PROFOUND IMPORTANCE OF LIFE. 505 



forests of the Tropics, such as those of Brazil and central Africa. The 

 difficulties of traversing such a region are almost incredible. The forests 

 are a tangle of vegetation, which it is possible to penetrate only by cutting 

 one's way. Under such conditions the rocks in the belt of weathering are 

 continuously acted upon in the most powerful way, under the most favorable 

 conditions of humidity and temperature, by the chemical products furnished 

 by the plants and animals, alive and dead. No one can realize the 

 abundance of plant and animal material, dead and alive, both above and 

 below the soil, in a dense forest region without traveling through one. The 

 soil for many feet below the sxirface seems to be mainly composed of vege- 

 tation. The individuals vary from great trees, through plants of medium 

 size, to the smaller plants, such as the mosses, fungi, and finally the 

 bacteria. To a large extent the living plants are feeding on the dead and 

 decomposing ones. At the surface the quantity of organic material is at a 

 maximum. Normally the quantity steadily decreases from the surface to 

 a depth of 12 to 15 meters, where it becomes unimportant. Thus there is 

 every gradation between places at the surface where the organic material 

 occupies the larger part of the space to a place below the surface where 

 the inorganic material occupies all the space. 



One who has studied regions where life is dense will i-eadily believe 

 that great geological consequences follow from the chemical activity of 

 compounds produced by animals and plants, both alive and dead. It is 

 not too much to say that life is an essential factor in far the larger part of 

 the chemical work of decomposition and solution of the rocks. 



Of the plants, the minute bacteria are among the most important. Of 

 the animals, earthworms are perhaps the most important. The prodigious 

 numbers of these compensate for their smallness. They furnish a striking 

 illustration of the principle that small numerous agents may be more potent 

 than large and more conspicuous but less numerous ones. 



From the foregoing it is plain that living and dead plants and animals 

 interact with one another and with the rocks in the production and main- 

 tenance of the soil. As shown on pages 452-453, chlorophyll-bearing plants, 

 and especially one group of them, the leguminosge, working in conjunction 

 with bacteria, are the agents which collect carbon and nitrogen from the 

 atmosphere and place them in the soil in a combined state. Dead plants 

 and animals are the materials upon which living bacteria, oxygen, and water 

 work to produce carbonic, nitric, and sulphuric acids. These form carbon- 



