130 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 ip reversal under continuous 
stimulation. 
