472 EVOLUTIONARY MORPHOLOGY 



smell is reduced below the level of the sense of sight, which 

 becomes the dominant sense in the body. The eyes of Tarsius 

 look forwards, and the fact that they have rotated onto the front 

 of the face necessitates the reduction of the nose and snout. 

 At the same time, the senses of hearing and touch are better 

 developed, together with their respective temporal and tactile 

 areas in the neopallium. The development of the tactile area 

 is important because it is associated with that area of the cerebral 

 cortex which is concerned with the performance of delicately 

 adjusted and skilful muscular movements. Such movements 

 are essential for an active arboreal animal, but there is another 

 reason for referring to this part of the neopallium, and that is 

 that a portion of it (the prefrontal region) is concerned with the 

 co-ordination of the movements of the two eyes. 



In the prefrontal area of Marsupials there are centres 

 which control the eye-muscles and therefore the movements of 

 the eyeball of the opposite side. The movements of the two 

 eyes are linked together in higher forms, and this is especially 

 significant in the Primates, where the visual axes of the eyes 

 become parallel. Further, whereas in lower vertebrates the 

 fibres from each eye all go to the other side of the brain (the 

 crossing at the optic chiasma is complete), in the mammals a 

 certain number of fibres remain uncrossed, and go to the same 

 side of the brain. Now, in the Platyrrhine stage of evolution, 

 represented by the Marmoset, the co-ordination between the 

 movements of the two eyes is perfected, and both eyes are able 

 to follow one and the same object. A consequence of this is 

 that " corresponding points " are developed in the retinae of 

 each eye, on which the images of one object are formed, and 

 the most important of these points is the macula lutea or spot 

 of optimum sensitiveness. 



Consequent on the power of making conjugate eye-move- 

 ments, the Anthropoidea have evolved a macula lutea, and this 

 still further increases the importance of the parietal (visual) 

 and prefrontal (skilled movement) areas, which features already 

 distinguish the brain of the Platyrrhine from that of Tarsius. 

 A continuation of the process of enlargement and perfection 

 of the parietal, prefrontal, and temporal areas can be gradually 



