308 PHYSIOLOGICAL DEVELOPMENT. 



outer disturbances would cause the sinking inwards, and 

 covering up, of the specially-sensitive area and the plexus 

 below it. But it is manifest that since these nervous struc 

 tures, at once all-important and easily injured, would be 

 safer if removed from the surface, survival of the fittest, con 

 tinually preserving those in which they were more deeply 

 seated, would tend to produce an arrangement in which all 

 parts but the actual receivers of external stimuli became 

 internal. 



Hence, contemplating generally these two fundamental 

 differentiations of inner from outer tissues, we may conclude 

 that though their first stages resulted from direct equilibra 

 tion, their subsequent and higher stages resulted from in 

 direct equilibration. 



