176 ARBOREAL MAN 



from side to side as the animal runs. When, with the 

 shortening of the face, the eyes are permanently brought 

 to the front, they both control one visual field. It is 

 noteworthy that in Tarsius spectrum the mobility of the 

 head seems almost to replace the mobility of the enormous 

 eyes, a change akin to that by which the mobility of the 

 head has already replaced the mobility of the ears (see 

 Fig. 70). Although in Man the two eyes see slightly 



PIG. 69. HUMAN CEREBRUM, TO SHOW THE MAIN CORTICAL 

 AREAS. 



Note the general change in position of the areas with cortical 

 growth and also their wide separation by " association " areas. 



different pictures when either is used alone, these pictures 

 overlap so greatly that with binocular vision they blend 

 into a common field in which stereoscopic effects are 

 produced. It is difficult to estimate the changes that this 

 has produced in the educational possibilities of the sense 

 of sight, but it is easy to realize that they have been great, 

 and I imagine it is to be appreciated crudely by studying 

 the different mental pictures produced by a simple photo- 

 graph and stereoscopic views examined through the 

 appropriate lenses. 



So far the arboreal animal has come to have a greater 

 dependence upon the impressions of sight and of hearing, 

 for the reason that its former guiding sense of smell is 



