THE PROBLEM OF LEARNING 1 89 



up of sensory impulses within the central nervous 

 system. Such an inter-acting relationship brings 

 every reflex act as well as instincts, habits and 

 learning under a general law, known as the in- 

 tegrative action of the nervous system. For the 

 reaction of an animal is not the result of a single 

 sensory impulse but an association of impulses of 

 varying degrees of intensity. While the earth- 

 worm was stimulated by direct sunlight, there 

 was also the absence of stimuli from its natural 

 burrow in the earth and the substitution of stimuli 

 from moist filter paper. All of Yerkes' "lessons" 

 with the earthworm were given under but partially 

 normal conditions. 



It is important to keep in mind the significance 

 of the functional divisions of the nervous system 

 which clearly proves that there are no breaks 

 in the entire circuit from receptor to effector. This 

 is especially evident in all vertebrates which places 

 a different interpretation on circumscribed areas 

 in the cerebrum than customarily given. The so- 

 called speech or hearing center is simply one of 

 the relay centers in the route that happens to lie 

 on the surface of the brain, — a group of nerve 

 cells readily accessible for experimental treatment. 

 They are neither isolated nor necessarily terminal 

 but connected by nerve tracts with other centers 



