1 68 The Animal Mind 



67. Instances of Photopathy and Phototaxis 



The phenomena of orientation to light in different groups 

 of animals suggest now one, now another of these questions. 

 In Protozoa, although attempts have been made to show that 

 orientation is produced by the direct effects of light on sym- 

 metrical points, according to the observations of Jennings 

 (206) and Mast (261), it seems to be due to negative reactions 

 given when the organism, in its ordinary swimming move- 

 ments, either passes into a region of greater or less illumina- 

 tion, or swings its anterior end " to ward or away from the 

 source of light, so that it is shaded at one moment and strongly 

 lighted at the next." That is, the reactions are caused, not 

 by the direction of the light rays as such, but by differences 

 in the intensity of illumination. Strasburger's results, in 

 which the swarm spores moved toward the light into a region 

 of less intense illumination, Jennings holds were due to the 

 fact that "turning the sensitive anterior end away from the 

 source of the light" would diminish the effective illumination 

 of the animal more than passing into the slightly less illu- 

 minated region. That is, the two ways of changing the inten- 

 sity 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 do not turn the head end from the light, 

 or rather they make the negative reaction when it is so 

 turned, and do move toward the shaded region. "If the 

 difference in intensity of light in different parts of the drop 

 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" 

 (211, p. 148). 



