118 ORGANISM AND MECHANISM 



and particularly of physical chemistry, has furnished ex- 

 planations of these processes. This is most certainly not 

 the case. What physical chemistry has helped us to do is 

 to obtain measures of the processes in the living body; but 

 the results of measurements have been to show with ever- 

 increasing clearness that the processes in the living body 

 do not correspond with our conceptions of those in non- 

 living structures, and that we are not remotely in sight of 

 mechanical explanations of the former. 



"As an example, I need only take the case of the exqui- 

 sitely thin and delicate living membrance which separates the 

 blood in the lung capillaries from the air in the alveoli or 

 air-cells of the lungs. A short time ago it was assumed 

 that this membrane plays only a passive part which we 

 regard a non-living membrane as playing, and allows oxygen 

 to diffuse through it just as a non-living membrane would. 

 On applying accurate methods of measurement we found 

 that, whenever there is need for an extra supply of oxygen, 

 as, for instance, during muscular exertion, the membrane 

 assumes an active role and pushes oxygen inwards, without 

 regard to the mechanical laws of diffusion. In this respect 

 the alveolar epithelium acts just like the epithelium of the 

 swim-bladder, or that of the kidney or any other gland, or 

 the alimentary canal. The progress of physical chemistry 

 is enabling us to distinguish sharply between physiological 

 activity and the processes occurring in non-living structures ; 

 and the establishment of the distinction is sweeping away 

 the easy-going mechanistic explanations which became cur- 

 rent during the latter half of last century." " On the whole, 

 there is no evidence of real progress towards a mechanistic 

 explanation of life." 



The inadequacy of the mechanical description is apparent 



