6l6 COMPARATIVE ELECTRO-PHYSIOLOGY 



molecular condition of the responding substance. There is 

 a school of investigators, again, who, appearing to discard the 

 theory of vital action, in accounting for these changes, have 

 substituted for it the hypothetical anabolic, or up-building, 

 and catabolic, or down-breaking, chemical changes. And 

 such assumptions have certainly the advantage of meeting 

 every emergency, whether it be an expected effect or its 

 direct opposite which occurs, for by their means it is always 

 possible to make a reference to the one process or the other, 

 whatever be the inconsistency involved. 



As regards the interminable controversy on the physical 

 versus chemical nature of response-phenomena, I have 

 already drawn attention to the fact that on the border-line 

 between Physics and Chemistry it is impossible to make any 

 sharp demarcation. Changes, in themselves undoubtedly 

 molecular or physical, may be attended by concomitant 

 changes of chemical activity. An example will perhaps 

 make this clear. We may take, for instance, the photo- 

 graphic action of light on a sensitive plate. This is re- 

 garded as due to chemical dissociation or break-down. If 

 this were so, however, the effect would be permanent. But ? 

 instead of this, the latent image is liable to disappear, and in 

 a Daguerreotype plate the after-effect of light that is to say, 

 the persistence of the image has a duration of a few hours 

 only. Such images, moreover, due to the action of light, 

 have been found to form themselves even on elementary and 

 inert chemical substances like gold. Here, any chemical 

 break-down, in the ordinary sense of the word, is out of the 

 question. 



Stimulus in general we have seen to induce molecular 

 distortion, the persistence of which is dependent on the 

 strength of the stimulus, and also on the power of self- 

 recovery characteristic of the given substance. We have 

 further seen a difference of electrical potential to be induced, 

 as between molecularly strained and unstrained areas. 

 When the substance, therefore, thus differentially acted upon, 

 is placed in a suitable electrolyte, volta-chemical actions are 



