THE REFLEX ARC 237 



simple excitation gradient, arising in direct response to 

 external excitation, is not as a whole an adaptive pattern. 

 The problem of the evolution of a true adaptive 

 reflex from the protoplasmic excitation gradient is a 

 problem of the specialization and adaptation of function 

 and structure. The establishment of definite axial 

 relations in the organism and so of definite relations to 

 the external world affords a physiological basis for the 

 localization and differentiation of definite receptors, 

 conductors, and effectors, and with the beginning of 

 the specialization of these as distinct organs, the reflex 

 in its essential physiological features is present. The 

 adaptive features of the reflex are chiefly in the central 

 organ, the adjuster, and the relations of the particular 

 arc to it. In this aspect the reflex arc becomes a 

 product of evolution, and its development is a historical 

 process associated with biological purpose. At present 

 we are concerned rather with the discovery of the 

 physiological basis of such adaptive evolution than 

 with the evolution itself. 



HISTORICAL AND CRITICAL 



If the physiological basis of the reflex arc is an 

 excitation gradient of the same sort as the gradients 

 which underlie axiate order and integration, we should 

 expect to find receptor, conductor, and effector associ- 

 ated with each other, not independent in their evolu- 

 tionary history. The relation between receptor and 

 conductor is self-evident and leaves little room for dis- 

 cussion, but as regards the relation of the effector to 

 the rest of the arc the physiological facts are not so 

 clear, and a wide difference of opinion has existed. 



