186 EXAMPLES OF TARSIUS FROM SARAWAK. 



But though Tarsius has binocular vision, it has no macula lutea 

 to enable it to appreciate the details of the objects seen, nor has 

 it any automatic mechanism for producing the conjugate move- 

 ments of the eyes necessary for bringing the two images of objects 

 seen on to the corresponding areas of the two retina?. In other 

 words, Tarsius has not yet acquired true stereoscopic vision. 

 But the wide range of movement of the head on the neck shows 

 that the co-ordination of the two eyes is becoming biologically 

 useful to the animal ; but as it has no mechanism for automatically 

 regulating the positions of the eyes the one to the other, it moves 

 its head as the cat does. 



These enhanced powers of observation opened the possibility 

 for one branch of the Eocene Tarsioidea to guide its hands with 

 greater precision foi- the performance of skilled movements, and 

 so incidental^ enhanced the sense of touch. Hence both the 

 tactile and the motor areas in the cortex underwent a great 

 expansion and elaboration ; and at the same time the prefrontal 

 cortex began to grow rapidly as a mechanism was built up for 

 co-ordinating the movements of the eyes. When this happened 

 specially sensitive areas (macula? lutea?) were differentiated in 

 the two retina?, and the optic tracts became rearranged for the 

 purpose of stereoscopic vision. These far-reaching changes led 

 to the transformation of the brain and converted a, Tarsioid into 

 a monkey. 



