2OO The Animal Mind 



end so as to cause gradations in the intensity of the light 

 reaching the vessel. When the light fell perpendicularly 

 through this screen, the distribution of the swarm spores 

 through the vessel was nearly uniform ; that is, the differ- 

 ences of intensity had no effect. When the screen was 

 removed, and the light fell at an angle, the spores imme- 

 diately oriented themselves to its direction, and preserved 

 this orientation even when the screen was replaced. They 

 would move toward the light even when by so doing they 

 passed into a region of less intense illumination. Jennings 

 suggested that these results were due to the fact that " turn- 

 ing the sensitive anterior end away from the source of the 

 light" would diminish the effective illumination of the 

 animal more than passing into a slightly less illuminated 

 region. That is, the two ways of changing the intensity 

 of the stimulus, moving forward into a darker region, and 

 turning the head end away from the light, are here opposed : 

 the latter effect is stronger than the former, hence the 

 organisms make the negative reaction when the head end is 

 turned from the light, and move toward the shaded region. 

 "If the difference in intensity of light in different parts 

 were increased till the change in illumination due to pro- 

 gression is greater than the change due to swinging the 

 anterior end away from the source of light, then the positive 

 organisms would gather in the more illuminated regions" 

 (378, page 148). 



59. Influences Affecting the Sense of Light Orientations 



In no class of animal responses to stimulation is the effect 

 more dependent upon the cooperation of a number of condi- 

 tions than in those involving orientation to light. Many 

 influences have been found to reverse the sense of light re- 



