i68 THE BOOK OF NATURE STUDY 



It should, however, be noted that the majority of such worms are 

 found to be in a diseased condition, and suffering from the attacks 

 of internal parasites. 



Uses. It is very doubtful if worms do any harm to the roots 

 of healthy plants, notwithstanding the general belief to the con- 

 trary. But it is certain that they perform a very useful function 

 in maintaining the soil in a state suitable to vegetation. Their 

 burrows serve as ventilating tubes down which the air can travel 

 and become available for respiration by the roots of plants ; at 

 the same time they open up drainage channels and prevent the 

 surface from becoming waterlogged. The roots too find an 

 easy path through the soil along the lines of the burrows even 

 after the walls have more or less collapsed ; moreover, the ex- 

 crement with which the burrows are lined is earth peculiarly 

 well suited to the root-fibres, being moist, loose, and chemically 

 fertile. But perhaps the most important work done is that of 

 tilling the soil, bringing up the deeper material and casting it 

 upon the surface to be exposed to and sweetened by the air. 

 Charles Darwin estimated that in every acre of soil of average 

 agricultural quality there are about 50,000 worms ; he further 

 ascertained that each worm casts up about 20 oz. of earth 

 annually. A simple calculation with these figures shows that 

 about 28 tons of soil are annually brought by worms to the surface 

 on every acre. When it is remembered that pasture land and all 

 land laid down to grass of any kind cannot from the nature of 

 the case be ploughed or digged by human husbandry, it is evident 

 that the debt of mankind to these humble tillers of the earth is 

 not small. 



We have also to thank worms for the preservation of many 

 objects of historic interest, such as coins, ornaments, weapons, 

 and even the floors and remains of ancient buildings that have in 

 course of time become buried by the soil thrown up as " casts." 

 The disappearance of such things is of course hastened by excava- 

 tions made by the worms below the surface, for the collapse of 

 the tunnels inevitably causes objects resting on the surface to 

 sink downwards. 



Again, the deepening of the subsoil and the general denuda- 

 tion of land surfaces is largely the result of the industry of these 



