The Mechanism of Adaptation 345 



to persist in the conditions in which they are found; those 

 that leave no offspring may be fit to live, but they are unfit 

 for the perpetuation of the species, and those that are in- 

 capable of beginning development are least fit of all. 



6. Trial and Error 



Such elimination of persons or cells or genes would not, 

 however, explain individual or contingent adaptations, which 

 are really only beneficial responses to environmental stimuli 

 in which no elimination of individuals occurs. It seems to 

 me that many, perhaps all, such adaptations may find a 

 mechanistic explanation in the further extension of the 

 selection principle to the physiological responses of organ- 

 isms. Herbert Spencer explained adaptive motions on the 

 principle of "overproduction of movements" and the per- 

 sistence of those that are beneficial. Darwin suggested this 

 method of explaining the apparently intelligent behavior of 

 the earthworm. Lloyd Morgan applied this principle to the 

 study of animal behavior under the designation of "trial and 

 error." In a series of masterly works Jennings has proved 

 that the beneficial responses shown by many lower organisms 

 may be reduced to this simple principle of "trial and error"; 

 in this way apparently purposive behavior which Binnet 

 supposed to be due to the relatively complex "psychic life 

 of micro-organisms" has been shown to be due to a few 

 simple motor reflexes, which are repeated indefinitely until 

 they bring the organism into a favorable environment. 

 Many recent investigators have shown that this principle is 

 applicable to the behavior of a large number of animals. 



This principle of overproduction of movements or of 

 "trial and error" is in reality the rejection, elimination, or 

 cessation of unfit responses and the persistence of beneficial 

 ones. It has hitherto been applied only to motor reactions, 



