STIMULATION AND TRANSMISSION 295 



yet it is of such a nature as to be initiated only by 

 physical changes proceeding at more than a certain 

 rate. 



We are thus led again to consider those special 

 properties of the living system which it possesses by 

 virtue of being living; i.e., metabolically and syn- 

 thetically active. The features above of stimulation 

 cannot be understood except by reference to what is 

 distinctive in vital processes as such. Yet it is to be 

 noted that the case of the living system is by no means 

 unexampled in the respect just considered. There are 

 many natural processes in which, if a certain effect is to 

 be produced, the effecting agent must act at more than 

 a certain minimal rate; if it acts slowly, the effect fails 

 entirely. For example, a swift current of air will extin- 

 guish a candle flame, while a slow one will not; a rapid 

 stroke will ignite a match, a rapid projectile penetrates 

 a plate, a rapid attack succeeds in war or on the football 

 field. The lighting of a match shows many analogies 

 with stimulation. Ignition occurs at a certain critical 

 temperature, which is attained when the match head is 

 drawn uniformly over a rough surface for a certain 

 time at more than a certain rate. If the movement is 

 too slow, ignition will never occur; in this case the 

 stationary condition at which the^ gain of heat from 

 friction is equal to that lost to the surroundings by 

 conduction and radiation is reached at a temperature 

 below that of ignition. A movement at a certain rate 

 must last for a certain time, which is shorter the more 

 rapid the rate. Summation phenomena and summation 

 intervals may also readily be demonstrated in this 

 system; i.e., a succession of brief strokes but not a 



