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 
