ORGANIC RESPONSE 245 



coagulating its proteids, although authentic instances 

 are known of lower plants and animals living in hot 

 springs at a very little below boiling temperature. 



Light. As we have seen, sunlight is an absolutely 

 essential condition of life for all green plants, and 

 hence secondarily for all organisms, and the effect 

 on plants of the alteration or absence of light is very 

 marked. But sunlight is a destructive agent for 

 many groups of molds and bacteria, particularly of 

 many pathogenic forms, and in consequence man 

 and the higher animals are very dependent on its 

 cleansing and purifying influence. Sunlight destroys 

 the " germs " that otherwise would threaten them 

 with extermination through disease. 



Chemical Environment. Since the nature of the 

 soil varies with its chemical composition, the organ- 

 isms living in the soil are affected so far as these 

 chemical substances are in solution. As combina- 

 tions and recombinations of chemical substances are 

 easily brought about and constantly occurring in the 

 soil, the organism in general is very delicately ad- 

 justed to its chemical environment and influenced 

 by it. 



In the air we have both chemical and physical 

 agencies to take into consideration. Not only its 

 pressure (some 15 Ibs. to the square inch), but more 

 particularly its movements, have an important 

 general influence on the organism, both plant and 

 animal. In addition, the composition of the atmos- 

 phere has an important and more significant bearing 



