20 FIELD, FOREST AND FARM 



the smooth rock in order to convert it into fertile 

 soil. It is from these heights that this fertile soil 

 has descended, little by little, washed down by the 

 rain, until it has fertilized the valleys. This work is 

 going on all the time ; in hilly regions plants of the 

 lowest order are constantly adding to the extent of 

 arable land. The little threads of rain-water that 

 furrow these regions carry away with them some of 

 this humus and bear it to the plains below. 



"What a worthy subject for our thoughtful study 

 is this formation of arable soil by these legions of 

 inferior plants, obscure workers indefatigably crum- 

 bling the rock ! What immense results obtained by 

 the simplest means!" 



