PHYSIOLOGICAL STATES AS DETERMINING FACTORS. 121 



again may be illustrated from the experiments on Stentor, described 

 above (p. 1 12). 



(3) Internal causes, not definable, may give rise to changes in physio- 

 logical condition. The result is spontaneous movement at intervals, 

 as described above for Vorticella and Hydra. As many authors have 

 pointed out, rhythmical spontaneous movements may be due to a steady, 

 non-rhythmical, internal change of condition. 



(4) The movement or reaction performed by the organism may 

 change the physiological condition. This is illustrated in one way by 

 the fact that after a single spontaneous contraction in Vorticella or 

 Hydra, the animal remains quiet for an interval, showing that the 

 original physiological condition was restored by the movement. The 

 fact that the reaction performed by the organism changes the physio- 

 logical condition of the latter is, of course, the basis of the formation of 

 habits in higher organisms ; in this case the performance of a reaction 

 once or repeatedly throws the organism into a condition where it is 

 more likely to react in the same way again. This particular method 

 of alteration of condition has perhaps not been clearly demonstrated 

 for unicellular organisms, though there is some indication of it in the 

 behavior of Vorticella, as described by Hodge & Aikins (1895), and 

 of Stentor as described by myself in the ninth of my studies. Thus, 

 Stentor responds to carmine in the water by a series of different reac- 

 tions, finally reaching the condition where it reacts by contracting at 

 once into its tube. If the stimulus is now repeated every time the 

 Stentor extends, it never gives its earlier method of reaction, but reacts 

 steadilv for a long time by contracting at each stimulus. Is this persist- 

 ence in the contraction reaction due partly to the fact that it has begun 

 on this reaction method, and therefore keeps it up, or is it due only to 

 the fact that the stimulus has been repeated many times? In the former 

 case the behavior would perhaps fall under our present point of view ; 

 in the latter it would not. Cases among the Protozoa where the repeated 

 performance of a reaction clearly makes the further performance of the 

 same reaction easier or more likely to occur, would be of much interest. 



NATURE OF REACTIONS TO STIMULI. 



The foregoing considerations evidently have a definite bearing on 

 the problem of the nature of reactions to stimuli. They lead, as set 

 forth briefly on page nS, to the following conception of the steps oc- 

 curring in a reaction to a stimulus: (i) The stimulus acting on the 

 organism causes a change in its physiological condition ; (2) this 

 change in physiological condition gives rise to the typical reaction. 



The evidence for this view is found scattered throughout the fore- 

 going discussion ; its main points may be briefly summarized here as 

 follows : 



