1 30 RESPONSE IN THE LIVING AND NON-LIVING 



The substance may next be in what we call the 

 normal condition. Successive uniform stimuli now 

 evoke uniform and equal positive responses, that is to 

 say, there is no fatigue. But after intense or long- 

 continued stimulation, the substance is overstrained. 

 The responses now undergo a change from positive to 

 less positive ; fatigue, that is to say, appears. 



Again, under very much prolonged stimulation the 

 response may decline to zero, or even undergo a reversal 

 to negative, a phenomenon which we shall find instanced 

 in the reversed response of retina under the long- 

 continued stimulus of light. 



We must then recognise that a substance may exist 

 in various molecular conditions, whether due to internal 

 changes or to the action of stimulus. The responses 

 give us indications of these conditions. A complete 

 cycle of molecular modifications can be traced, from 

 the abnormal negative to the normal positive, and then 

 again to negative seen w reversal under continuous 

 stimulation. 



