NATURAL SELECTION 13 



growth and nutrition of distant parts of the 

 body. Although the evidence is, owing to the 

 great chemical and experimental difficulties, 

 accumulating but slowly at present, we cannot 

 doubt that the growth and maintenance of 

 every part of a living organism is controlled 

 by the chemical stimuli derived from other 

 parts, or from the environment. 



There thus appears to be no difficulty in 

 regarding a living organism as a complex 

 system of physico-chemical mechanisms, each 

 of which is controlled by the rest in such a 

 way that the normal structure and activity of 

 the organism is, under ordinary conditions, 

 maintained. We can, moreover, verify the 

 existence of these mechanisms, one by one, by 

 exact experiment ; and their actual existence 

 has already been verified in a large number of 

 instances. Hence the co-ordination of growth 

 and activity, as found in living organisms, 

 presents, in itself, no real difficulties for the 

 mechanistic theory. 



We have still to account for the existence 

 of physico-chemical structures possessing the 

 marvellous characteristics of living organisms 

 so different from anything found in the 



