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TEE POPULAR SCIENCE MONTHLY 



tremes of heat and cold and if one end of a dish containing paramecia 

 is heated and the other end is cooled by ice, the paramecia collect in the 

 region somewhere between these two extremes (Fig. 19). Jennings, by 

 studying carefully the behavior of single individuals, established the fact 

 that this apparently intelligent action is due to differential sensitivity 

 and to the single motor reaction of the animal. If in the course of its 

 swimming a Paramecium comes into contact with an irritating substance 

 or condition, it backs a short distance, swerves toward its aboral side, 

 and goes ahead in a new path; if it again comes in contact with the irri- 



Fig. 21. Diagram of the Avoiding Reaction of Paramecium. A is a solid ob- 

 ject or other source of stimulation. 1-6, successive positions occupied by the animal. 

 The rotation on the long axis is not shown. (After Jennings.) 



tating conditions this reaction is repeated, and so on indefinitely until 

 finally a path is found in which the source of irritation is avoided alto- 

 gether. In short, Paramecium continually tries its environment, and 

 backs away from irritating substances or conditions. Its apparently 

 intelligent reactions are thus explained as due to a process of " trial and 

 error." 2 



The behavior of worms, star-fishes, crustaceans, mollusks, as well as 

 of fishes, frogs, reptiles, birds and mammals, have been studied and in 

 all cases it is found that their method of responding to stimuli is not at 

 first really purposive and intelligent but by the gradual elimination of 

 useless responses and the preservation (or remembering) of useful ones 

 the behavior may come to be purposive and intelligent. 



Thorndike found that when dogs, cats and monkeys were confined 



s In Paramcecium, there is certainly no consciousness of trial and error, 

 and probably no unconscious attempt on the part of the animal to attain certain 

 ends. Its responses are reflexes or tropisms, which are determined by the nature 

 of the animal, and the character of the stimulus. The fact that these responses 

 are in the main self -preservative is due to the teleological organization of Para- 

 moecium which has been evolved, according to current opinion, as the result of 

 long ages of the elimination of the unfit. If, in the opinion of any one, the ex- 

 pression ' ' trial and error ' ' necessarily involves a striving after ends, it would be 

 advisable to replace it in this ease by some such term as "useful or adaptive 

 reactions." 



