2^6 THE EVOLUTION OF MAN. 



differentiated sense-organs of all Intestinal Animals (J/e^azoa), 

 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 a 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 



