Читать книгу Intelligence in Plants and Animals - Thomas G. Gentry - Страница 11

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Earth-worms are found throughout the world. Though few in genera, and not many in species, yet they make up in individual numbers, for it has been estimated that they average about one hundred thousand to the acre. Our American species have never been monographed, which renders it impossible to judge of their probable number. Their castings may be seen on commons, so as to cover almost entirely their surface, where the soil is poor and the grass short and thin, and they are almost as numerous in some of our parks where the grass grows well and the soil appears rich. Even on the same piece of ground worms are much more frequent in some places than in others, although no visible difference in the nature of the soil is manifest. They abound in paved court-yards contiguous to houses, and on the sidewalks in country towns, and instances have been reported where they have burrowed through the floors of very damp cellars.

Beneath large trees few castings can be found during certain parts 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, an explanation which seems to be confirmed by the fact that such places may be observed covered with castings after the heavy autumnal rains. Although most coppices and woods support large numbers of worms, yet in forests of certain kinds of tree-growths, where the ground beneath is destitute of vegetation, not a casting is seen over wide reaches of ground, even during the autumn. In mountainous districts worms are mostly rare, it would seem, a circumstance which is perhaps owing to the close proximity of the subjacent rocks, into which it is impossible for them to burrow during the winter, so as to escape being frozen. But there are some exceptions to this rule, for they have been found at great altitudes in certain parts of the world, and especially is this so in India, where they have been observed to be quite numerous upon the mountains.

Though in one sense semi-aquatic animals, like the other members of the great class of Annelids to which they belong, yet it cannot be denied that earth-worms are terrestrial creatures. Their exposure to the dry air of a room for a single night proves fatal to them, while on the other hand they have been kept alive for nearly four months completely submerged in water. During the summer, when the ground is dry, they penetrate to a great depth and cease to work, just as they do in winter when the ground is frozen. They are nocturnal in their habits, and may be seen crawling about in large numbers at night, but generally with their tails still inserted in their burrows. By the expansion of this part of the body, and with the aid of the short reflexed bristles with which they are armed inferiorly, they hold so securely that they can seldom be withdrawn from the ground without being torn in pieces. But during the day, except at the time of pairing, when those which inhabit adjoining burrows expose the greater part of their bodies for an hour or two in the early morning, they remain in their burrows. Sick individuals, whose illness is caused by the parasitic larvæ of a fly, must also be excepted, as they wander about during the day and die on the surface. Astonishing numbers of dead worms may sometimes be seen lying on the ground after a heavy rain succeeding dry weather, no less than a half-hundred in a space of a few square yards, but these are doubtless worms that were already sick, whose deaths were merely hastened by the ground being flooded, for if they had been drowned it is probable, from the facts already given, that they would have perished in their burrows.

After there has been a heavy rain the film of mud or of very fine sand to be seen over gravel-walks in the morning is often distinctly marked with the tracks of worms. From May to August, inclusive, this has been noticed when the months have been wet. Very few dead worms are anywhere to be seen on these occasions, although the walks are marked with innumerable tracks, five tracks often being counted crossing a space of only an inch square, which could be traced either to or from the mouths of the burrows in the gravel-walks for distances varying from three to fifteen yards, but no two tracks being seen to lead to the same burrow. It is not likely, from what is known of the sense-organs of these animals, that a worm could find its way back to its burrow after having once left it. They leave their burrows, it would seem, on a voyage of discovery, and thus they find new sites for the exercise of their powers. For hours together they may often be seen lying almost motionless beneath the mouths of their burrows. But let the ejected earth or rubbish over their burrows be suddenly removed and the end of the worm’s body may be seen rapidly retreating.

This habit of lying near the surface leads to their destruction to an immense extent, for, at certain seasons of the year, the robins and blackbirds that visit our lawns in the country may be observed drawing out of their holes an astonishing number of worms, which could not be done unless they lay close to the surface. But what brings the worms to the surface? This is a question whose answer cannot be positively asserted. It is not probable that they behave in this manner for the purpose of breathing fresh air, for it has been seen that they can live a long time under water. That they are there for the sake of warmth, especially in the morning, is a more reasonable supposition, which seems to be confirmed by the fact that they often coat the mouths of their burrows with leaves, apparently to prevent their bodies from coming into contact with the cold, damp earth, and by the still other fact that they completely close their burrows during the winter.

Some remarks about the structure of the earth-worm now appear apropos. Its body consists of from one hundred to two hundred almost cylindrical rings, each provided with minute bristles. The muscular system is well developed, thus enabling these animals to crawl backwards as well as forwards, and to retreat by the help of their affixed tails into their burrows with extraordinary rapidity. Situated at the anterior end of the body is the mouth. It is furnished with a little projection, variously called the lobe or lip, which is used for prehension. Behind the mouth, internally located, is a strong pharynx, which is pushed forwards when the animal eats, corresponding, it is said, with the protrudable trunk of other Annelids. The pharynx conducts to the œsophagus, on each side of the lower part of which are placed three pairs of large glands, called calciferous glands, whose function is the secretion of carbonate of lime. These glands are very remarkable organs, and their like is not to be found in any other animal. Their use is connected in some way with the process of digestion. The œsophagus, in most of the species, 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, whose energetic action is most effectual in the trituration of the food, for these worms possess no jaws, or teeth of any kind. Grains of sand and small stones, from the one-twentieth to the one-tenth of an inch in size, are found in their gizzards and intestines, and these little stones, independently of those swallowed while excavating their burrows, most probably serve, like millstones, to triturate their food. The gizzard opens into the intestine—a most remarkable structure, an intestine within an intestine—which runs in a straight line to the vent at the posterior end of the body. But this curious structure, as shown by Claparède, merely consists of a deep longitudinal involution of the walls of the intestine, by which means an extensive absorbent surface is secured.

Worms have a well-developed circulating system. Their breathing is effected by the skin, and so they do not possess any special respiratory apparatus. Each individual unites the two sexes in its own body, but two individuals pair together. The nervous system is fairly well developed, the two nearly confluent cerebral ganglia being situated very close to the anterior extremity of the body.

Being destitute of eyes, we would naturally conclude that worms were quite insensible to light; but from many experiments that have been made by Darwin, Hofmeister and others, it is evident that light affects them, but only by its intensity and duration. It is the anterior extremity of the body, where the cerebral ganglia lie, that is affected, for if this part is shaded and other parts of the body are illuminated no effect will be produced. As these animals have no eyes, it is probable that the light passes through their skins and excites in some manner their cerebral ganglia. When worms are employed in dragging leaves into their burrows or in eating them, and even during the brief intervals of rest from their labors, they either do not perceive the light or are regardless of it, and this is even the case when the light is concentrated upon them through a large lens. Paired individuals will remain for an hour or two together out of their burrows, fully exposed to the morning light, but it appears, from what some writers have said, that a light will occasionally cause paired individuals to separate. When a worm is suddenly illuminated and dashes into its burrow, one is led to look at the action as a reflex one, the irritation of the cerebral ganglia apparently causing certain muscles to contract in an inevitable manner, without the exercise of the will or consciousness of the animal, as though it was an automaton. But the different effect which a light produces on different occasions, and especially the fact that a worm when in any way occupied, no matter what set of muscles and ganglia may be brought into play, is often regardless of light, are antagonistic 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 upon them, we ascribe this to their attention being then absorbed, and attention necessarily implies the presence of mind. Although worms cannot be said to possess the power of vision, yet their sensitiveness to light enables them to discriminate between day and night, and thus they escape the attacks of the many diurnal animals that would prey upon them. They are less sensitive to a moderate radiant heat than to a bright light, as repeated experiments have conclusively shown; and their disinclination to leave their burrows during a frost proves that they are sensitive to a low temperature.

Investigation fails to locate in worms any organ of hearing, from which must be concluded that they are insensible to sounds. The shrill notes of a metallic whistle sounded near them, and the deepest and loudest tones of a bassoon, failed to awaken the least notice. Although indifferent to modulations in the air, audible to human ears, yet they are extremely sensitive to vibrations in any solid object. Even the light and delicate tread of a robin affrights and sends them deep into their burrows. It has been said that if the ground is beaten, or otherwise made to tremble, that worms believe they are pursued by a mole and leave their burrows, but this does not stand the test of experiment, for the writer has frequently beaten the ground in many places where these creatures abounded, but not one emerged. A worm’s entire body is sensitive to contact, the slightest puff of air from the mouth causing an instant retreat. 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 object of touch, and there is good reason to believe that they are thus enabled to gain a general knowledge of the form of an object. Touch, including in this term the perception of a vibration, seems much the most highly developed of all their senses. The sense of smell is quite feeble, and is apparently confined to the perception of certain odors. They are quite indifferent to the human breath, even when tainted by tobacco, or to a pellet of cotton-wool with a few drops of Millefleur’s perfume when held by pincers and moved about within a few inches of them. The perception of such an unnatural odor would be of no service to them. Now, as such timid creatures would almost certainly exhibit some signs of any new impression, we may reasonably conclude that they did not perceive these odors. But when cabbage leaves and pieces of onion were employed, both of which are devoured with much relish by worms, the result was different. These, with bits of fresh raw meat, have been buried in pots beneath one-fourth of an inch of common garden soil, or sometimes laid on pieces of tin foil in the earth, the ground being pressed down slightly, so as not to prevent the emission of any odor, and yet they were always discovered by the worms that were placed in the pots, and removed after varying periods of time. These facts indicate that worms possess some power of smell, and that they discover by this means odoriferous and much-coveted kinds of food.

That all animals which feed on various substances possess the sense of taste, is a wise presumption. This is certainly the case with worms. Cabbage leaves are much liked by worms, and it would seem that they are able to distinguish between the different varieties, but this may perhaps be owing to differences in their texture. When leaves of the cabbage, horse-radish and onion were given together, they manifestly preferred the last to the others. Celery is preferred to the leaves of the cabbage, lime-tree, ampelopsis and parsnip, and the leaves of the wild cherry and carrots, especially the latter, to all the others. That the worms have a preference for one taste over another, is still further shown from what follows. Pieces of the leaves of cabbage, turnip, horse-radish and onion have been fed to the worms, mingled with the leaves of an Artemisia and of the culinary sage, thyme and mint, differing in no material degree in texture from the foregoing four, yet quite as strong in taste, but the latter were quite neglected excepting those of the mint, which were slightly nibbled, but the others were all attacked and had to be renewed.

There is little to be noted about the mental qualities of worms. They have been seen to be timid creatures. Their eagerness for certain kinds of food manifestly shows that they must enjoy the pleasure of eating. So strong is their sexual passion that they overcome for a time their dread of light. They seem to have a trace of social feeling, for they are not disturbed by crawling over each other’s bodies, and they sometimes lie in contact. Although remarkably deficient in the several sense-organs, yet this does not necessarily preclude intelligence, for it has been shown that when their attention is engaged they neglect impressions to which they would otherwise have attended, and attention, as is well known, indicates the presence of a mind of some kind. A few actions are performed instinctively, that is, all the individuals, including the young, perform each action in nearly the same manner. The various species of Perichæta eject their castings so as to construct towers, and the burrows of the Common Earth-worm—Lumbricus terrestris—are smoothly lined with fine earth and often with little stones, and the mouth with leaves. One of their strongest instincts is the plugging up of the mouths of their burrows with various objects, the very young worms acting in a similar manner. But some degree of intelligence is manifested, as will subsequently appear.

Almost everything is eaten by worms. They swallow enormous quantities of earth, from which they extract any digestible matter it may contain. Large numbers of half-decayed leaves of all kinds, excepting a few that are too tough and unpleasant to the taste, and likewise petioles, peduncles, and decayed flowers. Fresh leaves are consumed as well. Particles of sugar, licorice and starch, and bits of raw and roasted meat, and preferably raw fat, are eaten when they come into their possession, but the last article with a better relish than any other substance given to them. They are cannibals to a certain extent, and have been known to eat the dead bodies of their own companions.

The digestive fluid of worms, according to León Frédéricq, is analogous in nature to the pancreatic secretion of the higher animals, and this conclusion agrees perfectly with the kinds of food which they consume. Pancreatic juice emulsifies fat, dissolves fibrin, and worms greedily devour fat and eat raw meat. It converts starch into grape-sugar with wonderful rapidity, and the digestive fluid of worms acts upon the starch of leaves. But worms live chiefly on half-decayed leaves, and these would be useless to them unless they could digest the cellulose forming the cell-walls, for all other nutritious substances, as is well known, are almost completely withdrawn from leaves shortly before they fall off. It has been ascertained that cellulose, though very little or not at all attacked by the gastric juice of the higher animals, is acted on by that from the pancreas, and so worms eat the leaves as much for the cellulose as for the starch they contain. The half-decayed or fresh leaves which are intended for food 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, which has been assumed to hasten their decay, but which, from its alkaline nature, and from its acting both on the starch-granules and on the protoplasmic contents of the cells, is not of the nature of saliva, but a pancreatic secretion, and of the same kind as 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 the unarmed mouths of worms that they should first be moistened and softened, their disintegration being thereby the more readily effected. Fresh leaves, however soft and tender they may be, are similarly treated, probably from habit. Thus the leaves are partially digested before they are taken into the alimentary canal, an instance of extra-stomachal digestion, whose nearest analogy is to be found in such plants as Dionæa and Drosera, for in them animal matter is digested and converted into peptone, not within a stomach, but on the surfaces of the leaves.

But no portion of the economy of worms has been more the subject of speculation than the calciferous glands. About as many theories have been advanced on their utility as there have been observers. Judging from their size and from their rich supply of blood-vessels, they must be of vast importance to these animals. They consist of three pairs, which in the Common Earth-worm debouch into the alimentary canal in front of the gizzard, but posteriorly to it, in some genera. The two posterior pairs are formed by lamellæ, diverticula from the œsophagus, which 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, which are minute bodies, varying in diameter from two to six millimetres. They contain in their centres a small quantity of excessively fine granular matter, that looks so like oil globules that many scientists are deceived by its appearance. When treated with acetic acid they quickly dissolve with effervescence. An addition of oxalate of ammonia to the solution throws down a white precipitate, showing that the cells contain carbonate of lime. The two anterior glands differ a little in shape from the four posterior ones by being more oval, and 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 one and one-half millimetres in diameter. With respect to the function of the calciferous glands, it is likely that they primarily serve as organs of excretion, and secondarily as an aid to digestion. Worms consume many fallen leaves. 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, and worms would therefore be liable to become charged with this earth, unless there was some special apparatus for its excretion, and for this purpose the calciferous glands are ably adapted. On the other hand, 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. These half-decayed leaves, which are swallowed by worms in large quantities, would, therefore, after having been moistened and triturated in the alimentary canal, be apt to produce such acids, and in the case of several worms, whose alimentary canals were examined, their contents were plainly shown by litmus paper to be decidedly acid. This acidity cannot be attributed to the nature of the digestive fluid, for pancreatic juice is alkaline, and so also is the secretion which is poured out of the mouths of worms for the preparation of the leaves for consumption. With worms not only the contents of the intestines, but their ejected matter or the castings are generally acid. 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 necessarily alkaline, and 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. Therefore is seems probable that innumerable calciferous cells, which are emptied from the four posterior glands in the alimentary canal, serve to neutralize more or less completely the acids generated there by the half-decayed leaves. These cells, as has been seen, are instantly dissolved by a small quantity of acetic acid, and as they do not always suffice to render of no effect the contents of the upper part of the alimentary canal, it is probable that the lime is aggregated into concretions, in the anterior pair of glands, in order that some may be conveyed to the posterior parts of the intestine, where these concretions would be rolled about among the acid contents. The concretions found in the intestines and in the castings often present a worn appearance, but whether due to attrition or chemical corrosion it is impossible to say. That they are formed for the sake of acting as mill stones, as Claparède believed, and of thus assisting in the trituration of food, is not at all likely, as this object is already attained by the stones that are present in the gizzards and intestines.

In dragging leaves into their burrows worms generally seize the thin edge of a leaf with their mouths, between the projecting upper and lower lip, the thick and strong pharynx at the same time being pushed forwards within their bodies, so as to afford a point de resistance for the upper lip; but in the case of broad and flat objects the pointed anterior extremity of the body, after being brought into contact with an object of this kind, is drawn within the adjoining rings, so that it becomes truncated and as thick as the rest of the body. This part is then seen to swell a little, seemingly from the pharynx being pushed a little forwards. By a slight withdrawal of the pharynx, or by its expansion, a vacuum is produced beneath the truncated, slimy end of the body whilst in contact with the object, and by this means the two adhere firmly together. Worms can attach themselves to an object in the same manner under the water.

COMMON EARTH-WORMS.

Out on a Foraging Excursion.

As worms have no teeth, and their mouths consist of very soft tissue, it may be presumed that they consume by means of suction of the edges and 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. They not only seize leaves and other objects for purposes of food, but for plugging up the mouths of their burrows. Flower-peduncles, decayed twigs of trees, bits of paper, feathers, tufts of wool and horse-hair are some of the many things other than leaves that are dragged into their burrows for this purpose. Many hundred leaves of the pine-tree have been found drawn by their bases into burrows. Where fallen leaves are abundant, especially ordinary dicotyledonous leaves, many more than can be used are collected over the mouth of a burrow, 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 necessarily becomes much folded or crumpled, and when another is drawn in, this is done exteriorly to the first, and so on with succeeding leaves, till finally they all become closely folded and pressed together. Sometimes the mouth of a burrow is enlarged, or a fresh one is made close by, so that a larger number of leaves may be drawn in. Generally the interstices between the drawn-in leaves are filled with moist, viscid earth ejected from their bodies, thus rendering them doubly secure. Hundreds of such plugged burrows may be seen during the autumnal and early winter months.

When leaves, petioles, sticks, etc., cannot be obtained for the mouths of their burrows, heaps of stones, smooth, rounded pebbles, are utilized for protection. When the stones are removed and the surface of the ground is cleared for some inches round the burrow, the worms may be seen with their tails fixed in their burrows dragging the stones inward by the aid of their mouths, stones weighing as much as two ounces often being found in the little heaps, which goes to show how strong these apparently weak creatures are. Work of this kind is usually performed during the night, although objects have been occasionally known to be drawn into the burrows during the day. What advantage worms derive from plugging up the mouths of their burrows, or from piling stones over them, cannot be satisfactorily answered. They do not act in this manner when they eject much earth from their burrows, for then their castings serve to cover the mouth. Perhaps the plugs serve to protect them from the attacks of scolopenders, their most inveterate enemies, or to enable them to remain with safety with their heads close to the mouths of their burrows, which they like so well to do, but which, unless protected, costs many a fellow its life. Besides, 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? The last view of the matter seems especially well taken, because worms kept in pots where there is fire, having no cold air with which to contend, plug up their burrows in a slovenly manner, and because they often coat the upper part of their burrows with leaves, apparently to prevent their bodies from coming into contact with the cold, damp earth. But the plugging-up process may undoubtedly serve for all these purposes. Whatever the motive may be, it seems that worms much dislike leaving the mouths of their burrows open, yet, nevertheless, they will reopen them at night, whether or not they are able afterwards to close them.

Considerable intelligence is shown by worms in their manner of plugging up their burrows. If man had to plug up a cylindrical hole with such objects as leaves, petioles or twigs, he would push them in by their pointed ends, but if these were thin relatively to the size of the hole, he would probably insert some by their broader ends. Intelligence would certainly be his guide in such a case. But how worms would drag leaves into their burrows, whether by their tips, bases, or middle parts, has been a matter of interest to many. Darwin, who experimented upon the subject, found it especially desirable to experiment with plants not native to his country, for he conceived that although the habit of dragging leaves into their burrows is undoubtedly instinctive with worms, yet instinct could not teach them how to act in the case of leaves about which their progenitors knew nothing. Did they act solely through instinct, or an unvarying inherited impulse, they would draw all kinds of leaves into their burrows in the same manner. Having no such definite instinct, chance might be expected to determine whether the tip, base, or middle might be seized. If the worm in each case first tries many different methods, and follows that alone which proves possible or the most easy, then both instinct and chance are ruled out of the solution of the question. But to act in this manner, and to try different methods, makes what in man would be called intelligent action.

Three species of pine-leaves are mentioned by Darwin as being regularly drawn into the mouths of worm-burrows on the gravel-walk in his garden. These leaves consist of two needles, which are united to a common base, and it is by this point that they are almost invariably drawn into the burrows. As the sharply-pointed needles diverge somewhat, and as several are drawn into the same burrow, each tuft forms a perfect chevaux-de-frise. Many tufts were pulled up in the evening, but by the ensuing morning fresh leaves had taken their places, and the burrows again well protected. Impossible it would be to drag these leaves to any depth into the burrows, except by their bases, as a worm cannot seize hold of the two leaves at the same time, and if one alone were seized by the apex, the other would be pressed against the ground and resist the entry of the one that was seized. That the worms should do their work well, it was very essential that they drag the pine-leaves into their burrows by their bases, that is, where the two needles are conjoined. But how they are guided in this work was at first perplexing. The difficulty, however, was soon settled. With the assistance of his son Francis, the elder Darwin set to work to observe worms in confinement during several nights by the aid of a dim light, while they dragged the leaves of the aforementioned kinds into their burrows. They were seen to move the anterior extremities of their bodies about the leaves, and on several occasions when they touched the sharp end of the needle they suddenly withdrew as though they had been pricked, but it is doubtful that they were hurt, for they are indifferent to sharp objects, being known to swallow rose-thorns and small splinters of glass. It may be doubted whether the sharp end of the needle serves to tell them that is the wrong end to seize, for the points of many were cut off for the length of an inch, and these leaves were always drawn in by their bases and not by the cut-off ends. The worms, it seemed, almost instantly perceived as soon as they had seized a leaf in the proper manner. Many leaves were cemented together at the top, or tied together by fine thread, and these in the majority of instances were dragged in by their bases, which leads to the conclusion that there must be something attractive to worms in the base of pine-leaves, notwithstanding that few ordinary leaves are drawn in by their base or footstalk. Leaves of other plants, and also the petioles of some compound plants, as well as triangular bits of paper, dry and damp, were experimented with, and the manner of seizing the objects and bearing them into their burrows were as amusing as they were novel and interesting. The leaves and stems used were such as the worms had not been accustomed to in their respective haunts.

When the several cases experimented on are considered, one can hardly escape from the conclusion that some degree of intelligence is shown by worms in 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 labor having been saved by some being carried in by their broader ends. There is no doubt that worms are governed by instinct in plugging up their burrows, and it might be expected that they would have been taught in every particular instance how to act independently of intelligence. It is very difficult to judge when intelligence comes into play. The actions of animals, appearing due to intelligence, may be performed through inherited habit without any intelligence, although aboriginally acquired, or the habit may be acquired through the preservation and inheritance of some other action, and in the latter case the new habit will have been acquired independently of intelligence throughout the entire 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 and petioles of foreign plants, wholly unknown to the progenitors of the worms which have acted in the manner just described. Nor are their actions so unvarying or inevitable as are most true instincts.

As worms are not controlled 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 is that they try in many ways to draw in objects and finally succeed in some one way. It is surprising, however, that an animal so low in the scale as a worm should have the capacity to act in this way, as many higher animals have no such capacity, the instincts of the latter often being followed in a senseless or purposeless manner.

We can safely infer intelligence, as Mr. Romanes, who has specially studied animals, says, only when we see an individual profiting by his own experiences. That worms are able to judge either before or after having drawn an object close to the mouths of their burrows how best to drag it in, shows that they must have acquired 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 them as a tactile organ. Man, even when born blind and deaf, shows how perfect the sense of touch may become, and if worms, which also come into being in the same condition, have the power of acquiring some notion, however rude, of the shape of an object and their burrows, they deserve, it must seem to every sensible mind, to be called intelligent creatures, for they act in such a case in nearly the same manner as a man would under similar circumstances. That worms, which stand so low in the scale of organization, should possess some degree of intelligence, will doubtless strike everyone as very improbable. It may be doubted, however, whether we know enough about the nervous system of the lower animals to justify our natural distrust of such a conclusion. With regard to the small size of the cerebral ganglia, we would do well to 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.

Two ways are adopted by worms in excavating their burrows. Either the earth is pushed away on all sides or it is swallowed by the animal. In the former case the worm inserts the stretched-out and attenuated anterior extremity of its body into any little crevice or hole, and the pharynx is pushed forward into this part, which consequently swells and pushes away the earth on all sides, the anterior extremity thus acting as a wedge. When placed in loose mould a worm will bury itself in between two and three minutes, but in earth that is moderately pressed down it often requires as many as fifteen minutes for its disappearance. But whenever a worm burrows to a depth of several feet in undisturbed compact ground, it must form its passage by swallowing the earth, for it is impossible that the ground could yield on all sides to the pressure of the pharynx when pushed forward within the worm’s body. Great depths are reached only during continued dry weather and severe cold, the burrows sometimes attaining to a depth of from seven to eight feet. The burrows run down perpendicularly, or, more commonly, obliquely, and are sometimes said to branch. Generally, or invariably as I think, they are lined with fine, dark-colored earth voided by the worm, so that at first they must be made a little wider than their ultimate diameter. Little globular pellets of voided earth, still soft and viscid, often dot the walls of fresh burrows, and these are spread out on all sides by the worm as it travels up or down its burrow, the lining thus formed becoming very compact and smooth when nearly dry and closely fitting the worm’s body. Excellent points of support are thus afforded for the minute reflexed bristles which project in rows on all sides from the body, thus rendering the burrow 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, which would assuredly be the case when the burrows, as is occasionally observed, pass through a layer of sifted coal cinders. The burrows are thus seen to be not mere excavations, but may be compared with tunnels lined with cement. Those which run far down into the ground generally, or at least frequently, terminate in little chambers, where one or several worms pass the winter rolled up into a ball. Small pebbles and seeds as large as grains of mustard are carried down from the surface by being swallowed or within the mouths of worms, as well as bits of glass and tile, whose only use in their winter-quarters seems to be the prevention of their closely coiled-up bodies from coming into contiguity with the surrounding cold soil, for such contact would perhaps interfere with their respiration, which is effected by the skin alone.

After swallowing earth, whether for making its burrow or for food, the earth-worm soon comes to the surface to empty its body. The rejected matter is thoroughly mixed with the intestinal secretions, and is thus rendered viscid. After becoming dried, it sets hard. When in a very liquid state the earth is thrown out in little spurts, and when not so liquid by a slow peristaltic movement of the intestine. It is not cast indifferently on any side, but first on one and then on another, the tail being used almost like a trowel. The little heap being formed the worm seemingly avoids, for the sake of safety, the use of its tail, the earthy matter being forced up through the previously deposited soft mass. The mouth of the same burrow is used for this purpose for a considerable time. When a worm comes to the surface to eject earth, the tail protrudes, but when it collects leaves its head must protrude, and thus worms must have the power of performing the difficult feat, as it seems to us, of turning round in their closely-fitting burrows. Worms do not always eject their castings upon the surface of the ground, for when burrowing in newly turned-up earth, or between the stems of banked-up plants, they deposit their castings in such places, and even hollows beneath large stems lying on the surface of the ground are filled up with their ejections. Old burrows collapse in time. The fine earth voided by worms, if spread out uniformly, would form in many places a layer of one-fifth of an inch in thickness. But 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 the depth of ten inches or more, which in fifty years would grow into a hollow, unsupported place ten inches deep.

Hardly any animal is more universally distributed than worms. The earth-worm is found in all parts of the world, and some of the genera have an enormous range. They inhabit the most isolated islands, abounding in Iceland, and also being known to exist in the West Indies, St. Helena, Madagascar, New Caledonia and Tahiti. Worms from Kergulen Land in the Antarctic regions have been described by Ray Lankester, and Darwin has reported them as being found in the Falkland Islands. How they reach such isolated islands is quite unknown. They are easily killed by salt water, and it does not seem likely that young worms or their egg-capsules could be carried in earth adhering to the feet or beaks of land-birds, especially to Kergulen Land, for it is not now inhabited by any terrestrial bird.

We have seen that worms are found in nearly every part of the globe, that they are very numerous, as many as 348,480 having been found in an acre of rich ground in New Zealand, and that by the peculiar economy of their nature they are fitted to accomplish a great deal of good in the earth. They have played a more important part in the history of the world than most persons would at first suppose. In many parts of England, according to Darwin, a weight of more than ten tons of dry earth annually passes through their bodies and is brought to the surface in each acre of land, so that the entire superficial bed of vegetable mould passes through their bodies in the course of every few years; and in most parts of the forests and pasture-lands of Southern Brazil, where several species of earth-worms abound, the whole soil to a depth of a quarter of a metre looks as though it had passed through the intestines of worms, even where scarcely any castings are to be observed upon the surface. The upper crust is continually being eaten and ejected by them, thus aiding the fertility of the soil, as well as conveying water and air to the interior by the myriads of burrows which they drill. The vast quantities of leaves that they drag into their holes tend also to enrich the ground. Nor does their good end here. They cover up seeds, undermine rocks, burying them up, and to their labors is due the preservation of many ruins and ancient works of art. Numerous old-time Roman villas have been discovered beneath the ground in England, whose entombments were undoubtedly caused by the worms that undermined them and deposited their castings upon the floors, till finally, aided by other causes, they disappeared from sight.

When a wide, turf-covered expanse of earth is beheld, we would do well to remember that its smoothness, upon which so much of its beauty depends, is largely due to all the inequalities having been slowly levelled by worms. That all the surface-mould of any such expanse has passed, and will again pass, every few years through the bodies of worms is a marvellous reflection, and one which should not be lightly dismissed from the mind. The most ancient, as well as one of the most valuable of man’s inventions, is the plough. But long before man existed the land was in fact regularly ploughed, and still continues to be ploughed, by earth-worms. No other animal has played such a part in history as have these lowly-organized creatures. True it is that corals, which are still lower in the scale of animals, have performed more conspicuous work in the innumerable reefs and islands they have built in the great oceans, but their work is confined to the tropical zones, while that of the earth-worm is well-nigh universal. Verily it is by the little things in life that the Creator has erected the most stupendous monuments to show forth His infinite power and wisdom.