236 THE EVOLUTION OF MAN. 



differentiated sense-organs of all Intestinal Animals (Metazoa), 

 and, therefore, of all Vertebrates, originally developed. Start- 

 ing -from the consideration that necessarily only the most 

 superficial parts of the body, those immediately exposed to 

 the outer world, could have accomplished sensations, we 

 should be justified in conjecturing d priori that the organs of 

 sense also owe their origin to the same part. This is, indeed, 

 the fact. The most important part of all sense-organs 

 develops from the outermost germ-layer, from the skin- 

 sensory layer ; in part, directly from the horn-plate, and, in 

 part, from the brain, the foremost section of the medullary 

 tube, after this has separated from the horn-plate. On 

 comparing the individual development of the various organs 

 of sense, we see that at first they make their appearance in 

 the simplest conceivable form : only very gradually does 

 that wonderful perfect structure develop by which the 

 higher sense-organs eventually become the most remarkable 

 and the most complex mechanisms of the entire organiza- 

 tion. All organs of sense are, however, originally merely 

 portions of the external skin-covering, in which sensorial 

 nerves are distributed. Even these nerves were originally 

 homogeneous and undifferentiated in character. Gradually, 

 by division of labour, the various functions or " specific 

 energies" of the different sensorial nerves developed. Simul- 

 taneously the simple terminal expansions of these sense 

 nerves in the skin-covering developed into extremely com- 

 plex organs. 



The important bearings of these historic facts upon the 

 just appreciation of mental life will readily be perceived. 

 The whole philosophy of the future will assume another 



