INTRODUCTION. 7 



already alluded to, which appeared in 1877.* 

 Before entering on details with respect to the 

 castings, it will be advisable to give some 

 account of the habits of worms from my 

 own observations and from those of other 

 naturalists. 



* Mr. Darwin's attention was called by Professor Hensen to 

 P. E. Miiller's work on Humus in ' Tidsskrift for Skovbrug,' 

 Band iii. Heft 1 and 2, Copenhagen, 1878. He had, however, 

 no opportunity of consulting Miiller's work. Dr. Miiller pub- 

 lished 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 iiber die nattirlichen 

 Humusformen, unter deren Einwirkung auf Vegetation und 

 Boden,' 8vo., Berlin, 1887. 



[FIRST EDITION, 



October 10th, 1881.] 



