The formation of vegetable mould : through the action of worms with observations on their habits / By Charles Darwin ; 13th thousand with illustrations. Darwin, Charles, 1809-1882. Opisthopora. Humification. QL394 .D3 1897 575.01 DAF Includes index. London : Murray, 1904. 1904. 1897 Text viii, 298 p. : eng