Chap. VII. CONCLUSION. 309 



indefinitely long period every object, not 

 liable to decay, which is dropped on the 

 surface of the land, by burying it beneath 

 their castings. Thus, also, many elegant and 

 curious tesselated pavements and other ancient 

 remains have been preserved ; though no 

 doubt the worms have in these cases been 

 largely aided by earth washed and blown 

 from the adjoining land, especially when cul- 

 tivated. The old tesselated pavements have, 

 however, often suffered by having subsided 

 unequally from being unequally undermined 

 by the worms. Even old massive walls may 

 be undermined and subside ; and no building 

 is in this respect safe, unless the foundations 

 lie 6 or 7 feet beneath the surface, at a depth 

 at which worms cannot work. It is probable 

 that many monoliths and some old walls have 

 fallen down from having been undermined 

 by worms. 



Worms prepare the ground in an excellent 

 manner for the growth of fibrous-rooted 

 plants and for seedlings of all kinds. They 

 peiiodically expose the mould to the air, and 

 sift it so that no stones larger than the par- 



