THE EARTHWORM IN NATURE 37 



In the process of transition back to the soil, vegetation be- 

 comes humus. By impregnating, compounding, and combining 

 humus with the parent mineral soil, nature slowly builds topsoil. 

 Just as we have, from a practical standpoint, inexhaustible re- 

 sources of parent mineral soil, we also have practically inexhaust- 

 ible sources of vegetable material to draw upon for purposes of 

 soil-building, sources which have never heretofore been exploited 

 for the use of man. 



