158 THE NATURE AND ORIGIN OF LIFE 



rendered familiar to us ; and, on the contrary, they seem 

 to resist those external causes which are able to set moving 

 inanimate objects. A trout goes up stream, while a cork 

 goes down. 



The name of irritability has been given to the property 

 which living substances possess of moving under the influ- 

 ence of causes that seem incapable of stirring inanimate 

 objects. Claude Bernard says that irritability is the pro- 

 perty possessed by every anatomical element (that is, by the 

 protoplasm which constitutes it) of being put in activity 

 and of reacting in one certain way under the influence of 

 external stimuli. 



The same thing might be said of no matter what inanimate 

 body. Everything is put in activity in a way proper to 

 itself, by this or that external agent which would act differ- 

 ently on any other body. Claude Bernard's definition is 

 therefore harmful, when, as is done in most treatises of 

 physiology, it is pretended that irritability is the character- 

 istic property of life. But it is useful if it means simply that 

 the movements of living substances are not spontaneous, 

 but are provoked by causes of motion. But even in this case 

 it might be thought useless to create a new word ; it would 

 be enough to say that living bodies are inert just like others, 

 that is, they are incapable of changing by themselves their 

 state of repose or of movement. The word irritability is 

 borrowed from the psychological history of man and 

 gives the idea of some mysterious property opposed to 

 inertia. 



The same thing ought to be said of movement as of other 

 manifestations of vital activity in a given being A nothing 

 takes place determined by A itself. Every biological 

 phenomenon is the result of two factors the living body A 

 on the one hand and, on the other, the sum total B of exter- 



