100 ARBOREAL MAN 



the other cervical vertebrae ; but a wide range of movement 

 is permitted in the neighbourhood of the skull before the 

 other joints of the neck are involved. 



This ability to turn the head quickly in any direction 

 has had its influence upon the principal sense organs. 

 Both eyes may be directed immediately, and at the same 

 time, towards an object which attracts attention. In 

 Tarsius, which possesses a wonderfully mobile head poise, 

 there seems almost a tendency for the head movements 

 to replace the movements of the enormous eyes, but in 

 all other Primates, the head mobility merely supplements 

 and aids the mobility of the eyeballs. 



The head may be tilted into any conceivable position, 

 so as to be placed at the greatest advantage to catch a 

 sound proceeding from any direction. The sense organs 

 may be brought into greater harmony and their teachings 

 may be correlated by this mobility of the head, and, 

 indeed, it is this mobility which has replaced that seen 

 in lower animals in the pinna itself. An arboreal animal 

 Avhich has arrived at this stage does not " cock " or 

 " prick " its ears when it hears a sound, but turns its 

 mobile head so that it can catch the sound to greatest 

 advantage, and at the same time bring the cause of the 

 sound under the observation of its eyes. 



Man and the Anthropoids have lost all trace of the 

 useful movements of the external ear upon the scalp, 

 but he and the arboreal Primates have compensated for 

 this in the increased power to move the head — a power 

 permitted by the altered configuration of the skull con- 

 sequent upon the recession of the snout region. 



