i8o INDIVIDUALITY IN ORGANISMS 



the other. This interference is in certain respects 

 analogous to physical interference in the transmission 

 of water waves, sound waves, light waves, etc., but the 

 protoplasmic substratum in the organism represents a 

 factor not concerned in physical interference in non- 

 solid media. Undoubtedly a gradient which is originally 

 dynamic becomes more or less stably fixed or estab- 

 lished in the protoplasm as a gradient in irritabihty, 

 structure, or differentiation, because the effects of the 

 transmitted excitations modify the protoplasmic condi- 

 tion and this modification may become more or less 

 persistent. Temporary inhibition may result from 

 temporary interference between metabolic gradients, 

 but for permanent or long-enduring inhibition the 

 protoplasmic condition determined by one gradient 

 must be reduced or obliterated or its direction reversed 

 by the action on the protoplasm of another gradient. 

 In the cases of obliteration or reversal of the axial 

 gradients by other gradients this factor undoubtedly 

 plays a more or less important part, and the increasing 

 stability of the protoplasmic substratum with the prog- 

 ress of individual development and evolution^ deter- 

 mines that such obliteration and reversal occur much 

 more readily in the lower than in the higher organisms. 

 Since conduction in the nerve is apparently asso- 

 ciated with an axial gradient, it is at least an interesting 

 question whether nervous inhibition may not be funda- 

 mentally a similar relation of gradients, either in differ- 

 ent neurons or in the innervated organ. The mechanism 

 of nervous inhibition is still obscure, but if the nervous 



^ Child, Senescence and Rejuvenescence, 1915, pp. 50, 53, 194, 267, 

 463-65. 



