IMPRESSIONS OF SIGHT AND HEARING 175 



Then, again, there is that process which we have termed 

 the recession of the snout region, which effects so many 

 changes, and among them brings the two eyes to the 

 front of the face. This purely physical change produces 

 great and new possibilities of vision. In an animal in 

 which the snout region is prolonged, the eyes are lateral, 

 and the correlated vision of the two eyes is necessarily 

 imperfect, each eye possessing a more or less independent 



FIG. 68. CEREBRAL HEMISPHERE OF A MONKEY (Macacus), TO 

 SHOW THE CORTICAL AREAS AS DETERMINED BY BRODMANN. 

 (FROM DUCKWORTH.) 



A further advance is seen from the stage represented by the 

 lemur, especially in the development of the prefrontal area. 



field of vision; and the blending of the two visual fields 

 into stereoscopic effects can be only very partially effected. 

 There are7lproBably, not complete conditions of isolation 

 of the two visual fields in Mammals, although among the 

 Reptiles their complete separation is common enough, 

 but the separation in many mammalian forms must 

 approach completeness. 



The power to look directly forwards with both eyes at 

 once is present in all arboreal Mammals, but in many 

 terrestrial quadrupedal pronogrades it is very limited. 

 Even the dog is given to running with its head turned 

 somewhat sideways, a position which, affecting the car- 

 riage of the whole of its body, is alternated at intervals 



