SOIL OR SURFACE-MATTER. 227 



associated with them. It would appear, however, 

 that in these countries where soil has formed and a 

 protecting envelope of grass or the like has grown 

 on it, the increase downward is imperceptible, if any 

 at all takes place ; the increase that takes place being 

 upward from the vegetable growth, always assisted 

 by the work of worms, ants, and other earth-boring 

 animals. "We are aware that this opinion is con- 

 troverted, as Darwin seems to be of opinion that the 

 formation of soil is solely due to worm- work ; l but 

 after careful examination and consideration, we are 

 forced to come to conclusions different from those of 

 that eminent observer. A foreign substance placed 

 on grassland will be gradually covered up by a growth 

 of soil over it. This soil Darwin seems to believe to 

 be entirely due to the labour of earthworms, which 

 excavate in the ground under the foreign substances 

 and deposit over it ; so that the total thickness of the 

 soil is not increased upwards by mould formed from 

 vegetable decay; but all is taken from below the 

 foreign substance and placed above it ; thereby adding 

 to the thickness of the upper stratum of the mould, 

 and diminishing the thickness of the portion below 

 the foreign substance. To earthworms cannot be 

 due the formation of all the soil, for this reason 

 all worm-formed mould must be ejected by them ; 



1 "Transactions of the Geological Society of London," 2d Series, vol. 

 v. p. 505. 



