MOTOR IMPRESSIONS 163 



limbed Vertebrates, has responded to all the exacting 

 calls made upon its functions by the myriad promptings 

 of the complex human brain. We will, however, not 

 pursue this theme. 



It is the necessity for the close association of the func- 

 tions of sensation and mobility, which are subserved by 

 the emancipated hand, that is of interest in evolution 

 from the dawn stage we have pictured. We are con- 

 cerned only with the problem of the possible manner in 

 which these things have affected brain development. 

 In the present state of knowledge this problem is a highly 

 complex one, but there can be no doubt that, on broad 

 lines, fairly simple underlying processes act harmoniously 

 in the evolution of the brain. There has been enunciated, 

 by Dr. Ariens Kappers, of Amsterdam, a theory of one 

 such underlying principle to which he has given the name 

 of " neurobiotaxis." Put into simple language, the prin- 

 ciple involved is a calling of the nervous seat of origin 

 of the outgoing motor impulses towards the site to which 

 the associated incoming sensory impulses stream. Sup- 

 pose, for instance, the primitive nervous centre which is 

 associated with any definite sensibility to have a well- 

 defined anatomical position in the central nervous 

 system, then, in its immediate neighbourhood, and 

 attracted to it, will be the motor centre which governs 

 the movements of the parts most intimately related to 

 this particular sensibility. It may be that this particular 

 sensibility is intimately related to different movable parts 

 in different animals, and then in each will be found a 

 different motor centre appropriately attracted into the 

 closest anatomical relation to the sensory centre. This 

 general principle has been shown by Ariens Kappers to 

 hold good in the case of the ganglionic centres of the 

 cranial nerves, and to account for their otherwise inex- 

 plicable positions in the brain stem. This principle of 

 neurobiotaxis is, I believe, capable of extension from 

 the ganglionic masses in the brain stem, where Ariens 



