HISTORICAL REVIEW 49 



not a primitive condition, but a product of development." 

 " To a change leading away from the optimum (in either 

 plus or minus direction)" the organism responds in such a 

 way as to tend to return to the optimum. " Thus are pro- 

 duced the so-called positive and negative reactions." 



The essential characteristics in behavior, as analyzed by 

 Jennings, are clearly set forth in the following quotations 

 (1906, pp. 283-292). Internal factors: ** Activity does 

 not require present external stimulation. . . . Activity 

 may change without external cause. . . . Changes in 

 activity depend on changes in physiological states. . . . 

 Reactions to external agents depend on physiological 

 states. . . . The physiological state may be changed by 

 progressive internal processes, particularly those of metabo- 

 lism. . . . The physiological state may be changed by the 

 action of external agents. . . . The physiological state 

 may be changed by the activity of the organism. . . . 

 External agents cause reaction by changing the physio- 

 logical state of the organism. . . . The behavior of the 

 organism at any moment depends upon its physiological 

 state at that moment. . . . Physiological states change in 

 accordance with certain laws. . . . The resolution of one 

 physiological state into another becomes easier and more 

 rapid after it has taken place a number of times." 



Different factors on which behavior depends : "We have 

 seen that the behavior of the organism at a given moment 

 depends on its physiological state, and that it therefore 

 secondarily depends upon all the factors upon which the 

 physiological state depends. Hence we cannot expect the 

 behavior to be determined alone by the present external 

 stimulus, as is sometimes maintained, for this is only one 

 factor in determining the physiological state. The be- 

 havior at a given moment may depend on the following 

 factors, since these all affect the physiological state of the 

 organism : 



" I. The present external stimulus. 



''2. Former stimuli. 



