212 The Animal Mind 



negative reactions until the disturbance is removed by 

 proper orientation (378, p. 74). In animals with eyes, 

 however, there is reason to think that apparent rheotropism 

 is largely an affair of vision. Lyon's theory of rheotropism 

 in fishes is that the fish orients itself and swims in such 

 a way that its surroundings, the bottom of the stream, 

 for example, shall appear to the sense of sight to be at rest, 

 an hypothesis which, as we shall see, was adopted by Rddl 

 to explain the "hovering" of insects in one place (622). 

 Lyon supports it by experiments where the bottom or 

 sides of the aquarium were caused to move in the absence 

 of any current in the water, and the fish was found to follow 

 them. When the fish was placed in a revolving glass 

 cylinder, it followed the revolutions, although there was 

 a slow current, of course, in the same direction, against 

 which, on the pressure theory, the fish should have moved. 

 Still more decisive was the experiment where young fish 

 were placed in a corked bottle full of water which was sub- 

 merged and put near a wall covered with algae. When the 

 bottle was moved in one direction, all the fish went to the 

 opposite end, although no current could have been pro- 

 duced. Again, a wooden box with ends of wire netting, 

 the bottom covered with gravel and the sides with sea- 

 weed, was used ; fish (Fundulus) were placed in it, and the 

 box was held lengthwise in a strong current. The fish 

 oriented themselves, but as soon as the box was released 

 and allowed to float away, they lost their orientation, 

 though their relation to the current was in no way altered. 

 Blind fish, Lyon found, oriented themselves by touch, 

 sinking to the bottom. There does, however, appear to 

 be, in some cases, a genuine pressure reaction to current, 

 for when water is rushing through a small hole into a tank 

 containing blind fish, they keep their heads to the current 



