80 INTRODUCTION TO ZOOLOGY CHAP. 



(3) In a chalky district where the chalk was overlaid with 

 6 to 1 4 feet of stiff red clay and this covered by a few inches 

 of dark mould, a surface layer of broken chalk was added ; 

 after 29 years this layer was found 7 inches below the surface, 

 mould having, in this case also, been thrown up at the rate of 

 22 inch a year. 



It is obvious that worms affect the surface of the land in 

 two ways : they actually bring up soil as castings, and cover 

 with it the objects on the surface ; also they undermine the 

 ground below any object with their burrows and so cause 

 it to sink, so that there is an actual change in its level. In 

 this way they have doubtless played a considerable part in 

 causing the subsidence and burial of old Roman and other 

 remains. 1 . 



The number of worms living below a given area of ground 

 is very large. In garden soil, where they are specially numer- 

 ous, it has been calculated that there are over 50,000 to an 

 acre. In such a situation more than 10 tons of earth will pass 

 through their bodies and be cast up at the surface in one year, 

 and this in 10 years would form a layer of finely divided 

 surface soil at least 2 inches deep. The formation of new 

 surface soil by them does not, however, go on continuously at 

 this rate, for since earthworms find their food chiefly in the 

 richer surface soil, when they have burrowed in this and formed 

 a layer of 7 to 12 inches thick of their castings, they will 

 continue to burrow in the same soil, passing it over and over 

 again through their bodies. 



Worms also play some part in the breaking down of the 

 rocks of the sub-soil, for, owing to the digestion of the leaves, 

 acids similar to humic acids form in the intestine, pass out 

 with the castings, and are washed down by the rain on to the 

 rocks, disintegrating their surfaces. 



In many ways, therefore, worms affect the surface of the 

 earth, making it more suitable for the growth of plant-life, and 

 thus rendering a service of great value to man. As Darwin 

 says in his concluding paragraph : " When we behold a wide 

 turf-covered expanse ... it is a marvellous reflection that 

 the whole of the superficial mould over any such expanse has 

 passed, and will again pass, every few years, through the 

 body of worms. The plough is one of the most ancient and 

 1 Vegetable, Mould and Earthworms, chap, v. 



