Facts and Factors of Development 47 



these relatively simple motor reactions, Paramecium does many 

 things which seem to show intelligence and purpose. It avoids 

 many injurious substances, such as strong salts or acids, and it 

 collects in non-injurious or beneficial substances, such as weak 

 acids, masses of bacteria upon which it feeds, etc. It avoids ex- 

 tremes of heat and cold and if one end of a dish containing Para- 

 mecla is heated and the other end is cooled by ice, the Paramecia 

 collect in the region somewhere between these two extremes 

 (Fig. 17). Jennings, by studying carefully the behavior of sin- 

 gle individuals, established the fact that this apparently intelli- 

 gent action is due to differential sensitivity and to the single motor 

 reaction of the animal. If in the course of its swimming a Para- 

 mecium comes into contact with an irritating substance or con- 

 dition, 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 irritating conditions this reaction is repeated, and so on in- 

 definitely until finally a path is found in which the cause of irri- 

 tation is avoided altogether. In short, Parameciutn continually 

 tries its environment, and backs away from irritating substances 

 or conditions. Its apparently intelligent reactions are thus ex- 

 plained as due to a process of "trial and error."* 



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

 well as of fishes, frogs, reptiles, birds and mammals, has 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 



* In Paramecium, 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 teleo- 

 logical organization of Paramecium 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 expression "trial and error" necessarily 

 involves a striving after ends, it would be advisable to replace it in this 

 case by some such term as "useful or adaptive reactions." 



