REFLEX AND CONDITIONED ACTION 41 



Insulated fiber tracts but rather a more disperse 

 spreading of fibers; it is, accordingly, more impres- 

 sionable and more readily modified by practice; the 

 resulting behavior is more labile or plastic than that 

 mediated by the primary reflex centers alone. This 

 is the typical apparatus of the formation of so-called 

 associative memories, perceptual-motor learning 

 (Carr, 1925), learning by trial-and-error, and habit 

 formation by repetition, though it must not be forgot- 

 ten that these activities can be performed (less effi- 

 ciently) in the absence of correlation centers. 



Before proceeding with the discussion of the 

 thalamus and cerebral cortex as apparatus of correla- 

 tion, brief reference may be made to some interesting 

 steps in the differentiation of the primary sensori- 

 motor centers out of which these higher centers of 

 correlation have been elaborated. 



Primitive reactions to external stimulation are 

 generally mass reflexes, or total movements of the 

 body as a whole, such as swimming to or from the 

 source of the excitation. In fact, the simplest verte- 

 brates are so organized that there are very few things 

 that they can do in adjustment to changes in their 

 surroundings. A young salamander larva or a tadpole 

 Is well equipped with sense organs. He has nearly 

 as many ways of sensing what is going on around him 

 as we have, but the number of things that he can do 

 about it is surprisingly small. 



There Is a profound physiological principle behind 



