96 



TROPISMS 



problem to be solved is in both cases a purely physico- 

 chemical one. It was also found that the motions of 

 certain animals stop when they come suddenly from strong 

 light into weak light. This was observed in planarians 

 which as a consequence collect in greater density in spots 



f the space where the intensity of 



light is a relative minimum. 291 The 

 difference in the conduct of helio- 

 tropic organisms like Daphnia which 

 go to or from the light and animals 

 like planarians which come to rest 

 where the intensity of light is a rela- 

 tive minimum can be demonstrated 

 by putting them into a circular vessel 

 ( Fig. 33 ) . The positively heliotropic 

 animals collect at a, the negatively 

 heliotropic at &, while the planarians 

 collect at c and d where the intensity 

 of light is a minimum. Reactions de- 

 termined by the value Jj do not lead 

 to phenomena of orientation, though 

 such (improperly called) "fright 

 reactions " a occur in many helio- 

 tropic animals; they may lead, however, to collections 

 of animals. 



Jennings has maintained that all reactions of unicel- 

 lular organisms are due to "fright" or "avoiding reac- 



aThe reader should notice the difference in tlie treatment of animal 

 conduct from the point of view of the physicist and of the introspective psy- 

 chologist. What the physicist expresses correctly by the term the an- 

 thropomorphic biologist explains in terms of human analogy as " avoiding 

 reaction " or " fright reaction," a term which not only assumes the existence 

 of sensations without any adequate proof, but removes the problem from the 

 field of quantitative experimentation. 



FIG. 33. Difference 

 in place of gathering be- 

 tween heliotropic animals 

 and animals which come 

 to rest when reaching a 

 relative minimum in the 

 intensity of light. In a 

 circular vessel a c b d and 

 W W representing the 

 window, positively helio- 

 tropic animals will collect 

 at a, negatively heliotropic 

 animals at 6, and animals 

 which come to rest where 

 the intensity of light is a 

 relative minimum at c 

 and d. 



