196 THE EVOLUTION OF MAN. 



system and of the sense-organs. And yet, as we already 

 know from the germ-history of man, the latter is developed 

 from the former. Those organs of our body which discharge 

 the highest and most perfect functions of animal life — those 

 of sensation, volition, thought — in a word, the organs of 

 the psyche, of mental life — arise from the external skin- 

 covering. 



This remarkable fact, considered in itself alone, seems so 

 wonderful, inexplicable, and paradoxical, that the truth of the 

 fact was simply long denied. The most trustworthy embryo- 

 losfical observations were met with the erroneous statement 

 that the central nerve-system develops, not from the outer 

 germ-layer, but from a special cell-layer lying underneath 

 this. The ontogenetic fact would not, however, yield ; and, 

 now that Phylogeny has thrown light on the subject, the 

 fact seems perfectly natural and necessary. When we 

 reflect on the historic evolution of mind and sense activities, 

 we must necessarily conceive the cells, which accomplish 

 these, as originally situated on the outer surface of the 

 animal-body. Such externally placed elementary organs 

 could alone directly receive and deal with impressions from 

 the outer world. Afterwards, under the influence of 

 natural selection, the complex cell-masses which had become 

 especially " sensitive " gradually withdrew into the shelter of 

 the interior of the body, and there laid the first foundations 

 of a central nervous organ. As differentiation advanced, 

 the distance and distinction between the external skin- 

 covering and the central nervous system detached from this, 

 became continually greater, and finally the two were per- 

 manently connected merely by the conductive peripheric 

 nei-vcs^ 



