290 ADAPTATIONS TO SPACE AND MOTION 



roll smoothly in the orbit during their extensive eye-movements. Ordin- 

 arily, the eyelids impose no restriction upon the visual angle of the eye. 

 We can look up and see our eyebrows, which means that they are con- 

 cealing a part of space from us; but we cannot see our lid margins, even 

 as unfocused shadows. 



Another special case is that of the fish. The cornea having the same 

 refractive index as the water, it is optically eliminated. The lens then 

 takes over the control of the visual angle; but, being spherical, it imposes 

 no limitation at all and the visual angle is thus determined in the last 

 analysis by the angular extent of the retina. The strongly refractive fish 

 lens usually protrudes from the level of the surface of the head, and is 

 oftentimes able to place much more than 180° of space upon much less 

 than 180° of retina (Fig. 128, p. 376) — at least in the horizontal plane, 

 where an aphakic space often helps out considerably. 



Position of the Eyes in the Head — Many a careless writer has stated 

 that phylogenetically, 'from fish to man', there has been a gradual migra- 

 tion of the eyes from a position back-to-back to one in which the two 

 lines of sight are forward and parallel. Actually, a complete series of eye 

 positions can be arranged wholly within the fish group, another such 

 series within the birds, and a third within the mammals. Scattered species 

 elsewhere have the lines of sight parallel, but directed upward rather than 

 forward. The development of a frontal position of the eye from an initial 

 lateral one has taken place several times independently. Some cases of 

 'frontality', as for example in deep-sea fishes (see Fig. 138, p. 403) have 

 rather special interpretations. But by and large one finds a good 

 correlation with predacity: the hunters tend toward frontality so as to 

 have the best vision of the prey they are pursuing, while the hunted tend 

 to retain laterality of eye position so as to be able to detect an enemy 

 coming from any direction. The predaceous animal can afford not to 

 have such 'eyes in the back of his head', because his offensive weapons, 

 teeth and claws, give him immunity from stealthy attack. Carnivores 

 rarely make a habit of feeding upon other carnivores, for the risks are 

 too great and the meat is too tough. 



The most important effect of variations in the positions of the eyes 

 is to vary the extent of the binocular field and the direction in which it 

 lies — usually forward, but sometimes more or less upward. The binocular 

 visual field is simply the spatial cone or zone within which the separate 

 monocular fields overlap. Its value to the animal and the character of 



