STIMULATION AND TRANSMISSION 263 



especially to be noted that the two of the most general 

 features of stimulation-processes, viz., the susceptibihty 

 to the electric current, and the reversible modification or 

 suppression of irritability by the surface-active groups 

 of compounds (amesthesia or narcosis), indicate definitely 

 a dependence of protoplasmic activity on the polyphasic 

 structure of the system. The inference from such facts 

 is that the chemical reactions of protoplasm are controlled 

 by the peculiar conditions resident at the protoplasmic 

 interfaces or phase-boundaries; and the resemblance 

 between the conditions of activity of irritable proto- 

 plasmic systems and of the inorganic models just 

 described confirms this inference. 



Some of the more general features of the phenomena 

 of stimulation in living organisms have already been 

 discussed briefly. Since continued life depends on a 

 regulated interchange of material and energy with the 

 environment, it is to be assumed that all fundamental 

 vital activities are capable of varying in correlation 

 with, or ''in response to," environmental change; the 

 character and rate of the interaction of the living system 

 with its environment are thus controlled. Normally 

 the responses of any organism to stimulation are of such 

 a kind as to favor its continued or stable existence in 

 this environment. A certain difficulty in defining the 

 conception of stimulation arises here, since many cases 

 exist where physiological activities, which in themsch'es 

 are injurious or destructive to the living system as a 

 whole, may be induced by environmental change;^ such 



^ The oxidation rate of sea-urchin eggs may be increased l)y pure 

 NaCl solution to a degree which apparently is directly destrucli\e. 

 The case of fatigue carried to an injurious extreme is analogous. 



