CHAP. VII. CONCLUSION. 313 



periodically expose the mould to the air, and 

 sift it so that no stones larger than the par- 

 ticles which they can swallow are left in it 

 They mingle the whole intimately together, 

 like a gardener who prepares fine soil for his 

 choicest plants. In this state it is well fitted 

 to retain moisture and to absorb all soluble 

 substances, as well as for the process of nitri- 

 fication. The bones of dead animals, the 

 liarder parts of insects, the shells of land- 

 molluscs, leaves, twigs, &c., are before long 

 all buried beneath the accumulated castings of 

 worms, and are thus brought in a more or 

 less decayed state within reach of the roots 

 of plants. Worms likewise drag an infinite 

 number of dead leaves and other parts of 

 plants into their burrows, partly for the sake 

 of plugging them up and partly as food. 



The leaves which are dragged into the bur- 

 TOWS as food, after being torn into the finest 

 shreds, partially digested, and saturated with 

 .the intestinal and urinary secretions, are com- 

 mingled with much earth. This earth forms 

 the dark coloured, rich humus which almost 

 everywhere covers the surface of the land 

 with a fairly well-defined layer or mantle. 



