68 HEREDITY AND ENVIRONMENT 



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 

 irritating conditions this reaction is repeated, 

 and so on indefinitely until finally a path is 

 found in which the source of irritation is 

 avoided altogether. 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."* 



The behavior of worms, star-fishes, crusta- 

 ceans, mollusks, as well as of fishes, frogs, 

 reptiles, birds and mammals, has been studied 



* 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 re- 



