1 88 The Animal Mind 



one in which the object was moved ; if it was heading some- 

 what in the opposite direction already, it would turn farther 

 in that direction until parallel with the object's line of motion ; 

 if it was heading somewhat in the same direction as the object, 

 it would "back off hesitatingly," and reverse itself by a turn 

 in either direction, usually taking the way around toward 

 which it was already partially headed, if the object was rapidly 

 moved, but the other way around if the object's motion was 

 slow. At first sight this behavior seems to display an instinct 

 precisely opposite to that of keeping the visual field constant. 

 Yet the sticklebacks, when placed in a cylindrical glass tank 

 inside of a black and white striped vessel, moved with the 

 latter when it moved, proving that they possessed the usual 

 tendency shown by Lyon to be involved in rheotropism. 

 Garrey points out that movement in the opposite direction is 

 produced not when the whole visual field moves, but when it 

 is at rest, and one object in it moves. Can it be, he asks, that 

 the moving object "fixes the attention" of the fish and pro- 

 duces an apparent motion of the background in the opposite 

 direction, which motion the fish follows? (141.) 



Rheotropism in water arthropods may be similarly ac- 

 counted for, and in the opinion of Ra"dl, this same tendency 

 explains the habit swarms of insects have of hovering over the 

 same place, a phenomenon which Wheeler thought might be 

 due to odors emanating from the soil (435). Insects will often 

 be found to follow an object over or under which they are 

 grouped in the air, if it be moved (355). Swarms of insects 

 may be noted in the air over a country road, following its 

 windings and apparently oriented by the contrast between 

 the road and the dark banks on either side. When, however, 

 resting insects turn so as to keep their heads to the wind, the 

 reaction is evidently really due to the wind and not to their 

 visual surroundings (370). Probably the disturbance to their 



