32 EVOLUTION OF THE HUMAN EYE 



forward, so that at the third month of fetal life the optic 

 axes of each eye diverge 45 from the middle line; before 

 birth they become parallel. 



Wood Jones 1 has described how the alteration in the 

 position of the eyes in mammals is due to the recession of 

 the snout. He says: ' When an animal has a fully elongated 

 snout region, it may be said to possess a long face with an 

 eye situated upon each side of it ; but when the snout region 

 has undergone complete recession, it may be said to have a 

 flat face with two eyes situated upon the front of it. The 

 mere fact of the recession of the snout produces this change, 

 for the two eyes are turned to the front as the elongated 

 muzzle shrinks between them." 



He further points out this recession of the snout is the 

 result of (a) the substitution of the hands for it as the chief 

 means of tactile exploration; (b) the decrease in the use of 

 the jaws for seizing and tearing food; (c) the diminished 

 importance of the sense of smell. 



The three changes are all the outcome of the adoption 

 of arboreal life. It has resulted in the use of the forelimbs 

 for grasping purposes. From this they have gradually 

 acquired acute tactile sensibility, and have superseded the 

 snout as a means for exploration by the sense of touch; the 

 latter has, therefore, tended to atrophy from disuse. From 

 the grasping of boughs in arboreal life has arisen the power 

 .of grasping food with the hands to convey it to the mouth 

 instead of seizing it primarily with the jaws. This diminished 

 use of the jaws has led to their loss of prominence and power. 

 ''No trail of scent is laid among the branches of a tree' 1 

 (Wood Jones). 'Life amid the branches limits the use- 

 fulness of the olfactory organs' 3 (Elliot Smith). 



Binocular vision is produced by the overlapping of the 

 fields of vision in the two eyes. In Cetaceans so massive 



