486 JOHN LINCK ULRICH 



Every movement other than the customary one, results from 

 a change in the functional condition of the animal. Some objec- 

 tion may arise to this statement, when such complex behavior 

 is observed as the substitution of one method for another when 

 the first is ineffective. It may be contended that the rat "does 

 something," and that the movements of the rat are not strictly 

 determined by the functional condition of the organism. But 

 this substitution of movements is gradually facilitated, and does 

 not appear suddenly. A return to the plane and the use of 

 another method to plunge the plane occurs only in later trials 

 hi learning. It is easily observed to be determined by the 

 absence of sensory excitations which are usually aroused in the 

 organism when the plane is plunged, and which condition direct 

 progression to the opened door. Progression, when the plane 

 is plunged, is ended on the other side of the plane and a return 

 movement is facilitated, or progression around the problem box 

 takes place once or twice before the plane is plunged. Some- 

 tunes progression is continued ineffectively as far as the door and 

 not around the problem. At such times and when progression 

 is ended on the other side of the plane, wandering movements 

 in any direction appear. Effective posture for progression in a 

 definite direction does not take place, and eventually a return 

 movement is facilitated to the plane. But the plane is not 

 immediately plunged, for a facilitation of progression and the 

 use of another method must take place. One ineffective method 

 disturbs the established physiological sequence of movements 

 and another sequence is facilitated. Other movements com- 

 pensate for an ineffective sequence of movements. 



Another physiological condition besides the existing develop- 

 ment of the extensor thrusts influences the effective performance 

 of movements. Fluctuations in reflex excitability also appear 

 as well as fluctuations in the extensor thrusts. When fluctua- 

 tions appear in reflex excitability, movements may be more 

 rapidly, more effectively performed with an increase in excit- 

 ability, and more slowly and less effectively performed when it 

 becomes hyponormal. A change in a rat's movements is more 

 noticeable with a decrease than with an increase in reflex excit- 



