movement of
the particles of earth—A thick bed of mould checks the
disintegration of the underlying soil and rocks.
Particles of stone worn or triturated in the gizzards
of worms—Swallowed stones serve as mill-stones—The
levigated state of the castings—Fragments of brick in
the castings over ancient buildings well rounded.  The
triturating power of worms not quite insignificant
under a geological point of view.
                             CHAPTER VI.
               THE DENUDATION OF THE LAND—_continued_.
Denudation aided by recently ejected castings flowing          237–279
down inclined grass-covered surfaces—The amount of
earth which annually flows downwards—The effect of
tropical rain on worm castings—The finest particles of
earth washed completely away from castings—The
disintegration of dried castings into pellets, and
their rolling down inclined surfaces—The formation of
little ledges on hill-sides, in part due to the
accumulation of disintegrated castings—Castings blown
to leeward over level land—An attempt to estimate the
amount thus blown—The degradation of ancient
encampments and tumuli—The preservation of the crowns
and furrows on land anciently ploughed—The formation
and amount of mould over the Chalk formation.
                             CHAPTER VII.
                             CONCLUSION.
Summary of the part which worms have played in the             280–288
history of the world—Their aid in the disintegration of
rocks—In the denudation of the land—In the preservation
of ancient remains—In the preparation of the soil for
the growth of plants—Mental powers of worms—Conclusion.




INTRODUCTION.


THE share which worms have taken in the formation of the layer of
vegetable mould, which covers the whole surface of the land in every
moderately humid country, is the subject of the present volume.  This
mould is generally of a blackish colour and a few inches in thickness.
In different districts it differs but little in appearance, although it
may rest on various subsoils.  The uniform fineness of the particles of
which it is composed is one of its chief characteristic features; and
this may be well observed in any gravelly country, where a
recently-ploughed field immediately adjoins one which has long remained
undisturbed for pasture, and where the vegetable mould is exposed on the
sides of a ditch or hole.  The subject may appear an insignificant one,
but we shall see that it possesses some interest; and the maxim “de
minimis non curat lex,” does not apply to science.  Even Élie de
Beaumont, who generally undervalues small agencies and their accumulated
effects, remarks: {2} “La couche très-mince de la terre végétale est un
monument d’une haute antiquité, et, par le fait de sa permanence, un
objet digne d’occuper le géologue, et capable de lui fournir des
remarques intéressantes.”  Although the superficial layer of vegetable
mould as a whole no doubt is of the highest antiquity, yet in regard to
its permanence, we shall hereafter see reason to believe that its
component particles are in most cases removed at not a very slow rate,
and are replaced by others due to the disintegration of the underlying
materials.

As I was led to keep in my study during many months worms in pots filled
with earth, I became interested in them, and wished to learn how far they
acted consciously, and how much mental power they displayed.  I was the
more desirous to learn something on this head, as few observations of
this kind have been made, as far as I know, on animals so low in the
scale of organization and so poorly provided with sense-organs, as are
earth-worms.

In the year 1837, a short paper was read by me before the Geological
Society of London, {3} “On the Formation of Mould,” in which it was shown
that small fragments of burnt marl, cinders, &c., which had been thickly
strewed over the surface of several meadows, were found after a few years
lying at the depth of some inches beneath the turf, but still forming a
layer.  This apparent sinking of superficial bodies is due, as was first
suggested to me by Mr. Wedgwood of Maer Hall in Staffordshire, to the
large quantity of fine earth continually brought up to the surface by
worms in the form of castings.  These castings are sooner or later spread
out and cover up any object left on the surface.  I was thus led to
conclude that all the vegetable mould over the whole country has passed
many times through, and will again pass many times through, the
intestinal canals of worms.  Hence the term “animal mould” would be in
some respects more appropriate than that commonly used of “vegetable
mould.”

Ten years after the publication of my paper, M. D’Archiac, evidently
influenced by the doctrines of Élie de Beaumont, wrote about my
“singulière théorie,” and objected that it could apply only to “les
prairies basses et humides;” and that “les terres labourées, les bois,
les prairies élevées, n’apportent aucune preuve à l’appui de cette
manière de voir.” {4a}  But M. D’Archiac must have thus argued from inner
consciousness and not from observation, for worms abound to an
extraordinary degree in kitchen gardens where the soil is continually
worked, though in such loose soil they generally deposit their castings
in any open cavities or within their old burrows instead of on the
surface.  Hensen estimates that there are about twice as many worms in
gardens as in corn-fields. {4b}  With respect to “prairies élevées,” I do
not know how it may be in France, but nowhere in England have I seen the
ground so thickly covered with castings as on commons, at a height of
several hundred feet above the sea.  In woods again, if the loose leaves
in autumn are removed, the whole surface will be found strewed with
castings.  Dr. King, the superintendent of the Botanic Garden in
Calcutta, to whose kindness I am indebted for many observations on
earth-worms, informs me that he found, near Nancy in France, the bottom
of the State forests covered over many acres with a spongy layer,
composed of dead leaves and innumerable worm-castings.  He there heard
the Professor of “Aménagement des Forêts” lecturing to his pupils, and
pointing out this case as a “beautiful example of the natural cultivation
of the soil; for year after year the thrown-up castings cover the dead
leaves; the result being a rich humus of great thickness.”

In the year 1869, Mr. Fish {5} rejected my conclusions with respect to
the part which worms have played in the formation of vegetable mould,
merely on account of their assumed incapacity to do so much work.  He
remarks that “considering their weakness and their size, the work they
are represented to have accomplished is stupendous.”  Here we have an
instance of that inability to sum up the effects of a continually
recurrent cause, which has often retarded the progress of science, as
formerly in the case of geology, and more recently in that of the
principle of evolution.

Although these several objections seemed to me to have no weight, yet I
resolved to make more observations of the same kind as those published,
and to attack the problem on another side; namely, to weigh all the
castings thrown up within a given time in a measured space, instead of
ascertaining the rate at which objects left on the surface were buried by
worms.  But some of my observations have been rendered almost superfluous
by an admirable paper by Hensen, already alluded to, which appeared in
1877. {6}  Before entering on details with respect to the castings, it
will be advisable to give some account of the habits of worms from my own
observations and from those of other naturalists.

[FIRST EDITION,
            _October_ 10_th_, 1881.]




CHAPTER I.
HABITS OF WORMS.


Nature of the sites inhabited—Can live long under water—Nocturnal—Wander
about at night—Often lie close to the mouths of their burrows, and are
thus destroyed in large numbers by birds—Structure—Do not possess eyes,
but can distinguish between light and darkness—Retreat rapidly when
brightly illuminated, not by a reflex action—Power of attention—Sensitive
to heat and cold—Completely deaf—Sensitive to vibrations and to
touch—Feeble power of smell—Taste—Mental qualities—Nature of
food—Omnivorous—Digestion—Leaves before being swallowed, moistened with a
fluid of the nature of the pancreatic secretion—Extra-stomachal
digestion—Calciferous glands, structure of—Calcareous concretions formed
in the anterior pair of glands—The calcareous matter primarily an
excretion, but secondarily serves to neutralise the acids generated
during the digestive process.

EARTH-WORMS are distributed throughout the world under the form of a few
genera, which externally are closely similar to one another.  The British
species of Lumbricus have never been carefully monographed; but we may
judge of their probable number from those inhabiting neighbouring
countries.  In Scandinavia there are eight species, according to Eisen;
{8a} but two of these rarely burrow in the ground, and one inhabits very
wet places or even lives under the water.  We are here concerned only
with the kinds which bring up earth to the surface in the form of
castings.  Hoffmeister says that the species in Germany are not well
known, but gives the same number as Eisen, together with some strongly
marked varieties. {8b}

Earth-worms abound in England in many different stations.  Their castings
may be seen in extraordinary numbers on commons and chalk-downs, so as
almost to cover the whole surface, where the soil is poor and the grass
short and thin.  But they are almost or quite as numerous in some of the
London parks, where the grass grows well and the soil appears rich.  Even
on the same field worms are much more frequent in some places than in
others, without any visible difference in the nature of the soil.  They
abound in paved court-yards close to houses; and an instance will be
given in which they had burrowed through the floor of a very damp cellar.
I have seen worms in black peat in a boggy field; but they are extremely
rare, or quite absent in the drier, brown, fibrous peat, which is so much
valued by gardeners.  On dry, sandy or gravelly tracks, where heath with
some gorse, ferns, coarse grass, moss and lichens alone grow, hardly any
worms can be found.  But in many parts of England, wherever a path
crosses a heath, its surface becomes covered with a fine short sward.
Whether this change of vegetation is due to the taller plants being
killed by the occasional trampling of man and animals, or to the soil
being occasionally manured by the droppings from animals, I do not know.
{9b}  On such grassy paths worm-castings may often be seen.  On a heath
in Surrey, which was carefully examined, there were only a few castings
on these paths, where they were much inclined; but on the more level
parts, where a bed of fine earth had been washed down from the steeper
parts and had accumulated to a thickness of a few inches, worm-castings
abounded.  These spots seemed to be overstocked with worms, so that they
had been compelled to spread to a distance of a few feet from the grassy
paths, and here their castings had been thrown up among the heath; but
beyond this limit, not a single casting could be found.  A layer, though
a thin one, of fine earth, which probably long retains some moisture, is
in all cases, as I believe, necessary for their existence; and the mere
compression of the soil appears to be in some degree favourable to them,
for they often abound in old gravel walks, and in foot-paths across
fields.

Beneath large trees few castings can be found during certain seasons of
the year, and this is apparently due to the moisture having been sucked
out of the ground by the innumerable roots of the trees; for such places
may be seen covered with castings after the heavy autumnal rains.
Although most coppices and woods support many worms, yet in a forest of
tall and ancient beech-trees in Knole Park, where the ground beneath was
bare of all vegetation, not a single casting could be found over wide
spaces, even during the autumn.  Nevertheless, castings were abundant on
some grass-covered glades and indentations which penetrated this forest.
On the mountains of North Wales and on the Alps, worms, as I have been
informed, are in most places rare; and this may perhaps be due to the
close proximity of the subjacent rocks, into which worms cannot burrow
during the winter so as to escape being frozen.  Dr. McIntosh, however,
found worm-castings at a height of 1500 feet on Schiehallion in Scotland.
They are numerous on some hills near Turin at from 2000 to 3000 feet
above the sea, and at a great altitude on the Nilgiri Mountains in South
India and on the Himalaya.

Earth-worms must be considered as terrestrial animals, though they are
still in one sense semi-aquatic, like the other members of the great
class of annelids to which they belong.  M. Perrier found that their
exposure to the dry air of a room for only a single night was fatal to
them.  On the other hand he kept several large worms alive for nearly
four months, completely submerged in water. {11}  During the summer when
the ground is dry, they penetrate to a considerable depth and cease to
work, as they do during the winter when the ground is frozen.  Worms are
nocturnal in their habits, and at night may be seen crawling about in
large numbers, but usually with their tails still inserted in their
burrows.  By the expansion of this part of their bodies, and with the
help of the short, slightly reflexed bristles, with which their bodies
are armed, they hold so fast that they can seldom be dragged out of the
ground without being torn into pieces. {12}  During the day they remain
in their burrows, except at the pairing season, when those which inhabit
adjoining burrows expose the greater part of their bodies for an hour or
two in the early morning.  Sick individuals, which are generally affected
by the parasitic larvæ of a fly, must also be excepted, as they wander
about during the day and die on the surface.  After heavy rain succeeding
dry weather, an astonishing number of dead worms may sometimes be seen
lying on the ground.  Mr. Galton informs me that on one such occasion
(March, 1881), the dead worms averaged one for every two and a half paces
in length on a walk in Hyde Park, four paces in width.  He counted no
less than 45 dead worms in one place in a length of sixteen paces.  From
the facts above given, it is not probable that these worms could have
been drowned, and if they had been drowned they would have perished in
their burrows.  I believe that they were already sick, and that their
deaths were merely hastened by the ground being flooded.

It has often been said that under ordinary circumstances healthy worms
never, or very rarely, completely leave their burrows at night; but this
is an error, as White of Selborne long ago knew.  In the morning, after
there has been heavy rain, the film of mud or of very fine sand over
gravel-walks is often plainly marked with their tracks.  I have noticed
this from August to May, both months included, and it probably occurs
during the two remaining months of the year when they are wet.  On these
occasions, very few dead worms could anywhere be seen.  On January 31,
1881, after a long-continued and unusually severe frost with much snow,
as soon as a thaw set in, the walks were marked with innumerable tracks.
On one occasion, five tracks were counted crossing a space of only an
inch square.  They could sometimes be traced either to or from the mouths
of the burrows in the gravel-walks, for distances between 2 or 3 up to 15
yards.  I have never seen two tracks leading to the same burrow; nor is
it likely, from what we shall presently see of their sense-organs, that a
worm could find its way back to its burrow after having once left it.
They apparently leave their burrows on a voyage of discovery, and thus
they find new sites to inhabit.

Morren states {14} that worms often lie for hours almost motionless close
beneath the mouths of their burrows.  I have occasionally noticed the
same fact with worms kept in pots in the house; so that by looking down
into their burrows, their heads could just be seen.  If the ejected earth
or rubbish over the burrows be suddenly removed, the end of the worm’s
body may very often be seen rapidly retreating.  This habit of lying near
the surface leads to their destruction to an immense extent.  Every
morning during certain seasons of the year, the thrushes and blackbirds
on all the lawns throughout the country draw out of their holes an
astonishing number of worms, and this they could not do, unless they lay
close to the surface.  It is not probable that worms behave in this
manner for the sake of breathing fresh air, for we have seen that they
can live for a long time under water.  I believe that they lie near the
surface for the sake of warmth, especially in the morning; and we shall
hereafter find that they often coat the mouths of their burrows with
leaves, apparently to prevent their bodies from coming into close contact
with the cold damp earth.  It is said that they completely close their
burrows during the winter.

_Structure_.—A few remarks must be made on this subject.  The body of a
large worm consists of from 100 to 200 almost cylindrical rings or
segments, each furnished with minute bristles.  The muscular system is
well developed.  Worms can crawl backwards as well as forwards, and by
the aid of their affixed tails can retreat with extraordinary rapidity
into their burrows.  The mouth is situated at the anterior end of the
body, and is provided with a little projection (lobe or lip, as it has
been variously called) which is used for prehension.  Internally, behind
the mouth, there is a strong pharynx, shown in the accompanying diagram
(Fig. 1) which is pushed forwards when the animal eats, and this part
corresponds, according to Perrier, with the protrudable trunk or
proboscis of other annelids.  The pharynx leads into the œsophagus, on
each side of which in the lower part there are three pairs of large
glands, which secrete a surprising amount of carbonate of lime.  These
calciferous glands are highly remarkable, for nothing like them is known
in any other animal.  Their use will be discussed when we treat of the
digestive process.  In most of the species, the œsophagus is enlarged
into a crop in front of the gizzard.  This latter organ is lined with a
smooth thick chitinous membrane, and is surrounded by weak longitudinal,
but powerful transverse muscles.  Perrier saw these muscles in energetic
action; and, as he remarks, the trituration of the food must be chiefly
effected by this organ, for worms possess no jaws or teeth of any kind.
Grains of sand and small stones, from the 1/20 to a little more than the
1/10 inch in diameter, may generally be found in their gizzards and
intestines.  As it is certain that worms swallow many little stones,
independently of those swallowed while excavating their burrows, it is
probable that they serve, like mill-stones, to triturate their food.  The
gizzard opens into the intestine, which runs in a straight course to the
vent at the posterior end of the body.  The intestine presents a
remarkable structure, the typhlosolis, or, as the old anatomists called
it, an intestine within an intestine; and Claparède {17} has shown that
this consists of a deep longitudinal involution of the walls of the
intestine, by which means an extensive absorbent surface is gained.

[Picture: Fig. 1: Diagram of the alimentary canal of an earth-worm.  Fig.
                  2: Tower-like casting from near Nice]

The circulatory system is well developed.  Worms breathe by their skin,
as they do not possess any special respiratory organs.  The two sexes are
united in the same individual, but two individuals pair together.  The
nervous system is fairly well developed; and the two almost confluent
cerebral ganglia are situated very near to the anterior end of the body.

_Senses_.—Worms are destitute of eyes, and at first I thought that they
were quite insensible to light; for those kept in confinement were
repeatedly observed by the aid of a candle, and others out of doors by
the aid of a lantern, yet they were rarely alarmed, although extremely
timid animals.  Other persons have found no difficulty in observing worms
at night by the same means. {18a}

Hoffmeister, however, states {18b} that worms, with the exception of a
few individuals, are extremely sensitive to light; but he admits that in
most cases a certain time is requisite for its action.  These statements
led me to watch on many successive nights worms kept in pots, which were
protected from currents of air by means of glass plates.  The pots were
approached very gently, in order that no vibration of the floor should be
caused.  When under these circumstances worms were illuminated by a
bull’s-eye lantern having slides of dark red and blue glass, which
intercepted so much light that they could be seen only with some
difficulty, they were not at all affected by this amount of light,
however long they were exposed to it.  The light, as far as I could
judge, was brighter than that from the full moon.  Its colour apparently
made no difference in the result.  When they were illuminated by a
candle, or even by a bright paraffin lamp, they were not usually affected
at first.  Nor were they when the light was alternately admitted and shut
off.  Sometimes, however, they behaved very differently, for as soon as
the light fell on them, they withdrew into their burrows with almost
instantaneous rapidity.  This occurred perhaps once out of a dozen times.
When they did not withdraw instantly, they often raised the anterior
tapering ends of their bodies from the ground, as if their attention was
aroused or as if surprise was felt; or they moved their bodies from side
to side as if feeling for some object.  They appeared distressed by the
light; but I doubt whether this was really the case, for on two occasions
after withdrawing slowly, they remained for a long time with their
anterior extremities protruding a little from the mouths of their
burrows, in which position they were ready for instant and complete
withdrawal.

When the light from a candle was concentrated by means of a large lens on
the anterior extremity, they generally withdrew instantly; but this
concentrated light failed to act perhaps once out of half a dozen trials.
The light was on one occasion concentrated on a worm lying beneath water
in a saucer, and it instantly withdrew into its burrow.  In all cases the
duration of the light, unless extremely feeble, made a great difference
in the result; for worms left exposed before a paraffin lamp or a candle
invariably retreated into their burrows within from five to fifteen
minutes; and if in the evening the pots were illuminated before the worms
had come out of their burrows, they failed to appear.

From the foregoing facts it is evident that light affects worms by its
intensity and by its duration.  It is only the anterior extremity of the
body, where the cerebral ganglia lie, which is affected by light, as
Hoffmeister asserts, and as I observed on many occasions.  If this part
is shaded, other parts of the body may be fully illuminated, and no
effect will be produced.  As these animals have no eyes, we must suppose
that the light passes through their skins, and in some manner excites
their cerebral ganglia.  It appeared at first probable that the different
manner in which they were affected on different occasions might be
explained, either by the degree of extension of their skin and its
consequent transparency, or by some particular incident of the light; but
I could discover no such relation.  One thing was manifest, namely, that
when worms were employed in dragging leaves into their burrows or in
eating them, and even during the short intervals whilst they rested from
their work, they either did not perceive the light or were regardless of
it; and this occurred even when the light was concentrated on them
through a large lens.  So, again, whilst they are paired, they will
remain for an hour or two out of their burrows, fully exposed to the
morning light; but it appears from what Hoffmeister says that a light
will occasionally cause paired individuals to separate.

When a worm is suddenly illuminated and dashes like a rabbit into its
burrow—to use the expression employed by a friend—we are at first led to
look at the action as a reflex one.  The irritation of the cerebral
ganglia appears to cause certain muscles to contract in an inevitable
manner, independently of the will or consciousness of the animal, as if
it were an automaton.  But the different effect which a light produced on
different occasions, and especially the fact that a worm when in any way
employed and in the intervals of such employment, whatever set of muscles
and ganglia may then have been brought into play, is often regardless of
light, are opposed to the view of the sudden withdrawal being a simple
reflex action.  With the higher animals, when close attention to some
object leads to the disregard of the impressions which other objects must
be producing on them, we attribute this to their attention being then
absorbed; and attention implies the presence of a mind.  Every sportsman
knows that he can approach animals whilst they are grazing, fighting or
courting, much more easily than at other times.  The state, also, of the
nervous system of the higher animals differs much at different times, for
instance, a horse is much more readily startled at one time than at
another.  The comparison here implied between the actions of one of the
higher animals and of one so low in the scale as an earth-worm, may
appear far-fetched; for we thus attribute to the worm attention and some
mental power, nevertheless I can see no reason to doubt the justice of
the comparison.

Although worms cannot be said to possess the power of vision, their
sensitiveness to light enables them to distinguish between day and night;
and they thus escape extreme danger from the many diurnal animals which
prey on them.  Their withdrawal into their burrows during the day
appears, however, to have become an habitual action; for worms kept in
pots covered by glass plates, over which sheets of black paper were
spread, and placed before a north-east window, remained during the
day-time in their burrows and came out every night; and they continued
thus to act for a week.  No doubt a little light may have entered between
the sheets of glass and the blackened paper; but we know from the trials
with coloured glass, that worms are indifferent to a small amount of
light.

Worms appear to be less sensitive to moderate radiant heat than to a
bright light.  I judge of this from having held at different times a
poker heated to dull redness near some worms, at a distance which caused
a very sensible degree of warmth in my hand.  One of them took no notice;
a second withdrew into its burrow, but not quickly; the third and fourth
much more quickly, and the fifth as quickly as possible.  The light from
a candle, concentrated by a lens and passing through a sheet of glass
which would intercept most of the heat-rays, generally caused a much more
rapid retreat than did the heated poker.  Worms are sensitive to a low
temperature, as may be inferred from their not coming out of their
burrows during a frost.

Worms do not possess any sense of hearing.  They took not the least
notice of the shrill notes from a metal whistle, which was repeatedly
sounded near them; nor did they of the deepest and loudest tones of a
bassoon.  They were indifferent to shouts, if care was taken that the
breath did not strike them.  When placed on a table close to the keys of
a piano, which was played as loudly as possible, they remained perfectly
quiet.

Although they are indifferent to undulations in the air audible by us,
they are extremely sensitive to vibrations in any solid object.  When the
pots containing two worms which had remained quite indifferent to the
sound of the piano, were placed on this instrument, and the note C in the
bass clef was struck, both instantly retreated into their burrows.  After
a time they emerged, and when G above the line in the treble clef was
struck they again retreated.  Under similar circumstances on another
night one worm dashed into its burrow on a very high note being struck
only once, and the other worm when C in the treble clef was struck.  On
these occasions the worms were not touching the sides of the pots, which
stood in saucers; so that the vibrations, before reaching their bodies,
had to pass from the sounding board of the piano, through the saucer, the
bottom of the pot and the damp, not very compact earth on which they lay
with their tails in their burrows.  They often showed their sensitiveness
when the pot in which they lived, or the table on which the pot stood,
was accidentally and lightly struck; but they appeared less sensitive to
such jars than to the vibrations of the piano; and their sensitiveness to
jars varied much at different times.

It has often been said that if the ground is beaten or otherwise made to
tremble, worms believe that they are pursued by a mole and leave their
burrows.  From one account that I have received, I have no doubt that
this is often the case; but a gentleman informs me that he lately saw
eight or ten worms leave their burrows and crawl about the grass on some
boggy land on which two men had just trampled while setting a trap; and
this occurred in a part of Ireland where there were no moles.  I have
been assured by a Volunteer that he has often seen many large earth-worms
crawling quickly about the grass, a few minutes after his company had
fired a volley with blank cartridges.  The Peewit (_Tringa vanellus_,
Linn.) seems to know instinctively that worms will emerge if the ground
is made to tremble; for Bishop Stanley states (as I hear from Mr.
Moorhouse) that a young peewit kept in confinement used to stand on one
leg and beat the turf with the other leg until the worms crawled out of
their burrows, when they were instantly devoured.  Nevertheless, worms do
not invariably leave their burrows when the ground is made to tremble, as
I know by having beaten it with a spade, but perhaps it was beaten too
violently.

The whole body of a worm is sensitive to contact.  A slight puff of air
from the mouth causes an instant retreat.  The glass plates placed over
the pots did not fit closely, and blowing through the very narrow chinks
thus left, often sufficed to cause a rapid retreat.  They sometimes
perceived the eddies in the air caused by quickly removing the glass
plates.  When a worm first comes out of its burrow, it generally moves
the much extended anterior extremity of its body from side to side in all
directions, apparently as an organ of touch; and there is some reason to
believe, as we shall see in the next chapter, that they are thus enabled
to gain a general notion of the form of an object.  Of all their senses
that of touch, including in this term the perception of a vibration,
seems much the most highly developed.

In worms the sense of smell apparently is confined to the perception of
certain odours, and is feeble.  They were quite indifferent to my breath,
as long as I breathed on them very gently.  This was tried, because it
appeared possible that they might thus be warned of the approach of an
enemy.  They exhibited the same indifference to my breath whilst I chewed
some tobacco, and while a pellet of cotton-wool with a few drops of
millefleurs perfume or of acetic acid was kept in my mouth.  Pellets of
cotton-wool soaked in tobacco juice, in millefleurs perfume, and in
paraffin, were held with pincers and were waved about within two or three
inches of several worms, but they took no notice.  On one or two
occasions, however, when acetic acid had been placed on the pellets, the
worms appeared a little uneasy, and this was probably due to the
irritation of their skins.  The perception of such unnatural odours would
be of no service to worms; and as such timid creatures would almost
certainly exhibit some signs of any new impression, we may conclude that
they did not perceive these odours.

The result was different when cabbage-leaves and pieces of onion were
employed, both of which are devoured with much relish by worms.  Small
square pieces of fresh and half-decayed cabbage-leaves and of onion bulbs
were on nine occasions buried in my pots, beneath about ¼ of an inch of
common garden soil; and they were always discovered by the worms.  One
bit of cabbage was discovered and removed in the course of two hours;
three were removed by the next morning, that is, after a single night;
two others after two nights; and the seventh bit after three nights.  Two
pieces of onion were discovered and removed after three nights.  Bits of
fresh raw meat, of which worms are very fond, were buried, and were not
discovered within forty-eight hours, during which time they had not
become putrid.  The earth above the various buried objects was generally
pressed down only slightly, so as not to prevent the emission of any
odour.  On two occasions, however, the surface was well watered, and was
thus rendered somewhat compact.  After the bits of cabbage and onion had
been removed, I looked beneath them to see whether the worms had
accidentally come up from below, but there was no sign of a burrow; and
twice the buried objects were laid on pieces of tin-foil which were not
in the least displaced.  It is of course possible that the worms whilst
moving about on the surface of the ground, with their tails affixed
within their burrows, may have poked their heads into the places where
the above objects were buried; but I have never seen worms acting in this
manner.  Some pieces of cabbage-leaf and of onion were twice buried
beneath very fine ferruginous sand, which was slightly pressed down and
well watered, so as to be rendered very compact, and these pieces were
never discovered.  On a third occasion the same kind of sand was neither
pressed down nor watered, and the pieces of cabbage were discovered and
removed after the second night.  These several facts indicate that worms
possess some power of smell; and that they discover by this means
odoriferous and much-coveted kinds of food.

It may be presumed that all animals which feed on various substances
possess the sense of taste, and this is certainly the case with worms.
Cabbage-leaves are much liked by worms; and it appears that they can
distinguish between different varieties; but this may perhaps be owing to
differences in their texture.  On eleven occasions pieces of the fresh
leaves of a common green variety and of the red variety used for pickling
were given them, and they preferred the green, the red being either
wholly neglected or much less gnawed.  On two other occasions, however,
they seemed to prefer the red.  Half-decayed leaves of the red variety
and fresh leaves of the green were attacked about equally.  When leaves
of the cabbage, horse-radish (a favourite food) and of the onion were
given together, the latter were always, and manifestly preferred.  Leaves
of the cabbage, lime-tree, Ampelopsis, parsnip (Pastinaca), and celery
(Apium) were likewise given together; and those of the celery were first
eaten.  But when leaves of cabbage, turnip, beet, celery, wild cherry and
carrots were given together, the two latter kinds, especially those of
the carrot, were preferred to all the others, including those of celery.
It was also manifest after many trials that wild cherry leaves were
greatly preferred to those of the lime-tree and hazel (Corylus).
According to Mr. Bridgman the half-decayed leaves of _Phlox verna_ are
particularly liked by worms. {31}

Pieces of the leaves of cabbage, turnip, horse-radish and onion were left
on the pots during 22 days, and were all attacked and had to be renewed;
but during the whole of this time leaves of an Artemisia and of the
culinary sage, thyme and mint, mingled with the above leaves, were quite
neglected excepting those of the mint, which were occasionally and very
slightly nibbled.  These latter four kinds of leaves do not differ in
texture in a manner which could make them disagreeable to worms; they all
have a strong taste, but so have the four first mentioned kinds of
leaves; and the wide difference in the result must be attributed to a
preference by the worms for one taste over another.

_Mental Qualities_.—There is little to be said on this head.  We have
seen that worms are timid.  It may be doubted whether they suffer as much
pain when injured, as they seem to express by their contortions.  Judging
by their eagerness for certain kinds of food, they must enjoy the
pleasure of eating.  Their sexual passion is strong enough to overcome
for a time their dread of light.  They perhaps have a trace of social
feeling, for they are not disturbed by crawling over each other’s bodies,
and they sometimes lie in contact.  According to Hoffmeister they pass
the winter either singly or rolled up with others into a ball at the
bottom of their burrows. {32}  Although worms are so remarkably deficient
in the several sense-organs, this does not necessarily preclude
intelligence, as we know from such cases as those of Laura Bridgman; and
we have seen that when their attention is engaged, they neglect
impressions to which they would otherwise have attended; and attention
indicates the presence of a mind of some kind.  They are also much more
easily excited at certain times than at others.  They perform a few
actions instinctively, that is, all the individuals, including the young,
perform such actions in nearly the same fashion.  This is shown by the
manner in which the species of Perichæta eject their castings, so as to
construct towers; also by the manner in which the burrows of the common
earth-worm are smoothly lined with fine earth and often with little
stones, and the mouths of their burrows with leaves.  One of their
strongest instincts is the plugging up the mouths of their burrows with
various objects; and very young worms act in this manner.  But some
degree of intelligence appears, as we shall see in the next chapter, to
be exhibited in this work,—a result which has surprised me more than
anything else in regard to worms.

_Food and Digestion_.—Worms are omnivorous.  They swallow an enormous
quantity of earth, out of which they extract any digestible matter which
it may contain; but to this subject I must recur.  They also consume a
large number of half-decayed leaves of all kinds, excepting a few which
have an unpleasant taste or are too tough for them; likewise petioles,
peduncles, and decayed flowers.  But they will also consume fresh leaves,
as I have found by repeated trials.  According to Morren {33} they will
eat particles of sugar and liquorice; and the worms which I kept drew
many bits of dry starch into their burrows, and a large bit had its
angles well rounded by the fluid poured out of their mouths.  But as they
often drag particles of soft stone, such as of chalk, into their burrows,
I feel some doubt whether the starch was used as food.  Pieces of raw and
roasted meat were fixed several times by long pins to the surface of the
soil in my pots, and night after night the worms could be seen tugging at
them, with the edges of the pieces engulfed in their mouths, so that much
was consumed.  Raw fat seems to be preferred even to raw meat or to any
other substance which was given them, and much was consumed.  They are
cannibals, for the two halves of a dead worm placed in two of the pots
were dragged into the burrows and gnawed; but as far as I could judge,
they prefer fresh to putrid meat, and in so far I differ from
Hoffmeister.

Léon Fredericq states {34} that the digestive fluid of worms is of the
same nature as the pancreatic secretion of the higher animals; and this
conclusion agrees perfectly with the kinds of food which worms consume.
Pancreatic juice emulsifies fat, and we have just seen how greedily worms
devour fat; it dissolves fibrin, and worms eat raw meat; it converts
starch into grape-sugar with wonderful rapidity, and we shall presently
show that the digestive fluid of worms acts on starch. {35a}  But they
live chiefly on half-decayed leaves; and these would be useless to them
unless they could digest the cellulose forming the cell-walls; for it is
well known that all other nutritious substances are almost completely
withdrawn from leaves, shortly before they fall off.  It has, however,
now been ascertained that some forms of cellulose, though very little or
not at all attacked by the gastric secretion of the higher animals, are
acted on by that from the pancreas. {35b}

The half-decayed or fresh leaves which worms intend to devour, are
dragged into the mouths of their burrows to a depth of from one to three
inches, and are then moistened with a secreted fluid.  It has been
assumed that this fluid serves to hasten their decay; but a large number
of leaves were twice pulled out of the burrows of worms and kept for many
weeks in a very moist atmosphere under a bell-glass in my study; and the
parts which had been moistened by the worms did not decay more quickly in
any plain manner than the other parts.  When fresh leaves were given in
the evening to worms kept in confinement and examined early on the next
morning, therefore not many hours after they had been dragged into the
burrows, the fluid with which they were moistened, when tested with
neutral litmus paper, showed an alkaline reaction.  This was repeatedly
found to be the case with celery, cabbage and turnip leaves.  Parts of
the same leaves which had not been moistened by the worms, were pounded
with a few drops of distilled water, and the juice thus extracted was not
alkaline.  Some leaves, however, which had been drawn into burrows out of
doors, at an unknown antecedent period, were tried, and though still
moist, they rarely exhibited even a trace of alkaline reaction.

The fluid, with which the leaves are bathed, acts on them whilst they are
fresh or nearly fresh, in a remarkable manner; for it quickly kills and
discolours them.  Thus the ends of a fresh carrot-leaf, which had been
dragged into a burrow, were found after twelve hours of a dark brown
tint.  Leaves of celery, turnip, maple, elm, lime, thin leaves of ivy,
and, occasionally those of the cabbage were similarly acted on.  The end
of a leaf of _Triticum repens_, still attached to a growing plant, had
been drawn into a burrow, and this part was dark brown and dead, whilst
the rest of the leaf was fresh and green.  Several leaves of lime and elm
removed from burrows out of doors were found affected in different
degrees.  The first change appears to be that the veins become of a dull
reddish-orange.  The cells with chlorophyll next lose more or less
completely their green colour, and their contents finally become brown.
The parts thus affected often appeared almost black by reflected light;
but when viewed as a transparent object under the microscope, minute
specks of light were transmitted, and this was not the case with the
unaffected parts of the same leaves.  These effects, however, merely show
that the secreted fluid is highly injurious or poisonous to leaves; for
nearly the same effects were produced in from one to two days on various
kinds of young leaves, not only by artificial pancreatic fluid, prepared
with or without thymol, but quickly by a solution of thymol by itself.
On one occasion leaves of Corylus were much discoloured by being kept for
eighteen hours in pancreatic fluid, without any thymol.  With young and
tender leaves immersion in human saliva during rather warm weather, acted
in the same manner as the pancreatic fluid, but not so quickly.  The
leaves in all these cases often became infiltrated with the fluid.

Large leaves from an ivy plant growing on a wall were so tough that they
could not be gnawed by worms, but after four days they were affected in a
peculiar manner by the secretion poured out of their mouths.  The upper
surfaces of the leaves, over which the worms had crawled, as was shown by
the dirt left on them, were marked in sinuous lines, by either a
continuous or broken chain of whitish and often star-shaped dots, about 2
mm. in diameter.  The appearance thus presented was curiously like that
of a leaf, into which the larva of some minute insect had burrowed.  But
my son Francis, after making and examining sections, could nowhere find
that the cell-walls had been broken down or that the epidermis had been
penetrated.  When the section passed through the whitish dots, the grains
of chlorophyll were seen to be more or less discoloured, and some of the
palisade and mesophyll cells contained nothing but broken down granular
matter.  These effects must be attributed to the transudation of the
secretion through the epidermis into the cells.

The secretion with which worms moisten leaves likewise acts on the
starch-granules within the cells.  My son examined some leaves of the ash
and many of the lime, which had fallen off the trees and had been partly
dragged into worm-burrows.  It is known that with fallen leaves the
starch-grains are preserved in the guard-cells of the stomata.  Now in
several cases the starch had partially or wholly disappeared from these
cells, in the parts which had been moistened by the secretion; while it
was still well preserved in the other parts of the same leaves.
Sometimes the starch was dissolved out of only one of the two
guard-cells.  The nucleus in one case had disappeared, together with the
starch-granules.  The mere burying of lime-leaves in damp earth for nine
days did not cause the destruction of the starch-granules.  On the other
hand, the immersion of fresh lime and cherry leaves for eighteen hours in
artificial pancreatic fluid, led to the dissolution of the
starch-granules in the guard-cells as well as in the other cells.

From the secretion with which the leaves are moistened being alkaline,
and from its acting both on the starch-granules and on the protoplasmic
contents of the cells, we may infer that it resembles in nature not
saliva, {40} but pancreatic secretion; and we know from Fredericq that a
secretion of this kind is found in the intestines of worms.  As the
leaves which are dragged into the burrows are often dry and shrivelled,
it is indispensable for their disintegration by the unarmed mouths of
worms that they should first be moistened and softened; and fresh leaves,
however soft and tender they may be, are similarly treated, probably from
habit.  The result is that they are partially digested before they are
taken into the alimentary canal.  I am not aware of any other case of
extra-stomachal digestion having been recorded.  The boa-constrictor is
said to bathe its prey with saliva, but this is doubtful; and it is done
solely for the sake of lubricating its prey.  Perhaps the nearest analogy
may be found in such plants as Drosera and Dionæa; for here animal matter
is digested and converted into peptone not within a stomach, but on the
surfaces of the leaves.

_Calciferous Glands_.—These glands (see Fig. 1), judging from their size
and from their rich supply of blood-vessels, must be of much importance
to the animal.  But almost as many theories have been advanced on their
use as there have been observers.  They consist of three pairs, which in
the common earth-worm debouch into the alimentary canal in advance of the
gizzard, but posteriorly to it in Urochæta and some other genera. {41a}
The two posterior pairs are formed by lamellæ, which, according to
Claparède, are diverticula from the œsophagus. {41b}  These lamellæ are
coated with a pulpy cellular layer, with the outer cells lying free in
infinite numbers.  If one of these glands is punctured and squeezed, a
quantity of white pulpy matter exudes, consisting of these free cells.
They are minute, and vary in diameter from 2 to 6 _μ_.  They contain in
their centres a little excessively fine granular matter; but they look so
like oil globules that Claparède and others at first treated them with
ether.  This produces no effect; but they are quickly dissolved with
effervescence in acetic acid, and when oxalate of ammonia is added to the
solution a white precipitate is thrown down.  We may therefore conclude
that they contain carbonate of lime.  If the cells are immersed in a very
little acid, they become more transparent, look like ghosts, and are soon
lost to view; but if much acid is added, they disappear instantly.  After
a very large number have been dissolved, a flocculent residue is left,
which apparently consists of the delicate ruptured cell-walls.  In the
two posterior pairs of glands the carbonate of lime contained in the
cells occasionally aggregates into small rhombic crystals or into
concretions, which lie between the lamellæ; but I have seen only one
case, and Claparède only a very few such cases.

The two anterior glands differ a little in shape from the four posterior
ones, by being more oval.  They differ also conspicuously in generally
containing several small, or two or three larger, or a single very large
concretion of carbonate of lime, as much as 1½ mm. in diameter.  When a
gland includes only a few very small concretions, or, as sometimes
happens, none at all, it is easily overlooked.  The large concretions are
round or oval, and exteriorly almost smooth.  One was found which filled
up not only the whole gland, as is often the case, but its neck; so that
it resembled an olive-oil flask in shape.  These concretions when broken
are seen to be more or less crystalline in structure.  How they escape
from the gland is a marvel; but that they do escape is certain, for they
are often found in the gizzard, intestines, and in the castings of worms,
both with those kept in confinement and those in a state of nature.

Claparède says very little about the structure of the two anterior
glands, and he supposes that the calcareous matter of which the
concretions are formed is derived from the four posterior glands.  But if
an anterior gland which contains only small concretions is placed in
acetic acid and afterwards dissected, or if sections are made of such a
gland without being treated with acid, lamellæ like those in the
posterior glands and coated with cellular matter could be plainly seen,
together with a multitude of free calciferous cells readily soluble in
acetic acid.  When a gland is completely filled with a single large
concretion, there are no free cells, as these have been all consumed in
forming the concretion.  But if such a concretion, or one of only
moderately large size, is dissolved in acid, much membranous matter is
left, which appears to consist of the remains of the formerly active
lamellæ.  After the formation and expulsion of a large concretion, new
lamellæ must be developed in some manner.  In one section made by my son,
the process had apparently commenced, although the gland contained two
rather large concretions, for near the walls several cylindrical and oval
pipes were intersected, which were lined with cellular matter and were
quite filled with free calciferous cells.  A great enlargement in one
direction of several oval pipes would give rise to the lamellæ.

Besides the free calciferous cells in which no nucleus was visible, other
and rather larger free cells were seen on three occasions; and these
contained a distinct nucleus and nucleolus.  They were only so far acted
on by acetic acid that the nucleus was thus rendered more distinct.  A
very small concretion was removed from between two of the lamellæ within
an anterior gland.  It was imbedded in pulpy cellular matter, with many
free calciferous cells, together with a multitude of the larger, free,
nucleated cells, and these latter cells were not acted on by acetic acid,
while the former were dissolved.  From this and other such cases I am led
to suspect that the calciferous cells are developed from the larger
nucleated ones; but how this was effected was not ascertained.

When an anterior gland contains several minute concretions, some of these
are generally angular or crystalline in outline, while the greater number
are rounded with an irregular mulberry-like surface.  Calciferous cells
adhered to many parts of these mulberry-like masses, and their gradual
disappearance could be traced while they still remained attached.  It was
thus evident that the concretions are formed from the lime contained
within the free calciferous cells.  As the smaller concretions increase
in size, they come into contact and unite, thus enclosing the now
functionless lamellæ; and by such steps the formation of the largest
concretions could be followed.  Why the process regularly takes place in
the two anterior glands, and only rarely in the four posterior glands, is
quite unknown.  Morren says that these glands disappear during the
winter; and I have seen some instances of this fact, and others in which
either the anterior or posterior glands were at this season so shrunk and
empty, that they could be distinguished only with much difficulty.

With respect to the function of the calciferous glands, it is probable
that they primarily serve as organs of excretion, and secondarily as an
aid to digestion.  Worms consume many fallen leaves; and it is known that
lime goes on accumulating in leaves until they drop off the parent-plant,
instead of being re-absorbed into the stem or roots, like various other
organic and inorganic substances. {46}  The ashes of a leaf of an acacia
have been known to contain as much as 72 per cent. of lime.  Worms
therefore would be liable to become charged with this earth, unless there
were some special means for its excretion; and the calciferous glands are
well adapted for this purpose.  The worms which live in mould close over
the chalk, often have their intestines filled with this substance, and
their castings are almost white.  Here it is evident that the supply of
calcareous matter must be super-abundant.  Nevertheless with several
worms collected on such a site, the calciferous glands contained as many
free calciferous cells, and fully as many and large concretions, as did
the glands of worms which lived where there was little or no lime; and
this indicates that the lime is an excretion, and not a secretion poured
into the alimentary canal for some special purpose.

On the other hand, the following considerations render it highly probable
that the carbonate of lime, which is excreted by the glands, aids the
digestive process under ordinary circumstances.  Leaves during their
decay generate an abundance of various kinds of acids, which have been
grouped together under the term of humus acids.  We shall have to recur
to this subject in our fifth chapter, and I need here only say that these
acids act strongly on carbonate of lime.  The half-decayed leaves which
are swallowed in such large quantities by worms would, therefore, after
they have been moistened and triturated in the alimentary canal, be apt
to produce such acids.  And in the case of several worms, the contents of
the alimentary canal were found to be plainly acid, as shown by litmus
paper.  This acidity cannot be attributed to the nature of the digestive
fluid, for pancreatic fluid is alkaline; and we have seen that the
secretion which is poured out of the mouths of worms for the sake of
preparing the leaves for consumption, is likewise alkaline.  The acidity
can hardly be due to uric acid, as the contents of the upper part of the
intestine were often acid.  In one case the contents of the gizzard were
slightly acid, those of the upper intestines being more plainly acid.  In
another case the contents of the pharynx were not acid, those of the
gizzard doubtfully so, while those of the intestine were distinctly acid
at a distance of 5 cm. below the gizzard.  Even with the higher
herbivorous and omnivorous animals, the contents of the large intestine
are acid.  “This, however, is not caused by any acid secretion from the
mucous membrane; the reaction of the intestinal walls in the larger as in
the small intestine is alkaline.  It must therefore arise from acid
fermentations going on in the contents themselves . . .  In Carnivora the
contents of the coecum are said to be alkaline, and naturally the amount
of fermentation will depend largely on the nature of the food.” {49}

With worms not only the contents of the intestines, but their ejected
matter or the castings, are generally acid.  Thirty castings from
different places were tested, and with three or four exceptions were
found to be acid; and the exceptions may have been due to such castings
not having been recently ejected; for some which were at first acid, were
on the following morning, after being dried and again moistened, no
longer acid; and this probably resulted from the humus acids being, as is
known to be the case, easily decomposed.  Five fresh castings from worms
which lived in mould close over the chalk, were of a whitish colour and
abounded with calcareous matter; and these were not in the least acid.
This shows how effectually carbonate of lime neutralises the intestinal
acids.  When worms were kept in pots filled with fine ferruginous sand,
it was manifest that the oxide of iron, with which the grains of silex
were coated, had been dissolved and removed from them in the castings.

The digestive fluid of worms resembles in its action, as already stated,
the pancreatic secretion of the higher animals; and in these latter,
“pancreatic digestion is essentially alkaline; the action will not take
place unless some alkali be present; and the activity of an alkaline
juice is arrested by acidification, and hindered by neutralization.” {50}
Therefore it seems highly probable that the innumerable calciferous
cells, which are poured from the four posterior glands into the
alimentary canal of worms, serve to neutralise more or less completely
the acids there generated by the half-decayed leaves.  We have seen that
these cells are instantly dissolved by a small quantity of acetic acid,
and as they do not always suffice to neutralise the contents of even the
upper part of the alimentary canal, the lime is perhaps aggregated into
concretions in the anterior pair of glands, in order that some may be
carried down to the posterior parts of the intestine, where these
concretions would be rolled about amongst the acid contents.  The
concretions found in the intestines and in the castings often have a worn
appearance, but whether this is due to some amount of attrition or of
chemical corrosion could not be told.  Claparède believes that they are
formed for the sake of acting as mill-stones, and of thus aiding in the
trituration of the food.  They may give some aid in this way; but I fully
agree with Perrier that this must be of quite subordinate importance,
seeing that the object is already attained by stones being generally
present in the gizzards and intestines of worms.




CHAPTER II.
HABITS OF WORMS—_continued_.


Manner in which worms seize objects—Their power of suction—The instinct
of plugging up the mouths of their burrows—Stones piled over the
burrows—The advantages thus gained—Intelligence shown by worms in their
manner of plugging up their burrows—Various kinds of leaves and other
objects thus used—Triangles of paper—Summary of reasons for believing
that worms exhibit some intelligence—Means by which they excavate their
burrows, by pushing away the earth and swallowing it—Earth also swallowed
for the nutritious matter which it contains—Depth to which worms burrow,
and the construction of their burrows—Burrows lined with castings, and in
the upper part with leaves—The lowest part paved with little stones or
seeds—Manner in which the castings are ejected—The collapse of old
burrows—Distribution of worms—Tower-like castings in Bengal—Gigantic
castings on the Nilgiri Mountains—Castings ejected in all countries.

IN the pots in which worms were kept, leaves were pinned down to the
soil, and at night the manner in which they were seized could be
observed.  The worms always endeavoured to drag the leaves towards their
burrows; and they tore or sucked off small fragments, whenever the leaves
were sufficiently tender.  They generally seized the thin edge of a leaf
with their mouths, between the projecting upper and lower lip; the thick
and strong pharynx being at the same time, as Perrier remarks, pushed
forward within their bodies, so as to afford a point of resistance for
the upper lip.  In the case of broad flat objects they acted in a wholly
different manner.  The pointed anterior extremity of the body, after
being brought into contact with an object of this kind, was drawn within
the adjoining rings, so that it appeared truncated and became as thick as
the rest of the body.  This part could then be seen to swell a little;
and this, I believe, is due to the pharynx being pushed a little
forwards.  Then by a slight withdrawal of the pharynx or by its
expansion, a vacuum was produced beneath the truncated slimy end of the
body whilst in contact with the object; and by this means the two adhered
firmly together. {53}  That under these circumstances a vacuum was
produced was plainly seen on one occasion, when a large worm lying
beneath a flaccid cabbage leaf tried to drag it away; for the surface of
the leaf directly over the end of the worm’s body became deeply pitted.
On another occasion a worm suddenly lost its hold on a flat leaf; and the
anterior end of the body was momentarily seen to be cup-formed.  Worms
can attach themselves to an object beneath water in the same manner; and
I saw one thus dragging away a submerged slice of an onion-bulb.

The edges of fresh or nearly fresh leaves affixed to the ground were
often nibbled by the worms; and sometimes the epidermis and all the
parenchyma on one side was gnawed completely away over a considerable
space; the epidermis alone on the opposite side being left quite clean.
The veins were never touched, and leaves were thus sometimes partly
converted into skeletons.  As worms have no teeth and as their mouths
consist of very soft tissue, it may be presumed that they consume by
means of suction the edges and the parenchyma of fresh leaves, after they
have been softened by the digestive fluid.  They cannot attack such
strong leaves as those of sea-kale or large and thick leaves of ivy;
though one of the latter after it had become rotten was reduced in parts
to the state of a skeleton.

Worms seize leaves and other objects, not only to serve as food, but for
plugging up the mouths of their burrows; and this is one of their
strongest instincts.  They sometimes work so energetically that Mr. D. F.
Simpson, who has a small walled garden where worms abound in Bayswater,
informs me that on a calm damp evening he there heard so extraordinary a
rustling noise from under a tree from which many leaves had fallen, that
he went out with a light and discovered that the noise was caused by many
worms dragging the dry leaves and squeezing them into the burrows.  Not
only leaves, but petioles of many kinds, some flower-peduncles, often
decayed twigs of trees, bits of paper, feathers, tufts of wool and
horse-hairs are dragged into their burrows for this purpose.  I have seen
as many as seventeen petioles of a Clematis projecting from the mouth of
one burrow, and ten from the mouth of another.  Some of these objects,
such as the petioles just named, feathers, &c., are never gnawed by
worms.  In a gravel-walk in my garden I found many hundred leaves of a
pine-tree (_P. austriaca_ or _nigricans_) drawn by their bases into
burrows.  The surfaces by which these leaves are articulated to the
branches are shaped in as peculiar a manner as is the joint between the
leg-bones of a quadruped; and if these surfaces had been in the least
gnawed, the fact would have been immediately visible, but there was no
trace of gnawing.  Of ordinary dicotyledonous leaves, all those which are
dragged into burrows are not gnawed.  I have seen as many as nine leaves
of the lime-tree drawn into the same burrow, and not nearly all of them
had been gnawed; but such leaves may serve as a store for future
consumption.  Where fallen leaves are abundant, many more are sometimes
collected over the mouth of a burrow than can be used, so that a small
pile of unused leaves is left like a roof over those which have been
partly dragged in.

A leaf in being dragged a little way into a cylindrical burrow is
necessarily much folded or crumpled.  When another leaf is drawn in, this
is done exteriorly to the first one, and so on with the succeeding
leaves; and finally all become closely folded and pressed together.
Sometimes the worm enlarges the mouth of its burrow, or makes a fresh one
close by, so as to draw in a still larger number of leaves.  They often
or generally fill up the interstices between the drawn-in leaves with
moist viscid earth ejected from their bodies; and thus the mouths of the
burrows are securely plugged.  Hundreds of such plugged burrows may be
seen in many places, especially during the autumnal and early winter
months.  But, as will hereafter be shown, leaves are dragged into the
burrows not only for plugging them up and for food, but for the sake of
lining the upper part or mouth.

When worms cannot obtain leaves, petioles, sticks, &c., with which to
plug up the mouths of their burrows, they often protect them by little
heaps of stones; and such heaps of smooth rounded pebbles may frequently
be seen on gravel-walks.  Here there can be no question about food.  A
lady, who was interested in the habits of worms, removed the little heaps
of stones from the mouths of several burrows and cleared the surface of
the ground for some inches all round.  She went out on the following
night with a lantern, and saw the worms with their tails fixed in their
burrows, dragging the stones inwards by the aid of their mouths, no doubt
by suction.  “After two nights some of the holes had 8 or 9 small stones
over them; after four nights one had about 30, and another 34 stones.”
{58}  One stone—which had been dragged over the gravel-walk to the mouth
of a burrow weighed two ounces; and this proves how strong worms are.
But they show greater strength in sometimes displacing stones in a
well-trodden gravel-walk; that they do so, may be inferred from the
cavities left by the displaced stones being exactly filled by those lying
over the mouths of adjoining burrows, as I have myself observed.

Work of this kind is usually performed during the night; but I have
occasionally known objects to be drawn into the burrows during the day.
What advantage the worms derive from plugging up the mouths of their
burrows with leaves, &c., or from piling stones over them, is doubtful.
They do not act in this manner at the times when they eject much earth
from their burrows; for their castings then serve to cover the mouths.
When gardeners wish to kill worms on a lawn, it is necessary first to
brush or rake away the castings from the surface, in order that the
lime-water may enter the burrows. {59a}  It might be inferred from this
fact that the mouths are plugged up with leaves, &c., to prevent the
entrance of water during heavy rain; but it may be urged against this
view that a few, loose, well-rounded stones are ill-adapted to keep out
water.  I have moreover seen many burrows in the perpendicularly cut
turf-edgings to gravel-walks, into which water could hardly flow, as well
plugged as burrows on a level surface.  It is not probable that the plugs
or piles of stones serve to conceal the burrows from scolopendras, which,
according to Hoffmeister, {59b} are the bitterest enemies of worms, or
from the larger species of Carabus and Staphylinus which attack them
ferociously, for these animals are nocturnal, and the burrows are opened
at night.  May not worms when the mouth of the burrow is protected be
able to remain with safety with their heads close to it, which we know
that they like to do, but which costs so many of them their lives?  Or
may not the plugs check the free ingress of the lowest stratum of air,
when chilled by radiation at night, from the surrounding ground and
herbage?  I am inclined to believe in this latter view: firstly, because
when worms were kept in pots in a room with a fire, in which case cold
air could not enter the burrows, they plugged them up in a slovenly
manner; and secondarily, because they often coat the upper part of their
burrows with leaves, apparently to prevent their bodies from coming into
close contact with the cold damp earth.  Mr. E. Parfitt has suggested to
me that the mouths of the burrows are closed in order that the air within
them may be kept thoroughly damp, and this seems the most probable
explanation of the habit.  But the plugging-up process may serve for all
the above purposes.

Whatever the motive may be, it appears that worms much dislike leaving
the mouths of their burrows open.  Nevertheless they will reopen them at
night, whether or not they can afterwards close them.  Numerous open
burrows may be seen on recently-dug ground, for in this case the worms
eject their castings in cavities left in the ground, or in the old
burrows instead of piling them over the mouths of their burrows, and they
cannot collect objects on the surface by which the mouths might be
protected.  So again on a recently disinterred pavement of a Roman villa
at Abinger (hereafter to be described) the worms pertinaciously opened
their burrows almost every night, when these had been closed by being
trampled on, although they were rarely able to find a few minute stones
wherewith to protect them.

_Intelligence shown by worms in their manner of plugging up their
burrows_.—If a man had to plug up a small cylindrical hole, with such
objects as leaves, petioles or twigs, he would drag or push them in by
their pointed ends; but if these objects were very thin relatively to the
size of the hole, he would probably insert some by their thicker or
broader ends.  The guide in his case would be intelligence.  It seemed
therefore worth while to observe carefully how worms dragged leaves into
their burrows; whether by their tips or bases or middle parts.  It seemed
more especially desirable to do this in the case of plants not natives to
our country; for although the habit of dragging leaves into their burrows
is undoubtedly instinctive with worms, yet instinct could not tell them
how to act in the case of leaves about which their progenitors knew
nothing.  If, moreover, worms acted solely through instinct or an
unvarying inherited impulse, they would draw all kinds of leaves into
their burrows in the same manner.  If they have no such definite
instinct, we might expect that chance would determine whether the tip,
base or middle was seized.  If both these alternatives are excluded,
intelligence alone is left; unless the worm in each case first tries many
different methods, and follows that alone which proves possible or the
most easy; but to act in this manner and to try different methods makes a
near approach to intelligence.

In the first place 227 withered leaves of various kinds, mostly of
English plants, were pulled out of worm-burrows in several places.  Of
these, 181 had been drawn into the burrows by or near their tips, so that
the foot-stalk projected nearly upright from the mouth of the burrow; 20
had been drawn in by their bases, and in this case the tips projected
from the burrows; and 26 had been seized near the middle, so that these
had been drawn in transversely and were much crumpled.  Therefore 80 per
cent. (always using the nearest whole number) had been drawn in by the
tip, 9 per cent. by the base or foot-stalk, and 11 per cent. transversely
or by the middle.  This alone is almost sufficient to show that chance
does not determine the manner in which leaves are dragged into the
burrows.

Of the above 227 leaves, 70 consisted of the fallen leaves of the common
lime-tree, which is almost certainly not a native of England.  These
leaves are much acuminated towards the tip, and are very broad at the
base with a well-developed foot-stalk.  They are thin and quite flexible
when half-withered.  Of the 70, 79 per cent. had been drawn in by or near
the tip; 4 per cent. by or near the base; and 17 per cent. transversely
or by the middle.  These proportions agree very closely, as far as the
tip is concerned, with those before given.  But the percentage drawn in
by the base is smaller, which may be attributed to the breadth of the
basal part of the blade.  We here, also, see that the presence of a
foot-stalk, which it might have been expected would have tempted the
worms as a convenient handle, has little or no influence in determining
the manner in which lime leaves are dragged into the burrows.  The
considerable proportion, viz., 17 per cent., drawn in more or less
transversely depends no doubt on the flexibility of these half-decayed
leaves.  The fact of so many having been drawn in by the middle, and of
some few having been drawn in by the base, renders it improbable that the
worms first tried to draw in most of the leaves by one or both of these
methods, and that they afterwards drew in 79 per cent. by their tips; for
it is clear that they would not have failed in drawing them in by the
base or middle.

The leaves of a foreign plant were next searched for, the blades of which
were not more pointed towards the apex than towards the base.  This
proved to be the case with those of a laburnum (a hybrid between _Cytisus
alpinus_ and _laburnum_) for on doubling the terminal over the basal
half, they generally fitted exactly; and when there was any difference,
the basal half was a little the narrower.  It might, therefore, have been
expected that an almost equal number of these leaves would have been
drawn in by the tip and base, or a slight excess in favour of the latter.
But of 73 leaves (not included in the first lot of 227) pulled out of
worm-burrows, 63 per cent. had been drawn in by the tip; 27 per cent. by
the base, and 10 per cent. transversely.  We here see that a far larger
proportion, viz., 27 per cent. were drawn in by the base than in the case
of lime leaves, the blades of which are very broad at the base, and of
which only 4 per cent. had thus been drawn in.  We may perhaps account
for the fact of a still larger proportion of the laburnum leaves not
having been drawn in by the base, by worms having acquired the habit of
generally drawing in leaves by their tips and thus avoiding the
foot-stalk.  For the basal margin of the blade in many kinds of leaves
forms a large angle with the foot-stalk; and if such a leaf were drawn in
by the foot-stalk, the basal margin would come abruptly into contact with
the ground on each side of the burrow, and would render the drawing in of
the leaf very difficult.

Nevertheless worms break through their habit of avoiding the foot-stalk,
if this part offers them the most convenient means for drawing leaves
into their burrows.  The leaves of the endless hybridised varieties of
the Rhododendron vary much in shape; some are narrowest towards the base
and others towards the apex.  After they have fallen off, the blade on
each side of the midrib often becomes curled up while drying, sometimes
along the whole length, sometimes chiefly at the base, sometimes towards
the apex.  Out of 28 fallen leaves on one bed of peat in my garden, no
less than 23 were narrower in the basal quarter than in the terminal
quarter of their length; and this narrowness was chiefly due to the
curling in of the margins.  Out of 36 fallen leaves on another bed, in
which different varieties of the Rhododendron grew, only 17 were narrower
towards the base than towards the apex.  My son William, who first called
my attention to this case, picked up 237 fallen leaves in his garden
(where the Rhododendron grows in the natural soil) and of these 65 per
cent. could have been drawn by worms into their burrows more easily by
the base or foot-stalk than by the tip; and this was partly due to the
shape of the leaf and in a less degree to the curling in of the margins:
27 per cent. could have been drawn in more easily by the tip than by the
base: and 8 per cent. with about equal ease by either end.  The shape of
a fallen leaf ought to be judged of before one end has been drawn into a
burrow, for after this has happened, the free end, whether it be the base
or apex, will dry more quickly than the end imbedded in the damp ground;
and the exposed margins of the free end will consequently tend to become
more curled inwards than they were when the leaf was first seized by the
worm.  My son found 91 leaves which had been dragged by worms into their
burrows, though not to a great depth; of these 66 per cent. had been
drawn in by the base or foot-stalk; and 34 per cent. by the tip.  In this
case, therefore, the worms judged with a considerable degree of
correctness how best to draw the withered leaves of this foreign plant
into their burrows; notwithstanding that they had to depart from their
usual habit of avoiding the foot-stalk.

On the gravel-walks in my garden a very large number of leaves of three
species of Pinus (_P. austriaca_, _nigricans_ and _sylvestris_) are
regularly drawn into the mouths of worm burrows.  These leaves consist of
two so-called needles, which are of considerable length in the two first
and short in the last named species, and are united to a common base; and
it is by this part that they are almost invariably drawn into the
burrows.  I have seen only two or at most three exceptions to this rule
with worms in a state of nature.  As the sharply pointed needles diverge
a little, and as several leaves are drawn into the same burrow, each tuft
forms a perfect _chevaux de frise_.  On two occasions many of these tufts
were pulled up in the evening, but by the following morning fresh leaves
had been pulled in, and the burrows were again well protected.  These
leaves could not be dragged into the burrows to any depth, except by
their bases, as a worm cannot seize hold of the two needles at the same
time, and if one alone were seized by the apex, the other would be
pressed against the ground and would resist the entry of the seized one.
This was manifest in the above mentioned two or three exceptional cases.
In order, therefore, that worms should do their work well, they must drag
pine-leaves into their burrows by their bases, where the two needles are
conjoined.  But how they are guided in this work is a perplexing
question.

This difficulty led my son Francis and myself to observe worms in
confinement during several nights by the aid of a dim light, while they
dragged the leaves of the above named pines into their burrows.  They
moved the anterior extremities of their bodies about the leaves, and on
several occasions when they touched the sharp end of a needle they
withdrew suddenly as if pricked.  But I doubt whether they were hurt, for
they are indifferent to very sharp objects, and will swallow even
rose-thorns and small splinters of glass.  It may also be doubted,
whether the sharp ends of the needles serve to tell them that this is the
wrong end to seize; for the points were cut off many leaves for a length
of about one inch, and fifty-seven of them thus treated were drawn into
the burrows by their bases, and not one by the cut-off ends.  The worms
in confinement often seized the needles near the middle and drew them
towards the mouths of their burrows; and one worm tried in a senseless
manner to drag them into the burrow by bending them.  They sometimes
collected many more leaves over the mouths of their burrows (as in the
case formerly mentioned of lime-leaves) than could enter them.  On other
occasions, however, they behaved very differently; for as soon as they
touched the base of a pine-leaf, this was seized, being sometimes
completely engulfed in their mouths, or a point very near the base was
seized, and the leaf was then quickly dragged or rather jerked into their
burrows.  It appeared both to my son and myself as if the worms instantly
perceived as soon as they had seized a leaf in the proper manner.  Nine
such cases were observed, but in one of them the worm failed to drag the
leaf into its burrow, as it was entangled by other leaves lying near.  In
another case a leaf stood nearly upright with the points of the needles
partly inserted into a burrow, but how placed there was not seen; and
then the worm reared itself up and seized the base, which was dragged
into the mouth of the burrow by bowing the whole leaf.  On the other
hand, after a worm had seized the base of a leaf, this was on two
occasions relinquished from some unknown motive.

As already remarked, the habit of plugging up the mouths of the burrows
with various objects, is no doubt instinctive in worms; and a very young
one, born in one of my pots, dragged for some little distance a
Scotch-fir leaf, one needle of which was as long and almost as thick as
its own body.  No species of pine is endemic in this part of England, it
is therefore incredible that the proper manner of dragging pine-leaves
into the burrows can be instinctive with our worms.  But as the worms on
which the above observations were made, were dug up beneath or near some
pines, which had been planted there about forty years, it was desirable
to prove that their actions were not instinctive.  Accordingly,
pine-leaves were scattered on the ground in places far removed from any
pine-tree, and 90 of them were drawn into the burrows by their bases.
Only two were drawn in by the tips of the needles, and these were not
real exceptions, as one was drawn in for a very short distance, and the
two needles of the other cohered.  Other pine-leaves were given to worms
kept in pots in a warm room, and here the result was different; for out
of 42 leaves drawn into the burrows, no less than 16 were drawn in by the
tips of the needles.  These worms, however, worked in a careless or
slovenly manner; for the leaves were often drawn in to only a small
depth; sometimes they were merely heaped over the mouths of the burrows,
and sometimes none were drawn in.  I believe that this carelessness may
be accounted for either by the warmth of the air, or by its dampness, as
the pots were covered by glass plates; the worms consequently did not
care about plugging up their holes effectually.  Pots tenanted by worms
and covered with a net which allowed the free entrance of air, were left
out of doors for several nights, and now 72 leaves were all properly
drawn in by their bases.

It might perhaps be inferred from the facts as yet given, that worms
somehow gain a general notion of the shape or structure of pine-leaves,
and perceive that it is necessary for them to seize the base where the
two needles are conjoined.  But the following cases make this more than
doubtful.  The tips of a large number of needles of _P. austriaca_ were
cemented together with shell-lac dissolved in alcohol, and were kept for
some days, until, as I believe, all odour or taste had been lost; and
they were then scattered on the ground where no pine-trees grew, near
burrows from which the plugging had been removed.  Such leaves could have
been drawn into the burrows with equal ease by either end; and judging
from analogy and more especially from the case presently to be given of
the petioles of _Clematis montana_, I expected that the apex would have
been preferred.  But the result was that out of 121 leaves with the tips
cemented, which were drawn into burrows, 108 were drawn in by their
bases, and only 13 by their tips.  Thinking that the worms might possibly
perceive and dislike the smell or taste of the shell-lac, though this was
very improbable, especially after the leaves had been left out during
several nights, the tips of the needles of many leaves were tied together
with fine thread.  Of leaves thus treated 150 were drawn into burrows—123
by the base and 27 by the tied tips; so that between four and five times
as many were drawn in by the base as by the tip.  It is possible that the
short cut-off ends of the thread with which they were tied, may have
tempted the worms to drag in a larger proportional number by the tips
than when cement was used.  Of the leaves with tied and cemented tips
taken together (271 in number) 85 per cent. were drawn in by the base and
15 per cent. by the tips.  We may therefore infer that it is not the
divergence of the two needles which leads worms in a state of nature
almost invariably to drag pine-leaves into their burrows by the base.
Nor can it be the sharpness of the points of the needles which determines
them; for, as we have seen, many leaves with the points cut off were
drawn in by their bases.  We are thus led to conclude, that with
pine-leaves there must be something attractive to worms in the base,
notwithstanding that few ordinary leaves are drawn in by the base or
foot-stalk.

_Petioles_.—We will now turn to the petioles or foot-stalks of compound
leaves, after the leaflets have fallen off.  Those from _Clematis
montana_, which grew over a verandah, were dragged early in January in
large numbers into the burrows on an adjoining gravel-walk, lawn, and
flower-bed.  These petioles vary from 2½ to 4½ inches in length, are
rigid and of nearly uniform thickness, except close to the base where
they thicken rather abruptly, being here about twice as thick as in any
other part.  The apex is somewhat pointed, but soon withers and is then
easily broken off.  Of these petioles, 314 were pulled out of burrows in
the above specified sites; and it was found that 76 per cent. had been
drawn in by their tips, and 24 per cent by their bases; so that those
drawn in by the tip were a little more than thrice as many as those drawn
in by the base.  Some of those extracted from the well-beaten gravel-walk
were kept separate from the others; and of these (59 in number) nearly
five times as many had been drawn in by the tip as by the base; whereas
of those extracted from the lawn and flower-bed, where from the soil
yielding more easily, less care would be necessary in plugging up the
burrows, the proportion of those drawn in by the tip (130) to those drawn
in by the base (48) was rather less than three to one.  That these
petioles had been dragged into the burrows for plugging them up, and not
for food, was manifest, as neither end, as far as I could see, had been
gnawed.  As several petioles are used to plug up the same burrow, in one
case as many as 10, and in another case as many as 15, the worms may
perhaps at first draw in a few by the thicker end so as to save labour;
but afterwards a large majority are drawn in by the pointed end, in order
to plug up the hole securely.

The fallen petioles of our native ash-tree were next observed, and the
rule with most objects, viz., that a large majority are dragged into the
burrows by the more pointed end, had not here been followed; and this
fact much surprised me at first.  These petioles vary in length from 5 to
8½ inches; they are thick and fleshy towards the base, whence they taper
gently towards the apex, which is a little enlarged and truncated where
the terminal leaflet had been originally attached.  Under some ash-trees
growing in a grass-field, 229 petioles were pulled out of worm burrows
early in January, and of these 51.5 per cent. had been drawn in by the
base, and 48.5 per cent. by the apex.  This anomaly was however readily
explained as soon as the thick basal part was examined; for in 78 out of
103 petioles, this part had been gnawed by worms, just above the
horse-shoe shaped articulation.  In most cases there could be no mistake
about the gnawing; for ungnawed petioles which were examined after being
exposed to the weather for eight additional weeks had not become more
disintegrated or decayed near the base than elsewhere.  It is thus
evident that the thick basal end of the petiole is drawn in not solely
for the sake of plugging up the mouths of the burrows, but as food.  Even
the narrow truncated tips of some few petioles had been gnawed; and this
was the case in 6 out of 37 which were examined for this purpose.  Worms,
after having drawn in and gnawed the basal end, often push the petioles
out of their burrows; and then drag in fresh ones, either by the base for
food, or by the apex for plugging up the mouth more effectually.  Thus,
out of 37 petioles inserted by their tips, 5 had been previously drawn in
by the base, for this part had been gnawed.  Again, I collected a handful
of petioles lying loose on the ground close to some plugged-up burrows,
where the surface was thickly strewed with other petioles which
apparently had never been touched by worms; and 14 out of 47 (_i.e._
nearly one-third), after having had their bases gnawed had been pushed
out of the burrows and were now lying on the ground.  From these several
facts we may conclude that worms draw in some petioles of the ash by the
base to serve as food, and others by the tip to plug up the mouths of
their burrows in the most efficient manner.

The petioles of _Robinia pseudo-acacia_ vary from 4 or 5 to nearly 12
inches in length; they are thick close to the base before the softer
parts have rotted off, and taper much towards the upper end.  They are so
flexible that I have seen some few doubled up and thus drawn into the
burrows of worms.  Unfortunately these petioles were not examined until
February, by which time the softer parts had completely rotted off, so
that it was impossible to ascertain whether worms had gnawed the bases,
though this is in itself probable.  Out of 121 petioles extracted from
burrows early in February, 68 were imbedded by the base, and 53 by the
apex.  On February 5 all the petioles which had been drawn into the
burrows beneath a Robinia, were pulled up; and after an interval of
eleven days, 35 petioles had been again dragged in, 19 by the base, and
16 by the apex.  Taking these two lots together, 56 per cent. were drawn
in by the base, and 44 per cent. by the apex.  As all the softer parts
had long ago rotted off, we may feel sure, especially in the latter case,
that none had been drawn in as food.  At this season, therefore, worms
drag these petioles into their burrows indifferently by either end, a
slight preference being given to the base.  This latter fact may be
accounted for by the difficulty of plugging up a burrow with objects so
extremely thin as are the upper ends.  In support of this view, it may be
stated that out of the 16 petioles which had been drawn in by their upper
ends, the more attenuated terminal portion of 7 had been previously
broken off by some accident.

_Triangles of paper_.—Elongated triangles were cut out of moderately
stiff writing-paper, which was rubbed with raw fat on both sides, so as
to prevent their becoming excessively limp when exposed at night to rain
and dew.  The sides of all the triangles were three inches in length,
with the bases of 120 one inch, and of the other 183 half an inch in
length.  These latter triangles were very narrow or much acuminated. {79}
As a check on the observations presently to be given, similar triangles
in a damp state were seized by a very narrow pair of pincers at different
points and at all inclinations with reference to the margins, and were
then drawn into a short tube of the diameter of a worm-burrow.  If seized
by the apex, the triangle was drawn straight into the tube, with its
margins infolded; if seized at some little distance from the apex, for
instance at half an inch, this much was doubled back within the tube.  So
it was with the base and basal angles, though in this case the triangles
offered, as might have been expected, much more resistance to being drawn
in.  If seized near the middle the triangle was doubled up, with the apex
and base left sticking out of the tube.  As the sides of the triangles
were three inches in length, the result of their being drawn into a tube
or into a burrow in different ways, may be conveniently divided into
three groups: those drawn in by the apex or within an inch of it; those
drawn in by the base or within an inch of it; and those drawn in by any
point in the middle inch.

In order to see how the triangles would be seized by worms, some in a
damp state were given to worms kept in confinement.  They were seized in
three different manners in the case of both the narrow and broad
triangles: viz., by the margin; by one of the three angles, which was
often completely engulfed in their mouths; and lastly, by suction applied
to any part of the flat surface.  If lines parallel to the base and an
inch apart, are drawn across a triangle with the sides three inches in
length, it will be divided into three parts of equal length.  Now if
worms seized indifferently by chance any part, they would assuredly seize
on the basal part or division far oftener than on either of the two other
divisions.  For the area of the basal to the apical part is as 5 to 1, so
that the chance of the former being drawn into a burrow by suction, will
be as 5 to 1, compared with the apical part.  The base offers two angles
and the apex only one, so that the former would have twice as good a
chance (independently of the size of the angles) of being engulfed in a
worm’s mouth, as would the apex.  It should, however, be stated that the
apical angle is not often seized by worms; the margin at a little
distance on either side being preferred.  I judge of this from having
found in 40 out of 46 cases in which triangles had been drawn into
burrows by their apical ends, that the tip had been doubled back within
the burrow for a length of between 1/20 of an inch and 1 inch.  Lastly,
the proportion between the margins of the basal and apical parts is as 3
to 2 for the broad, and 2½ to 2 for the narrow triangles.  From these
several considerations it might certainly have been expected, supposing
that worms seized hold of the triangles by chance, that a considerably
larger proportion would have been dragged into the burrows by the basal
than by the apical part; but we shall immediately see how different was
the result.

Triangles of the above specified sizes were scattered on the ground in
many places and on many successive nights near worm-burrows, from which
the leaves, petioles, twigs, &c., with which they had been plugged, were
removed.  Altogether 303 triangles were drawn by worms into their
burrows: 12 others were drawn in by both ends, but as it was impossible
to judge by which end they had been first seized, these are excluded.  Of
the 303, 62 per cent. had been drawn in by the apex (using this term for
all drawn in by the apical part, one inch in length); 15 per cent. by the
middle; and 23 per cent. by the basal part.  If they had been drawn
indifferently by any point, the proportion for the apical, middle and
basal parts would have been 33.3 per cent. for each; but, as we have just
seen, it might have been expected that a much larger proportion would
have been drawn in by the basal than by any other part.  As the case
stands, nearly three times as many were drawn in by the apex as by the
base.  If we consider the broad triangles by themselves, 59 per cent.
were drawn in by the apex, 25 per cent. by the middle, and 16 per cent.
by the base.  Of the narrow triangles, 65 per cent. were drawn in by the
apex, 14 per cent, by the middle, and 21 per cent. by the base; so that
here those drawn in by the apex were more than 3 times as many as those
drawn in by the base.  We may therefore conclude that the manner in which
the triangles are drawn into the burrows is not a matter of chance.

In eight cases, two triangles had been drawn into the same burrow, and in
seven of these cases, one had been drawn in by the apex and the other by
the base.  This again indicates that the result is not determined by
chance.  Worms appear sometimes to revolve in the act of drawing in the
triangles, for five out of the whole lot had been wound into an irregular
spire round the inside of the burrow.  Worms kept in a warm room drew 63
triangles into their burrows; but, as in the case of the pine-leaves,
they worked in a rather careless manner, for only 44 per cent. were drawn
in by the apex, 22 per cent. by the middle, and 33 per cent. by the base.
In five cases, two triangles were drawn into the same burrow.

It may be suggested with much apparent probability that so large a
proportion of the triangles were drawn in by the apex, not from the worms
having selected this end as the most convenient for the purpose, but from
having first tried in other ways and failed.  This notion was
countenanced by the manner in which worms in confinement were seen to
drag about and drop the triangles; but then they were working carelessly.
I did not at first perceive the importance of this subject, but merely
noticed that the bases of those triangles which had been drawn in by the
apex, were generally clean and not crumpled.  The subject was afterwards
attended to carefully.  In the first place several triangles which had
been drawn in by the basal angles, or by the base, or a little above the
base, and which were thus much crumpled and dirtied, were left for some
hours in water and were then well shaken while immersed; but neither the
dirt nor the creases were thus removed.  Only slight creases could be
obliterated, even by pulling the wet triangles several times through my
fingers.  Owing to the slime from the worms’ bodies, the dirt was not
easily washed off.  We may therefore conclude that if a triangle, before
being dragged in by the apex, had been dragged into a burrow by its base
with even a slight degree of force, the basal part would long retain its
creases and remain dirty.  The condition of 89 triangles (65 narrow and
24 broad ones), which had been drawn in by the apex, was observed; and
the bases of only 7 of them were at all creased, being at the same time
generally dirty.  Of the 82 uncreased triangles, 14 were dirty at the
base; but it does not follow from this fact that these had first been
dragged towards the burrows by their bases; for the worms sometimes
covered large portions of the triangles with slime, and these when
dragged by the apex over the ground would be dirtied; and during rainy
weather, the triangles were often dirtied over one whole side or over
both sides.  If the worms had dragged the triangles to the mouths of
their burrows by their bases, as often as by their apices, and had then
perceived, without actually trying to draw them into the burrow, that the
broader end was not well adapted for this purpose—even in this case a
large proportion would probably have had their basal ends dirtied.  We
may therefore infer—improbable as is the inference—that worms are able by
some means to judge which is the best end by which to draw triangles of
paper into their burrows.

The percentage results of the foregoing observations on the manner in
which worms draw various kinds of objects into the mouths of their
burrows may be abridged as follows:—

   Nature of       Drawn into the   Drawn in, by or   Drawn in, by or
    Object.        burrows, by or       near the       near the base.
                   near the apex.       middle.
Leaves of                       80                11                 9
various kinds
—of the Lime,                   79                17                 4
basal margin of
blade broad,
apex acuminated
—of a Laburnum,                 63                10                27
basal part of
blade as narrow
as, or
sometimes
little narrower
than the apical
part
—of the                         34        ...                       66
Rhododendron,
basal part of
blade often
narrower than
the apical part
—of Pine-trees,         ...               ...                      100
consisting of
two needles
arising from a
common base
Petioles of a                   76        ...                       24
Clematis,
somewhat
pointed at the
apex, and blunt
at the base
—of the Ash,                  48.5        ...                     51.5
the thick basal
end often drawn
in to serve as
food
—of Robinia,                    44        ...                       56
extremely thin,
especially
towards the
apex, so as to
be ill-fitted
for plugging up
the burrows
Triangles of                    62                15                23
paper, of the
two sizes
—of the broad                   59                25                16
ones alone
—of the narrow                  65                14                21
ones alone

If we consider these several cases, we can hardly escape from the
conclusion that worms show some degree of intelligence in their manner of
plugging up their burrows.  Each particular object is seized in too
uniform a manner, and from causes which we can generally understand, for
the result to be attributed to mere chance.  That every object has not
been drawn in by its pointed end, may be accounted for by labour having
been saved through some being inserted by their broader or thicker ends.
No doubt worms are led by instinct to plug up their burrows; and it might
have been expected that they would have been led by instinct how best to
act in each particular case, independently of intelligence.  We see how
difficult it is to judge whether intelligence comes into play, for even
plants might sometimes be thought to be thus directed; for instance when
displaced leaves re-direct their upper surfaces towards the light by
extremely complicated movements and by the shortest course.  With
animals, actions appearing due to intelligence may be performed through
inherited habit without any intelligence, although aboriginally thus
acquired.  Or the habit may have been acquired through the preservation
and inheritance of beneficial variations of some other habit; and in this
case the new habit will have been acquired independently of intelligence
throughout the whole course of its development.  There is no _à priori_
improbability in worms having acquired special instincts through either
of these two latter means.  Nevertheless it is incredible that instincts
should have been developed in reference to objects, such as the leaves of
petioles of foreign plants, wholly unknown to the progenitors of the
worms which act in the described manner.  Nor are their actions so
unvarying or inevitable as are most true instincts.

As worms are not guided by special instincts in each particular case,
though possessing a general instinct to plug up their burrows, and as
chance is excluded, the next most probable conclusion seems to be that
they try in many different ways to draw in objects, and at last succeed
in some one way.  But it is surprising that an animal so low in the scale
as a worm should have the capacity for acting in this manner, as many
higher animals have no such capacity.  For instance, ants may be seen
vainly trying to drag an object transversely to their course, which could
be easily drawn longitudinally; though after a time they generally act in
a wiser manner, M. Fabre states {89a} that a Sphex—an insect belonging to
the same highly-endowed order with ants—stocks its nest with paralysed
grass-hoppers, which are invariably dragged into the burrow by their
antennæ.  When these were cut off close to the head, the Sphex seized the
palpi; but when these were likewise cut off, the attempt to drag its prey
into the burrow was given up in despair.  The Sphex had not intelligence
enough to seize one of the six legs or the ovipositor of the grasshopper,
which, as M. Fabre remarks, would have served equally well.  So again, if
the paralysed prey with an egg attached to it be taken out of the cell,
the Sphex after entering and finding the cell empty, nevertheless closes
it up in the usual elaborate manner.  Bees will try to escape and go on
buzzing for hours on a window, one half of which has been left open.
Even a pike continued during three months to dash and bruise itself
against the glass sides of an aquarium, in the vain attempt to seize
minnows on the opposite side. {89b}  A cobra-snake was seen by Mr. Layard
{90} to act much more wisely than either the pike or the Sphex; it had
swallowed a toad lying within a hole, and could not withdraw its head;
the toad was disgorged, and began to crawl away; it was again swallowed
and again disgorged; and now the snake had learnt by experience, for it
seized the toad by one of its legs and drew it out of the hole.  The
instincts of even the higher animals are often followed in a senseless or
purposeless manner: the weaver-bird will perseveringly wind threads
through the bars of its cage, as if building a nest: a squirrel will pat
nuts on a wooden floor, as if he had just buried them in the ground: a
beaver will cut up logs of wood and drag them about, though there is no
water to dam up; and so in many other cases.

Mr. Romanes, who has specially studied the minds of animals, believes
that we can safely infer intelligence, only when we see an individual
profiting by its own experience.  By this test the cobra showed some
intelligence; but this would have been much plainer if on a second
occasion he had drawn a toad out of a hole by its leg.  The Sphex failed
signally in this respect.  Now if worms try to drag objects into their
burrows first in one way and then in another, until they at last succeed,
they profit, at least in each particular instance, by experience.

But evidence has been advanced showing that worms do not habitually try
to draw objects into their burrows in many different ways.  Thus
half-decayed lime-leaves from their flexibility could have been drawn in
by their middle or basal parts, and were thus drawn into the burrows in
considerable numbers; yet a large majority were drawn in by or near the
apex.  The petioles of the Clematis could certainly have been drawn in
with equal ease by the base and apex; yet three times and in certain
cases five times as many were drawn in by the apex as by the base.  It
might have been thought that the foot-stalks of leaves would have tempted
the worms as a convenient handle; yet they are not largely used, except
when the base of the blade is narrower than the apex.  A large number of
the petioles of the ash are drawn in by the base; but this part serves
the worms as food.  In the case of pine-leaves worms plainly show that
they at least do not seize the leaf by chance; but their choice does not
appear to be determined by the divergence of the two needles, and the
consequent advantage or necessity of drawing them into their burrows by
the base.  With respect to the triangles of paper, those which had been
drawn in by the apex rarely had their bases creased or dirty; and this
shows that the worms had not often first tried to drag them in by this
end.

If worms are able to judge, either before drawing or after having drawn
an object close to the mouths of their burrows, how best to drag it in,
they must acquire some notion of its general shape.  This they probably
acquire by touching it in many places with the anterior extremity of
their bodies, which serves as a tactile organ.  It may be well to
remember how perfect the sense of touch becomes in a man when born blind
and deaf, as are worms.  If worms have the power of acquiring some
notion, however rude, of the shape of an object and of their burrows, as
seems to be the case, they deserve to be called intelligent; for they
then act in nearly the same manner as would a man under similar
circumstances.

To sum up, as chance does not determine the manner in which objects are
drawn into the burrows, and as the existence of specialized instincts for
each particular case cannot be admitted, the first and most natural
supposition is that worms try all methods until they at last succeed; but
many appearances are opposed to such a supposition.  One alternative
alone is left, namely, that worms, although standing low in the scale of
organization, possess some degree of intelligence.  This will strike
every one as very improbable; but it may be doubted whether we know
enough about the nervous system of the lower animals to justify our
natural distrust of such a conclusion.  With respect to the small size of
the cerebral ganglia, we should remember what a mass of inherited
knowledge, with some power of adapting means to an end, is crowded into
the minute brain of a worker-ant.

_Means by which worms excavate their burrows_.—This is effected in two
ways; by pushing away the earth on all sides, and by swallowing it.  In
the former case, the worm inserts the stretched out and attenuated
anterior extremity of its body into any little crevice, or hole; and
then, as Perrier remarks, {93} the pharynx is pushed forwards into this
part, which consequently swells and pushes away the earth on all sides.
The anterior extremity thus serves as a wedge.  It also serves, as we
have before seen, for prehension and suction, and as a tactile organ.  A
worm was placed on loose mould, and it buried itself in between two and
three minutes.  On another occasion four worms disappeared in 15 minutes
between the sides of the pot and the earth, which had been moderately
pressed down.  On a third occasion three large worms and a small one were
placed on loose mould well mixed with fine sand and firmly pressed down,
and they all disappeared, except the tail of one, in 35 minutes.  On a
fourth occasion six large worms were placed on argillaceous mud mixed
with sand firmly pressed down, and they disappeared, except the extreme
tips of the tails of two of them, in 40 minutes.  In none of these cases,
did the worms swallow, as far as could be seen, any earth.  They
generally entered the ground close to the sides of the pot.

A pot was next filled with very fine ferruginous sand, which was pressed
down, well watered, and thus rendered extremely compact.  A large worm
left on the surface did not succeed in penetrating it for some hours, and
did not bury itself completely until 25 hrs. 40 min. had elapsed.  This
was effected by the sand being swallowed, as was evident by the large
quantity ejected from the vent, long before the whole body had
disappeared.  Castings of a similar nature continued to be ejected from
the burrow during the whole of the following day.

As doubts have been expressed by some writers whether worms ever swallow
earth solely for the sake of making their burrows, some additional cases
may be given.  A mass of fine reddish sand, 23 inches in thickness, left
on the ground for nearly two years, had been penetrated in many places by
worms; and their castings consisted partly of the reddish sand and partly
of black earth brought up from beneath the mass.  This sand had been dug
up from a considerable depth, and was of so poor a nature that weeds
could not grow on it.  It is therefore highly improbable that it should
have been swallowed by the worms as food.  Again in a field near my house
the castings frequently consist of almost pure chalk, which lies at only
a little depth beneath the surface; and here again it is very improbable
that the chalk should have been swallowed for the sake of the very little
organic matter which could have percolated into it from the poor
overlying pasture.  Lastly, a casting thrown up through the concrete and
decayed mortar between the tiles, with which the now ruined aisle of
Beaulieu Abbey had formerly been paved, was washed, so that the coarser
matter alone was left.  This consisted of grains of quartz, micaceous
slate, other rocks, and bricks or tiles, many of them from 1/20 to 1/10
inch in diameter.  No one will suppose that these grains were swallowed
as food, yet they formed more than half of the casting, for they weighed
19 grains, the whole casting having weighed 33 grains.  Whenever a worm
burrows to a depth of some feet in undisturbed compact ground, it must
form its passage by swallowing the earth; for it is incredible that the
ground could yield on all sides to the pressure of the pharynx when
pushed forwards within the worm’s body.

That worms swallow a larger quantity of earth for the sake of extracting
any nutritious matter which it may contain than for making their burrows,
appears to me certain.  But as this old belief has been doubted by so
high an authority as Claparède, evidence in its favour must be given in
some detail.  There is no _à priori_ improbability in such a belief, for
besides other annelids, especially the _Arenicola marina_, which throws
up such a profusion of castings on our tidal sands, and which it is
believed thus subsists, there are animals belonging to the most distinct
classes, which do not burrow, but habitually swallow large quantities of
sand; namely, the molluscan Onchidium and many Echinoderms. {97}

If earth were swallowed only when worms deepened their burrows or made
new ones, castings would be thrown up only occasionally; but in many
places fresh castings may be seen every morning, and the amount of earth
ejected from the same burrow on successive days is large.  Yet worms do
not burrow to a great depth, except when the weather is very dry or
intensely cold.  On my lawn the black vegetable mould or humus is only
about 5 inches in thickness, and overlies light-coloured or reddish
clayey soil: now when castings are thrown up in the greatest profusion,
only a small proportion are light coloured, and it is incredible that the
worms should daily make fresh burrows in every direction in the thin
superficial layer of dark-coloured mould, unless they obtained nutriment
of some kind from it.  I have observed a strictly analogous case in a
field near my house where bright red clay lay close beneath the surface.
Again on one part of the Downs near Winchester the vegetable mould
overlying the chalk was found to be only from 3 to 4 inches in thickness;
and the many castings here ejected were as black as ink and did not
effervesce with acids; so that the worms must have confined themselves to
this thin superficial layer of mould, of which large quantities were
daily swallowed.  In another place at no great distance the castings were
white; and why the worms should have burrowed into the chalk in some
places and not in others, I am unable to conjecture.

Two great piles of leaves had been left to decay in my grounds, and
months after their removal, the bare surface, several yards in diameter,
was so thickly covered during several months with castings that they
formed an almost continuous layer; and the large number of worms which
lived here must have subsisted during these months on nutritious matter
contained in the black earth.

The lowest layer from another pile of decayed leaves mixed with some
earth was examined under a high power, and the number of spores of
various shapes and sizes which it contained was astonishingly great; and
these crushed in the gizzards of worms may largely aid in supporting
them.  Whenever castings are thrown up in the greatest number, few or no
leaves are drawn into the burrows; for instance the turf along a
hedgerow, about 200 yards in length, was daily observed in the autumn
during several weeks, and every morning many fresh castings were seen;
but not a single leaf was drawn into these burrows.  These castings from
their blackness and from the nature of the subsoil could not have been
brought up from a greater depth than 6 or 8 inches.  On what could these
worms have subsisted during this whole time, if not on matter contained
in the black earth?  On the other hand, whenever a large number of leaves
are drawn into the burrows, the worms seem to subsist chiefly on them,
for few earth-castings are then ejected on the surface.  This difference
in the behaviour of worms at different times, perhaps explains a
statement by Claparède, namely, that triturated leaves and earth are
always found in distinct parts of their intestines.

Worms sometimes abound in places where they can rarely or never obtain
dead or living leaves; for instance, beneath the pavement in well-swept
courtyards, into which leaves are only occasionally blown.  My son Horace
examined a house, one corner of which had subsided; and he found here in
the cellar, which was extremely damp, many small worm-castings thrown up
between the stones with which the cellar was paved; and in this case it
is improbable that the worms could ever have obtained leaves.  Mr. A. C.
Horner confirms this account, as he has seen castings in the cellars of
his house, which is an old one at Tonbridge.

But the best evidence, known to me, of worms subsisting for at least
considerable periods of time solely on the organic matter contained in
earth, is afforded by some facts communicated to me by Dr. King.  Near
Nice large castings abound in extraordinary numbers, so that 5 or 6 were
often found within the space of a square foot.  They consist of fine,
pale-coloured earth, containing calcareous matter, which after having
passed through the bodies of worms and being dried, coheres with
considerable force.  I have reason to believe that these castings had
been formed by species of Perichæta, which have been naturalized here
from the East. {101}  They rise like towers, with their summits often a
little broader than their bases, sometimes to a height of above 3 and
often to a height of 2½ inches.  The tallest of those which were measured
was 3.3 inches in height and 1 inch in diameter.  A small cylindrical
passage runs up the centre of each tower, through which the worm ascends
to eject the earth which it has swallowed, and thus to add to its height.
A structure of this kind would not allow leaves being easily dragged from
the surrounding ground into the burrows; and Dr. King, who looked
carefully, never saw even a fragment of a leaf thus drawn in.  Nor could
any trace be discovered of the worms having crawled down the exterior
surfaces of the towers in search of leaves; and had they done so, tracks
would almost certainly have been left on the upper part whilst it
remained soft.  It does not, however, follow that these worms do not draw
leaves into their burrows during some other season of the year, at which
time they would not build up their towers.

From the several foregoing cases, it can hardly be doubted that worms
swallow earth, not only for the sake of making their burrows, but for
obtaining food.  Hensen, however, concludes from his analyses of mould
that worms probably could not live on ordinary vegetable mould, though he
admits that they might be nourished to some extent by leaf-mould. {102}
But we have seen that worms eagerly devour raw meat, fat, and dead worms;
and ordinary mould can hardly fail to contain many ova, larvæ, and small
living or dead creatures, spores of cryptogamic plants, and micrococci,
such as those which give rise to saltpetre.  These various organisms,
together with some cellulose from any leaves and roots not utterly
decayed, might well account for such large quantities of mould being
swallowed by worms.  It may be worth while here to recall the fact that
certain species of Utricularia, which grow in damp places in the tropics,
possess bladders beautifully constructed for catching minute subterranean
animals; and these traps would not have been developed unless many small
animals inhabited such soil.

_The depth to which worms penetrate_, _and the construction of their
burrows_.—Although worms usually live near the surface, yet they burrow
to a considerable depth during long-continued dry weather and severe
cold.  In Scandinavia, according to Eisen, and in Scotland, according to
Mr. Lindsay Carnagie, the burrows run down to a depth of from 7 to 8
feet; in North Germany, according to Hoffmeister, from 6 to 8 feet, but
Hensen says, from 3 to 6 feet.  This latter observer has seen worms
frozen at a depth of 1½ feet beneath the surface.  I have not myself had
many opportunities for observation, but I have often met with worms at
depths of 3 to 4 feet.  In a bed of fine sand overlying the chalk, which
had never been disturbed, a worm was cut into two at 55 inches, and
another was found here at Down in December at the bottom of its burrow,
at 61 inches beneath the surface.  Lastly, in earth near an old Roman
Villa, which had not been disturbed for many centuries, a worm was met
with at a depth of 66 inches; and this was in the middle of August.

The burrows run down perpendicularly, or more commonly a little
obliquely.  They are said sometimes to branch, but as far as I have seen
this does not occur, except in recently dug ground and near the surface.
They are generally, or as I believe invariably, lined with a thin layer
of fine, dark-coloured earth voided by the worms; so that they must at
first be made a little wider than their ultimate diameter.  I have seen
several burrows in undisturbed sand thus lined at a depth of 4 ft. 6 in.;
and others close to the surface thus lined in recently dug ground.  The
walls of fresh burrows are often dotted with little globular pellets of
voided earth, still soft and viscid; and these, as it appears, are spread
out on all sides by the worm as it travels up or down its burrow.  The
lining thus formed becomes very compact and smooth when nearly dry, and
closely fits the worm’s body.  The minute reflexed bristles which project
in rows on all sides from the body, thus have excellent points of
support; and the burrow is rendered well adapted for the rapid movement
of the animal.  The lining appears also to strengthen the walls, and
perhaps saves the worm’s body from being scratched.  I think so because
several burrows which passed through a layer of sifted coal-cinders,
spread over turf to a thickness of 1½ inch, had been thus lined to an
unusual thickness.  In this case the worms, judging from the castings,
had pushed the cinders away on all sides and had not swallowed any of
them.  In another place, burrows similarly lined, passed through a layer
of coarse coal-cinders, 3½ inches in thickness.  We thus see that the
burrows are not mere excavations, but may rather be compared with tunnels
lined with cement.

The mouths of the burrow are in addition often lined with leaves; and
this is an instinct distinct from that of plugging them up, and does not
appear to have been hitherto noticed.  Many leaves of the Scotch-fir or
pine (_Pinus sylvestris_) were given to worms kept in confinement in two
pots; and when after several weeks the earth was carefully broken up, the
upper parts of three oblique burrows were found surrounded for lengths of
7, 4, and 3½ inches with pine-leaves, together with fragments of other
leaves which had been given the worms as food.  Glass beads and bits of
tile, which had been strewed on the surface of the soil, were stuck into
the interstices between the pine-leaves; and these interstices were
likewise plastered with the viscid castings voided by the worms.  The
structures thus formed cohered so well, that I succeeded in removing one
with only a little earth adhering to it.  It consisted of a slightly
curved cylindrical case, the interior of which could be seen through
holes in the sides and at either end.  The pine-leaves had all been drawn
in by their bases; and the sharp points of the needles had been pressed
into the lining of voided earth.  Had this not been effectually done, the
sharp points would have prevented the retreat of the worms into their
burrows; and these structures would have resembled traps armed with
converging points of wire, rendering the ingress of an animal easy and
its egress difficult or impossible.  The skill shown by these worms is
noteworthy and is the more remarkable, as the Scotch pine is not a native
of this district.

After having examined these burrows made by worms in confinement, I
looked at those in a flower-bed near some Scotch pines.  These had all
been plugged up in the ordinary manner with the leaves of this tree,
drawn in for a length of from 1 to 1½ inch; but the mouths of many of
them were likewise lined with them, mingled with fragments of other kinds
of leaves, drawn in to a depth of 4 or 5 inches.  Worms often remain, as
formerly stated, for a long time close to the mouths of their burrows,
apparently for warmth; and the basket-like structures formed of leaves
would keep their bodies from coming into close contact with the cold damp
earth.  That they habitually rested on the pine-leaves, was rendered
probable by their clean and almost polished surfaces.

The burrows which run far down into the ground, generally, or at least
often, terminate in a little enlargement or chamber.  Here, according to
Hoffmeister, one or several worms pass the winter rolled up into a ball.
Mr. Lindsay Carnagie informed me (1838) that he had examined many burrows
over a stone-quarry in Scotland, where the overlying boulder-clay and
mould had recently been cleared away, and a little vertical cliff thus
left.  In several cases the same burrow was a little enlarged at two or
three points one beneath the other; and all the burrows terminated in a
rather large chamber, at a depth of 7 or 8 feet from the surface.  These
chambers contained many small sharp bits of stone and husks of
flax-seeds.  They must also have contained living seeds, for on the
following spring Mr. Carnagie saw grass-plants sprouting out of some of
the intersected chambers.  I found at Abinger in Surrey two burrows
terminating in similar chambers at a depth of 36 and 41 inches, and these
were lined or paved with little pebbles, about as large as mustard seeds;
and in one of the chambers there was a decayed oat-grain, with its husk.
Hensen likewise states that the bottoms of the burrows are lined with
little stones; and where these could not be procured, seeds, apparently
of the pear, had been used, as many as fifteen having been carried down
into a single burrow, one of which had germinated. {108}  We thus see how
easily a botanist might be deceived who wished to learn how long deeply
buried seeds remained alive, if he were to collect earth from a
considerable depth, on the supposition that it could contain only seeds
which had long lain buried.  It is probable that the little stones, as
well as the seeds, are carried down from the surface by being swallowed;
for a surprising number of glass beads, bits of tile and of glass were
certainly thus carried down by worms kept in pots; but some may have been
carried down within their mouths.  The sole conjecture which I can form
why worms line their winter-quarters with little stones and seeds, is to
prevent their closely coiled-up bodies from coming into close contact
with the surrounding cold soil; and such contact would perhaps interfere
with their respiration which is effected by the skin alone.

A worm after swallowing earth, whether for making its burrow or for food,
soon comes to the surface to empty its body.  The ejected earth is
thoroughly mingled with the intestinal secretions, and is thus rendered
viscid.  After being dried it sets hard.  I have watched worms during the
act of ejection, and when the earth was in a very liquid state it was
ejected in little spurts, and by a slow peristaltic movement when not so
liquid.  It is not cast indifferently on any side, but with some care,
first on one and then on another side; the tail being used almost like a
trowel.  When a worm comes to the surface to eject earth, the tail
protrudes, but when it collects leaves its head must protrude.  Worms
therefore must have the power of turning round in their closely-fitting
burrows; and this, as it appears to us, would be a difficult feat.  As
soon as a little heap has been formed, the worm apparently avoids, for
the sake of safety, protruding its tail; and the earthy matter is forced
up through the previously deposited soft mass.  The mouth of the same
burrow is used for this purpose for a considerable time.  In the case of
the tower-like castings (see Fig. 2) near Nice, and of the similar but
still taller towers from Bengal (hereafter to be described and figured),
a considerable degree of skill is exhibited in their construction.  Dr.
King also observed that the passage up these towers hardly ever ran in
the same exact line with the underlying burrow, so that a thin
cylindrical object such as a haulm of grass, could not be passed down the
tower into the burrow; and this change of direction probably serves in
some manner as a protection.

Worms do not always eject their castings on the surface of the ground.
When they can find any cavity, as when burrowing in newly turned-up
earth, or between the stems of banked-up plants, they deposit their
castings in such places.  So again any hollow beneath a large stone lying
on the surface of the ground, is soon filled up with their castings.
According to Hensen, old burrows are habitually used for this purpose;
but as far as my experience serves, this is not the case, excepting with
those near the surface in recently dug ground.  I think that Hensen may
have been deceived by the walls of old burrows, lined with black earth,
having sunk in or collapsed; for black streaks are thus left, and these
are conspicuous when passing through light-coloured soil, and might be
mistaken for completely filled-up burrows.

It is certain that old burrows collapse in the course of time; for as we
shall see in the next chapter, the fine earth voided by worms, if spread
out uniformly, would form in many places in the course of a year a layer
0.2 of an inch in thickness; so that at any rate this large amount is not
deposited within the old unused burrows.  If the burrows did not
collapse, the whole ground would be first thickly riddled with holes to a
depth of about ten inches, and in fifty years a hollow unsupported space,
ten inches in depth, would be left.  The holes left by the decay of
successively formed roots of trees and plants must likewise collapse in
the course of time.

The burrows of worms run down perpendicularly or a little obliquely, and
where the soil is at all argillaceous, there is no difficulty in
believing that the walls would slowly flow or slide inwards during very
wet weather.  When, however, the soil is sandy or mingled with many small
stones, it can hardly be viscous enough to flow inwards during even the
wettest weather; but another agency may here come into play.  After much
rain the ground swells, and as it cannot expand laterally, the surface
rises; during dry weather it sinks again.  For instance, a large flat
stone laid on the surface of a field sank 3.33 mm. whilst the weather was
dry between May 9th and June 13th, and rose 1.91 mm, between September
7th and 19th of the same year, much rain having fallen during the latter
part of this time.  During frosts and thaws the movements were twice as
great.  These observations were made by my son Horace, who will hereafter
publish an account of the movements of this stone during successive wet
and dry seasons, and of the effects of its being undermined by worms.
Now when the ground swells, if it be penetrated by cylindrical holes,
such as worm-burrows, their walls will tend to yield and be pressed
inwards; and the yielding will be greater in the deeper parts (supposing
the whole to be equally moistened) from the greater weight of the
superincumbent soil which has to be raised, than in the parts near the
surface.  When the ground dries, the walls will shrink a little and the
burrows will be a little enlarged.  Their enlargement, however, through
the lateral contraction of the ground, will not be favoured, but rather
opposed, by the weight of the superincumbent soil.

_Distribution of Worms_.—Earth-worms are found in all parts of the world,
and some of the genera have an enormous range. {113}  They inhabit the
most isolated islands; they abound in Iceland, and are known to exist in
the West Indies, St. Helena, Madagascar, New Caledonia and Tahiti.  In
the Antarctic regions, worms from Kerguelen Land have been described by
Ray Lankester; and I found them in the Falkland Islands.  How they reach
such isolated islands is at present quite unknown.  They are easily
killed by salt-water, and it does not appear probable that young worms or
their egg-capsules could be carried in earth adhering to the feet or
beaks of land-birds.  Moreover Kerguelen Land is not now inhabited by any
land-bird.

In this volume we are chiefly concerned with the earth cast up by worms,
and I have gleaned a few facts on this subject with respect to distant
lands.  Worms throw up plenty of castings in the United States.  In
Venezuela, castings, probably ejected by species of Urochæta, are common
in the gardens and fields, but not in the forests, as I hear from Dr.
Ernst of Caracas.  He collected 156 castings from the court-yard of his
house, having an area of 200 square yards.  They varied in bulk from half
a cubic centimeter to five cubic centimeters, and were on an average
three cubic centimeters.  They were, therefore, of small size in
comparison with those often found in England; for six large castings from
a field near my house averaged 16 cubic centimeters.  Several species of
earth-worms are common in St. Catharina in South Brazil, and Fritz Müller
informs me “that in most parts of the forests and pasture-lands, the
whole soil, to a depth of a quarter of a metre, looks as if it had passed
repeatedly through the intestines of earth-worms, even where hardly any
castings are to be seen on the surface.”  A gigantic but very rare
species is found there, the burrows of which are sometimes even two
centimeters or nearly 0.8 of an inch in diameter, and which apparently
penetrate the ground to a great depth.

In the dry climate of New South Wales, I hardly expected that worms would
be common; but Dr. G. Krefft of Sydney, to whom I applied, after making
inquiries from gardeners and others, and from his own observations,
informs me that their castings abound.  He sent me some collected after
heavy rain, and they consisted of little pellets, about 0.15 inch in
diameter; and the blackened sandy earth of which they were formed still
cohered with considerable tenacity.

The late Mr. John Scott of the Botanic Gardens near Calcutta made many
observations for me on worms living under the hot and humid climate of
Bengal.  The castings abound almost everywhere, in jungles and in the
open ground, to a greater degree, as he thinks, than in England.  After
the water has subsided from the flooded rice-fields, the whole surface
very soon becomes studded with castings—a fact which much surprised Mr.
Scott, as he did not know how long worms could survive beneath water.
They cause much trouble in the Botanic garden, “for some of the finest of
our lawns can be kept in anything like order only by being almost daily
rolled; if left undisturbed for a few days they become studded with large
castings.”  These closely resemble those described as abounding near
Nice; and they are probably the work of a species of Perichæta.  They
stand up like towers, with an open passage in the centre.

   [Picture: Fig. 3: A tower-like casting.  Fig. 4: A casting from the
                            Nilgiri Mountains]

A figure of one of these castings from a photograph is here given (Fig.
3).  The largest received by me was 3½ inches in height and 1.35 inch in
diameter; another was only ¾ inch in diameter and 2¾ in height.  In the
following year, Mr. Scott measured several of the largest; one was 6
inches in height and nearly 1½ in diameter: two others were 5 inches in
height and respectively 2 and rather more than 2½ inches in diameter.
The average weight of the 22 castings sent to me was 35 grammes (1¼ oz.);
and one of them weighed 44.8 grammes (or 2 oz.). All these castings were
thrown up either in one night or in two.  Where the ground in Bengal is
dry, as under large trees, castings of a different kind are found in vast
numbers: these consist of little oval or conical bodies, from about the
1/20 to rather above 1/10 of an inch in length.  They are obviously
voided by a distinct species of worms.

The period during which worms near Calcutta display such extraordinary
activity lasts for only a little over two months, namely, during the cool
season after the rains.  At this time they are generally found within
about 10 inches beneath the surface.  During the hot season they burrow
to a greater depth, and are then found coiled up and apparently
hybernating.  Mr. Scott has never seen them at a greater depth than 2½
feet, but has heard of their having been found at 4 feet.  Within the
forests, fresh castings may be found even during the hot season.  The
worms in the Botanic garden, during the cool and dry season, draw many
leaves and little sticks into the mouths of their burrows, like our
English worms; but they rarely act in this manner during the rainy
season.

Mr. Scott saw worm-castings on the lofty mountains of Sikkim in North
India.  In South India Dr. King found in one place, on the plateau of the
Nilgiris, at an elevation of 7000 feet, “a good many castings,” which are
interesting for their great size.  The worms which eject them are seen
only during the wet season, and are reported to be from 12 to 15 inches
in length, and as thick as a man’s little finger.  These castings were
collected by Dr. King after a period of 110 days without any rain; and
they must have been ejected either during the north-east or more probably
during the previous south-west monsoon; for their surfaces had suffered
some disintegration and they were penetrated by many fine roots.  A
drawing is here given (Fig. 4) of one which seems to have best retained
its original size and appearance.  Notwithstanding some loss from
disintegration, five of the largest of these castings (after having been
well sun-dried) weighed each on an average 89.5 grammes, or above 3 oz.;
and the largest weighed 123.14 grammes, or 4⅓ oz.,—that is, above a
quarter of a pound!  The largest convolutions were rather more than one
inch in diameter; but it is probable that they had subsided a little
whilst soft, and that their diameters had thus been increased.  Some had
flowed so much that they now consisted of a pile of almost flat confluent
cakes.  All were formed of fine, rather light-coloured earth, and were
surprisingly hard and compact, owing no doubt to the animal matter by
which the particles of earth had been cemented together.  They did not
disintegrate, even when left for some hours in water.  Although they had
been cast up on the surface of gravelly soil, they contained extremely
few bits of rock, the largest of which was only 0.15 inch in diameter.

Dr. King saw in Ceylon a worm about 2 feet in length and ½ inch in
diameter; and he was told that it was a very common species during the
wet season.  These worms must throw up castings at least as large as
those on the Nilgiri Mountains; but Dr. King saw none during his short
visit to Ceylon.

Sufficient facts have now been given, showing that worms do much work in
bringing up fine earth to the surface in most or all parts of the world,
and under the most different climates.




CHAPTER III.
THE AMOUNT OF FINE EARTH BROUGHT UP BY WORMS TO THE SURFACE.


Rate at which various objects strewed on the surface of grass-fields are
covered up by the castings of worms—The burial of a paved path—The slow
subsidence of great stones left on the surface—The number of worms which
live within a given space—The weight of earth ejected from a burrow, and
from all the burrows within a given space—The thickness of the layer of
mould which the castings on a given space would form within a given time
if uniformly spread out—The slow rate at which mould can increase to a
great thickness—Conclusion.

WE now come to the more immediate subject of this volume, namely, the
amount of earth which is brought up by worms from beneath the surface,
and is afterwards spread out more or less completely by the rain and
wind.  The amount can be judged of by two methods,—by the rate at which
objects left on the surface are buried, and more accurately by weighing
the quantity brought up within a given time.  We will begin with the
first method, as it was first followed.

Near Mael Hall in Staffordshire, quick-lime had been spread about the
year 1827 thickly over a field of good pasture-land, which had not since
been ploughed.  Some square holes were dug in this field in the beginning
of October 1837; and the sections showed a layer of turf, formed by the
matted roots of the grasses, ½ inch in thickness, beneath which, at a
depth of 2½ inches (or 3 inches from the surface), a layer of the lime in
powder or in small lumps could be distinctly seen running all round the
vertical sides of the holes.  The soil beneath the layer of lime was
either gravelly or of a coarse sandy nature, and differed considerably in
appearance from the overlying dark-coloured fine mould.  Coal-cinders had
been spread over a part of this same field either in the year 1833 or
1834; and when the above holes were dug, that is after an interval of 3
or 4 years, the cinders formed a line of black spots round the holes, at
a depth of 1 inch beneath the surface, parallel to and above the white
layer of lime.  Over another part of this field cinders had been strewed,
only about half-a-year before, and these either still lay on the surface
or were entangled among the roots of the grasses; and I here saw the
commencement of the burying process, for worm-castings had been heaped on
several of the smaller fragments.  After an interval of 4¾ years this
field was re-examined, and now the two layers of lime and cinders were
found almost everywhere at a greater depth than before by nearly 1 inch,
we will say by ¾ of an inch.  Therefore mould to an average thickness of
0.22 of an inch had been annually brought up by the worms, and had been
spread over the surface of this field.

Coal-cinders had been strewed over another field, at a date which could
not be positively ascertained, so thickly that they formed (October,
1837) a layer, 1 inch in thickness at a depth of about 3 inches from the
surface.  The layer was so continuous that the over-lying dark vegetable
mould was connected with the sub-soil of red clay only by the roots of
the grasses; and when these were broken, the mould and the red clay fell
apart.  In a third field, on which coal-cinders and burnt marl had been
strewed several times at unknown dates, holes were dug in 1842; and a
layer of cinders could be traced at a depth of 3½ inches, beneath which
at a depth of 9½ inches from the surface there was a line of cinders
together with burnt marl.  On the sides of one hole there were two layers
of cinders, at 2 and 3½ inches beneath the surface; and below them at a
depth in parts of 9½, and in other parts of 10½ inches there were
fragments of burnt marl.  In a fourth field two layers of lime, one above
the other, could be distinctly traced, and beneath them a layer of
cinders and burnt marl at a depth of from 10 to 12 inches below the
surface.

  [Picture: Fig. 5: Section of the vegetable mould in a field.  Fig. 6:
                  Traverse section across a large stone]

A piece of waste, swampy land was enclosed, drained, ploughed, harrowed
and thickly covered in the year 1822 with burnt marl and cinders.  It was
sowed with grass seeds, and now supports a tolerably good but coarse
pasture.  Holes were dug in this field in 1837, or 15 years after its
reclamation, and we see in the accompanying diagram (Fig. 5), reduced to
half of the natural scale, that the turf was ½ inch thick, beneath which
there was a layer of vegetable mould 2½ inches thick.  This layer did not
contain fragments of any kind; but beneath it there was a layer of mould,
1½ inch in thickness, full of fragments of burnt marl, conspicuous from
their red colour, one of which near the bottom was an inch in length; and
other fragments of coal-cinders together with a few white quartz pebbles.
Beneath this layer and at a depth of 4½ inches from the surface, the
original black, peaty, sandy soil with a few quartz pebbles was
encountered.  Here therefore the fragments of burnt marl and cinders had
been covered in the course of 15 years by a layer of fine vegetable
mould, only 2½ inches in thickness, excluding the turf.  Six and a half
years subsequently this field was re-examined, and the fragments were now
found at from 4 to 5 inches beneath the surface.  So that in this
interval of 6½ years, about 1½ inch of mould had been added to the
superficial layer.  I am surprised that a greater quantity had not been
brought up during the whole 21½ years, for in the closely underlying
black, peaty soil there were many worms.  It is, however, probable that
formerly, whilst the land remained poor, worms were scanty; and the mould
would then have accumulated slowly.  The average annual increase of
thickness for the whole period is 0.19 of an inch.

Two other cases are worth recording.  In the spring of 1835, a field,
which had long existed as poor pasture and was so swampy that it trembled
slightly when stamped on, was thickly covered with red sand so that the
whole surface appeared at first bright red.  When holes were dug in this
field after an interval of about 2½ years, the sand formed a layer at a
depth of ¾ in. beneath the surface.  In 1842 (i.e., 7 years after the
sand had been laid on) fresh holes were dug, and now the red sand formed
a distinct layer, 2 inches beneath the surface, or 1½ inch beneath the
turf; so that on an average, 0.21 inch of mould had been annually brought
to the surface.  Immediately beneath the layer of red sand, the original
substratum of black sandy peat extended.

A grass field, likewise not far from Maer Hall, had formerly been thickly
covered with marl, and was then left for several years as pasture; it was
afterwards ploughed.  A friend had three trenches dug in this field 28
years after the application of the marl, {126} and a layer of the marl
fragments could be traced at a depth, carefully measured, of 12 inches in
some parts, and of 14 inches in other parts.  This difference in depth
depended on the layer being horizontal, whilst the surface consisted of
ridges and furrows from the field having been ploughed.  The tenant
assured me that it had never been turned up to a greater depth than from
6 to 8 inches; and as the fragments formed an unbroken horizontal layer
from 12 to 14 inches beneath the surface, these must have been buried by
the worms whilst the land was in pasture before it was ploughed, for
otherwise they would have been indiscriminately scattered by the plough
throughout the whole thickness of the soil.  Four-and-a-half years
afterwards I had three holes dug in this field, in which potatoes had
been lately planted, and the layer of marl-fragments was now found 13
inches beneath the bottoms of the furrows, and therefore probably 15
inches beneath the general level of the field.  It should, however, be
observed that the thickness of the blackish sandy soil, which had been
thrown up by the worms above the marl-fragments in the course of 32½
years, would have measured less than 15 inches, if the field had always
remained as pasture, for the soil would in this case have been much more
compact.  The fragments of marl almost rested on an undisturbed
substratum of white sand with quartz pebbles; and as this would be little
attractive to worms, the mould would hereafter be very slowly increased
by their action.

We will now give some cases of the action of worms, on land differing
widely from the dry sandy or the swampy pastures just described.  The
chalk formation extends all round my house in Kent; and its surface, from
having been exposed during an immense period to the dissolving action of
rain-water, is extremely irregular, being abruptly festooned and
penetrated by many deep well-like cavities. {128}  During the dissolution
of the chalk, the insoluble matter, including a vast number of unrolled
flints of all sizes, has been left on the surface and forms a bed of
stiff red clay, full of flints, and generally from 6 to 14 feet in
thickness.  Over the red clay, wherever the land has long remained as
pasture, there is a layer a few inches in thickness, of dark-coloured
vegetable mould.

A quantity of broken chalk was spread, on December 20, 1842, over a part
of a field near my house, which had existed as pasture certainly for 30,
probably for twice or thrice as many years.  The chalk was laid on the
land for the sake of observing at some future period to what depth it
would become buried.  At the end of November, 1871, that is after an
interval of 29 years, a trench was dug across this part of the field; and
a line of white nodules could be traced on both sides of the trench, at a
depth of 7 inches from the surface.  The mould, therefore, (excluding the
turf) had here been thrown up at an average rate of 0.22 inch per year.
Beneath the line of chalk nodules there was in parts hardly any fine
earth free of flints, while in other parts there was a layer, 2¼ inches
in thickness.  In this latter case the mould was altogether 9¼ inches
thick; and in one such spot a nodule of chalk and a smooth flint pebble,
both of which must have been left at some former time on the surface,
were found at this depth.  At from 11 to 12 inches beneath the surface,
the undisturbed reddish clay, full of flints, extended.  The appearance
of the above nodules of chalk surprised me, much at first, as they
closely resembled water-worn pebbles, whereas the freshly-broken
fragments had been angular.  But on examining the nodules with a lens,
they no longer appeared water-worn, for their surfaces were pitted
through unequal corrosion, and minute, sharp points, formed of broken
fossil shells, projected from them.  It was evident that the corners of
the original fragments of chalk had been wholly dissolved, from
presenting a large surface to the carbonic acid dissolved in the
rain-water and to that generated in soil containing vegetable matter, as
well as to the humus-acids. {131}  The projecting corners would also,
relatively to the other parts, have been embraced by a larger number of
living rootlets; and these have the power of even attacking marble, as
Sachs has shown.  Thus, in the course of 29 years, buried angular
fragments of chalk had been converted into well-rounded nodules.

Another part of this same field was mossy, and as it was thought that
sifted coal-cinders would improve the pasture, a thick layer was spread
over this part either in 1842 or 1843, and another layer some years
afterwards.  In 1871 a trench was here dug, and many cinders lay in a
line at a depth of 7 inches beneath the surface, with another line at a
depth of 5½ inches parallel to the one beneath.  In another part of this
field, which had formerly existed as a separate one, and which it was
believed had been pasture-land for more than a century, trenches were dug
to see how thick the vegetable mould was.  By chance the first trench was
made at a spot where at some former period, certainly more than forty
years before, a large hole had been filled up with coarse red clay,
flints, fragments of chalk, and gravel; and here the fine vegetable mould
was only from 4⅛ to 4⅜ inches in thickness.  In another and undisturbed
place, the mould varied much in thickness, namely, from 6½ to 8½ inches;
beneath which a few small fragments of brick were found in one place.
From these several cases, it would appear that during the last 29 years
mould has been heaped on the surface at an average annual rate of from
0.2 to 0.22 of an inch.  But in this district when a ploughed field is
first laid down in grass, the mould accumulates at a much slower rate.
The rate, also, must become very much slower after a bed of mould,
several inches in thickness, has been formed; for the worms then live
chiefly near the surface, and burrow down to a greater depth so as to
bring up fresh earth from below, only during the winter when the weather
is very cold (at which time worms were found in this field at a depth of
26 inches) and during summer, when the weather is very dry.

A field, which adjoins the one just described, slopes in one part rather
steeply (viz., at from 10° to 15°); this part was last ploughed in 1841,
was then harrowed and left to become pasture-land.  For several years it
was clothed with an extremely scant vegetation, and was so thickly
covered with small and large flints (some of them half as large as a
child’s head) that the field was always called by my sons “the stony
field.”  When they ran down the slope the stones clattered together, I
remember doubting whether I should live to see these larger flints
covered with vegetable mould and turf.  But the smaller stones
disappeared before many years had elapsed, as did every one of the larger
ones after a time; so that after thirty years (1871) a horse could gallop
over the compact turf from one end of the field to the other, and not
strike a single stone with his shoes.  To anyone who remembered the
appearance of the field in 1842, the transformation was wonderful.  This
was certainly the work of the worms, for though castings were not
frequent for several years, yet some were thrown up month after month,
and these gradually increased in numbers as the pasture improved.  In the
year 1871 a trench was dug on the above slope, and the blades of grass
were cut off close to the roots, so that the thickness of the turf and of
the vegetable mould could be measured accurately.  The turf was rather
less than half an inch, and the mould, which did not contain any stones,
2½ inches in thickness.  Beneath this lay coarse clayey earth full of
flints, like that in any of the neighbouring ploughed fields.  This
coarse earth easily fell apart from the overlying mould when a spit was
lifted up.  The average rate of accumulation of the mould during the
whole thirty years was only .083 inch per year (i.e., nearly one inch in
twelve years); but the rate must have been much slower at first, and
afterwards considerably quicker.

The transformation in the appearance of this field, which had been
effected beneath my eyes, was afterwards rendered the more striking, when
I examined in Knole Park a dense forest of lofty beech-trees, beneath
which nothing grew.  Here the ground was thickly strewed with large naked
stones, and worm-castings were almost wholly absent.  Obscure lines and
irregularities on the surface indicated that the land had been cultivated
some centuries ago.  It is probable that a thick wood of young
beech-trees sprung up so quickly, that time enough was not allowed for
worms to cover up the stones with their castings, before the site became
unfitted for their existence.  Anyhow the contrast between the state of
the now miscalled “stony field,” well stocked with worms, and the present
state of the ground beneath the old beech-trees in Knole Park, where
worms appeared to be absent, was striking.

A narrow path running across part of my lawn was paved in 1843 with small
flagstones, set edgeways; but worms threw up many castings and weeds grew
thickly between them.  During several years the path was weeded and
swept; but ultimately the weeds and worms prevailed, and the gardener
ceased to sweep, merely mowing off the weeds, as often as the lawn was
mowed.  The path soon became almost covered up, and after several years
no trace of it was left.  On removing, in 1877, the thin overlying layer
of turf, the small flag-stones, all in their proper places, were found
covered by an inch of fine mould.

Two recently published accounts of substances strewed on the surface of
pasture-land, having become buried through the action of worms, may be
here noticed.  The Rev. H. C. Key had a ditch cut in a field, over which
coal-ashes had been spread, as it was believed, eighteen years before;
and on the clean-cut perpendicular sides of the ditch, at a depth of at
least seven inches, there could be seen, for a length of 60 yards, “a
distinct, very even, narrow line of coal-ashes, mixed with small coal,
perfectly parallel with the top-sward.” {136a}  This parallelism and the
length of the section give interest to the case.  Secondly, Mr. Dancer
states {136b} that crushed bones had been thickly strewed over a field;
and “some years afterwards” these were found “several inches below the
surface, at a uniform depth.”

The Rev. Mr. Zincke informs me that he has lately had an orchard dug to
the unusual depth of 4 feet.  The upper 18 inches consisted of
dark-coloured vegetable mould, and the next 18 inches of sandy loam,
containing in the lower part many rolled pieces of sandstone, with some
bits of brick and tile, probably of Roman origin, as remains of this
period have been found close by.  The sandy loam rested on an indurated
ferruginous pan of yellow clay, on the surface of which two perfect celts
were found.  If, as seems probable, the celts were originally left on the
surface of the land, they have since been covered up with earth 3 feet in
thickness, all of which has probably passed through the bodies of worms,
excepting the stones which may have been scattered on the surface at
different times, together with manure or by other means.  It is difficult
otherwise to understand the source of the 18 inches of sandy loam, which
differed from the overlying dark vegetable mould, after both had been
burnt, only in being of a brighter red colour, and in not being quite so
fine-grained.  But on this view we must suppose that the carbon in
vegetable mould, when it lies at some little depth beneath the surface
and does not continually receive decaying vegetable matter from above,
loses its dark colour in the course of centuries; but whether this is
probable I do not know.

Worms appear to act in the same manner in New Zealand as in Europe; for
Professor J. von Haast has described {138a} a section near the coast,
consisting of mica-schist, “covered by 5 or 6 feet of loess, above which
about 12 inches of vegetable soil had accumulated.”  Between the loess
and the mould there was a layer from 3 to 6 inches in thickness,
consisting of “cores, implements, flakes, and chips, all manufactured
from hard basaltic rock.”  It is therefore probable that the aborigines,
at some former period, had left these objects on the surface, and that
they had afterwards been slowly covered up by the castings of worms.

Farmers in England are well aware that objects of all kinds, left on the
surface of pasture-land, after a time disappear, or, as they say, work
themselves downwards.  How powdered lime, cinders, and heavy stones, can
work down, and at the same rate, through the matted roots of a
grass-covered surface, is a question which has probably never occurred to
them. {138b}

_The Sinking of great Stones through the Action of Worms_.—When a stone
of large size and of irregular shape is left on the surface of the
ground, it rests, of course, on the more protuberant parts; but worms
soon fill up with their castings all the hollow spaces on the lower side;
for, as Hensen remarks, they like the shelter of stones.  As soon as the
hollows are filled up, the worms eject the earth which they have
swallowed beyond the circumference of the stones; and thus the surface of
the ground is raised all round the stone.  As the burrows excavated
directly beneath the stone after a time collapse, the stone sinks a
little. {139}  Hence it is, that boulders which at some ancient period
have rolled down from a rocky mountain or cliff on to a meadow at its
base, are always somewhat imbedded in the soil; and, when removed, leave
an exact impression of their lower surfaces in the underlying fine mould.
If, however, a boulder is of such huge dimensions, that the earth beneath
is kept dry, such earth will not be inhabited by worms, and the boulder
will not sink into the ground.

A lime-kiln formerly stood in a grass-field near Leith Hill Place in
Surrey, and was pulled down 35 years before my visit; all the loose
rubbish had been carted away, excepting three large stones of quartzose
sandstone, which it was thought might hereafter be of some use.  An old
workman remembered that they had been left on a bare surface of broken
bricks and mortar, close to the foundations of the kiln; but the whole
surrounding surface is now covered with turf and mould.  The two largest
of these stones had never since been moved; nor could this easily have
been done, as, when I had them removed, it was the work of two men with
levers.  One of these stones, and not the largest, was 64 inches long, 17
inches broad, and from 9 to 10 inches in thickness.  Its lower surface
was somewhat protuberant in the middle; and this part still rested on
broken bricks and mortar, showing the truth of the old workman’s account.
Beneath the brick rubbish the natural sandy soil, full of fragments of
sandstone was found; and this could have yielded very little, if at all,
to the weight of the stone, as might have been expected if the sub-soil
had been clay.  The surface of the field, for a distance of about 9
inches round the stone, gradually sloped up to it, and close to the stone
stood in most places about 4 inches above the surrounding ground.  The
base of the stone was buried from 1 to 2 inches beneath the general
level, and the upper surface projected about 8 inches above this level,
or about 4 inches above the sloping border of turf.  After the removal of
the stone it became evident that one of its pointed ends must at first
have stood clear above the ground by some inches, but its upper surface
was now on a level with the surrounding turf.  When the stone was
removed, an exact cast of its lower side, forming a shallow crateriform
hollow, was left, the inner surface of which consisted of fine black
mould, excepting where the more protuberant parts rested on the
brick-rubbish.  A transverse section of this stone, together with its
bed, drawn from measurements made after it had been displaced, is here
given on a scale of ½ inch to a foot (Fig. 6).  The turf-covered border
which sloped up to the stone, consisted of fine vegetable mould, in one
part 7 inches in thickness.  This evidently consisted of worm-castings,
several of which had been recently ejected.  The whole stone had sunk in
the thirty-five years, as far as I could judge, about 1½ inch; and this
must have been due to the brick-rubbish beneath the more protuberant
parts having been undermined by worms.  At this rate the upper surface of
the stone, if it had been left undisturbed, would have sunk to the
general level of the field in 247 years; but before this could have
occurred, some earth would have been washed down by heavy rain from the
castings on the raised border of turf over the upper surface of the
stone.

The second stone was larger that the one just described, viz., 67 inches
in length, 39 in breadth, and 15 in thickness.  The lower surface was
nearly flat, so that the worms must soon have been compelled to eject
their castings beyond its circumference.  The stone as a whole had sunk
about 2 inches into the ground.  At this rate it would have required 262
years for its upper surface to have sunk to the general level of the
field.  The upwardly sloping, turf-covered border round the stone was
broader than in the last case, viz., from 14 to 16 inches; and why this
should be so, I could see no reason.  In most parts this border was not
so high as in the last case, viz., from 2 to 2½ inches, but in one place
it was as much as 5½.  Its average height close to the stone was probably
about 3 inches, and it thinned out to nothing.  If so, a layer of fine
earth, 15 inches in breadth and 1½ inch in average thickness, of
sufficient length to surround the whole of the much elongated slab, must
have been brought up by the worms in chief part from beneath the stone in
the course of 35 years.  This amount would be amply sufficient to account
for its having sunk about 2 inches into the ground; more especially if we
bear in mind that a good deal of the finest earth would have been washed
by heavy rain from the castings ejected on the sloping border down to the
level of the field.  Some fresh castings were seen close to the stone.
Nevertheless, on digging a large hole to a depth of 18 inches where the
stone had lain, only two worms and a few burrows were seen, although the
soil was damp and seemed favourable for worms.  There were some large
colonies of ants beneath the stone, and possibly since their
establishment the worms had decreased in number.

The third stone was only about half as large as the others; and two
strong boys could together have rolled it over.  I have no doubt that it
had been rolled over at a moderately recent time, for it now lay at some
distance from the two other stones at the bottom of a little adjoining
slope.  It rested also on fine earth, instead of partly on brick-rubbish.
In agreement with this conclusion, the raised surrounding border of turf
was only 1 inch high in some parts, and 2 inches in other parts.  There
were no colonies of ants beneath this stone, and on digging a hole where
it had lain, several burrows and worms were found.

At Stonehenge, some of the outer Druidical stones are now prostrate,
having fallen at a remote but unknown period; and these have become
buried to a moderate depth in the ground.  They are surrounded by sloping
borders of turf, on which recent castings were seen.  Close to one of
these fallen stones, which was 17 ft long, 6 ft. broad, and 28½ inches
thick, a hole was dug; and here the vegetable mould was at least 9½
inches in thickness.  At this depth a flint was found, and a little
higher up on one side of the hole a fragment of glass.  The base of the
stone lay about 9½ inches beneath the level of the surrounding ground,
and its upper surface 19 inches above the ground.

A hole was also dug close to a second huge stone, which in falling had
broken into two pieces; and this must have happened long ago, judging
from the weathered aspect of the fractured ends.  The base was buried to
a depth of 10 inches, as was ascertained by driving an iron skewer
horizontally into the ground beneath it.  The vegetable mould forming the
turf-covered sloping border round the stone, on which many castings had
recently been ejected, was 10 inches in thickness; and most of this mould
must have been brought up by worms from beneath its base.  At a distance
of 8 yards from the stone, the mould was only 5½ inches in thickness
(with a piece of tobacco pipe at a depth of 4 inches), and this rested on
broken flint and chalk which could not have easily yielded to the
pressure or weight of the stone.

A straight rod was fixed horizontally (by the aid of a spirit-level)
across a third fallen stone, which was 7 feet 9 inches long; and the
contour of the projecting parts and of the adjoining ground, which was
not quite level, was thus ascertained, as shown in the accompanying
diagram (Fig. 7) on a scale of ½ inch to a foot.  The turf-covered border
sloped up to the stone on one side to a height of 4 inches, and on the
opposite side to only 2½ inches above the general level.  A hole was dug
on the eastern side, and the base of the stone was here found to lie at a
depth of 4 inches beneath the general level of the ground, and of 8
inches beneath the top of the sloping turf-covered border.

                                * * * * *

Sufficient evidence has now been given showing that small objects left on
the surface of the land where worms abound soon get buried, and that
large stones sink slowly downwards through the same means.  Every step of
the process could be followed, from the accidental deposition of a single
casting on a small object lying loose on the surface, to its being
entangled amidst the matted roots of the turf, and lastly to its being
embedded in the mould at various depths beneath the surface.  When the
same field was re-examined after the interval of a few years, such
objects were found at a greater depth than before.  The straightness and
regularity of the lines formed by the imbedded objects, and their
parallelism with the surface of the land, are the most striking features
of the case; for this parallelism shows how equably the worms must have
worked; the result being, however, partly the effect of the washing down
of the fresh castings by rain.  The specific gravity of the objects does
not affect their rate of sinking, as could be seen by porous cinders,
burnt marl, chalk and quartz pebbles, having all sunk to the same depth
within the same time.  Considering the nature of the substratum, which at
Leith Hill Place was sandy soil including many bits of rock, and at
Stonehenge, chalk-rubble with broken flints; considering, also, the
presence of the turf-covered sloping border of mould round the great
fragments of stone at both these places, their sinking does not appear to
have been sensibly aided by their weight, though this was considerable.
{147}

_On the number of worms which live within a given space_.—We will now
show, firstly, what a vast number of worms live unseen by us beneath our
feet, and, secondly, the actual weight of the earth which they bring up
to the surface within a given space and within a given time.  Hensen, who
has published so full and interesting an account of the habits of worms,
{148} calculates, from the number which he found in a measured space,
that there must exist 133,000 living worms in a hectare of land, or
53,767 in an acre.  This latter number of worms would weigh 356 pounds,
taking Hensen’s standard of the weight of a single worm, namely, three
grams.  It should, however, be noted that this calculation is founded on
the numbers found in a garden, and Hensen believes that worms are here
twice as numerous as in corn-fields.  The above result, astonishing
though it be, seems to me credible, judging from the number of worms
which I have sometimes seen, and from the number daily destroyed by birds
without the species being exterminated.  Some barrels of bad ale were
left on Mr. Miller’s land, {149} in the hope of making vinegar, but the
vinegar proved bad, and the barrels were upset.  It should be premised
that acetic acid is so deadly a poison to worms that Perrier found that a
glass rod dipped into this acid and then into a considerable body of
water in which worms were immersed, invariably killed them quickly.  On
the morning after the barrels had been upset, “the heaps of worms which
lay dead on the ground were so amazing, that if Mr. Miller had not seen
them, he could not have thought it possible for such numbers to have
existed in the space.”  As further evidence of the large number of worms
which live in the ground, Hensen states that he found in a garden
sixty-four open burrows in a space of 14½ square feet, that is, nine in 2
square feet.  But the burrows are sometimes much more numerous, for when
digging in a grass-field near Maer Hall, I found a cake of dry earth, as
large as my two open hands, which was penetrated by seven burrows, as
large as goose-quills.

_Weight of the earth ejected from a single burrow_, _and from all the
burrows within a given space_.—With respect to the weight of the earth
daily ejected by worms, Hensen found that it amounted, in the case of
some worms which he kept in confinement, and which he appears to have fed
with leaves, to only 0.5 gram, or less than 8 grains per diem.  But a
very much larger amount must be ejected by worms in their natural state,
at the periods when they consume earth as food instead of leaves, and
when they are making deep burrows.  This is rendered almost certain by
the following weights of the castings thrown up at the mouths of single
burrows; the whole of which appeared to have been ejected within no long
time, as was certainly the case in several instances.  The castings were
dried (excepting in one specified instance) by exposure during many days
to the sun or before a hot fire.

 WEIGHT OF THE CASTINGS ACCUMULATED AT THE MOUTH OF A SINGLE BURROW.
(1.)  Down, Kent (sub-soil red clay, full of flints,              3.98
over-lying the chalk).  The largest casting which I could
find on the flanks of a steep valley, the sub-soil being
here shallow.  In this one case, the casting was not well
dried
(2.)  Down.—Largest casting which I could find (consisting        3.87
chiefly of calcareous matter), on extremely poor pasture
land at the bottom of the valley mentioned under (1.)
(3.)  Down.—A large casting, but not of unusual size, from        1.22
a nearly level field, poor pasture, laid down in a grass
about 35 years before
(4.)  Down.  Average weight of 11 not large castings               0.7
ejected on a sloping surface on my lawn, after they had
suffered some loss of weight from being exposed during a
considerable length of time to rain
(5.)  Near Nice in France.—Average weight of 12 castings of       1.37
ordinary dimensions, collected by Dr. King on land which
had not been mown for a long time and where worms abounded,
viz., a lawn protected by shrubberies near the sea; soil
sandy and calcareous; these castings had been exposed for
some time to rain, before being collected, and must have
lost some weight by disintegration, but they still retained
their form
(6.)  The heaviest of the above twelve castings                   1.76
(7.)   Lower Bengal.—Average weight of 22 castings,               1.24
collected by Mr. J. Scott, and stated by him to have been
thrown up in the course of one or two nights
(8.)  The heaviest of the above 22 castings                       2.09
(9.)  Nilgiri Mountains, S. India; average weight of the 5        3.15
largest castings collected by Dr. King.  They had been
exposed to the rain of the last monsoon, and must have lost
some weight
(10.)  The heaviest of the above 5 castings                       4.34

In this table we see that castings which had been ejected at the mouth of
the same burrow, and which in most cases appeared fresh and always
retained their vermiform configuration, generally exceeded an ounce in
weight after being dried, and sometimes nearly equalled a quarter of a
pound.  On the Nilgiri mountains one casting even exceeded this latter
weight.  The largest castings in England were found on extremely poor
pasture-land; and these, as far as I have seen, are generally larger than
those on land producing a rich vegetation.  It would appear that worms
have to swallow a greater amount of earth on poor than on rich land, in
order to obtain sufficient nutriment.

With respect to the tower-like castings near Nice (Nos. 5 and 6 in the
above table), Dr. King often found five or six of them on a square foot
of surface; and these, judging from their average weight, would have
weighed together 7½ ounces; so that the weight of those on a square yard
would have been 4 lb. 3½ oz.  Dr. King collected, near the close of the
year 1872, all the castings which still retained their vermiform shape,
whether broken down or not, from a square foot, in a place abounding with
worms, on the summit of a bank, where no castings could have rolled down
from above.  These castings must have been ejected, as he judged from
their appearance in reference to the rainy and dry periods near Nice,
within the previous five or six months; they weighed 9½ oz., or 5 lb. 5½
oz. per square yard.  After an interval of four months, Dr. King
collected all the castings subsequently ejected on the same square foot
of surface, and they weighed 2½ oz., or 1 lb. 6½ oz. per square yard.
Therefore within about ten months, or we will say for safety’s sake
within a year, 12 oz. of castings were thrown up on this one square foot,
or 6.75 pounds on the square yard; and this would give 14.58 tons per
acre.

In a field at the bottom of a valley in the chalk (see No. 2 in the
foregoing table), a square yard was measured at a spot where very large
castings abounded; they appeared, however, almost equally numerous in a
few other places.  These castings, which retained perfectly their
vermiform shape, were collected; and they weighed when partially dried, 1
lb. 13½ oz.  This field had been rolled with a heavy agricultural roller
fifty-two days before, and this would certainly have flattened every
single casting on the land.  The weather had been very dry for two or
three weeks before the day of collection, so that not one casting
appeared fresh or had been recently ejected.  We may therefore assume
that those which were weighed had been ejected within, we will say, forty
days from the time when the field was rolled,—that is, twelve days short
of the whole intervening period.  I had examined the same part of the
field shortly before it was rolled, and it then abounded with fresh
castings.  Worms do not work in dry weather during the summer, or in
winter during severe frosts.  If we assume that they work for only half
the year—though this is too low an estimate—then the worms in this field
would eject during the year, 8.387 pounds per square yard; or 18.12 tons
per acre, assuming the whole surface to be equally productive in
castings.

In the foregoing cases some of the necessary data had to be estimated,
but in the two following cases the results are much more trustworthy.  A
lady, on whose accuracy I can implicitly rely, offered to collect during
a year all the castings thrown up on two separate square yards, near
Leith Hill Place, in Surrey.  The amount collected was, however, somewhat
less than that originally ejected by the worms; for, as I have repeatedly
observed, a good deal of the finest earth is washed away, whenever
castings are thrown up during or shortly before heavy rain.  Small
portions also adhered to the surrounding blades of grass, and it required
too much time to detach every one of them.

On sandy soil, as in the present instance, castings are liable to crumble
after dry weather, and particles were thus often lost.  The lady also
occasionally left home for a week or two, and at such times the castings
must have suffered still greater loss from exposure to the weather.
These losses were, however, compensated to some extent by the collections
having been made on one of the squares for four days, and on the other
square for two days more than the year.

A space was selected (October 9th, 1870) for one of the squares on a
broad, grass-covered terrace, which had been mowed and swept during many
years.  It faced the south, but was shaded during part of the day by
trees.  It had been formed at least a century ago by a great accumulation
of small and large fragments of sandstone, together with some sandy
earth, rammed down level.  It is probable that it was at first protected
by being covered with turf.  This terrace, judging from the number of
castings on it, was rather unfavourable for the existence of worms, in
comparison with the neighbouring fields and an upper terrace.  It was
indeed surprising that as many worms could live here as were seen; for on
digging a hole in this terrace, the black vegetable mould together with
the turf was only four inches in thickness, beneath which lay the level
surface of light-coloured sandy soil, with many fragments of sandstone.
Before any castings were collected all the previously existing ones were
carefully removed.  The last day’s collection was on October 14th, 1871.
The castings were then well dried before a fire; and they weighed exactly
3½ lbs.  This would give for an acre of similar land 7.56 tons of dry
earth annually ejected by worms.

The second square was marked on unenclosed common land, at a height of
about 700 ft. above the sea, at some little distance from Leith Hill
Tower.  The surface was clothed with short, fine turf, and had never been
disturbed by the hand of man.  The spot selected appeared neither
particularly favourable nor the reverse for worms; but I have often
noticed that castings are especially abundant on common land, and this
may, perhaps, be attributed to the poorness of the soil.  The vegetable
mould was here between three and four inches in thickness.  As this spot
was at some distance from the house where the lady lived, the castings
were not collected at such short intervals of time as those on the
terrace; consequently the loss of fine earth during rainy weather must
have been greater in this than in the last case.  The castings moreover
were more sandy, and in collecting them during dry weather they sometimes
crumbled into dust, and much was thus lost.  Therefore it is certain that
the worms brought up to the surface considerably more earth than that
which was collected.  The last collection was made on October 27th, 1871;
i.e., 367 days after the square had been marked out and the surface
cleared of all pre-existing castings.  The collected castings, after
being well dried, weighed 7.453 pounds; and this would give, for an acre
of the same kind of land, 16.1 tons of annually ejected dry earth.

                 SUMMARY OF THE FOUR FOREGOING CASES.
(1.)  Castings ejected near Nice within about a year, collected by Dr.
King on a square foot of surface, calculated to yield per acre 14.58
tons.
(2.)  Castings ejected during about 40 days on a square yard, in a
field of poor pasture at the bottom of a large valley in the Chalk,
calculated to yield annually per acre 18.12 tons.
(3.)  Castings collected from a square yard on an old terrace at Leith
Hill Place, during 369 days, calculated to yield annually per acre
7.56 tons.
(4.)  Castings collected from a square yard on Leith Hill Common
during 367 days, calculated to yield annually per acre 16.1 tons.

_The thickness of the layer of mould_, _which castings ejected during a
year would form if uniformly spread out_.—As we know, from the two last
cases in the above summary, the weight of the dried castings ejected by
worms during a year on a square yard of surface, I wished to learn how
thick a layer of ordinary mould this amount would form if spread
uniformly over a square yard.  The dry castings were therefore broken
into small particles, and whilst being placed in a measure were well
shaken and pressed down.  Those collected on the Terrace amounted to
124.77 cubic inches; and this amount, if spread out over a square yard,
would make a layer 0.9627 inch in thickness.  Those collected on the
Common amounted to 197.56 cubic inches, and would make a similar layer
0.1524 inch in thickness.

These thicknesses must, however, be corrected, for the triturated
castings, after being well shaken down and pressed, did not make nearly
so compact a mass as vegetable mould, though each separate particle was
very compact.  Yet mould is far from being compact, as is shown by the
number of air-bubbles which rise up when the surface is flooded with
water.  It is moreover penetrated by many fine roots.  To ascertain
approximately by how much ordinary vegetable mould would be increased in
bulk by being broken up into small particles and then dried, a thin
oblong block of somewhat argillaceous mould (with the turf pared off) was
measured before being broken up, was well dried and again measured.  The
drying caused it to shrink by 1/7 of its original bulk, judging from
exterior measurements alone.  It was then triturated and partly reduced
to powder, in the same manner as the castings had been treated, and its
bulk now exceeded (notwithstanding shrinkage from drying) by 1/16 that of
the original block of damp mould.  Therefore the above calculated
thickness of the layer, formed by the castings from the Terrace, after
being damped and spread over a square yard, would have to be reduced by
1/16; and this will reduce the layer to 0.09 of an inch, so that a layer
0.9 inch in thickness would be formed in the course of ten years.  On the
same principle the castings from the Common would make in the course of a
single year a layer 0.1429 inch, or in the course of 10 years 1.429 inch,
in thickness.  We may say in round numbers that the thickness in the
former case would amount to nearly 1 inch, and in the second case to
nearly 1½ inch in 10 years.

In order to compare these results with those deduced from the rates at
which small objects left on the surfaces of grass-fields become buried
(as described in the early part of this chapter), we will give the
following summary:—

  SUMMARY OF THE THICKNESS OF THE MOULD ACCUMULATED OVER OBJECTS LEFT
          STREWED ON THE SURFACE, IN THE COURSE OF TEN YEARS.
The accumulation of mould during 14¾ years on the surface of a dry,
sandy, grass-field near Maer Hall, amounted to 2.2 inches in 10 years.
The accumulation during 21½ years on a swampy field near Maer Hall,
amounted to nearly 1.9 inch in 10 years.
The accumulation during 7 years on a very swampy field near Maer Hall
amounted to 2.1 inches in 10 years.
The accumulation during 29 years, on good, argillaceous pasture-land
over the Chalk at Down, amounted to 2.2 inches in 10 years.
The accumulation during 30 years on the side of a valley over the
Chalk at Down, the soil being argillaceous, very poor, and only just
converted into pasture (so that it was for some years unfavourable for
worms), amounted to 0.83 inch in 10 years.

In these cases (excepting the last) it may be seen that the amount of
earth brought to the surface during 10 years is somewhat greater than
that calculated from the castings which were actually weighed.  This
excess may be partly accounted for by the loss which the weighed castings
had previously undergone through being washed by rain, by the adhesion of
particles to the blades of the surrounding grass, and by their crumbling
when dry.  Nor must we overlook other agencies which in all ordinary
cases add to the amount of mould, and which would not be included in the
castings that were collected, namely, the fine earth brought up to the
surface by burrowing larvæ and insects, especially by ants.  The earth
brought up by moles generally has a somewhat different appearance from
vegetable mould; but after a time would not be distinguishable from it.
In dry countries, moreover, the wind plays an important part in carrying
dust from one place to another, and even in England it must add to the
mould on fields near great roads.  But in our country these latter
several agencies appear to be of quite subordinate importance in
comparison with the action of worms.

We have no means of judging how great a weight of earth a single
full-sized worm ejects during a year.  Hensen estimates that 53,767 worms
exist in an acre of land; but this is founded on the number found in
gardens, and he believes that only about half as many live in
corn-fields.  How many live in old pasture land is unknown; but if we
assume that half the above number, or 26,886 worms live on such land,
then taking from the previous summary 15 tons as the weight of the
castings annually thrown up on an acre of land, each worm must annually
eject 20 ounces.  A full-sized casting at the mouth of a single burrow
often exceeds, as we have seen, an ounce in weight; and it is probable
that worms eject more than 20 full-sized castings during a year.  If they
eject annually more than 20 ounces, we may infer that the worms which
live in an acre of pasture land must be less than 26,886 in number.

Worms live chiefly in the superficial mould, which is usually from 4 or 5
to 10 and even 12 inches in thickness; and it is this mould which passes
over and over again through their bodies and is brought to the surface.
But worms occasionally burrow into the subsoil to a much greater depth,
and on such occasions they bring up earth from this greater depth; and
this process has gone on for countless ages.  Therefore the superficial
layer of mould would ultimately attain, though at a slower and slower
rate, a thickness equal to the depth to which worms ever burrow, were
there not other opposing agencies at work which carry away to a lower
level some of the finest earth which is continually being brought to the
surface by worms.  How great a thickness vegetable mould ever attains, I
have not had good opportunities for observing; but in the next chapter,
when we consider the burial of ancient buildings, some facts will be
given on this head.  In the two last chapters we shall see that the soil
is actually increased, though only to a small degree, through the agency
of worms; but their chief work is to sift the finer from the coarser
particles, to mingle the whole with vegetable débris, and to saturate it
with their intestinal secretions.

Finally, no one who considers the facts given in this chapter—on the
burying of small objects and on the sinking of great stones left on the
surface—on the vast number of worms which live within a moderate extent
of ground on the weight of the castings ejected from the mouth of the
same burrow—on the weight of all the castings ejected within a known time
on a measured space—will hereafter, as I believe, doubt that worms play
an important part in nature.




CHAPTER IV.
THE PART WHICH WORMS HAVE PLAYED IN THE BURIAL OF ANCIENT BUILDINGS.


The accumulation of rubbish on the sites of great cities independent of
the action of worms—The burial of a Roman villa at Abinger—The floors and
walls penetrated by worms—Subsidence of a modern pavement—The buried
pavement at Beaulieu Abbey—Roman villas at Chedworth and Brading—The
remains of the Roman town at Silchester—The nature of the débris by which
the remains are covered—The penetration of the tesselated floors and
walls by worms—Subsidence of the floors—Thickness of the mould—The old
Roman city of Wroxeter—Thickness of the mould—Depth of the foundations of
some of the Buildings—Conclusion.

ARCHÆOLOGISTS are probably not aware how much they owe to worms for the
preservation of many ancient objects.  Coins, gold ornaments, stone
implements, &c., if dropped on the surface of the ground, will infallibly
be buried by the castings of worms in a few years, and will thus be
safely preserved, until the land at some future time is turned up.  For
instance, many years ago a grass-field was ploughed on the northern side
of the Severn, not far from Shrewsbury; and a surprising number of iron
arrow-heads were found at the bottom of the furrows, which, as Mr.
Blakeway, a local antiquary, believed, were relics of the battle of
Shrewsbury in the year 1403, and no doubt had been originally left
strewed on the battle-field.  In the present chapter I shall show that
not only implements, &c., are thus preserved, but that the floors and the
remains of many ancient buildings in England have been buried so
effectually, in large part through the action of worms, that they have
been discovered in recent times solely through various accidents.  The
enormous beds of rubbish, several yards in thickness, which underlie many
cities, such as Rome, Paris, and London, the lower ones being of great
antiquity, are not here referred to, as they have not been in any way
acted on by worms.  When we consider how much matter is daily brought
into a great city for building, fuel, clothing and food, and that in old
times when the roads were bad and the work of the scavenger was
neglected, a comparatively small amount was carried away, we may agree
with Élie de Beaumont, who, in discussing this subject, says, “pour une
voiture de matériaux qui en sort, on y en fait entrer cent.” {166a}  Nor
should we overlook the effects of fires, the demolition of old buildings,
and the removal of rubbish to the nearest vacant space.

_Abinger_, _Surrey_.—Late in the autumn of 1876, the ground in an old
farm-yard at this place was dug to a depth of 2 to 2½ feet, and the
workmen found various ancient remains.  This led Mr. T. H. Farrer of
Abinger Hall to have an adjoining ploughed field searched.  On a trench
being dug, a layer of concrete, still partly covered with tesseræ (small
red tiles), and surrounded on two sides by broken-down walls, was soon
discovered.  It is believed, {166b} that this room formed part of the
atrium or reception-room of a Roman villa.  The walls of two or three
other small rooms were afterwards discovered.  Many fragments of pottery,
other objects, and coins of several Roman emperors, dating from 133 to
361, and perhaps to 375 A.D., were likewise found.  Also a half-penny of
George I., 1715.  The presence of this latter coin seems an anomaly; but
no doubt it was dropped on the ground during the last century, and since
then there has been ample time for its burial under a considerable depth
of the castings of worms.  From the different dates of the Roman coins we
may infer that the building was long inhabited.  It was probably ruined
and deserted 1400 or 1500 years ago.

I was present during the commencement of the excavations (August 20,
1877) and Mr. Farrer had two deep trenches dug at opposite ends of the
atrium, so that I might examine the nature of the soil near the remains.
The field sloped from east to west at an angle of about 7°; and one of
the two trenches, shown in the accompanying section (Fig. 8) was at the
upper or eastern end.  The diagram is on a scale of 1/20 of an inch to an
inch; but the trench, which was between 4 and 5 feet broad, and in parts
above 5 feet deep, has necessarily been reduced out of all proportion.
The fine mould over the floor of the atrium varied in thickness from 11
to 16 inches; and on the side of the trench in the section was a little
over 13 inches.  After the mould had been removed, the floor appeared as
a whole moderately level; but it sloped in parts at an angle of 1°, and
in one place near the outside at as much as 8° 30′.  The wall surrounding
the pavement was built of rough stones, and was 23 inches in thickness
where the trench was dug.  Its broken summit was here 13 inches, but in
another part 15 inches, beneath the surface of the field, being covered
by this thickness of mould.  In one spot, however, it rose to within 6
inches of the surface.  On two sides of the room, where the junction of
the concrete floor with the bounding walls could be carefully examined,
there was no crack or separation.  This trench afterwards proved to have
been 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.

   [Picture: Fig. 8: Section through the foundations of a buried Roman
                                  villa]

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 6½ 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 tesseræ torn up, and
the tops of the old walls knocked down.

When the concrete and tesseræ 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 tesseræ.  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 48½, and another at 51½ inches beneath
the surface.  A fresh humus-lined burrow was also met with at a depth of
57 and another at 65½ 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 6¾,
10 and 11½ 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 5½
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 2¾ inches, and this rested on a
layer from 8¾ 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. {183}  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°; 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 1½ mile in
compass, now surrounds a space of about 100 acres of cultivated land, on
which a farm-house and a church stand. {187}  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 tesseræ, 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.

 [Picture: Fig. 7: Section through one of the fallen Druidical stones at
Stonehenge.  Fig. 9: Section within a room in the Basilica at Silchester]

A section across the middle of another hall in the Basilica, 32 feet 6
inches in length, called the Ærarium, 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.

 [Picture: Fig. 10: Section within a hall in the Basilica of Silchester]

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.

 [Picture: Fig. 11: Section in a block of buildings in the middle of the
                           town of Silchester]

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 débris, the lower one of which
rested on a floor with tesseræ.  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.

 [Picture: Fig. 12: Section in the centre of the Basilica at Silchester]

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 tesseræ 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
tesseræ.  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 49½ 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 5¾ 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 tesseræ appeared as if drawn
a little away from the walls; whilst in other places they were still in
close contact with them.

 [Picture: Fig. 14: A north and south section through the subsided floor
                              of a corridor]

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 ¾ of an inch above its level.  The field, which was in
pasture, here sloped from north to south, at an angle of 30°, 40′.  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
débris, 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 7¾ 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.

          [Picture: Fig. 15: Section through the subsided floor]

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 28½ inches, and at the eastern end by a
thickness of only 11½ 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 3° 40′, 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 (25½ 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 25½-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 1¾
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.
1.  At a depth of 36 inches                                         20
undisturbed sand was reached
2.  At a depth of 33 inches                                         21
concrete was reached
3. At a depth of 9 inches                                            9
concrete was reached

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°.

                                    Thickness of mould in inches.
4.  Summit of field, trench 45                                      40
inches deep
5.  Close to summit of field,                                       26
trench 36 inches deep
6.  Close to summit of field,                                       28
trench 28 inches deep
7.  Near summit of field, trench                                    24
36 inches deep
8.  Near summit of field, trench                                    24
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
9.  Trench close to the last, 28                                    24
inches in depth
10.  Lower part of same field,                                      15
trench 30 inches deep
11.  Lower part of same field,                                      17
trench 31 inches deep
12.  Lower part of same field,                                      28
trench 36 inches deep, at which
depth undisturbed sand was
reached
13.  In another part of same                                        9½
field, trench 9½ inches deep
stopped by concrete
14.  In another part of same                                         9
field, trench 9 inches deep,
stopped by concrete
15.  In another part of the same                                    16
field, trench 24 inches deep,
when sand was reached
16.  In another part of same                                        13
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

Small field between “Old Works” and “Shop Leasows,” I believe nearly as
high as the upper part of the latter field.

                                    Thickness of mould in inches.
17.  Trench 26 inches deep                                          24
18.  Trench 10 inches deep, and                                     10
then came upon a causeway
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.

                                    Thickness of mould in inches.
21.  Trench 28 inches deep, when                                    16
undisturbed sand was reached
22.  Trench 29 inches deep, when                                    15
undisturbed sand was reached
23.  Trench 14 inches deep, and                                     14
then came upon a building

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°, 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, {210} 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.” {211}  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. {212}  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 {213} 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. {215}  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. {217a}  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; {217b} 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 Thénard
azohumic, are enabled to dissolve colloid silica in proportion to the
nitrogen which they contain. {220}  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. {222}  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. {223}  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. {224a}  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. {224b}  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, {225a} as “une
véritable armature.”  The gizzard is surrounded by powerful transverse
muscles, which, according to Claparède, 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. {225b}  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. {226}

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,
{227} 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
tenuissimâ terrâ, veluti in pulverem redactâ.” {228a}  Perrier also
speaks of “l’état de pâte excessivement fine à laquelle est réduite la
terre qu’ils rejettent,” &c. {228b}

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 tesseræ on one of the buried floors of the Roman villa at
Abinger, there were many particles (from ½ 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 “pâte excessivement fine,”—of which the castings largely
consist, is in part due to the mechanical action of the gizzard; {234}
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.” {236}  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° and 11° 30′ 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 7½
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°;
and where it was less than 1° 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° 30′ and 17° 30′; the mean of the
eleven inclinations being 9° 26′. 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 ⅛ 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° 19′, 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° 5′.

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° 26′.  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. {244}

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 ⅓ of the earth
brought up can be considered to flow downwards; hence ⅓ 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 ⅓ of 0.2 × 36, 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 1½ 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° 20′; 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° 30′, 3° and 6°; 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 2½ inches, where the slope was 7°, 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°.

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 ¾ 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 2½ 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 1½ 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° to 60° 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° 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°, 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. {259}

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° and 15°, 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°
45′, 1°, 3° and 3° 30′ (mean 2° 45′) 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 2¾ to 1; whereas we have seen that
with several castings which had flowed down slopes having a mean
inclination of 9° 26′, and with three castings where the inclination was
above 12°; 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.  É. 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. {265}  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.” {266}

_Ancient encampments and tumuli_.—É. 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° or
50° 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. {269}  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 ½ 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° to
the north-east, the depth of the furrows (only 7 feet apart) was
carefully measured, and was found to be about 4½ 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 2½ to 3½ 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 2½ inches, and in the lower part 4½ 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° to
10 °, 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 2½ 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 3½ 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°; 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° and 20°, 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° and 3°, 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 ¼ 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 2½ to 3½ 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 9½ inches.  On the embankment itself the mould was only 1 to 1½ inch
in thickness; and within the ditch at the bottom it varied from 2½ to 3½,
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°, the
mould was very thin, namely, from 1½ to 2½ inches; whilst near the base,
where the slope was only 3° to 4°, 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° 26′, 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 11½ 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.

                                * * * * *

Archæologists 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 {284} 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 {285} 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.


{2}  ‘Leçons de Géologie Pratique,’ tom. i. 1845, p. 140.

{3}  ‘Transactions Geolog. Soc.’ vol. v. p. 505.  Read November 1, 1837.

{4a}  ‘Histoire des progrès de la Géologie,’ tom. i. 1847, p. 224.

{4b}  ‘Zeitschrift für 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.
Müller’s work on Humus in ‘Tidsskrift for Skovbrug,’ Band iii. Heft 1 and
2, Copenhagen, 1878.  He had, however, no opportunity of consulting
Müller’s work.  Dr. Müller 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 über die natürlichen
Humusformen, unter deren Einwirkung auf Vegetation und Boden,’ 8vo.,
Berlin, 1887.

{8a}  ‘Bidrag till Skandinaviens Oligochætfauna,’ 1871.

{8b}  ‘Die bis jetzt bekannten Arten aus der Familie der Regenwürmer,’
1845.

{9b}  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.”

{11}  I shall have occasion often to refer to M. Perrier’s admirable
memoir, ‘Organisation des Lombriciens terrestres’ in ‘Archives de Zoolog.
expér.’ 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.

{12}  Morren, ‘De Lumbrici terrestris Hist. Nat.’ &c., 1829, p. 67.

{14}  ‘De Lumbrici terrestris Hist. Nat.’ &c., p. 14.

{17}  Histolog.  Untersuchungen über die Regenwürmer.  ‘Zeitschrift für
wissenschaft.  Zoologie,’ B. xix., 1869, p. 611.

{18a}  For instance, Mr. Bridgman and Mr. Newman (‘The Zoologist,’ vol.
vii. 1849, p. 2576), and some friends who observed worms for me.

{18b}  ‘Familie der Regenwürmer,’ 1845, p. 18.

{31}  ‘The Zoologist,’ vol. vii. 1849, p. 2576.

{32}  ‘Familie der Regenwürmer,’ 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.”

{33}  ‘De Lumbrici terrestris Hist. Nat.’ p. 19.

{34}  ‘Archives de Zoologie expérimentale,’ 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.

{35a}  On the action of the pancreatic ferment, see ‘A Text-Book of
Physiology,’ by Michael Foster, 2nd edit. pp. 198–203.  1878.

{35b}  Schmulewitsch, ‘Action des Sucs digestifs sur la Cellulose.’
Bull. de l’Acad. Imp. de St. Pétersbourg, tom. xxv. p. 549.  1879.

{40}  Claparède doubts whether saliva is secreted by worms: see
‘Zeitschrift für wissenschaft.  Zoologie,’ B. xix. 1869, p. 601.

{41a}  Perrier, ‘Archives de Zoolog. expér.’ July, 1874, pp. 416, 419.

{41b}  ‘Zeitschrift für wissenschaft.  Zoologie,’ B. xix, 1869, pp.
603–606.

{46}  De Vries, ‘Landwirth. Jahrbücher,’ 1881, p. 77.

{49}  M. Foster, ‘A Text-Book of Physiology,’ 2nd edit. 1878, p. 243.

{50}  M. Foster, _ut sup._ p. 200.

{53}  Claparède remarks (‘Zeitschrift für wisseuschaft.  Zoolog.’ B. 19,
1869, p. 602) that the pharynx appears from its structure to be adapted
for suction.

{58}  An account of her observations is given in the ‘Gardeners’
Chronicle,’ March 28th, 1868, p. 324.

{59a}  London’s ‘Gard. Mag.’ xvii. p. 216, as quoted in the ‘Catalogue of
the British Museum Worms,’ 1865, p. 327.

{59b}  ‘Familie der Regenwürmer,’ p. 19.

{79}  In these narrow triangles the apical angle is 9° 34′, and the basal
angles 85° 13′.  In the broader triangles the apical angle is 19° 10′ and
the basal angles 80° 25′.

{89a}  See his interesting work, ‘Souvenirs entomologiques,’ 1879, pp.
168–177.

{89b}  Möbius, ‘Die Bewegungen der Thiere,’ &c., 1873, p. 111.

{90}  ‘Annals and Mag. of N. History,’ series ii. vol. ix. 1852, p. 333.

{93}  ‘Archives de Zoolog. expér.’ tom. iii. 1874, p. 405.

{97}  I state this on the authority of Semper, ‘Reisen im Archipel der
Philippinen,’ Th. ii. 1877, p. 30.

{101}  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
_Perichæta 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 Perichæta
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 Perichæta abound.

{102}  ‘Zeitschrift für wissenschaft.  Zoolog.’  B. xxviii. 1877, p. 364.

{108}  ‘Zeitschrift für wissenschaft.  Zoolog.’ B. xxviii. 1877, p. 356.

{113}  Perrier, ‘Archives de Zoolog. expér.’ tom. 3, p. 378, 1874.

{126}  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.

{128}  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 be 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.

{131}  S. W. Johnson, ‘How Crops Feed,’ 1870, p. 139.

{136a}  ‘Nature,’ November 1877, p. 28.

{136b}  ‘Proc. Phil. Soc.’ of Manchester, 1877, p. 247.

{138a}  ‘Trans. of the New Zealand Institute,’ vol. xii., 1880, p. 152.

{138b}  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.”

{139}  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.

{147}  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.”

{148}  ‘Zeitschrift für wissensch. Zoolog.’ Bd. xxviii., 1877, p. 360.

{149}  See Mr. Dancer’s paper in ‘Proc. Phil. Soc. of Manchester,’ 1877,
p. 248.

{166a}  ‘Leçons de Géologie pratique,’ 1845, p. 142.

{166b}  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.

{183}  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.

{187}  These details are taken from the ‘Penny Cyclopædia,’ article
Hampshire.

{210}  “On the denudation of South Wales,” &c., ‘Memoirs of the
Geological Survey of Great Britain,’ vol. 1., p. 297, 1846.

{211}  ‘Geological Magazine,’ October and November, 1867, vol. iv. pp.
447 and 483.  Copious references on the subject are given in this
remarkable memoir.

{212}  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.

{213}  “An account of the fine dust which often falls on Vessels in the
Atlantic Ocean,” Proc. Geolog. Soc. of London, June 4th, 1845.

{215}  For La Plata, see my ‘Journal of Researches,’ during the voyage of
the _Beagle_, 1845, p. 133.  Élie de Beaumont has given (‘Leçons de
Géolog. 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.

{217a}  These statements are taken from Hensen in ‘Zeitschrift für
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.

{217b}  I have given some facts on the climate necessary or favourable
for the formation of peat, in my ‘Journal of Researches,’ 1845, p. 287.

{220}  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.

{222}  See, for references on this subject, S. W. Johnson, ‘How Crops
Feed,’ 1870, p. 326.

{223}  This statement is taken from Mr. Julien, ‘Proc. American Assoc.
Science,’ vol.  xxviii., 1879, p. 330.

{224a}  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.

{224b}  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.

{225a}  ‘Archives de Zoolog. expér.’ tom. iii. 1874, p. 409.

{225b}  ‘Nouvelles Archives du Muséum,’ tom. viii. 1872, pp.  95, 131.

{226}  Morren, in speaking of the earth in the alimentary canals of
worms, says, “præsepè cum lapillis commixtam vidi:” ‘De Lumbrici
terrestris Hist. Nat.’ &c., 1829, p. 16.

{227}  Perrier, ‘Archives de Zoolog. expér.’ tom. iii. 1874, p. 419.

{228a}  Morren, ‘De Lumbrici terrestris Hist. Nat.’ &c., p. 16.

{228b}  ‘Archives de Zoolog. expér.’ tom. iii. 1874, p. 418.

{234}  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.

{236}  Anniversary Address: ‘The Quarterly Journal of the Geological
Soc.’ May 1880, p. 59.

{244}  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.

{259}  ‘Elements of Geology,’ 1865, p. 20.

{265}  ‘Leçons de Géologie pratique, 1845; cinquième Leçon.  All Élie 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.

{266}  ‘Illustrations of the Huttonian Theory of the Earth,’ p. 107.

{269}  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’ (Hochäcker, and Heidenäcker) 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.”

{284}  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.

{285}  ‘Zeitschrift für wissenschaft. Zoolog.’ B. xxviii. 1877, p. 360.




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