82 DEVELOPMENT AND HEREDITY. 



of locomotion and nutrition, it is only tiirough the 

 mediation of the nervous system. The nervous 

 system is of all parts of the body the most mobile, 

 and most sensitive to the action force : it controls 

 the nutrition, assimilation, and growth; therefore we 

 conclude that it is through this method of physio- 

 logical activity which we know as nervous action, 

 that the forces of the environment act upon the 

 growth and development of the individual and of 

 the race. 



We may suppose that when the kinetic energy 

 which acts from the environment has once passed 

 the p2riphery and entered the mass of living matter, 

 it no longer acts according to the laws of heat or 

 light, or whatever its external form may have been, 

 but, operating upon a mass of different nature, it 

 assumes a new form, and acts according to the 

 laws of nervous activity. Therefore in order to 

 understand the effects of the forces of the environ- 

 ment we must know the laws of nervous activity. 

 For this purpose we must resort to the methods of 

 investigation of modern psychology. It is among 

 the higher animals, possessing well-developed 

 nervous structures, that we must look for the most 

 perfect display of nervous activity. Where the 

 activity is displayed by certain definite organs, we 

 can separate and analyse its phases and investigate 

 its laws. 



