308 



ADAPTATIONS TO SPACE AND MOTION 



with the two central foveae, and binocularly straight ahead with the two 

 temporal foveae (Figs. 114, 115). A substantial part of the whole visual 

 field of the bird is thus subtended by highly superior receptor areas. The 

 sacrifice of lateral and posterior visual field entailed by the frontality of 

 the eyes is easily made by the hawk (which fears no enemy whether he 

 can see it approach or not) and by the swallow, which expects to outfly 

 any challenger. 



Except in the eagles and in Apus apus, the temporal foveae are inferior 

 in construction to the central ones. This seems to hint that in birds 

 (unlike ourselves) binocular resolving power is higher than monocular 



Fig. 1 14 — Dissected head of a hawk, with eye bisected equatorially. After Rochon-Duvigneaud. 

 cf- central (nasal) fovea; tf- temporal fovea; p- pecten. 



— being teamed with its mate in the other eye, the temporal fovea per- 

 haps does not need the structure of the central one which works alone. 

 A hawk prefers to turn the head to follow objects binocularly, and can 

 rotate the head on the neck through a full half-circle. But if the head is 

 held, the hawk will 'follow' monocularly within the narrow limits of its 

 ability to swing the eye in the orbit. 



The owls have only the temporal fovea. It is an academic question 

 whether this was once a second fovea and the original, central one has 

 disappeared, or whether a one-and-only £ovea migrated temporally as the 

 eyes became more and more frontally aimed, during the evolution of the 



