196 The Animal Mind 



positively to the light, that is, turn towards the light, 

 and suppose a ray of Ught strikes it obliquely from 

 the right. Evidently the point a receives a greater in- 

 tensity of the stimulus than the point a'. Now if the 

 animal is positive to light, Loeb would suppose that its 

 chemical constitution is such that light causes, either by 

 direct action on the muscles or reflexly through the eyes, 

 a contraction of the muscles. Hence the muscles at point a, 

 or controlled through point a, would contract more strongly 

 than those at point a' : the animal in consequence would 

 turn towards the right, that is, towards the Ught, and 

 would continue so turning until the light struck a and a' 

 with equal intensity, that is, until it directly faced the 

 light. All subsequent movement would have to be directed 

 straight towards the light. If the animal is negative in its 

 response to light, then it is so chemically constituted that 

 light causes a relaxation of the muscles. In such a case, 

 the point least strongly stimulated would produce the 

 strongest muscular contractions : the animal would turn 

 towards that side, and would continue turning until 

 opposite points were equally stimulated, that is, until it 

 headed directly away from the light : all subsequent move- 

 ment would have to be in this direction. 



Now Jennings (373), has suggested that the oriented 

 reactions of certain organisms, at least, are reaUy due to 

 changes in the intensity of the light, brought about by the 

 animal's own movements. This view would, if generalized, 

 put all directed light reactions in the "sensibility to differ- 

 ence " class given to changes in intensity : the effect of con- 

 tinuous light would be limited to photokinesis. Let us see 

 how an oriented response may be conceived to result from 

 reactions to changes in light intensity. In the Protozoa, ac- 

 cording to Jennings (373) and Mast (463), the orientation is 



