258 Animal Instincts and Tropisms 



eyes (or the photosensitive elements of the skin). The 

 mass of photochemical reaction products formed in 

 the retina (or its homologues) influences the central 

 nervous system and through this the tension or energy 

 production of the muscles. If the rate of photo- 

 chemical reaction is equal in both eyes this effect 

 on the symmetrical muscles is equal, and the muscles 

 of both sides of the body work with equal energy; as a 

 consequence the animal will not be deviated from the 

 direction in which it was moving. This happens when 

 the axis or plane of symmetry of the animal goes through 

 the source of light, provided only one source of light 

 be present. If, however, the light falls sidewise upon 

 the animal, the rate of photochemical reaction will be 

 unequal in both eyes and the rate at which the sym- 

 metrical muscles of both sides of the body work will 

 no longer be equal; as a consequence the direction 

 in which the animal moves will change. This change 

 will take place in one of two ways, according as the 

 animal is either positively or negatively heliotropic ; in 

 the positively heliotropic animal the resulting motion 

 will be toward, in the negatively heliotropic from, the 

 light. Where we have no central nervous system, as 

 in plants or lower animals, the tension of the contractile 

 or turgid organs is influenced in a different way, which 

 we need not discuss here. 



The reader will perceive that according to the 

 writer's theory two agencies are to be considered in 



