112 ORGANISM AND ENVIRONMENT 



Perhaps the case of the respiratory centre or of 

 the kidney illustrates as well as anything else the ob- 

 jections to vitalism. We have seen with what marvel- 

 lous exactitude the respiratory centre regulates the 

 hydrogen ion concentration of the blood, but also that 

 the response of the centre is nevertheless dependent on, 

 and proportional to, an increase, however small, in the 

 hydrogen ion concentration of the blood. If our 

 methods of measurement had been less exact, if, for 

 instance, we had employed rougher methods of gas 

 analysis in investigating the alveolar air, or if we had 

 been compelled to rely simply on the methods, delicate 

 as they seem to a chemist, which are at present avail- 

 able for measuring hydrogen ion concentration, it 

 might have seemed as if the respiratory centre acted 

 without a stimulus, guided by an outside agency, just 

 as a locomotive is guided by the driver, who shuts off 

 or turns on steam according to requirements, and thus 

 keeps his train up to time in spite of various accidental 

 hindrances. Vitalism is a theory of this kind: it 

 ignores the participation of the environment in the 

 regulation, and consequently does not correspond to 

 the observed facts, and is thus of little use as a work- 

 ing hypothesis in actual investigation. Its only real 

 merit is that it serves as a means of expressing facts 

 relating to organic regulation, and the defects of 

 mechanistic theories. These facts are registered by 

 referring them to the vital principle or entelechy. 



The further physiology seems to advance in the 

 direction of mechanistic explanations the more ob- 

 viously it is driven into vitalism. For advance in 



