io8 Studies in Animal Behavior 



generalizations regarding the reversal of tropisms 

 because the same factors often cause quite opposite 

 effects in different organisms. Increase of tempera- 

 ture, as we have seen, may change phototaxis from 

 positive to negative in some forms and from nega- 

 tive to positive in others. Contact stimuli act in 

 the same way as shown by their different influence 

 on the light reactions of Cypridopsis and Daphnia 

 or Ranatra. Exposure to darkness tends to make 

 some species positive and others negative, and ex- 

 posure to strong light produces similar varied re- 

 sults. These apparently contradictory forms of be- 

 havior will probably be reconciled when we have 

 acquired a deeper insight into the physiological 

 mechanisms involved in tropic reactions. Thus far 

 we have had few serious attempts to explain the 

 phenomena of reversal, owing probably to the diffi- 

 culties and perplexities of the undertaking. 



Dr. B. Moore in the course of a paper on the 

 light reactions of certain marine organisms has de- 

 veloped a view which he expresses as follows: 

 "Both the positive and the negative behavior to 

 light may be explained on the basis of one chemi- 

 cal action of the cell (a katabolic one). The posi- 

 tive state indicates that the speed of reactions in 

 the cell lies below a certain value, which may be 

 called the optimal value, and the negative state cor- 

 responds to a speed of reactions in the cell above the 

 optimal value. In the former case the sentient sur- 

 faces are turned into the light to increase velocity 



