244 LIGHT AND THE BEHAVIOR OF ORGANISMS 



forms, particularly in the early stages of development, aggre- 

 gate at the surface of the water in this way. They do not 

 have a response similar to the avoiding reaction in many 

 unicellular forms, nor do they become negative even if the 

 light intensity is increased to such an extent that it is 

 undoubtedly injurious. Conditions can readily be so ar- 

 ranged that they will go toward the light into chemical 

 solutions, temperatures and concentrations where they are 

 killed. Still their reactions under natural conditions are 

 highly adaptive. Owing to their strong positive reaction 

 they are brought to the surface of the water and scattered 

 far and wide. Under natural conditions they apparently 

 never or at least seldom experience light of such an intensity 

 that it is injurious, or a combination of other environmental 

 conditions such that their positive reactions to light prove 

 fatal. The power to become negative in excessively high 

 light intensity would consequently be of no special benefit 

 to these organisms. 



d. Orientation and movement directly toward the place 

 where the organism comes to rest. — Most of the organisms 

 with well-developed eyes, together with some few without, 

 may be placed under this head. They go directly to a 

 given place and remain because they come to rest there. 

 Thus we find according to Bohn (1908) that Littorina and 

 some starfishes under certain conditions go directly toward 

 rocks and other objects and come to rest in their shadows. 

 Butterflies and various other forms, according to Parker 

 (1903) and Cole (1907), go directly toward large luminous 

 areas and come to rest there. And frogs in direct sunlight 

 were found by Torelle (1903) to go directly toward shadows, 

 even if in so doing they had to move perpendicular to the 

 direction of the light rays. But after they were in the 

 shadow they turned so as to face the light, an adaptive reac- 

 tion throughout; for under the conditions of the experiment 

 It was probably advantageous for the frogs to be pro- 

 tected by the shade and to face the light when in it, since 

 thus they were most likely to see both food and enemies. 



