THE PROBLEM OF LEARNING I93 



one making its own contribution to the moving 

 impulses until it finally became a motor message. 

 If such is the history through which habits and 

 instincts have passed, then we have a foundation 

 upon which modifications of habits can be con- 

 structed. 



It required numerous trials to teach the earth- 

 worm the difference between success and error. 

 The same combination of reactions had to be done 

 over and over. When the earthworm grew a new 

 brain, this in turn had to be trained. When one 

 contrasts the structural possibilities of such a 

 simple animal as the earthworm with the higher 

 vertebrates, there is an almost endless multiplica- 

 tion of associational routes over which sensory 

 impulses and the attendant reflexes may pass. In 

 the highest group of all the vertebrates, the mam- 

 mals, there is in addition to the regular brain the 

 so-called "new brain" which receives impulses 

 from all over the body. This is the first time in 

 the history of animals that one single region be- 

 comes the regulating center for all activities. 

 From it impulses travel vrhich Influence every 

 structure in the body. Here are made possible 

 associations which are not found elsewhere in the 

 nervous system. In order that it should come to 

 have such a dominating influence, the powers that 

 it possesses must hav^e come from other parts of 

 the brain. Some of the activities that were once 



