14 TROPISMS 



locomotor apparatus permit. If, however, the velocity 

 of chemical reactions in one side of the body, e.g., in one 

 eye of an insect, is increased, the physiological symmetry 

 of both sides of the brain and as a consequence the equality 

 of tension of the symmetrical muscles no longer exist. The 

 muscles connected with the more strongly illuminated eye 

 are thrown into a stronger tension, a and if now impulses 

 for locomotion originate in the central nervous system, 

 they will no longer produce an equal response in the 

 symmetrical muscles, but a stronger one in the muscles 

 turning the head and body of the animal to the source 

 of light. The animal will thus be compelled to change the 

 direction of its motion and to turn to the source of light. 

 As soon as the plane of symmetry goes through the source 

 of light, both eyes receive again equal illumination, the 

 tension (or tonus) of symmetrical muscles becomes equal 

 again, and the impulses for locomotion will now produce 

 equal activity in the symmetrical muscles. As a conse- 

 quence, the animal will move in a straight line to the 

 source of light until some other asymmetrical disturbance 

 once more changes the direction of motion. 



What has been stated for light holds true also if light 

 is replaced by any other form of energy. Motions caused 

 by light or other agencies appear to the layman as expres- 

 sions of will and purpose on the part of the animal, 

 whereas in reality the animal is forced to go where carried 

 by its legs. For the conduct of animals consists of forced 

 movements. 



The term forced movements is borrowed from brain 

 physiology, where it designates the fact that certain ani- 

 mals are no longer able to move in a straight line when 



We are speaking of positively heliotropic animals exposed to only 

 one source of light. 



