132 



TROPISMS 



the same object fixed, an apparent forward motion of the 

 fixed object changes the muscles of the fins in such a 

 sense as to cause the animal to follow the fixed object 

 automatically. 



When such rheotropic fishes were kept in an aquarium 

 and a white sheet of paper with black stripes was moved 

 constantly in front of the aquarium the fish oriented them- 



Fig. 39. — Influence of motion of the hand of an observer on the direction of the motion 

 of a swarm of sticklebacks in an aquarium. The arrows indicate the direction in which the 

 hand was moved. The swarm of fish moves always in the opposite direction in which the 

 hand is moved. (After Garrey.) 



selves against the direction in which the paper and its 

 stripes moved. The phenomenon was more marked in 

 young than in older specimens. 



Alllhe phenom ena of rheotropism ceased in the dark 

 or when the fish were blind. 



Wheeler 508 has observed a phenomenon of anemotrop- 

 ism, namely that certain insects have a tendency to put 

 the axis of their body in the direction of and against the 

 wind. He considers this analogous to the phenomenon of 

 rheotropism in fishes. The cause is also in all probability 

 the tendency toward fixation of the moving retina image. 



A very pretty demonstration of the orienting effect 

 of moving retina images was discovered by Garrey in 



