RESPONSE IN THE LIVING AND NON-LIVING 89 



the animal machine it is necessary to face numerous difficulties 

 which at present seem formidable. 



Then follows ' a comparative study of the curve of 

 molecular reaction of inorganic and living substances.' 

 First a curve from magnetic oxide of iron (Fe 3 O 4 ), slightly 

 warmed, and then following it, one of the usual muscle 

 curves, showing - a striking general resemblance to the 

 former. 



This leads to further study of the behaviour of the iron 

 oxide in comparison with that of muscle : (i) of the effect 

 of a superposition of maximum excitations ; (2) that of 

 summation of moderate excitations slowly succeeding each 

 other ; and (3) that of rapidly succeeding stimuli. 'Alike for 

 mineral and muscle, these effects are extraordinarily similar, 

 and their curves correspond so closely in fact that either 

 may be taken for the other. And in detail : (i) when the first 

 excitation is at maximum, no effect is in either case ob- 

 servable from a second stimulus ; (2) moderate excitations 

 are summated ; and when in slow succession, the effect of 

 each shock can be distinguished as steps in the ascending 

 curve ; (3) when the stimuli are very rapid, the effects are 

 combined, and the phenomenon known as tetanus appears 

 in both alike. 



He also found that in many inorganic substances, when 

 ordinary stimulus produces the normal ' negative ' effect, 

 a feeble stimulus elicits the very opposite, i.e. positive. 

 He was long puzzled by the dual result, not simply as 

 being new to physics, but as yet without parallel in the 

 observed response of living tissues. But, he asked himself, 

 is this a real contrast between non-living and living ? 

 or may not farther experiment disclose an analogous dual 

 reaction in living things ? The inquiry led him to the dis- 

 covery of certain living reactions of high significance. These 

 will be treated later in greater detail. 



Iron oxide, when warmed, gives an enhanced response 

 under stimulus ; and recovery is also much quickened ; 

 but only up to a certain level, when both are again 



