NATUBAL SYSTEM OF PSYCHOLOGY. 237 



form as soon as Psychology has gained an accurate know- 

 ledge of these genetic facts, and has made them the basis of 

 its speculations. 



If the psychological teachings, published by the best- 

 known speculative philosophers, and stUl generally received, 

 are impartially studied, the simplicity with which the authors 

 bring foi-ward their airy metaphysical speculations, regardless 

 of all the significant ontogenetic facts by which theii 

 doctrines are clearly refuted, cannot fail to causie great sur- 

 prise. And yet the history of evolution, in conjunction 

 with the rapidly advancing Comparative Anatomy and 

 Physiology of the sense-organs, afibrds the only safe founda- 

 tion for the natural theory of the mind. 



With reference to the terminal expansions of the 

 sensory nerves, the human organs of sense may be distri- 

 buted into three groups, corresponding to three different 

 Btages of development. The first group includes those 

 sense-organs, the nerves of which distribute themselves 

 simply in the free surface of the skin-covering (organs of 

 the sense of pressure, of heat, and of the sexual sense). In 

 the second group, the nerves distribute themselves in the 

 mucous membrane of cavities, which are originally grooves 

 or inversions of the skin-covering (organs of taste and of 

 smell). Finally, the third group is constituted by those 

 very highly developed sense-organs, the nerves of which 

 distribute themselves over an internal vesicle detached from 

 the skin-covering (organs of sight and hearing). This 

 remarkable genetic relation is represented in the following 

 table : — 



