684 



THE EYE TN EVOLUTION 



Eagle 



Colubrids the binocular field is very variable {Coluber, 20° ; Tarbophis, 

 24°; Zamenis, 28-32°; Tropidonotus, 34-42°; Malpolon, 38°; Uromacer, 

 40° ; Dispholidus, 42°), as also in the more primitive Boidae {Constrictor, 

 34°), while, as we have noted, the active tree-snakes {Dryophis, 

 Passerita) have the maximal binocular field of 46°. 



The binocular fields of birds may be classified into two distinct 

 types — that of birds with narrow heads and laterally directed eyes 

 with a central fovea, which have a wide panoramic field of about 300° 

 and a relatively small binocular field varying from 10° or less (6° in 



BLIND AREA- 



BINOCULAR 

 FIELD 40° 



Fig. 828. — The Visual Trident of Birds of Prey. 



The foveal projections in the hawk, c and c, the projections of the central 

 fovese for panoramic searching, t, the projections of the two temporal fovese 

 for stereoscopic vision in attack (after Rochon-Duvigneaud). 



parrots) to 30° (Figs. 820 and 823), and that of birds with rounded 

 heads and frontally directed eyes which have a relatively small total 

 field of about 180° with a relatively large binocular segment varying 

 from 35° or 40° to 60° or 70° (Figs. 821-2 and 824). As occurs in most 

 species of animal the former are timorous in type and granivorous in 

 habit ; their survival depends on early awareness of an enemy and 

 rapid flight ; typical examples are the song-birds or the pigeon. Those 

 with an extensive binocularity are the predators — the swallows, the 

 falcons, the hawks, the eagles, the owls, and so on — and in these, while 

 the laterally -looking central fovese are ideal for searching, the temporal 

 fovese have a common projection straight ahead in the binocular field 

 so that their judgment of distances for swooping on their prey while 

 in rapid flight attains an accuracy which can only be described as 

 extraordinary (the visual trident of Rochon-Duvigneaud, 1933) 

 (Figs. 825 to 828). 



