102 THE EVOLUTION OF MAN 



subject to restrictions which continually recall the material 



background on which it rests. 



In the evolution of the nervous system of man as presented 

 in this brief outline we see a succession of steps rather than a 

 gradual ascent and at each step a fundamental change suddenly 

 appears, mutation-like in its character, to borrow a term from 

 the geneticists. Each step is a new phase in organization, but 

 not necessarily a change in the kind of elementary materials 

 involved, and with these changes in organization come changes 

 in the degrees of freedom of reaction which enable us to bridge 

 over the gulf that lies between the relatively circumscribed 

 activity in ordinary chemical operations and the greater free- 

 dom seen in the voluntary and responsible acts of human 

 beings. Something of this view of the nature and the possi- 

 bilities of living protoplasm has been put forward by Haldane 

 under the name of organicism, but with perhaps less reliance 

 on the material side of the problem than has been suggested 

 in this lecture. Yet interesting and important as it is to push, 

 to the extreme, speculation as to the relation of our mental life 

 to the materials of our body, it nevertheless must be remem- 

 bered that, with all our progress, we are still not far from the 

 position described by Vesalius in 1543 when he wrote, "How 

 the brain performs its functions in imagination, in reasoning, 

 in thinking, and in memory, I can form no opinion whatever." 



