120 HABITS OF WOKMS. Chap. II. 



the surface of the ground. When they can 

 find any cavity, as when burrowing in newly 

 turned-up earth, or between the stems of 

 banked-up plants, they deposit their castings 

 in such places. So again any hollow beneath 

 a large stone lying on the surface of the 

 ground, is soon filled up with their castings. 

 According to Hensen, old burrows are habitu- 

 ally used for this purpose ; but as far as my 

 experience serves, this is not the case, except- 

 ing with those near the surface in recently dug 

 ground. I think that Hensen may have been 

 deceived by the walls of old burrows, lined 

 with black earth, having sunk in or collapsed ; 

 for black streaks are thus left, and these are 

 conspicuous when passing through light- 

 coloured soil, and might be mistaken for 

 completely filled-up burrows. 



It is certain that old burrows collapse in 

 the course of time ; for as we shall see in the 

 next chapter, the fine earth voided by worms, 

 if spread out uniformly, would form in many 

 places in the course of a year a layer ^ of an 

 inch in thickness ; so that at any rate this large 

 amount is not deposited within the old unused 

 burrows. If the burrows did not collapse, 



