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BRADLEY M. PATTEN 



therefore, no way in which the animal can 'select' from among the 

 different intensities produced by its change of position, as the 

 'trial and error' interpretation demands. 



Mast believes that the shading of the sensitive region by 

 contraction, alternating with its exposure by extension provides 

 changes of intensity which are effective in bringing about orien- 

 tation. This explanation does not apply under conditions of 

 bilateral illumination, for the change of effective intensity would 

 be the same in whatever direction the anterior end was extended. 



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Fig. 18 Trails made by an asymmetrically sensitive larva under non-directive 

 light; a, the test trails to horizontal lights, showing the asymmetry of response; 

 b and c, trails of the same larva started in the orienting lights and subjected to 

 vertical illuminating at the points marked by the arrows. 



A def nite orientation by the method of 'trial and error' where 

 the 'trials' all result in the same amount of stimulation is hardly 

 possible. Apparently the only explanation available is some 

 form of the hypothesis advanced by Loeb, that equal intensity 

 operating on symmetrically placed photosensitive areas produce a 

 symmetrically distributed response. There is considerable experi- 

 mental evidence that may be advanced in support of this view. 

 Herms ('11, p. 207) blackened one side of the sensitive anterior 

 region and found that when the animals so treated were subjected 

 to non-directive light from above, typical circus movements were 



