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.,