60 DEVELOPMENT AND HEREDITY. 



When we look for the immediate effects of the 

 external forces, we find that they are not the move- 

 ments and changes of shape and growth of the 

 organism. We find that there is an intermediate 

 link which physiology describes as nervous activity. 

 In the same way, when we look for the immediate 

 causes of movement and change of shape and 

 growth, we find that they are not the external 

 forces, but consist in this same nervous activity. 

 This idea, from the psychologist's point of view, 

 has been expressed by Professor Ladd as follows : — 

 "The forces of external nature continually storm 

 the peripheral parts of the animal's body. In 

 order that any of these forces may act as the stimuli 

 of sensations, they must be converted into molecular 

 motions within the tissues of the body. In order, 

 further, that the masses of the body may constantly 

 be readjusted to the external changes of which 

 the sensations are signs, the molecular motions 

 must, in turn, be converted into movements of 

 these masses. In other words a process of constant 

 interchange must take place between the animal 

 organism and external nature." ^ 



We know that those forces which act from with- 

 out upon living matter, have their immediate action 

 chiefly or wholly upon the nervous organisation. 



1 Ladd's Elements of Physiological Psychology, p. 220. 



