60 TEOPISMS 



bility of producing a different sensitiveness of the two 

 eyes and corresponding differences in the muscle tonus 

 with asymmetry of position, and in physiological action 

 of the muscles of the tw r o sides of the body when the 

 two eyes were equally illuminated. Such an experiment 

 constitutes a crucial test of the tonus theory of helio- 

 tropism. It succeeded beyond our greatest expectations. 

 Asphalt black was applied to the right eye of several 

 specimens of Proctacantlius. In two or three days the 

 paint had formed a brittle shell. During this time the 

 blackened eye had become 'dark adapted.' When such a 

 fly is exposed to light, it tilts and circles to the left. If 

 now the brittle shell is cracked off the right eye by care- 

 fully pinching with fine forceps, the exposure of this very 

 sensitive eye to light results in a reversal of the whole 

 picture; the fly circles toward the side from which the 

 black was removed. Although the illumination of the 

 two eyes is of equal intensity, what was the normal eye 

 now becomes relatively a darkened eye owing to its lesser 

 sensitiveness. A differential effect results, probably due 

 to a difference in the rate of photochemical change in the 

 two eyes. This reversal of the muscle tonus and of forced 

 motions may persist for an hour or two or even longer, 

 until the two eyes become, as they ultimately do, of equal 

 sensitiveness and the fly behaves like a normal animal. 



"These experiments are not only incompatible with 

 any 'avoidance' idea, for after removal of the black there 

 is nothing to avoid, but they are also incompatible with the 

 conception of 'habit formation,' for 'habit' in the per- 

 formance of the circling movements is of no avail when 

 light is admitted to the darkened eye the animals circle 

 to that side because the tonus of the muscles is such that 

 they are forced to do so. 



