474 EVOLUTIONARY MORPHOLOGY 



co-presence, the ape does not " see " the solution to the 

 problem. 



Perhaps the most important of all the consequences of the 

 perfection of the sense of sight in the Primates is the fact that 

 it is the neopallium which undergoes commensurate develop- 

 ment in the brain, and the neopallium is the physical companion 

 of memory, of the ability to profit by experience, and of the 

 arbitrator of possible responses, known as the will. There 

 is also to be noticed here the importance of remaining un- 

 specialised. For if the great development of the sense of sight 

 had taken place earlier in evolution, in an ancestor of the 

 mammals, it would have been not the neopallium, but the 

 optic lobes which would have undergone specialisation, and 

 for a number of reasons these are unsuited for the development 

 of the higher mental faculties. The success of man is therefore 

 also due to the fact that his ancestors did not shoot their bolt 

 of specialisation prematurely. 



A consequence of binocular vision and conjugate move- 

 ment is the power to converge the eyes on an object. In the 

 first place, this enables an estimate of distance to be made, which 

 is important in leaping from branch to branch. Feeling of 

 the degree of convergence is conveyed by stimuli from pro- 

 prioceptive sense-organs in the eye-muscles by afferent fibres 

 in the eye-muscle nerves. When, however, the eyes are 

 converged on an object, that object occupies the attention of 

 the animal, and the stereoscopic vision which it now enjoys 

 enables it to become aware of the true geometrical and spatial 

 relations of the objects in the world around it. 



To return to the face, it is obvious that when the nose and 

 snout are reduced as a result of the eyes coming on to the 

 front of the face, the mouth itself can no longer so easily be 

 used as a food-obtaining organ, as it is in lower forms. Here, 

 the hands come to the rescue, and being five-fingered and 

 with opposable thumbs, capable of pronation and supination, 

 they undertake the function of carrying food to the mouth. 

 At the same time, the development of the prefrontal area of 

 the neopallium enables delicate movements to be made, in the 

 course of which the animal acquires skill. It is an interesting 



