THE MANIFESTATIONS OF LIFE 37 



plasm, irritability returns, but if further disturbance is 

 effected, exhaustion and death may ensue. 



Since all the reactions of irritability are associated with 

 more or less marked expenditures of energy, they all 

 result in metabolic disturbances, and are all associated 

 with chemical changes. 



Certain agents cold, chloroform, ether, chloral, etc. 

 inhibit the irritative phenomena. These are called 

 depressants and anesthetics, and are quite well known 

 experimentally though they are not known to play any 

 part in the normal vital processes. 



Irritable Reactions Toward Stimuli of Unknown Nature. 

 Among these are included those stimuli by which the 

 development of the organism is governed, and its auto- 

 matic behavior determined. A superficial acquaintance 

 with embryology is sufficient to show that under appro- 

 priate conditions the germinal cells of plants and animals 

 invariably develop according to a fixed plan. Further- 

 more, the developed animal behaves according to a fixed 

 plan of conduct inherited from its ancestors. In ignor- 

 ance of the character of the impulses thus engaged, we 

 look upon them as of physico-chemical nature. 



Irritable Reactions Occasioned by Stimuli of Known 

 Nature. These external stimuli embrace all those agents 

 by which the behavior of the organism in its relations 

 to the external world is determined. The irritable re- 

 sponses to these agents are frequently referred. to as 

 tropisms, and receive special denominations according to 

 their respective qualities. 



THERMOTROPISM OR RESPONSE TO THERMAL 

 STIMULATION. 



No living substance is indifferent to variations of 

 temperature. It is temperature that makes the con- 

 ditions under which life is possible, and it is tempera- 

 ture that stimulates activity when the proper conditions 

 obtain. Thus, active life is impossible without water by 



