Footnotes:
{1} 'Lecons de Geologie Pratique,' tom. i. 1845, p. 140.
{2} 'Transactions Geolog. Soc.' vol. v. p. 505. Read November 1, 1837.
{3} 'Histoire des progres de la Geologie,' tom. i. 1847, p. 224.
{4} 'Zeitschrift fur 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. 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.
{7} 'Bidrag till Skandinaviens Oligochaetfauna,' 1871.
{8} 'Die bis jetzt bekannten Arten aus der Familie der Regenwurmer,' 1845.
{9} 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."
{10} 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.
{11} Morren, 'De Lumbrici terrestris Hist. Nat.' &c., 1829, p. 67.
{12} 'De Lumbrici terrestris Hist. Nat.' &c., p. 14.
{13} Histolog. Untersuchungen uber die Regenwurmer. 'Zeitschrift fur wissenschaft. Zoologie,' B. xix., 1869, p. 611.
{14} For instance, Mr. Bridgman and Mr. Newman ('The Zoologist,' vol. vii. 1849, p. 2576), and some friends who observed worms for me.
{15} 'Familie der Regenwurmer,' 1845, p. 18.
{16} 'The Zoologist,' vol. vii. 1849, p. 2576.
{17} '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."
{18} 'De Lumbrici terrestris Hist. Nat.' p. 19.
{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.
{20} On the action of the pancreatic ferment, see 'A Text-Book of Physiology,' by Michael Foster, 2nd edit. pp. 198-203. 1878.
{21} Schmulewitsch, 'Action des Sucs digestifs sur la Cellulose.' Bull. de l'Acad. Imp. de St. Petersbourg, tom. xxv. p. 549. 1879.
{22} Claparede doubts whether saliva is secreted by worms: see 'Zeitschrift fur wissenschaft. Zoologie,' B. xix. 1869, p. 601.
{23} Perrier, 'Archives de Zoolog. exper.' July, 1874, pp. 416, 419.
{24} 'Zeitschrift fur wissenschaft. Zoologie,' B. xix, 1869, pp. 603-606.
{25} De Vries, 'Landwirth. Jahrbucher,' 1881, p. 77.
{26} M. Foster, 'A Text-Book of Physiology,' 2nd edit. 1878, p. 243.
{27} M. Foster, ut sup. p. 200.
{28} Claparede remarks ('Zeitschrift fur wisseuschaft. Zoolog.' B. 19, 1869, p. 602) that the pharynx appears from its structure to be adapted for suction.
{29} An account of her observations is given in the 'Gardeners' Chronicle,' March 28th, 1868, p. 324.
{30} London's 'Gard. Mag.' xvii. p. 216, as quoted in the 'Catalogue of the British Museum Worms,' 1865, p. 327.
{31} 'Familie der Regenwurmer,' p. 19.
{32} 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.
{33} See his interesting work, 'Souvenirs entomologiques,' 1879, pp. 168-177.
{34} Mobius, 'Die Bewegungen der Thiere,' &c., 1873, p. 111.
{35} 'Annals and Mag. of N. History,' series ii. vol. ix. 1852, p. 333.
{36} 'Archives de Zoolog. exper.' tom. iii. 1874, p. 405.
{37} I state this on the authority of Semper, 'Reisen im Archipel der Philippinen,' Th. ii. 1877, p. 30.
{38} 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.