96 DEVELOPMENT AND HEREDITY. 



sitated in an organism by external influence, is 

 performed with greater facility after frequent repe- 

 tition than before. 



We have learned that each action of an organism 

 must have its sufficient stimulus ^ meaning by 

 " stimulus," the action of force upon the nervous 

 organisation. We are compelled to this belief by 

 the law of the conservation of energy. No molec- 

 ular, or atomic change or action can originate itself, 

 but must be part of the universal series of cause and 

 effect. The molecules of an organism must be sub- 

 ject to those laws of molecular mechanics, which, we 

 have reason to believe, apply universally. We must 

 therefore seek the cause of the actions of organisms 

 in all the forces which operate actively on the organ- 

 isms from the outside, or which may be stored as 

 potential energy within ; and this latter potential 

 energy, we must remember, can have had ultimately 

 no other source than the world outside the organ- 

 ism. If we undertake to treat of the forces acting 

 upon organisms and the changes caused thereby, in 

 the terms of force, molecular change, etc., we are 

 quickly brought to a standstill. But if we treat of 

 them in. the correlative terms of stimulus, physiologi- 

 cal reaction, and nervous or psychical response, we may 

 proceed with some certainty, though not with mathe- 

 matical accuracy. As every molecular change de- 



