THE WAYS OF LIFE 247 



Of great interest are the experiments made by Yerkes 

 on ' dancing mice '. These fascinating creatures represent 

 a peculiar variety, of unknown origin, which has been the 

 subject of artificial selection. They are characterized by 

 the inabiUty to move far in a straight line without whirling 

 or circhng about with extreme rapidity. They are quite 

 deaf, except sometimes during the third week of life. 

 Their power of discriminating differences in brightness is 

 acute, but their colour- vision, in the strict sense, is poor. 

 They are quick to perceive movements, but make little 

 of form. They have considerable powers of learning and 

 can remember an acquired habit for 2-8 weeks after disuse. 

 What has been forgotten is more quickly re-learned. 



Dr. Yerkes arranged in their cage two passages, with doors 

 which bore movable cards differing in colour or in surface. 

 One passage led to food, the other to a slight electric shock. 

 The food was sometimes to the right and sometimes to the 

 left, but the door which led to it was marked with the 

 same kind of card. When there were many changes the 

 mice hesitated a good deal, going from one to another 

 and touching the cards. 



This point is of great interest, and must be emphasized. 

 When the mouse found the right-hand door to be the path 

 to food and freedom several times in succession, it tried 

 the plan of keeping to that door. When it found that 

 the cards were being alternated, it learned also to alternate. 

 When it found that this did not work — when the changes 

 were irregular — then it brought all its powers of discrimina- 

 tion to bear on the problem. The learning how to ' choose ' 

 aright was quickest when the difference in the illumination 

 of the two cards was most marked (colour in itself does 

 not seem to count), and it was also noteworthy that when 



