PHYSICS, BIOLOGY, AND PSYCHOLOGY. 149 



There are thus no real grounds for the contention 

 that life must, in ultimate analysis, be capable of inter- 

 pretation as a mechanical process. We must base our 

 working conception of life on actual observation of 

 living organisms, and certainly not on mechanical 

 conceptions. Even from the purely physical standpoint, 

 these are no longer adequate, but only provisional 

 working hypotheses, useful for certain limited practical 

 purposes, like the gas laws in either their original or 

 amended form. 



Empirical observations with regard to the behaviour 

 of living organisms point clearly to the conclusion that 

 in each detail of organic structure, composition, environ- 

 ment, and activity there is a manifestation or expression 

 of the life of the organism regarded as a whole which 

 tends to persist. It is this manifestation which dis- 

 tinguishes biological phenomena; and through all the 

 temporary variations of structure, activity, composition, 

 and environment it can be traced more and more clearly 

 with every year of advance in biological investigation. 

 We can trace it through the ordinary metabolic pheno- 

 mena in living organisms, as well as through the pheno- 

 mena of senescence, death, and reproduction. As it 

 seems to me, it is only through the central working 

 hypothesis or category of life that we can bring unity 

 and intelligibility into the group of phenomena with 

 which biology deals ; and it is because the biological 

 working hypothesis is for the present absent in our 

 ordinary conceptions of physical and chemical phenomena 

 that we must treat physical and biological categories 

 as radically different. The popular and completely 

 natural distinction between the living and non-living 



