108 ROBERT M. YERKES 



which apparently are perfectly natural to the anthropoid apes 

 and to man are rarely employed by the lower primates. 



Hammer and Nail Test 



One day I happened to observe Skirrl playing with a staple 

 in his cage. He had found it on the floor where it had fallen 

 and was intently prodding himself with the sharp points, ap- 

 parently enjoying the unusual sensations which he got from 

 sticking the staple into the skin in various portions of his body, 

 and especially into the prepuce. 



A few days later I saw him playing in similar fashion with a 

 nail which he had found, and still later he was seen to be using 

 a stick to pound the nail with. This suggested to me the hammer 

 and nail test. 



A heavy spike was driven into an old hammer to serve as 

 an indestructible handle. This hammer, along with a number 

 of large wire nails and a piece of redwood board, was then 

 placed in the monkey's cage. Skirrl immediately took up the 

 hammer, grasping the middle of the handle with his left hand, 

 and with his right hand taking up a nail. He then sat down 

 on the board, examined the nail, placed the pointed end on the 

 board, and with well directed strokes by the use of the head of 

 the hammer drove the nail into the board for the distance of 

 at least an inch. He then tried to pull it out, but was forced 

 to knock it several times with the hammer before he could do so. 



This performance, during the next few minutes, was repeated 

 several times with variations. Often the side of the hammer 

 was used instead of the head, and occasionally, as is shown 

 in figure 8 of plate H, he seized the hammer well up toward 

 the juncture of the same with the spike. This figure does 

 justice to the performance. At the moment the picture was 

 taken, Skirrl's attention had been attracted by a monkey in 

 an adjoining cage, and he had momentarily looked up from 

 his task, the while holding nail and hammer perfectly still. 



This test was repeated on various days, and almost uniformly 

 Skirrl showed intense interest in hammer and nails and used them 

 more or less persistently in the manner described. Occasionally, 

 apparently for the sake of variety, he would put the blunt end 

 of the nail on the board and hammer on the point. Again, he 

 would try persistently to drive the nail into the cement floor, 



