A SIGHT REFLEX SHOWN BY STICKLEBACKS. 8 1 



or to the left, according to whether the stimulating object be 

 moved to the left or to the right respectively. Figs. 2 and 3 

 illustrate this feature also, if the rectangle represent the top of 

 the aquarium instead of its face. 



By using two moving objects for the purpose of stimulation 

 the most fascinating maneuvers may be elicited. A group col- 

 lected at the center of the aquarium (Fig. 4, o) may be broken 

 into two platoons which move toward the ends when the stimu- 

 lating objects approach the center from opposite directions. 

 These platoons may be made to turn up or down, to face about 



FIG. 4. FIG. 5. 



and by quickly moving the stimulating objects out from the 

 center to charge upon one another and intermingle at the center 

 again (Fig. 5). These reflex acts are most striking in their regu- 

 larity and are executed with a military precision which never fails. 



Another experiment may be performed as follows : The school 

 of fish is brought to the center of the aquarium and the stimu- 

 lating object is moved horizontally toward or away from it. In 

 the former case some of the fish dart swiftly away to the back of 

 the aquarium as if in fright, the others turn some to the right 

 others to the left, thus forming two platoons as in the previous 

 experiment, but when the object again recedes they swing about 

 and collect in the center. In this experiment the animals which 

 are in a direct line with the moving object are the ones to scurry 

 away, but those outside this line are stimulated as if by some 

 lateral change in the position of the moving body. 



That there may be such an apparent lateral change can be 

 seen by a glance at Fig. 6, in which the movement of an object 

 from A to B has an apparent lateral motion, for example from C 

 to B if viewed from a point outside the line of actual motion. 

 The fish is stimulated accordingly and oriented as indicated in 

 the diagram. 



