ASYMMETRICAL ANIMALS 71 



until its plane of symmetry goes through the source of 

 light. All that the inherited or accidental asymmetry 

 does is to cause the animal to move in a path which is not 

 a mathematically straight line; but this deviation will 

 be marked only in a case of very pronounced or excessive 

 asymmetry. 



We have already described the behavior of a dog whose 

 left cerebral hemisphere has been injured and who has a 

 tendency to deviate to the left. When such a dog is shown 

 a piece of meat it moves toward it in a fairly straight line, 

 its tendency to deviate to the left being compensated by 

 the orienting effect of the retina image of the piece of 

 meat. If the dog deviates to the left, the piece of meat is 

 apparently dislocated to the right of the dog and this 

 dislocation alters the tension of the muscles on the two 

 sides of the animal in such a way as to make it turn back 

 to the right. In this way the dog reaches the piece of 

 meat in a fairly straight line, though with a greater 

 amount of labor, since the tendency to deviate to the left 

 is constantly compensated automatically by a stronger 

 contraction of the muscles turning the animal to the right. 



The writer showed many years ago that many insects 

 have a tendency to creep upward, and that this is due to 

 an orienting effect of gravity upon the animal. When a 

 perfectly symmetrical insect is put on a vertical stick 

 it walks upward in a straight line. What will happen when 

 such an animal is made asymmetrical 1 ? Garrey has per- 

 formed this experiment by using flies in which one eye 

 was blackened. As we have seen, such organisms are 

 rendered asymmetrical not only in regard to the eyes 

 but also in regard to their apparatus of locomotion, since 

 in one side of the body the tension of the flexors, in the 



