LIVING THINGS AND MACHINES 115 



has the power, after a time of rest, to return to its 

 former condition, so that on the stimulus being 

 repeated it will be found that the muscle can re- 

 spond to it as it did on the previous occasion. 

 The machine, on the contrary has no power of 

 repairing injuries or wear and tear which may 

 take place in it as the result of its ordinary use ; it 

 cannot lubricate itself or clean itself or make itself 

 ready for new action. 



And thirdly, no machine can beget a new 

 machine nor has any one succeeded in constructing 

 anything which, by division, will shape itself into 

 two instruments where but one existed before. It 

 is only and properly so in a living organism that 

 we talk of irritability, of stimuli, of reflex actions, 

 and it is a hopeless task to attempt to explain an 

 organism on mechanical principles. 



In a machine the movements of the rollers, 

 wheels, levers and other parts can be explained on 

 purely mechanical principles, but in the organism 

 the operations of a chemical character, extra- 

 ordinarily varied as they are, cannot be explained 

 in this way. 



Whilst in the machine the movements of the 

 various parts are firmly united to one another by 

 their construction, so that they cannot alter inde- 

 pendently of one another, the living organism can 

 alter its structure and at the same time set aside 



