4^2 H. S. Jennings. 



internal processes, so that the changes in behavior cease, and the 

 reheving condition is thus retained. 



It is clear that regulation taking place in this way does not 

 require that the. end or purpose of the action shall function in any 

 way as part of its cause, as some vitalistic theories hold. There 

 is no objective evidence that a final aim is guiding the organism. 

 None of the factors above mentioned appear to include anything 

 differing in essential principle from such laws of causality as we 

 find in the inorganic world. 



Now an additional factor enters the problem. By the process 

 which we have just considered, the organism reaches in time a 

 movement that brings relief from the interfering conditions. 

 This relieving response becomes fixed through the operation of a 

 certain law which appears to hold throughout organic activities. 

 This law may be stated as follows: An action performed or a 

 physiological state reached, is performed or reached more readily 

 after one or more repetitions, so that in time it becomes " habitual." 

 The statement of this law just given is in reality not adequate, and 

 it may be well to dwell upon it a moment, developing it farther, 

 and pointing out some of the phenomena in which it is expressed. 

 In previous papers, including the one immediately preceding the 

 present, I have pointed out the fact that the behavior and reac- 

 tions of an organism depend largely on "physiological states;" 

 the same point has recently been emphasized by Bohn ('05). 

 We may distinguish at least two great classes of physiological 

 states — those depending on the metabolic processes of the organ- 

 ism, treated in detail in the preceding paper, and those otherwise 

 determined. The physiological states of organisms change in 

 accordance with certain laws. The changes in the metabolic 

 states of course depend on the laws of metabolism. In the physio- 

 logical states not directly dependent upon metabolism, but rather 

 upon stimulation and upon the activity of the organism, such as are 

 found in Stentor and Planaria (see Jennings, '04), we find certain 

 fairly well defined laws of change that are of a peculiar character. 



In the organisms just mentioned, and in many others, the fol- 

 lowing phenomena have been found. Under certain external 

 conditions the organism reacts in a certain way. These con- 

 ditions continuing, the organism changes its first reaction for a 

 second, and then perhaps for a third and fourth. Later the 

 same external conditions recur, and now the organism at once 



