{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.