82 



TROPISMS 



cal photosensitive elements on the two sides of the body 

 alters the tension of symmetrical muscles, and as a con- 

 sequence the animal is, when moving, compelled to change 

 its direction of motion until it is oriented in such a way 

 to the light that symmetrical elements receive the same 

 illumination. In this case the tension of symmetrical 

 muscles is equal again and the animal is compelled to 

 move in this direction. 



It has been suggested by the anthropomorphic inter- 

 preters of animal conduct that the motion of an animal 

 to a source of light is the same phenomenon as when a 

 human being who has lost his way in the dark is attracted 

 by an illuminated human habitation. As Bohn pointed 

 out, the definite path in which a positively heliotropic 

 animal moves when under the influence of two lights, 

 shows that the anthropomorphic interpretation is as 

 erroneous in this as in any other case. A human being 

 would go to one of two illuminated houses and not toward 

 a point between them, determined by the relative intensity 

 of the two lights. 66 



