PITHECUS RHESUS, MACACUS RHESUS 355 



with those of previous observers. This no doubt was in consequence of more 

 systematic, longer study, and experimental methods better suited to reveal 

 the problem-solving ability on the part of the monkey. Yerkes' interest 

 centered on the question of the ability of the animal to learn the solution of 

 relational problems of varying difliculties. On the basis of these obser- 

 vations, he constructed curves of learning. The Pithecus monkeys which he 

 employed yielded evidence of ideation, and Yerkes agrees with Thorndyke 

 that free ideas are scanty in these animals; though ideas may develop, they 

 are rather concrete and definitely attached than free. He believes that the 

 general conclusions of previous experimental observers have done no real 

 injustice to the ideational ability of monkeys. It is clear, however, that there 

 are extreme differences in the mental development of different species of 

 monkeys. The slow process of monkeys in the solution of their problems is 

 quite surprising, however, but their success in such solutions is really less 

 rapid than that of many of the lower mammals, such for example as the pig. 



Measurements and Indices of Macacus Rhesus 



Total length of the animal 1260 mm. 



Length of tail 330 mm. 



Total length of the skull 145 • 4 mm. 



Occipito-nasal length ^^5-5 mm. 



Breadth 96 . 3 mm. 



Intertemporal width 50 mm. 



Brain case 67 . 6 mm. 



The brain is distinctly gyrencephalic in type, with the fissure of Sylvius, 

 the fissure of Rolando and the sulcus simiarum constituting the chief fissural 

 landmarks upon the convexity. As compared with baboon, the convolutions 

 and fissures are about equal in prominence and size. 



