OF VITAL PHENOMENA 153 



Jennings (1906) has shown that the movements of certain 

 protbzoa, that had been considered tropisms, are complex activ- 

 ities in which sensibility to change takes a leading part. Parame- 

 cium, when stimulated sufficiently by a change in the environment, 

 reverses some of its cilia, so that it moves a short distance back- 

 ward and the spiral course of its movement is widened. Being 

 removed from the acute stimulation, it starts forward again from 

 some segment of the spiral. The fact that the spiral is widened 

 during the backward movement causes the course to be altered 

 when the forward course is resumed. If this new course leads 

 to a second stimulation the whole process is repeated. This repe- 

 tition is continued until the animal ceases to be stimulated. 

 Jennings calls this a case of "trial and error," thus showing 

 a relation to the reactions of man. The distinction between 

 tropism and trial and error is a philosophical one. There is no 

 trial in the teleological sense and the cause of error will not be 

 known until the mechanism of the reaction is understood. Trial 

 and error has the same significance as chance in the laws of 

 chance. Chance is merely a name to express ignorance of un* 

 known factors. Only where these factors have some constant 

 numerical relation to one another, is there a law of chance. 



The question arises whether in Paramecium the stimulus acts 

 directly on the motor organs, cilia. Since any strong stimulus, 

 no matter from what direction, causes the same motor response, 

 it would seem that there is some coordinating mechanism for 

 the ciliary movement, or at least that the stimulus is propagated 

 over the body of the Paramecium. Since the stimulus is propa- 

 gated over the cells of sensitive plants without a special con- 

 ducting mechanism, there is no reason to suppose that the same 

 thing might not be true of animals. Nemec describes a conduct- 

 ing mechanism in plants, but this has been shown to be an arte- 

 fact (McClendon, 1910 b). Neresheimer describes a conducting 

 mechanism in infusoria, but this also is doubted. 



The ameboid movements just described might properly be 

 classed as tropisms, but even in the ameba there seems to be 

 some evidence of the transmission of stimuli over the plasma 

 membrane (McClendon, 1909 a). 



The reactions of protozoa to constant electric currents seem 

 to harmonize with the idea that tropisms are forced movements. 



