378 JOHN LINCK ULRICH 



It is accordingly not a bit surprising that many modifications 

 of the original working concept of trial and error and pleasure-pain 

 hi learning have been made. Such modifications, after introduc- 

 ing additional physiological explanations, have either discredited 

 the concept of "pleasure-pain" or, have modestly retained it. 

 These other explanations throw the burden of fixation of the 

 successful movements partly or wholly upon the nervous system. 

 With the occurrence of the successful movements, physiological 

 changes of importance are supposed to result in a selection of 

 pathways, or in a differentiation of them in the central nervous 

 system. Sensory excitations, which were at first sent diffusely 

 through many channels producing many movements of a "ran- 

 dom" kind are, in consequence of the performances of the suc- 

 cessful act, conducted, in the main, through one channel or com- 

 mon path. Such views state that either originally no preformed 

 paths were present, or that reflex arcs were not selected, or that 

 no passage through definite synapses, or reflex arc, existed as an 

 outlet for a definite sensory excitation to produce the successful 

 movement. These paths or passages are established in a degree 

 when a successful act is once performed. Then definite path- 

 ways, through opposite (hypothetical), semi-impermeable mem- 

 branes, the synapses, are established, by the reduction of the 

 resistance through them for excitation, and this resistance les- 

 sens with a repetition of the successful act; or, definite pathways, 

 and reflex arcs are selected or established because of a repetition 

 of this same act. "Pleasure-pain" is, or is not, regarded as of 

 significance hi the fixation of either of the pathways, or hi the es- 

 tablishment of reflex arcs. 



In accordance with an idea of the existence of different degrees 

 of development of reflex arcs or pathways in the nervous system, 

 sometimes it is said that learning, leading to habit formation, 

 occurs in two ways. Learning is rapid, immediately effective, 

 when groupings of instinctive responses and successful move- 

 ments soon establish a habit; it is slow, laborious, ineffective for 

 a time, when instinctive responses result in the production of 

 "random unconnected diffuse responses," and it requires the 

 appearance of the successful act which by a longer repetition 



