672 THE EYE IN EVOLUTION 



Ungulates such as the horse increase the extent of their field in the 

 horizontal meridian, as does the pear-like elongation of the pupillary 

 aperture in the aphakic area in some teleostean fishes ^ or the key-hole 

 pupil of some arboreal snakes. ^ 



The following estimations have been made of the uniocular visual fields 

 (Figs. 798-803) : 



Teleostean fishes— 110°-170° (Verrier, 1930). 

 Lizards — slightly less than 180° (Kahmann, 1932). 

 Birds— pigeon, 165° ; owl, 110° (Rochon-Duvigneaud, 1921-23). 

 Mammals — guinea-pig, 135° ; cat, over 200° ; cattle, 205° ; horse, average 

 190°-195° with a transverse extension to 215° or more (Rochon- 

 Duvigneaud, 1943 ; Bresson, 1955). 



The upper visual field of under-water fishes deserves special 

 mention (Fig. 804). When looking directly upwards the fish sees 

 through a "window" into the air ; but in a slantingly upwards direction 

 a progressively greater degree of refraction occurs at the water-air 

 interface until the critical angle is reached (48-8° in fresh water) when 

 the rays of light run horizontally along the surface ; objects in this 

 hemispherical aerial field therefore become progressively smaller, 

 dimmer and foreshortened as the periphery of the " window " is reached. 

 Once the critical angle has been exceeded rays suffer total reflection 

 so that outside his circular " window " the fish must see the bottom 

 mirrored on the surface of the water. 



THE BINOCULAR VISUAL FIELDS OF VERTEBRATES 



Since the angle subtended by the uniocular field is relatively 

 constant, the extent of the binocular field is determined almost 

 entirely by the position of the eyes in the head. It is often stated that 

 there is a tendency for the eyes to swing from the lateral to the frontal 

 position during the course of evolution so that binocular vision as it 

 is seen in the Primates eventually becomes possible. This, of course, 

 is not the case, for the swing forward in the visual axes has occurred 

 independently many times within the vertebrate phylum, depending 

 on the habits and requirements of different species. Thus most 

 freely swimming fishes have laterally placed eyes but the flat-fishes 

 which lie on the sea-bottom have upward-looking eyes, and in some 

 deep-sea fishes they are directed frontally (Figs. 376, 379) ; the same 

 variation is seen in the Birds which show similar gradations between 

 laterality and frontality, and again in the Mammals. The extent of uni- 

 ocularity is determined rather by the need of a wide panoramic field for 

 the hunted animal whether it be fish, bird or mammal, for its existence de- 

 pends on the early detection of enemies in whatever direction, and rapid 

 escape from them (Figs. 805 and 806) ; the extent of binocularity, on 



1 p. 304. 2 p. 674. 



