236 THE ORIGIN OF THE NERVOUS SYSTEM 



because the specialization may alter the primary rela- 

 tions, e.g., an effector at a certain level, although 

 subordinate to the receptor-conductor portion of the 

 system, may show under excitation a more intense 

 activity than levels of the gradient above it. An 

 excitation system in which local specialization of receptor, 

 conductor, and effector, or of some of them, has begun 

 may then be distinguished physiologically from a simple 

 gradient as an excitation arc. Morphologically as well 

 as physiologically, of course, it may differ from the 

 simple gradient in an indefinite number of ways. Except 

 as regards the highly adaptive character and the central 

 complications in the reflexes of higher animals, such an 

 excitation arc is already similar physiologically to a 

 reflex arc, though it is not necessarily associated with 

 visibly differentiated nervous structure. With the evo- 

 lution of the adaptive character, i.e., the preferential 

 development, persistence, or low "functional resistance" 

 of certain arcs which serve a biological purpose, the arc 

 may be conceived, as Dewey (1893) suggests, as physio- 

 logically a circuit. 



It is perhaps necessary to call attention to the dis- 

 tinction between organismic and protoplasmic pattern 

 involved in what has been said concerning adaptation in 

 the reflex arc. Even the simplest organism capable of 

 responding in any way to the action of an external 

 factor has unquestionably undergone adaptation as 

 regards its protoplasmic pattern, but a physiological 

 gradient in it may be a matter of direct response of this 

 protoplasm to external factors. This gradient pattern 

 itself is not adaptive in character, though the protoplasmic 

 pattern on which it is based may be highly adaptive. A 



