CHAP. III. ANNUALLY ACCUMULATED. 177 



lower level some of the finest earth which is 

 continually being brought to the surface by- 

 worms. How great a thickness vegetable 

 mould ever attains, I have not had good 

 opportunities for observing; but in the next 

 chapter, when we consider the burial of 

 ancient buildings, some facts will be given on 

 this head. In the two last chapters we 

 shall see that the soil is actually increased, 

 though only to a small degree, through the 

 agency of worms; but .their chief work is 

 to sift the finer from the coarser particles, to 

 mingle the whole with, vegetable debris, and 

 to saturate it with their intestinal secretions. 



Finally, no one who considers the facts 

 given in this chapter on the burying of 

 small objects and on the sinking of great 

 stones left on the surface on the vast 

 number of worms which live within a 

 moderate extent of ground on the weight of 

 the castings ejected from the mouth of the 

 same burrow on the weight of all the cast- 

 ings ejected within a known time on a measured 

 space will hereafter, as I believe, doubt that 

 worms play an important part in nature. 



