356 THE BIOLOGY OF THE FROG chap. 



to a tank of water, into which the frog is naturally desirous of 

 getting. Near A the box is divided so that a choice of two 

 paths is given. If the frog passes to the right, its course is 

 blocked off by the partition P. Near the other end of the 

 box two alternatives are also presented, in that the frog can 

 go either to the left, where its course is cut off by the glass 

 plate G, or to the right path, which leads to the water. The 

 sides of the box were fitted so that colored cardboard could 

 be placed in the positions marked W and R, and the color 

 of the sides of the labyrinth could thus be varied at will. 

 The partition P was also movable and could be shifted to 

 the other side of the box so as to reverse the closed and 

 free passages. A frog entering the box at A usually does 

 not go at first by the most direct route to the water, but 

 after several trials it comes to avoid the closed passages and 

 travels to the water by the shortest route. The frog learns 

 this path very slowly, as it was found to take from fifty to 

 one hundred trials before it would take the direct route 

 without being liable to make a mistake. Associations once 

 formed, however, were found to persist for over a month. 

 If, after the frog had learned to go to the water by the 

 nearest path, the colored cardboards lining the sides of 

 the box were exchanged, so that the side that was red 

 before was made white, the animal would become con- 

 fused and frequently take the wrong route. Yerkes comes 

 to the conclusion that the frog is guided by color vision 

 as well as by "complex sensations of turning." 



Fear exercises a strong inhibiting effect on the formation 

 of associations. The frogs experimented with by Yerkes, 

 although they " gave little evidence of fear by movements, 

 after being kept in the laboratory for a few weeks, they 

 were really very timid, and the presence of any strange 

 object influenced all their reactions. Quiescence, it is to be 



