12 MANUAL OF ELEMENTARY ZOOLOGY 



is brought about in gunpowder by heating it. Secondly, 

 that the extent of the internal change bears no relation to 

 that of the external one which acts as its stimulus. Thus 

 when, in response to a command, a man lifts a heavy load, 

 the energy of the sound-waves which call forth this reaction 

 is immeasurably smaller than that of the work done by the 

 man, and either may be greater or less without a corre- 

 sponding alteration in the other. 



Activity, however, is not necessarily associated with 



irritability. It is sometimes claimed that the 

 matism.' activity of living animals is characterised by 



a feature which is in its essence the very 

 opposite of irritability. This feature is called automatism, 

 the word meaning that certain actions of the beings of which 

 it is used arise independently of external stimuli. Now 

 in a certain sense it is true that this is the case. That is to 

 say, in the living animal there do arise actions which are 

 not the direct result of any stimulus from without. The 

 simplest instance of this is the beating of the heart, but 

 we are well aware that some of our more complicated 

 actions are at least not due to any stimuli which we can 

 trace. In view of the great number of stimuli which the 

 body is always receiving, it is necessary to be cautious in 

 attributing an automatic character to any of its actions, 

 but there is little doubt that automatism, in the sense in 

 which we have used the word, is characteristic of the 

 living animal. 1 It must be borne in mind, however, by 

 those who are interested in the philosophical bearings of 

 this conclusion, that automatism does not consist in any 

 action being completely independent of the outer world, 

 since the very constitution of the animal which acts is due 

 in part to the effect upon it of its surroundings. 



The living animal has not come into being, as it could not exist, in- 



1 An action is none the less automatic because it is modified by 

 stimuli. It is well known, for instance, that the beating of the heart 

 may be hastened or slowed by stimuli such as those which are afforded 

 by an event which arouses great joy or fear. Nor, again, does an 

 action cease to be automatic because it cannot be maintained unless a 

 supply of nourishment be provided for the body in which it takes place, 

 as the beating of the heart is dependent on the quality of the blood 

 which nourishes it. 



