THE PLACE OF BIOLOGY. 43 



physico-chemical theory in accepting the physico- 

 chemical interpretation of all the individual facts which 

 we can observe and measure in connection with life ; 

 but they also maintain, and very rightly, that the phe- 

 nomena, taken as a whole, are of such a nature that no 

 merely physico-chemical explanation of them is con- 

 ceivable. To fill up the gap, they make the assumption 

 that a non-physical influence — the " vital force " of older 

 writers, or the " entelechy " of Driesch and his followers 

 — exercises a guiding influence on the physical and 

 chemical processes occurring within living matter. In 

 Bergson's writings the same ideas take a more subtle 

 philosophical form, and he maintains that we can only 

 grasp the elan vital by intuition, while perception limits 

 us to the physico-chemical world. 



I think it must already be evident that the doctrine I 

 have been defending is just as inconsistent with vitalism 

 as with the physico-chemical theory of life. The defect 

 of vitalism is that it seeks to set a limit on ordinary 

 physiological investigation, while as a matter of fact the 

 passing of any such limit is only a matter of further in- 

 vestigation. When we discovered the marvellous accur- 

 acy with which the respiratory centre governs the lung 

 ventilation so as to maintain with various rates of dis- 

 charge of CO 2 a certain partial pressure of carbon dioxide 

 in the alveolar air and arterial blood, we might have 

 pointed to this as an instance of the interference of the 

 guiding " entelechy " ; and had we been content with 

 relatively inaccurate methods of gas analysis, the varying 

 stimulus which controls the action of the centre might 

 not have been detected. The action of the kidneys in 

 secreting water is another similar example. It is of no 



