LIMITS OF NATURAL SELECTION. 15 



— that the psychic action moved the molecules. 

 But unless the fundamental conclusion of physiolog- 

 ical psychology be wrong, psychic action is simul- 

 taneous and co-existent with change in the material 

 particles of the brain. Thus we may consider the 

 one as indicative of the other. 



According to this principle, an animal which for 

 a lifetime has been meeting with new experiences, 

 new conditions, and new demands upon its ingenuity, 

 and acquiring new habits, must have at the end 

 a more developed and more complicated nervous 

 structure than it had at the beginning of its life. 

 In such an animal there have arisen new nervous 

 co-ordinations, new actions are possible, its dis- 

 charges of nervous force are more concentrated and 

 efficient, and the purely reflex mechanical actions 

 are performed in a more perfect way. The ner- 

 vous mechanism of the animal has been improved, 

 and the brain, the material substratum of its psychic 

 life, has been changed in structure. Moreover, we 

 know that this development, viewed as a physical 

 change, must have occurred according to the laws 

 of matter in motion and the law of the conservation 

 of energy. The material change, the new arrange- 

 ment of the molecules or atoms, is the effect caused 

 by the action of the stimuli or forces of the en- 

 vironment upon the animal. Were these same 



