2IO The Animal Mind 



reactions to other stimuli" (378, p. 150). In the mol- 

 lusks observed by Bohn, the tendency in ascending or 

 descending the rocks is to orient the body in the line of the 

 greatest slope. When light and gravity are acting together 

 upon the animal, its movement seems to be a resultant of 

 the two, but if the moUusk is made to move on a vertical 

 plane, gravity thus exerting its maximal force, the influ- 

 ence of the light disappears altogether ; and if the animal 

 is put in an upside-down position by further tipping of the 

 surface, the sense of its phototropism is reversed ; that is, 

 it may be repelled instead of attracted by a dark screen (80). 

 The fairy shrimp, Branchipus, is positively geotropic in 

 light, negatively geotropic in darkness (454) . 



A curious tendency has been noted by many observers in 

 insects with both eyes blinded ; namely, to fly straight up 

 into the air. Forel thought they did so because in no other 

 direction could they escape obstacles (231) ; but this fact 

 they would have to learn by experience, for which, in some 

 cases at least, they do not take time. Plateau believed the 

 rising into the air was due to sensations produced by 

 the action of the light on the surface of the body, leading the 

 insects in the direction of the strongest light, which usually 

 comes from above. He supported this view by showing 

 experimentally that a blinded insect would not rise if set 

 free at night, while on the other hand, if liberated in a 

 lighted room, it would, in spite of the blinding, fly toward 

 the light or the lightest part of the ceiling (596, 599). 

 In the butterfly Vanessa, Parker thinks the rising due 

 to negative geotropism, as the insect flew upward in 

 a darkened room (537). Axenfeld suggested that it 

 might be caused by light penetrating the integument 

 of the head (9). 



