iv MENTAL DEVELOPMENT 47 



which depend directly on the physical structure and are 

 therefore part of the hereditary equipment. These actions 

 may occur in response to external stimuli, and as such we 

 shall meet them again in the next section under the name 

 of reflexes, or they may proceed from internal forces main- 

 taining by their interaction a rhythm of change. In the 

 latter case, however, which we take first, the structural 

 activity may often be increased, arrested or modified by 

 external events and it may need something external to work 

 upon. It is not therefore easy in practice to keep the two 

 types distinct. Digestion is a process determined by the 

 correlated action of a number of internal forces, but that 

 digestion may begin there must of course be food taken 

 in from without and the character of the food will affect 

 details of the digestive process. So for respiration there 

 must be an atmosphere, and the amount of oxygen in the 

 atmosphere will affect the respiratory process, and through 

 it the details of the circulation which is otherwise much 

 nearer to a true c automatic ' function. Still, among our- 

 selves, the vital processes as a whole are determined mainly 

 by the interacting forces 1 involved in the structure of 

 heart, lungs, alimentary canal and nervous system, and 

 form together the going concern, the self-maintaining 

 process which is life. Far from being purely dependent 

 on external things to set it going, if this process does not 

 meet with the elements of the environment appropriate 

 to it, e.g. oxygen or food, it sets up cravings and ultimately 

 movements of the entire body which tend to supply the 

 deficiency. These movements, again, are movements of 

 limbs, arms and legs, hands and Feet, determined in type 

 by their structure. So, though the precise direction of the 

 movement may differ indefinitely according to circum- 

 stances, there is even for highly developed behaviour a 

 generic basis in the physical organisation which is part of 

 the hereditary equipment. 



Such type reactions are readily verifiable in the lowest 



1 It must be borne in mind that these forces may be set in motion by 

 internal stimuli, and in that sense be reflex. This, however, would not 

 destroy their independence of external stimuli which is for the moment 

 in question. 



