Chap. II. EAETH SWALLOWED AS FOOD. 105 



be seen every morning, and the amount of 

 earth ejected from the same burrow on succes- 

 sive days is large. Yet worms do not burrow 

 to a great depth, except when the weather 

 is very dry or intensely cold. On my lawn 

 the black vegetable mould or humus is only 

 about 5 inches in thickness, and overlies light- 

 coloured or reddish clayey soil: now when 

 castings are thrown up in the greatest 

 profusion, only a small proportion are light 

 coloured, and it is incredible that the worms 

 should daily make fresh burrows in every 

 direction in the thin superficial layer of 

 dark-coloured mould, unless they obtained 

 nutriment of some kind from it. I have ob- 

 served a strictly analogous case in a field near 

 my house where bright red clay lay close 

 beneath the surface. Again on one part of 

 the Downs near Winchester the vegetable 

 mould overlying the chalk was found to be 

 only from 3 to 4 inches in thickness ; and the 

 many castings here ejected were as black as 

 ink and did not effervesce with acids ; so that 

 the worms must have confined themselves to 

 this thin superficial layer of mould, of which 

 large quantities were daily swallowed. In 



