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diff --git a/2355-0.txt b/2355-0.txt new file mode 100644 index 0000000..a3a2218 --- /dev/null +++ b/2355-0.txt @@ -0,0 +1,6083 @@ +The Project Gutenberg eBook of The Formation of Vegetable Mould, by Charles Darwin + +This eBook is for the use of anyone anywhere in the United States and +most other parts of the world at no cost and with almost no restrictions +whatsoever. You may copy it, give it away or re-use it under the terms +of the Project Gutenberg License included with this eBook or online at +www.gutenberg.org. If you are not located in the United States, you +will have to check the laws of the country where you are located before +using this eBook. + +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. 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