18 THE PROBLEMS OF EVOLUTION 



acteristics are more important for the purposes of 

 our inquiry than any details. The recognition of 

 three phases of environment is by no means new,^ 

 and by no means difficult to grasp. In its life the 

 organism encounters a number of physical and 

 chemical factors which cannot fail to influence it. 

 The importance of water and carbon dioxide, for 

 example, is beautifully expressed by Henderson. ^° 

 Since the perfection of the hydrogen ion method 

 of determining acidity, this condition has been 

 shown to exert an important influence on the dis- 

 tribution of organisms. Temperature, especially 

 daily and seasonal fluctuations thereof, is an 

 unavoidable part of any environment which must 

 be of importance to the organism. The nature of 

 the soil, air or water pressure incidental to bathy- 

 metric distribution, the inorganic constituents of 

 the environment, motion of the air, all of these 

 things influence life and constitute the physical or 

 non-living environment to which organisms must 

 adjust themselves. 



The physical environment is very important to 

 the green plants, since they depend for their food 

 upon inorganic materials. To them water and 

 carbon dioxide are the fundamental necessities of 

 life. Together with inorganic salts and the radiant 



^Osborn, H. F., "Tetraplasy," Jn. Acad. Nat. Sci. Phil., Special Volume, 

 pp. 275-309, 1912; Osborn, H. F., Origin and Evolution of Life, p. 18, et 

 seg., 1917. 



*° The Fitness of the Environment, 1913. 



