Animal Instincts and Tropisms 259 



these reactions: first, the symmetrical arrangement of 

 the photosensitive and the contractile organs, and sec- 

 ond, the relative masses of the photochemical reaction 

 products produced in both retinae or photosensitive 

 organs at the same time. If a positively heliotropic 

 animal is struck by light from one side, the effect on 

 tension or energy production of muscles connected 

 with this eye will be such that an automatic turning 

 of the head and the whole animal towards the source of 

 light takes place ; as soon as both eyes are illuminated 

 equally the photochemical reaction velocity will be 

 the same in both eyes, the symmetrical muscles of the 

 body will work equally, and the animal will continue 

 to move in this direction. In the case of the nega- 

 tively heliotropic animal the picture is the same except 

 that if only one eye is illuminated the muscles connected 

 with this eye will work less energetically. The theory 

 can be nicely tested for negatively heliotropic animals 

 in the larvae of the blowfly when they are fully grown, 

 and for positively heliotropic animals on the larvae of 

 Balanus, and many other organisms. 



One of the difficulties in identifying the motions 

 of animals to or from the light with the positive and 

 negative heliotropism of plants consisted in the fact 

 that plants are mostly sessile (and respond to a one- 

 sided illumination with heliotropic curvatures to or 

 from the light), while most animals are free moving 

 and respond to the one-sided illumination by being 



