2 LIGHT AND THE BEHAVIOR OF ORGANISMS 



behave in light of different colors? What are the factors 

 involved in orientation, i.e., in attaining a definite axial 

 position with reference to the source of stimulation? How 

 do organisms regulate the direction of movement; how do 

 they remain oriented? What is the cause of reversal in 

 the sense of orientation? What controls variability and 

 modifiability in reactions to light? Are the reactions 

 adaptive? These are the principal problems before us, 

 problems which cannot fail to be of interest to all who are 

 in any way concerned with the activities of organisms. 



Various solutions of these problems have been offered 

 by different investigators. Some say that motile plants 

 and animals orient and collect in light of a given intensity 

 because the particular intensity in which they congregate 

 pleases them more than any other, implying that there are 

 psychic phenamena involved in the process and indicating 

 that it is difference of light intensity in the field which 

 controls the direction of movement. Others say the re- 

 actions are not fundamentally adaptive and can be ex- 

 plained mechanically; that the movements of organisms 

 are, with few exceptions, regulated by the direction in 

 which the rays of light penetrate the tissue or by the angle 

 which the rays make with the sensitive surface or by the 

 relative intensity on symmetrical opposite sides. Light is 

 supposed by these investigators to act constantly as a 

 directive stimulus. The organisms are automatically con- 

 trolled by external factors. Still other authors claim that 

 the reactions to light are in general useful to the organism, 

 but that they can be accounted for mechanically and that 

 the essential controlling factor is a change of intensity on 

 the surface of the organism; that the other external factors 

 mentioned are of Importance only in so far as they make 

 such a change possible; that light does not act constantly 

 as an orienting stimulus, and that internal physiological 

 processes have much to do with the reactions. Some 

 maintain that only the more refrangible rays of the spec- 

 trum, those toward the violet end, are efficient in stimu- 



