600 THE EYE IN EVOLUTION 



heat with astonishing accuracy even when the snake is bhndfolded 

 or in total darkness.^ 



The general tendency to rely increasingly upon vision, however, 

 becomes much more marked in birds among which the sense of vision 

 comes fully into its own ; with the other senses poorly developed, 

 particularly olfaction, the intense activity of bird-life is dominated 

 almost entirely by visual impressions and their eyes can attain an 

 order of excellence unmatched in any other species not excepting man. 



Fig. 736. — The Giant Ant-eater, MmMEvoriiAGA TuiDAcirLA 

 Note the very small eyes and the long exploring tongue which takes 

 over the visual functions in seeking otit ants in their nest (Zool. Soc, London). 



It is only because the brain of the bird is so much inferior to that 

 of the higher Mammals that its visual interpretation may be less 

 effective. 



MAMMALS are in a different case. The early Mammals, small and 

 generally timid creatures leading a restricted life of nocturnal habits, 

 derived from an ancient reptilian stock, had little use for vision even 

 although in them for the first time the structural basis for conjugate 

 eye-movements became laid.- In Monotremes, nocturnal habits in 



1 Tlie facial pit of crotalid vipers (rattlesnake, moccasin, etc.) which responds to 

 a difference in temperature as mmute as 0-L C between a small object and its back- 

 ground, shows its highest sensitivity to infra-red wave-lengths between 2,000 and 3,000 

 m/x ; moreover, its capacity for directional analysis is very accurate (Lynn, 1931 ; 

 Noble and Schmidt, 1937 ;' Bullock and Gowles, 1952 ; Bullock and Diecke, 1956). 



" p. 697. 



