THE PLACE OF BIOLOGY. 39 



correspond to the constancy of action of the supposed 

 secreting mechanism. 



Whatever organ in the body we turn to we shall cer- 

 tainly meet with similar delicacy and constancy of 

 reaction when we investigate closely and quantitatively 

 the function of the organ. The blood forms a common 

 internal environment for the living cells of the body, 

 and we know from experiment that the constancy in 

 reaction of the different organs of the body depends 

 upon the constancy in composition of the blood. But 

 evidently this is no explanation, for, as we have already 

 seen, the constancy of the composition of the blood 

 is itself the outcome of the activity of the organs of the 

 constancy of whose reactions we are seeking an explana- 

 tion. It is only a game of battledore and shuttlecock 

 to attempt to explain one of these facts by the other. 



We are thus driven back to the external environment 

 for an explanation ; and at first sight this may seem a 

 hopeful direction in which to look. The nutrition of an 

 organism, with all that this implies, depends evidently 

 on the supply of nutriment, water, and oxygen ; and it 

 is equally true that the behaviour of the organism de- 

 pends on the sum of the stimuli acting upon it. We can 

 verify these facts experimentally ; and those who uphold 

 the mechanistic theory of life have often pointed to this 

 verification as a justification of their hypothesis. But 

 when we look at the facts a little more closely, and par- 

 ticularly when we investigate them quantitatively, we 

 soon see that we are no further forward. For the organ- 

 ism itself determines the stimuli to which it responds 

 and the ratio between physical stimulus and actual 

 response. Whether food is abundantly available or just 



