Chap. III. ANNUALLY ACCUMULATED. 175 



which in all ordinary cases add to the 

 amount of mould, and which would not be 

 included in the castings that were collected, 

 namely, the fine earth brought up to the 

 surface by burrowing larvas and insects, espe- 

 cially by ants. The earth brought up by moles 

 generally has a somewhat different appearance 

 from vegetable mould ; but after a time would 

 not be distinguishable from it. In dry coun- 

 tries, moreover, the wind plays an important 

 part in carrying dust from one place to another, 

 and even in England it must add to the mould 

 on fields near great roads. But in our country 

 these latter several agencies appear to be of 

 quite subordinate importance in comparison 

 with the action of worms. 



We have no means of judging how great a 

 weight of earth a single full-sized worm ejects 

 during a year. Hensen estimates that 53,767 

 worms exist in an acre of land; but this is 

 founded on the number found in gardens, and 

 he believes that only about half as many live 

 in corn-fields. How many live in old pasture 

 land is unknown ; but if we assume that half 

 the above number, or 26,886 worms live on 

 such land, then taking from the previous 



