RELATION OF PHYSIOLOGY TO PHYSICS. 11 



physiology is not to obtain piecemeal physico-chemical 

 explanations of physiological processes, but to discover 

 by observation and experiment the relatedness to one 

 another of all the details of structure and activity in 

 each organism as expressions of its nature as an organism. 



The first step in physiological or morphological dis- 

 covery is to observe the bare sensuous fact of some 

 detail of physical or chemical change, or of composition 

 or structure, in connection with an organism. It is 

 only, however, when we find that this detail is not 

 accidental that it becomes of biological interest. We 

 can observe its constancy or otherwise in the same 

 organism or similar organisms that is to say, the 

 constancy of its relations to other details of structure 

 and activity. We can also by experiment search for the 

 element of constancy when it is at first sight hidden 

 from our view. In so far as we find this, it seems to 

 me that we reach physiological or biological explana- 

 tion ; but evidently the process of reaching it is at 

 any stage in knowledge only imperfectly realised, since 

 new details of activity and structure are constantly 

 being revealed. 



Concrete examples will make the matter clearer, and 

 I shall first take as an example the progress of know- 

 ledge in relation to animal heat. It was of course 

 common knowledge from early times that in the higher 

 animals a certain amount of warmth in the body is 

 present during life. With the invention of the ther- 

 mometer the body-temperature could be measured, 

 and its extraordinary constancy observed. When La- 

 voisier measured the heat-production of an animal, and 

 compared the output of heat with the output of 



