176 The Animal Mind 



fluences have been found to reverse the sense of light reactions, 

 transforming negatively phototropic into positively photo- 

 tropic animals, and vice versa. That such reversal should 

 occur in response to increase or decrease of the intensity of 

 the light is what one would naturally expect ; if a certain in- 

 tensity of illumination is favorable to the life processes of an 

 animal, it would seem appropriate for it to seek light of that 

 intensity but avoid light of greater intensity. Many animals, 

 like Gonionemus, are positive to light of moderate intensity 

 and negative to strong light (451). The females of the crusta- 

 cean Labidocera migrate to the surface of the water at night- 

 fall because, like the earthworm, they react positively to faint 

 light ; and move downward at sunrise because they are nega- 

 tive in their response to intenser light (304). On the other 

 hand, Holmes observed that Orchestia agilis, an amphipod 

 crustacean, would, if brought from strong to weaker light, be- 

 come negative for a short time ; the meaning of such a change 

 it is difficult to conjecture (181). Sudden reduction of light 

 causes a temporary negative phase also in Convoluta roscof- 

 fensis (140). 



Prolonged action of^Jight__ma,y alter phototropism : the 

 "depth migrations," that is, the periodical movements toward 

 and away from the surface of the water, in the free- swimming 

 larvae of the barnacle, Balanus, are due apparently to the fact 

 that an exposure of several hours to light will make positive 

 animals negative, even though the light at the end of the 

 period of exposure is decidedly fainter than it was at the be- 

 ginning (153). The positive reactions of the water insect 

 Ranatra increase in violence the longer the light acts ; on the 

 other hand, after being kept in darkness for several hours, 

 Ranatra is negative on first being taken out (186). Daphnias 

 kept in darkness for a time become decidedly negative to dif- 

 fused daylight, whereas if kept in light they would have been 



