RESPONSIVE LIFE OF ORGANISMS 457 



extinction, unless, perchance, some line of descent should 

 break away from traditional habits, by developing some new 

 paths of action. 



Brain action is not necessarily a whit less automatic than 

 the action of a reflex arc. The moth has a brain, but it does 

 not learn in this instance by experience. If past experience 

 is reproduced at all in memory, the impulses arising from it 

 are powerless to check those that arise from the next percep- 

 tion of light. Control here is based on racial — not at all on 

 individual — experience. 



A mechanism for adaptation in the individual. — The 

 growth of the control centers in the nervous system has ever 

 meant a multiplication of new channels of intercom- 

 munication between the added neurones in them. It has 

 meant the development of accessory circuits not directly 

 responsible for the ordinary activities of the body. It has 

 meant more and ever more by-paths, which peripheral 

 stimuli might or might not traverse. 



The primary function of reflex response not being required 

 of these accessory circuits, they have taken on new functions 

 of their own, and have assumed new powers of control. 

 Especially in the cerebral hemispheres of the vertebrate 

 brain, where they reach their best development, they have 

 come to preside over most of the activities of the body. 



The neurones of the body are by no means to be con- 

 sidered merely as mechanical agents of intercommunication; 

 they are all living cells, having their own metabolism, con- 

 suming food and developing energy, which may manifest 

 itself in more than one way. They act upon each other as 

 upon the other tissues of the body, in and of themselves, 

 whether stimulated from without or not; and it is natural, 

 therefore, that the masses of neurones that make up the con- 

 trol centers should manifest themselves in new ways. 

 Their dendrites are combined in innumerable paths, and 



