The Mind of the Simplest Animals 51 



it continues on its way ; if not, repeated contact with the stim- 

 ulus causes a second reaction, the Paramecium always turning 

 in the same direction, so that ultimately it avoids the source 

 of stimulation (194, 211) (Fig. 4). Differing strengths of 

 stimulus produce the reaction with different degrees of vio- 

 lence. When a very strong stimulus is encountered, the 

 animals "respond first by swimming a long way backward, 

 thus removing themselves as far as possible from the source 

 of stimulation. Then they turn directly toward the aboral 



FIG. 4. Negative reaction of Paramecium. A is the source of stimulation. 

 1-6 are the successive positions of the animal. After Jennings (211). 



side, the rotation on the long axis completely ceasing. 

 In this way the animal may turn directly away from the drop 

 [the stimulus] and retrace its course" (211, p. 50). On the 

 other hand, when the stimulus is very weak the reaction may 

 be reduced to the following form: the Paramecium " merely 

 stops, or progresses more slowly, and begins to swing its 

 anterior end about in a circle." As long as it does not thus 

 get out of range of the stimulus, the movement is continued. 

 "When the anterior end is finally pointed in a direction from 

 which no more of the stimulating agent comes, the Paramecium 

 swims forward " (211, p. 51). Evidently, however, these are 



