The Problem of Orientation 77 



ulated again, after which it again turns sharply from 

 the source of light. This process is repeated until 

 the organism has turned to such an extent that the 

 anterior end is practically as much exposed when it 

 turns in one direction as when it turns in the other. 

 The great preponderance of lateral movements 

 from the source of light and direct orientation in 

 diffuse light therefore do not indicate that fly larvae 

 have the power of differential response to localized 

 stimulation." 



Loeb, who sets little store by such proximate 

 categories of explanation as trial and error, has con- 

 tended against the doctrine that orientation is ef- 

 fected by this method. In speaking of the doctrine 

 that orientation of an organism in strong light may 

 be direct, while in weak light it may take place by 

 the indirect method described above, he says: "If 

 the photosensitiveness of the animal is lessened the 

 animal may deviate for a longer period from the 

 direction of the light rays. Such animals do event- 

 ually reach the lighted side of the vessel, but they 

 no longer go straight toward it, moving instead in 

 zig-zag lines or very irregularly. It is therefore not 

 a case of qualitative, but of a quantitative, difference 

 in the behavior of heliotropic animals under greater 

 or lesser illumination, and it is therefore erroneous 

 to assert that heliotropism determines the movements 

 of animals toward the source of light only under 

 strong illumination, but that under weaker illumina- 

 tion an essentially different condition exists." 



