306 ADAPTATIONS TO SPACE AND MOTION 



In Sphenodon, and in the lizards except the monitors, the binocular 

 field is so small, and the fovea so nearly central, that any binocular em- 

 ployment of the fovea (such as can occur in some fishes) is out of the 

 question. Lizards on the whole rely entirely upon monocular fixation, 

 with the two eyes wholly independent in their voluntary movements. 

 Monocular fixative and exploratory movements are especially conspic- 

 uous in alert and active lizards such as the agamids, iguanids, and 

 Z.onurus. But independent spontaneous movements of the eyes reach 

 their zenith in the chameleons (which are so frequently stated to be the 

 only vertebrates whose eyes move independently). The extraordinary 

 mobility of the chameleon eye is the resultant of several factors : the lid 

 crater around the small cornea restricts the external visual field of the 

 eye; the visual axis is long and the retinal image relatively large; and the 

 retina, away from the fovea, falls off rapidly in quality of construction 

 for high resolving power. The insectivorous feeding habit, in so slow- 

 moving an animal, requires perfect judgment of distance, necessitating 

 that the eyes be capable of enough convergence to give the foveae a 

 common point of aim. The chameleon's eye bulges quite a bit from the 

 head, enabling the animal to sweep the visual line through a wide angle, 

 turret-fashion; and it can employ the eyes independently for its perpetual 

 exploration of the surroundings or, at will, associate them for foveal bin- 

 ocularity when a prey insect is spotted. The eye can be turned through 

 180° horizontally, 90° vertically, and one eye may be made to aim back- 

 ward while the other looks straight forward. By way of comparison, 

 Lacerta viridis (a typical lizard) has but 40 of eye movement. 



We have seen that only the teleosts can use a temporal fovea mon- 

 ocular ly. The chameleon is also exceptional, in that it can use a central 

 fovea binocularly. The movements of its body are slow in the extreme — 

 reminding one of the sea-horses (which also have central foveae and 

 prehensile tails, and better deserve to be called the 'chameleons of the 

 sea' than other fishes which have been given that appellation) — but the 

 sticky tongue is shot out with lightning speed at any insect that settles 

 within range. As Rochon-Duvigneaud has so well put it : "S'il y a encore 

 des cameleons, c'est que leur oeil est infaillible." 



Many of the more sluggish, less eye-minded lizards, such as the Gila 

 monster, have fixed eyes; and in the snakes there is but little spontaneous 

 mobility. It is because of this that the static binocular field of snakes is 

 wider than that of the lizards whose eyes can move and converge. What 

 movements the snake's eyes do make are either independent or, in con- 



