RHEOTROPISM 133 



sticklebacks. 170 When a swarm of such fish was kept in 

 an aquarium it was noticed that all the fish were oriented 

 with the long axes parallel and that the whole school swam 

 in a course parallel, but in a direction opposite, to that 

 of the moving observer. If the observer remains station- 

 ary opposite the aquarium and moves an object, prefer- 

 ably white, which is held in the hand, the little fish at once 

 respond by moving slowly and oppositely to that of the 

 moving object. They can be thus made to move up or 

 down or to the right or left (Fig. 39). 



By experiments which space forbids us to report in 

 detail Garrey has reached the conclusion that the motion 

 of a near object causes an apparent motion of the whole 

 horizon in the opposite direction and this apparent motion 

 the fish tries to compensate by the motions of its body. 

 This brings the observations on the stickleback into har- 

 mony with the general influence of moving retina images, 

 consisting in a compensatory motion of the fish. 



"We have already referred to the fact that the influence 

 of a moving retina image is capable of compensating the 

 forced movement of a dog after a one-sided lesion of 

 the cerebral hemispheres. 



