32 CRITICISM OF MECHANISTIC THEORY 



sophical character, the main strength of the 

 mechanistic theory arises from the fact that all 

 physiological activity is apparently dependent 

 on physical or chemical 'causes.' Whether 

 the cause be physical disturbance, heat, light, 

 or the presence of one or other of numerous 

 chemical substances, it is always something 

 definite. The physiologist, however, usually 

 calls the cause a ' stimulus,' and it must in the 

 first place be pointed out that in identifying 

 stimulus and response with physical or chemi 

 cal cause and effect the mechanistic theory 

 makes a gigantic leap in the dark. 



To make this matter clear, we may take one 

 or two examples. A minute and scarcely 

 measurable increase in the hydrogen ion con 

 centration of the blood excites the respiratory 

 centre of a normal warm-blooded animal to 

 intense activity. Similar minute alterations 

 in the concentration of water, or sugar, or 

 sodium chloride, or hydrogen ions, have a 

 corresponding influence on the secretory 

 action of the kidney ; and cases of a similar 

 kind, where both growth and physiological 

 activity are similarly influenced, may be mul 

 tiplied indefinitely. 



