262 PROTOPLASMIC ACTION AND NERVOUS ACTION 



irritable systems with highly developed transmissive 

 properties, e.g., the nerve fibers and muscle cells of 

 higher organisms, the character and intensity of the 

 response are quite independent of those of the stimulus, 

 provided the latter attains the threshold value. A full 

 response, involving the whole irritable element, results 

 from either a ''weak" or a "strong" stimulus; this is 

 the ''all or none" type of behavior, which is found also 

 in many physical systems in unstable equilibrium, and 

 also in explosive systems or others in which chemical 

 change is rapidly transmitted; e.g., the passive iron 

 system. In all such cases there is a "release" of stored 

 energy, and the work performed by the releasing agent 

 has no definite relation to the energy transformed in 

 the resulting process.' 



The phenomena of stimulation in living organisms 

 are so various that one hesitates to regard them all as 

 determined by conditions of the same physico-chemical 

 kind. Nevertheless, it is a striking fact that whatever 

 the special peculiarities of the organic activity or response 

 in different living systems may be, the conditions of 

 initiation and control are remarkably uniform. The 

 universal susceptibility to the electric current, to mechani- 

 cal disturbance, and to certain kinds of chemical influ- 

 ence, especially the influence of inorganic salts and the 

 lipoid-solvent or surface-active group of organic com- 

 pounds, indicates that the fundamental structural and 

 chemical conditions underlying the response to stimula- 

 tion are the same in all forms of protoplasm. It is 



* The typical case is one of "trigger action," which is a characteristic 

 feature of all modes of organic response (cf. the interesting discussion of 

 Lotka: "Natural Selection as a Physical Principle," Proceedings of the 

 National Academy of Science, VIII [1922], 151). 



