Modification by Experience 283 



right, some species of ants are aided in their return to the 

 nest by repeating all the turnings they took on the out- 

 ward journey. Watson reports the following observation 

 on the terns of the Tortugas. On an occasion after he 

 had trained one of these birds to use a nest raised a hundred 

 centimeters above the ground, he moved the nest a hundred 

 centimeters to the eastward ; the bird, returning, hovered 

 "in space, attempting to adjust to the nest in the air at its 

 former position and height" (769, page 226). Rock- 

 well (639) relates that a ground squirrel had made inside 

 a cabin, a nest which it was accustomed to reach by climb- 

 ing up the leg of a cot that stood in one corner of the 

 cabin. When the cot was removed, the squirrel, entering 

 the cabin, ran to the place formerly occupied by the cot, 

 and went through the motions of trying to climb the non- 

 existent leg. 



It must further be noted that useless movements not in- 

 frequently get organized into movement systems, and thus 

 their elimination is delayed. Animals in running a maze 

 form habits of going wrong which greatly interfere with 

 the reduction of their time records. In the case of some 

 salamanders which the writer vainly tried to teach a fairly 

 complicated maze, each individual acquired quite an elab- 

 orate habit of making wrong turnings, and remained true 

 to it for some time. Whenever the situation does not in- 

 volve strongly prepotent movements, whenever, that is, 

 the " motive" is weak, the natural tendency of movements to 

 organize into systems may take the place of the tendency 

 to drop off the unnecessary ones. Such an influence as 

 this is very likely one reason why errors in the maze are 

 not eliminated in the exact order of their distance from the 

 final, beneficial, and pleasurable goal. 



The conscious accompaniment of the formation of sue- 



