THE ORIGIN OF FERTILE SOIL 19 



of fertilizing material. Next come ferns, which re- 

 quire a richer soil, and after that a few tufts of 

 grass; then some brambles, some meager shrubs; 

 and thus with each succeeding year the fertile soil is 

 added to from the new remnants of lava and mold 

 left by the preceding generation of plants that have 

 gone to decay. It is in this way that gradually a 

 lava-bed finally becomes covered with a forest. 



"Our own arable land had a similar origin. Ster- 

 ile rocks, hard as they are, contributed the mineral 

 part by being reduced to dust through the combined 

 action of water, air, and frost; and the successive 

 generations of plant-life, beginning with the sim- 

 plest, furnished the mold. 



"Notice how admirably, in the processes of nature, 

 the smallest of created beings perform their part 

 and contribute as best they can to the general har- 

 mony. To produce fertile soil there is needed some- 

 thing more than the frosts and thaws that crumble 

 the hardest rock: there is need of plants hardy 

 enough to live on this sterile soil, such as tough 

 grasses, mosses, lichens, which gnaw the stone. It 

 is through the medium of these rudimentary plants, 

 so pitiful in appearance and yet so hardy, that the 

 dust of the rocks is enriched with mold and converted 

 into a soil capable of bearing other and more delicate 

 plants. 



"It is not in cultivated fields that you will find 

 those thick carpets of mosses and lichens, valiant 

 disintegrators of stone; it is on the mountain-tops 

 that they can be seen at their work of crusting over 



