EARTHWORM. 211 
worms have been ploughers before the plough. Darwin 
calculated that there were on an average over 53,000 
earthworms in an acre of garden ground, that ro tons of 
Fic, 110,—Earthworms, 
soil per acre pass annually through their bodies, and that 
they cover the surface with earth at the rate of 3 in. in 
fifteen years. He was therefore led to the conclusion that 
earthworms have been the great soil- makers, or, more 
