228 THE EVOLUTION OF MAN. 



higher and lower Apes than between the higher Apes and 

 Man. This statement is, indeed, equally true of all the 

 other parts of the body. But the fact that it is true of the 

 central marrow is especially important. This does not 

 become fully evident unless these morphological facts are 

 considered in connection with the corresponding physio- 

 logical phenomena; until we consider that every mental 

 activity requires for its complete and normal exercise the 

 complete and normal condition of the corresponding brain- 

 structure. The extremely complex and perfect active 

 phenomena within the nerve -cells, summed up in the word 

 "mental life," can no more exist without their organs in 

 the vertebrates, including man, than can the circulation of 

 the blood without a heart or blood. As, however, the 

 central marrow of Man has developed from the same 

 medullary tube as in all other Vertebrates, so also must the 

 mental life of Man have had the same origin. 



All this is of course true of the conductive marrow, or 

 the so-called "peripheric nervous system." This consists 

 of the sensitive nervous fibres which convey the impressions 

 of sensation from the skin and the organs of the senses in 

 a centripetal direction to the central marrow ; as well as 

 of the motor nervous fibres, which, reversely, convey the 

 movements of volition from the central marrow, in a cen- 

 trifugal direction to the muscles. By far the greater part 

 of these peripheric conductive nerves originates from the 

 skin-fibrous layer, by peculiar local differentiation of the 

 rows of cells into the respective organs. 



The membranous coverings and blood-vessels of the 

 central marrow are identical in origin with the greater part 

 of the conductive marrow; these membranous coverings 



