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 



of the space where the intensity of 



,v 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 

 w T here 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 b, while the planarians 

 collect at c and d where the intensity 

 of light is a minimum. Eeactions de- 

 termined by the value j t 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- 



Fio. 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 b, and animals 

 which come to rest where 

 the intensity of light is a 

 relative minimum at c 

 and d. 



a The reader should notice the difference in the 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 



dt 



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. 



