20 EVOLUTION OF THE HUMAN EYE 



vision in dim lights, rather superior to that of their victims, 

 is essential for success in their pursuits. 



Mammals like the Simioe, who have found safety from 

 carnivorous foes by life in the trees, have in learning to 

 climb acquired the capacity of using their forelimbs for pre- 

 hensile purposes. They require sight which will enable 

 them to swing and spring with accuracy from bough to 

 bough. Assuming the semierect posture, they grasp their 

 food with their hands and convey it by them to their mouths. 

 Their visual organs have, therefore, to be adapted for con- 

 siderable accuracy in the judgment of distances, varying 

 considerably in degree; hence not only binocular vision but 

 stereoscopic vision is required, together with accurately 

 associated powers of accommodation and convergence. 

 As the food of these arboreal mammals consists of fruits 

 and insects,. which they pick up with their fingers, they re- 

 quire color-sense and a high degree of acuity of vision for small 

 objects. Safe in their arboreal resorts, away from the dangers 

 which prowl by night in the land below, vision in dim lights 

 is not so essential for their safety as it is for some other 

 classes of mammals. 



Man, having descended from the trees and become a 

 hunter and also exposed to the attacks of carnivorous foes, 

 has succeeded in maintaining his existence by the assumption 

 of the erect posture, which has entirely freed his forelimbs 

 for the use of weapons of offence and defence. The manu- 

 facture and use of such weapons has necessitated increased 

 precision in the judgment of distances and of concentration 

 in the thoughtful recognition of detail. The elevation of 

 his head, due to his erect posture, has increased his range 

 of circumferential vision. Having acquired during arboreal 

 life a more acute central form-sense than any other terrestrial 

 mammal, together with the most highly-developed stereo- 



