34 THE NATURE OF LIFE 



comes to rest. In many cells no contrac- 

 tion is ever observed yet there is no ques- 

 tion that they are alive. Evidently it is 

 not one of the essential characteristics 

 of living matter. 



IRRITABILITY 



Let us now consider irritability as a 

 criterion of life. Irritability is regarded 

 as one of the most fundamental charac- 

 teristics of living matter, since it enables 

 the organism to adjust itself continually 

 to its environment. 



Irritability is often defined as response 

 to a stimulus. Let us see what is meant 

 by this definition. 



For the sake of simplicity let us con- 

 sider a purely physical stimulus, such as 

 a blow. We might compare the effect 

 upon a man with that upon such an ob- 

 ject as a billiard ball. 



In the first place the response of the 



