88 BIOLOGY AND PHYSICAL SCIENCES 



dissociated in the blood, play an important 

 part in helping this regulation. 



The idea which gives unity and coherence 

 to the whole of the physiology of respiration 

 is that of the organic determination of the 

 phenomena. The same idea has to a greater 

 or less extent already given, or is in process of 

 giving, unity and coherence to the phenomena 

 of nutrition, secretion, and circulation. It is 

 an idea which guides us at every turn in 

 physiological work, and constantly suggests 

 new lines of investigation. To leave it out of 

 account in physiology, or to treat it as a mere 

 * heuristic principle ' of very uncertain value, 

 seems to me about as foolish as it would be to 

 reject the idea of mass in chemistry, and retain 

 the phlogiston theory, as Priestley and Caven 

 dish actually did till their deaths. By regard 

 ing the structure and activities of a living 

 organism as the expression of organic unity 

 we arm ourselves with a theory which is 

 just as useful in biology as the idea of mass 

 in chemistry. Neither the idea of mass nor 

 that of organism will enable us to predict 

 everything in the chemical and biological 

 worlds respectively, but they both help us 



