THE CIRCULATION OF FOOD MATERIALS 301 



the utmost importance in the formation of rocks and in determining the 

 configuration of the surface of the globe, for both substances are universally 

 present, and the decomposition and reconstruction of silicious and car- 

 boniferous rocks are processes which are perpetually recurring. 



The distribution of diffusible and transportable substances is a purely 

 physical problem, as is also the continual readjustment of differences of 

 potential. From this standpoint it is of the highest importance that 

 oxygen and carbon dioxide should be gases which are readily and rapidly 

 distributed throughout the atmosphere. Water, on the other hand, will 

 wash out all the saline constituents of a sandy soil, and hence arises the 

 great importance of the absorptive and retentive powers of a soil rich in humus 

 (Sect. 28). When the covering of vegetation disappears from a localized 

 area, the unavoidable trifling loss of nitrogenous compounds and ash con- 

 stituents is counterbalanced by fresh supplies derived from the water of the 

 soil, from rain and dust, and from the weathering of rocky fragments, so that 

 by means of the absorptive powers of the humus the nourishment necessary 

 to permit of the reappearance of vegetation is retained for a time. These 

 agencies are insufficient, however, to make up for the large quantities 

 of ash constituents and nitrogenous compounds which are removed from 

 the soil by the annual harvest, and hence gradual impoverishment ensues 

 unless the constituents which are removed are replaced by means of a supply 

 of appropriate manure. 



By the weathering of rock, and of the rocky particles in the soil, 

 a continual supply of the essential ash constituents may be provided for 

 for a very long, but not for an indefinite, time, and indeed every river 

 annually carries a certain amount of the soluble saline constituents of 

 the soil to the sea. The gradual sinking and rising of land which is 

 always occurring, is perhaps quite sufficient to counterbalance this loss, 

 for the soil of a new continent rising from the floor of the ocean may 

 contain all the constituents removed by rivers from pre-existent continents 

 and carried to the sea. The nitrogen of the nitrogenous compounds 

 washed away from the soil may return to dry land in the form of volatile 

 compounds, such as ammonia and oxides of nitrogen, or may be set free 

 as atmospheric nitrogen. Our knowledge of life and the conditions 

 of existence in the depths of the ocean is very incomplete ' ; we do 

 not know whether all the organic food for deep-sea life is derived from 

 the surface, or whether certain of the deep-sea organisms can produce, 

 by means of chemical energy, the organic material required by themselves 

 and by the organisms which may prey upon them, or whether organisms 



1 For the lowest depths at which Bacteria have been found, see Dieudonne, Biol. Centralbl., 

 1895, p. 108 ; C. Schroter, Vegetation des Bodensees, 1896, p. 16. 



