Spatially Determined Reactions 195 



Plateau attempted to test the responses of certain Dip- 

 tera to the size of an opening admitting light, by placing them 

 in a dark room, into which light entered from two sources. 

 One was a single orifice large enough to let the insects out ; 

 the other was covered with a net whose meshes were too fine 

 to allow them to pass. The amount of light from the two 

 sources could be made equal. When this was done, the 

 insects, which were positively phototropic, sought the two 

 equally often ; if the light from either was made more intense, 

 they went to that one. Plateau concluded both that the flies 

 could not see the netting and that the area of the light source 

 did not affect them (328). On the other hand, Parker 

 found that the mourning-cloak butterfly did discriminate 

 areas, flying to the larger of two sources of equally intense 

 light (307). 



This method of testing the image-forming power of an 

 animal's eyes has recently been elaborated by L. J. Cole. 

 He subjected animals with decided positive or negative 

 phototropism to the influence of two lights made equally 

 intense but differing in area, one coming through a piece of 

 ground glass 41 cm. square, the other a mere point. Eye- 

 less animals, the earthworm, for example, reacted equally 

 often to each light. Animals whose eyes from their structure 

 have been judged capable of perceiving merely the direction 

 of light rays, such as the planarian Bipalium, confirmed the 

 argument from structure by showing little more discrimination 

 than the eyeless ones. On the other hand, animals with 

 well-developed compound or camera eyes, for example certain 

 insects and frogs, did distinguish between the lights, going, 

 if positively phototropic, toward the one of larger area; 

 if negatively phototropic, away from it (80). 



Discrimination of boxes differing in size, but alike in form, 

 placed in a row along a board, food having been put in one, 



