THE PLACE OF BIOLOGY. 41 



have already tried to explain in detail, that we are simply 

 passing backwards and forwards in an absolutely help- 

 less and aimless manner. 



Someone will perhaps say, I find 200 c.c. measured at 

 0° C. and 760 mm. barometric pressure of carbon dioxide 

 given off in one minute in the expired air. Is not this a 

 solid, indisputable, and independent fact, whatever be 

 the nature of the organism which expired the carbon 

 dioxide ? The true answer to this question is a blunt 

 and bold negative. The statement about the carbon 

 dioxide is no statement of immediately evident fact, but 

 a highly theoretical interpretation of certain very limited 

 sense data. When we extend our observations we see 

 that what we interpreted at first as a mere flow of gas- 

 molecules is one of the expressions of the life of an 

 organism. The interpretation in terms of the flow of a 

 certain volume or mass of gas molecules was only a 

 preliminary interpretation, satisfactory so far as it went, 

 but also untrue, because it involved a physical inter- 

 pretation inconsistent, as we have just seen, with the 

 physiological facts taken as a whole. If we start our 

 physiological investigations with such assumptions as 

 that the flow of a certain mass of gas in a certain time 

 is a certain, solid, and independent fact, we are inevitably 

 carried forward by actual investigation up to the point 

 where we finally realise that our interpretation cannot 

 interpret the facts. 



We can now get a clearer view of what the true aim of 

 physiology is. We can see, also, that this aim reveals 

 itself to us in actual physiological observation and ex- 

 periment. In the crude sense-material which presents 

 itself to us we are tracing the thread of constancy in 



