'Lecons de Geologie Pratique,' tom. i. 1845, p. 140.
 'Transactions Geolog. Soc.' vol. v. p. 505. Read November 1, 1837.
 'Histoire des progres de la Geologie,' tom. i. 1847, p. 224.
 'Zeitschrift fur wissenschaft. Zoologie,' B. xxviii. 1877, p. 361.
 'Gardeners' Chronicle,' April 17, 1869, p. 418.
 Mr. Darwin's attention was called by Professor Hensen to P. E. Muller's work on Humus in 'Tidsskrift for Skovbrug,' Band iii. Heft 1 and 2, Copenhagen, 1878. He had, however, no opportunity of consulting Muller's work. Dr. Muller 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 uber die naturlichen Humusformen, unter deren Einwirkung auf Vegetation und Boden,' 8vo., Berlin, 1887.
 'Bidrag till Skandinaviens Oligochaetfauna,' 1871.
 'Die bis jetzt bekannten Arten aus der Familie der Regenwurmer,' 1845.
 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."
 I shall have occasion often to refer to M. Perrier's admirable memoir, 'Organisation des Lombriciens terrestres' in 'Archives de Zoolog. exper.' 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.
 Morren, 'De Lumbrici terrestris Hist. Nat.' 1829, p. 67.
 'De Lumbrici terrestris Hist. Nat.' p. 14.
 Histolog. Untersuchungen uber die Regenwurmer. 'Zeitschrift fur wissenschaft. Zoologie,' B. xix., 1869, p. 611.
 For instance, Mr. Bridgman and Mr. Newman ('The Zoologist,' vol. vii. 1849, p. 2576), and some friends who observed worms for me.
 'Familie der Regenwurmer,' 1845, p. 18.
 'The Zoologist,' vol. vii. 1849, p. 2576.
 'Familie der Regenwurmer,' 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."
 'De Lumbrici terrestris Hist. Nat.' p. 19.
 'Archives de Zoologie experimentale,' 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.
 On the action of the pancreatic ferment, see 'A Text-Book of Physiology,' by Michael Foster, 2nd edit. pp. 198-203. 1878.
 Schmulewitsch, 'Action des Sucs digestifs sur la Cellulose.' Bull. de l'Acad. Imp. de St. Petersbourg, tom. xxv. p. 549. 1879.
 Claparede doubts whether saliva is secreted by worms: see 'Zeitschrift fur wissenschaft. Zoologie,' B. xix. 1869, p. 601.
 Perrier, 'Archives de Zoolog. exper.' July, 1874, pp. 416, 419.
 'Zeitschrift fur wissenschaft. Zoologie,' B. xix, 1869, pp. 603-606.
 De Vries, 'Landwirth. Jahrbucher,' 1881, p. 77.
 M. Foster, 'A Text-Book of Physiology,' 2nd edit. 1878, p. 243.
 M. Foster, ut sup. p. 200.
 Claparede remarks ('Zeitschrift fur wisseuschaft. Zoolog.' B. 19, 1869, p. 602) that the pharynx appears from its structure to be adapted for suction.
 An account of her observations is given in the 'Gardeners' Chronicle,' March 28th, 1868, p. 324.
 London's 'Gard. Mag.' xvii. p. 216, as quoted in the 'Catalogue of the British Museum Worms,' 1865, p. 327.
 'Familie der Regenwurmer,' p. 19.
 In these narrow triangles the apical angle is 9 degrees 34 seconds, and the basal angles 85 degrees 13 seconds. In the broader triangles the apical angle is 19 degrees 10 seconds and the basal angles 80 degrees 25 seconds.
 See his interesting work, 'Souvenirs entomologiques,' 1879, pp. 168-177.
 Mobius, 'Die Bewegungen der Thiere,' 1873, p. 111.
 'Annals and Mag. of N. History,' series ii. vol. ix. 1852, p. 333.
 'Archives de Zoolog. exper.' tom. iii. 1874, p. 405.
 I state this on the authority of Semper, 'Reisen im Archipel der Philippinen,' Th. ii. 1877, p. 30.
 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 Perichaeta 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 Perichaeta 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 Perichaeta abound.
 'Zeitschrift fur wissenschaft. Zoolog.' B. xxviii. 1877, p. 364.
 'Zeitschrift fur wissenschaft. Zoolog.' B. xxviii. 1877, p. 356.
 Perrier, 'Archives de Zoolog. exper.' tom. 3, p. 378, 1874.
 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.
 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 he 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.
 S. W. Johnson, 'How Crops Feed,' 1870, p. 139.
 'Nature,' November 1877, p. 28.
 'Proc. Phil. Soc.' of Manchester, 1877, p. 247.
 'Trans. of the New Zealand Institute,' vol. xii., 1880, p. 152.
 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."
 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.
 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."
 'Zeitschrift fur wissensch. Zoolog.' Bd. xxviii., 1877, p. 360.
 See Mr. Dancer's paper in 'Proc. Phil. Soc. of Manchester,' 1877, p. 248.
 'Lecons de Geologie pratique,' 1845, p. 142.
 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.
 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.
 These details are taken from the 'Penny Cyclopaedia,' article Hampshire.
 "On the denudation of South Wales," 'Memoirs of the Geological Survey of Great Britain,' vol. 1., p. 297, 1846.
 'Geological Magazine,' October and November, 1867, vol. iv. pp. 447 and 483. Copious references on the subject are given in this remarkable memoir.
 A. Tylor "On changes of the sea-level," ' 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.
 "An account of the fine dust which often falls on Vessels in the Atlantic Ocean," Proc. Geolog. Soc. of London, June 4th, 1845.
 For La Plata, see my 'Journal of Researches,' during the voyage of the Beagle, 1845, p. 133. Elie de Beaumont has given ('Lecons de Geolog. 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.
 These statements are taken from Hensen in 'Zeitschrift fur 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.
 I have given some facts on the climate necessary or favourable for the formation of peat, in my 'Journal of Researches,' 1845, p. 287.
 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.
 See, for references on this subject, S. W. Johnson, 'How Crops Feed,' 1870, p. 326.
 This statement is taken from Mr. Julien, 'Proc. American Assoc. Science,' vol. xxviii., 1879, p. 330.
 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.
 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.
 'Archives de Zoolog. exper.' tom. iii. 1874, p. 409.
 'Nouvelles Archives du Museum,' tom. viii. 1872, pp. 95, 131.
 Morren, in speaking of the earth in the alimentary canals of worms, says, "praesepe cum lapillis commixtam vidi:" 'De Lumbrici terrestris Hist. Nat.' 1829, p. 16.
 Perrier, 'Archives de Zoolog. exper.' tom. iii. 1874, p. 419.
 Morren, 'De Lumbrici terrestris Hist. Nat.' p. 16.
 'Archives de Zoolog. exper.' tom. iii. 1874, p. 418.
 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, which browse on the living corals.
 Anniversary Address: 'The Quarterly Journal of the Geological Soc.' May 1880, p. 59.
 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.
 'Elements of Geology,' 1865, p. 20.
 'Lecons de Geologie pratique, 1845; cinquieme Lecon. All Elie 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.
 'Illustrations of the Huttonian Theory of the Earth,' p. 107.
 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' (Hochacker, and Heidenacker) 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."
 White of Selborne has some good remarks on the service performed by worms in loosening, the soil. Edit, by L. Jenyns, 1843, p. 281.
 'Zeitschrift fur wissenschaft. Zoolog.' B. xxviii. 1877, p. 360.