Modification by Experience 257 



reach full expansion after having contracted at a touch, will 

 respond to the second touch just as it did to the first ; the 

 stimuli, to exert any influence on later reactions, must come 

 in quick succession. On the other hand, in the responses of 

 mollusks to shadows, the experiences of one day appear to 

 extend their effects to the following day (520, 588, 590). 

 Here we are dealing with a new type of modification by 

 experience, though one which develops directly out of sen- 

 sory adaptation ; namely, the relatively permanent dropping 

 off of useless movements. 



73. Modifications Due to Relatively Permanent Effects 



of Stimuli 



In true learning, the conscious experience and the be- 

 havior of an animal suffer changes so lasting, relatively 

 speaking, that they cannot be set down as due merely to 

 adaptation of the sense organ, muscular fatigue, hunger, 

 satiety, or any other variable physiological state of the 

 organism. On the other hand, as we saw in Chapter II, 

 the modifications must occur rapidly enough so that there 

 is not time for actual changes in the animal's muscular 

 structure to be produced. In animals which possess nerv- 

 ous systems, true learning is probably always the result 

 of alterations in the connections between the elements of 

 that system, such that the nervous process is able to pass 

 easily in a direction where it originally encountered high 

 resistances. 



The fundamental law of all learning is the Law of Repeti- 

 tion, whereby when a nervous process traverses a certain 

 pathway in the nervous system, it leaves the resistances in 

 that pathway less than it found them. This is the law 

 in accordance with which, when we wish to learn anything, 



