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 10 tons of 



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 



