CHAPTER IV. 



THE MANIFESTATIONS OF LIFE. 

 IRRITABILITY. 



Irritability is that property of living substance by 

 virtue of which it responds to stimulation. It is a 

 universal and fundamental characteristic, and forms the 

 starting-point of all vital manifestation. The behavior 

 of living substance is determined by the stimulations 

 it receives, and without stimulation it does nothing and 

 cannot be recognized as living. When matter is no 

 longer irritable, and when it fails to respond to stimula- 

 tion, we declare it to be dead, or no longer living. 



The stimuli that excite the irritable reactions, and thus 

 govern and determine the vital manifestations are of 

 two kinds: 1. Intrinsic, inherited, and regulating, and 2. 

 extrinsic and modifying. 



Stimuli of the first class determine that the living 

 thing, regardless of its simplicity or complexity, shall 

 conform to a certain type, perform certain functions, pass 

 through a definite cycle of existence, and impart a cer- 

 tain amount of its substance to new individuals of its 

 own kind by whom it may be survived. Those of the 

 second class initiate, accelerate, retard, or modify these 

 effects. 



Every living thing is thus the creature of circumstance, 

 dominated and controlled by inheritance and environ- 

 ment. 



The action of stimuli may be continuous, intermittent, 

 or occasional, as to time; normal, deficient, or excessive 

 as to intensity, and beneficial or injurious according to 

 duration, intensity, and quality. 



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