THE EARTHWORM 221 



out more or less evenly is one fifth of an inch a year. In the 

 course of comparatively few years this is sufficient to conceal 

 from sight objects of considerable size. Indeed, Darwin wit- 

 nessed a sterile, stony field with flints " as large as a child's 

 head " transformed into a fertile, grass-covered pasture, " so 

 that after thirty years (1871) a horse could gallop over the 

 compact turf from one end of the field to the other, and not 

 strike a single stone with its shoes. This was certainly the 

 work of worms, for, though castings were not frequent for 

 several years, yet some were thrown up month after month, 

 and these gradually increased in numbers as the pasture 

 improved." 



Estimates which Darwin based on careful observations 

 indicate that over fifty thousand earthworms find plenty of 

 working-room in an acre of ground, and that these bring 

 to the surface annually from fourteen to eighteen tons of 

 earth. 



