Harper, Behavior of Corethra. 453 



whose stimulation leads to the intensified negative response, essen- 

 tial to safety. The highly developed eucone eyes, rarely found in 

 larvae, are highly adaptive to the open water habitat. 



With the set of adaptions already mentioned, further adapta- 

 tion to environment in changing physiological states may be 

 secured by movements in a vertical path, and these movements are 

 regulated under the combined influence of light and gravity. 



The mode of orientation in the negative reaction and in the 

 positive light response were shown to be unconformable to the 

 conventional tropism schema, but adapted to the peculiar action 

 system of this animal. 



The great majority of the movements appear as spontaneous, 

 overflow movements adapted to the needs of the animal by their 

 special character, to bring it into contact with as wide an environ- 

 ment as possible. Such movements may be called trial move- 

 ments. Finally in comparing the two kinds, the externally directed 

 and the spontaneous, it was seen that each is purposive in its 

 nature, but in a different way. The internally incited are varied 

 so as to bring the animal into wider contact with the environ- 

 ment. The externally directed are not varied in direction, but 

 precisely oriented and indicate the animal's capacity to regulate 

 its movements beneficially by orienting itself immediately with 

 reference to the direction of an external localized stimulus. The 

 trial and error principle does not inhere in the externally directed 

 movements. The basis of the trial and error theory of orienta- 

 tion lies in the assumptions contained in the phrase "varied move- 

 ments under stimulation." 



In the undifferentiated movements of the Protozoa there may 

 be a blending of internal and external factors in the same move- 

 ment, which can only be interpreted by analysis into its compo- 

 nents. Results derived from the Protozoa should therefore be 

 checked by comparison with animals showing a more differenti- 

 ated type of behavior in which the various reactions, taking food, 

 avoiding injury, spontaneous movements, etc., take place sepa- 

 rately and may be recognized as such. 



It will then be seen that the directive influence of external stim- 

 uli and the trial and error principle in behavior are not mutually 

 exclusive, but complementary. But to make the trial and error 

 principle include all behavior is as false as the tendency to regard 

 reactions to external stimuli as the foundation of all behavior. 



