250 GENERAL ZOOLOGY 



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 in- 

 creased 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. 



