OBSTACLES TO LIFE 339 



all the energy which living beings constantly use. The amount 

 of living matter that can maintain itself on a given territory 

 depends largely upon the amount of light available. The tropics, 

 in addition to being warmer, also receive more sunlight and are 

 therefore more closely occupied by living beings than the frigid 

 zones. There is an almost continuous gradation in the density 

 of population 1 between the equator and the poles. 



On the other hand, extreme intensity of light is itself a 

 serious obstacle to the normal processes of living protoplasm. 

 Light interferes with the growing process (p. 38) and may be 

 destructive to protoplasm. We see again, then, that a form of 

 energy that is essential to life may be a source of danger to it. 



393. Salts and life. The various mineral salts found in the 

 ocean, in other bodies of water, and in the soil are ordinarily 

 absorbed by living beings through the process of osmosis, and 

 many of the salts take active parts in the processes that go 

 on in living protoplasm. Many of them are apparently indif- 

 ferent in their action, being neither helpful nor injurious ; a 

 few are injurious ; and of those that are essential, some are 

 injurious in large quantities. On the other hand, a scarcity of 

 particular elements, or of compounds containing these ele- 

 ments, will absolutely prevent the growth and development of 

 living things. The kind of life that is possible in each of two 

 regions that are substantially alike as to temperature, moisture, 

 and light will in many cases be determined by the chemical 

 condition of the substratum. 



394. Excess of air. The air, which is necessary to practically all 

 living beings either directly, as an immediate source of oxygen, or 

 indirectly, as a more remote source of oxygen (for plants and animals 

 living in the water) and as a source of carbon dioxid, never seems to 

 be injurious when in excess. Indeed, we do not know of any situa- 

 tion where the air is in excess. If we consider high atmospheric pres- 

 sure in deep holes in the earth as such situations, we may not be sure 



1 Population refers here, of course, to all plants and animals and not 

 merely to human beings. The statement is not strictly true for human beings. 



