176 THICKNESS OF THE MOULD Chap. III. 



summary 15 tons as the weight of the castings 

 annually thrown up on an acre of land, each 

 worm must annually eject 20 ounces. A full- 

 sized casting at the mouth of a single burrow 

 often exceeds, as we have seen, an ounce in 

 weight ; and it is probable that worms eject 

 more than 20 full-sized castings during a 

 year. If they eject annually more than 20 

 ounces, we may infer that the worms which 

 live in an acre of pasture land must be less 

 than 26,886 in number. 



Worms live chiefly in the superficial mould, 

 which is usually from 4 or 5 to 10 and even 

 1 2 inches in thickness ; and it is this mould 

 which passes over and over again through 

 their bodies and is brought to the surface. 

 But worms occasionally burrow into the sub- 

 soil to a much greater depth, and on such 

 occasions they bring up earth from this 

 greater depth ; and this process has gone on 

 for countless ages. Therefore the superficial 

 layer of mould would ultimately attain, 

 though at a slower and slower rate, a thick- 

 ness equal to the depth to which worms 

 ever burrow, were there not other opposing 

 agencies at work which carry away to a 



