The Memory Idea 277 



profoundly affected by the original stimulus than it was in 

 either of the before-mentioned cases. What characteristics 

 of a stimulus would determine how strongly and deeply it 

 would affect the nervous substance through which its energy 

 passed ? Its intensity, the quantity of that energy, of course ; 

 but still more emphatically the length of time the energy 

 remained in the centres concerned, without being drained 

 off into motor paths and transformed into bodily movement. 

 Not merely the strength, but the duration of the current de- 

 termines how deep a path it shall dig out for itself. 



Now, as we have seen, stimuli that are in a position to help 

 or harm an organism at the instant of their contact with its 

 body are stimuli demanding immediate motor reaction. In 

 such cases, the energy of the stimulus is deflected at once 

 into the appropriate motor path ; it is not delayed long enough 

 in the sensory regions to produce any. permanent change there. 

 But where the animal possesses a capacity to be affected by 

 light and sound, which cannot help or harm at the moment 

 of their action upon its body, then reaction may be postponed ; 

 then the current of energy sent by the stimulus into the ner- 

 vous substance is not at once drained off, but may linger 

 sufficiently long to produce whatever alteration, whatever 

 impress upon sensory centres, is needful to insure their 

 subsequent functioning as the basis of a memory image. 

 The delay between stimulus and reaction, made possible by 

 sensitiveness of the organism to stimuli only indirectly affect- 

 ing its welfare, may then supply time for the nervous modi- 

 fication to be produced that is later to underlie the memory 

 image, as the delay occupied in waiting for an expected 

 stimulus offers a chance to bring this modification into play 

 and call the image to consciousness. The same principle 

 also helps to explain why the human mind gets its clearest 

 and most controllable memory images from the senses whose 



