674 



THE EYE IN EVOLUTION 



deep-sea fishes with tubular eyes or in such species as the owl or Tarsius. To 

 such animals a significant lowering of the light-threshold may be of considerable 

 survival-value, while the loss of the panoramic field is compensated by the 

 security of darkness. 



Apart from the positioning of the eyes in the head, several devices 

 have been adopted to increase the extent of the binocular field. Most 

 of these we have already noted. Some of them concern the configura- 

 tion of the eye — ^the prominence of the corneal curvature (or the 



Fig. 



Fig. 807. 



Fig. 807. — The Emerald Tree-Snake, Passerita. 



Showing the deep facial grooves to allow accurate 

 binocular vision (the long body of the animal is coiled up 

 behind the head) (photograph by Michael Soley). 



Fig. 808. — The Key-hole Shaped Pupil of the Tree- 

 snake, Drtophis. 

 To show the aphakic area, the aperture being designed 

 to direct light onto the temporal fovea in the interests of 

 binocular vision. 



lenticular curvature in Fishes) ; the occurrence of a horizontally oval 

 pupil as in Ungulates or some snakes and fishes so that the overlap of 

 the two fields is increased in the horizontal plane ; the nasal shift of 

 the lens by the transversalis muscle in turtles, lizards and some snakes 

 (Dryophis) on accommodation so that the visual axes are directed 

 forwards more nearly parallel to the axis of the body when the eyes 

 are converging on near objects in front (Fig. 808) ; and the marked 

 nasal asymmetry of the eye in so many types (many Fishes and lizards, 

 all Bii-ds, Ungulates and Carnivores) whereby the ciliary region is 

 narrowed and the visual retina is allowed to advance far forwards on 

 the temporal side while the cornea and lens are tilted nasally so that 

 the visual axes are encouraged to intersect towards the mid-line. This 



