1 66 The Animal Mind 



rinth; it may be the eyes. In any case, what shall we say 

 about the sensation quality involved ? Perhaps the reactions 

 produced are wholly reflex. Perhaps the statolith or the 

 canal fluid produces a specific sensation quality. Or perhaps, 

 as Verworn thinks, the sensation quality is merely that of 

 pressure (415). Whatever its nature, spatial perception, 

 the perception of the spatial relations between several stimuli 

 simultaneously apprehended, plays no part in the orientation 

 of animals to gravity. 



66. Orientation to Light: Photopathy and Phototaxis 

 One of the first facts that confronts the student of the 

 ways and means by which animals become oriented to visual 

 stimuli is the distinction which Loeb drew between what he 

 called heliotropism and sensibility to difference (Unterschieds- 

 empfindlichkeit) (239) ; and to indicate which the terms 

 " phototaxis " and " photopathy " have also been applied. 

 The phenomena are as follows. Strasburger, working on 

 the swarm spores of certain plants, thought he had evidence 

 that their reactions to light evinced not so much a tendency 

 to seek a certain intensity of illumination, as a susceptibility 

 to the direction from which the light came. He placed over 

 the vessel containing them an India ink screen, thicker at 

 one 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 differences 

 of intensity had no effect. When the screen was removed, 

 and the light fell at an angle, the spores immediately oriented 

 themselves to its direction, and preserved this orientation 

 even when the screen was replaced (392). The word 

 phototaxis, instead of being used to designate any reaction 

 to light, has been narrowed to designate the tendency to 



