CHAPTER XIII 



FACTORS INVOLVED IN REGULATING REACTIONS TO LIGHT 

 — VARIABILITY AND MODIFIABILITY IN BEHAVIOR 



Everyone who has ever attempted observations on the 

 behavior of organisms with precise methods, knows that 

 variability even in the lower forms under constant external 

 conditions is one of the striking characteristics in reactions. 

 There are internal as well as external factors involved in 

 determining what the organism is to do. Just what these 

 are and how they influence reactions is a question of pri- 

 mary importance. 



Many organisms turn or move toward a source of light 

 under certain conditions and away from it under other 

 conditions. They may be either positive or negative; that 

 is, the sense of orientation and response, in general, may be 

 reversed. Nearly all organisms turn through i8o° when 

 the sense of orientation changes so that they always move 

 with the same end ahead. There are however some excep- 

 tions. Radl (1903, p. 91) claims that Daphnia may swim 

 about in various directions with the back constantly facing 

 the source of light. Bohn (1905, p. 8) found that young 

 European lobster larvae always swim with the posterior end 

 directed toward the source of light, so that when they are 

 positive this end is ahead, and when negative it is behind. 

 Hadley (1908, p. 260) observed the same in the larvae of 

 the American lobster, as did also Lyon (1906) in several 

 larval stages of Palaemon. I observed similar methods of 

 locomotion in the larvae of several other decapod Crustacea. 

 In many of the lower forms orientation results from re- 

 sponses to changes in light Intensity. When these forms are 

 negative they respond only to an Increase of. intensity, and 

 when they are positive only to a decrease. It will be our 



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