EVOLUTION OF BINOCULAR VISION 327 



Starts to hike to a mountain which looks two miles away and is nearer 

 twenty. 



But long before it has had time to learn any of these lessons, an infant, 

 not yet able to speak, can employ binocular parallax to reach accurately 

 for a toy. A chick reacts correctly to distance as soon as it is hatched. 

 Considering their greatly inferior mental equipment, were not the lower 

 animals fortunate to hit upon a cue to distance which required no learn- 

 ing for its successful employment, but merely a reflex coordination of 

 the muscles of locomotion with the muscles of the eyes? 



Of course, in many animals which give every evidence of depending 

 upon binocularity, the eyes are so close together that they cannot pos- 

 sibly have much parallactic 'leverage' — the angle between the lines of 

 sight, at any great distance, is so small that the binocular cue to distance 

 seems of low value as compared with our own. And, their two views of 

 an object at any great distance are so nearly alike that their stereopsis 

 can only be relatively weak. But — an intelligent lion, looking at our (to 

 his mind) small heads and ridiculously small interpupillary distance, 

 might say the same unkind things about the usefulness of our binocular 

 vision. After all, a small animal may have descended from a larger one, 

 retaining the same facial conformation. A half-pint galago has the same 

 frontality as a dreadnought gorilla, but only a fraction of the gorilla's 

 interpupillary distance. Neither of these species represents the size of the 

 extinct primate which originated primate frontality. Then too, small 

 animals feed on small objects; and, their absolute speed being low, only 

 small distances mean much to them from moment to moment of their 

 existence. Within these small distances, the angle between the lines of 

 sight of their close-set eyes may be just as great as the one between our 

 own visual axes when we look yards ahead at an object in our own path. 

 And it is this angle, not the linear interpupillary distance,* which really 

 counts. 



We can set up a rather complex series of *ifs', as follows : 



(a) If vertebrates have sacrificed the ancient periscopy to evolve bin- 

 ocularity, it must be because it offers advantages; but 



(b) If they have binocular vision of an object, they would gain abso 

 lutely nothing from binocularity if they saw the object diplopically; so 



*The interpupillary distances of some of the larger animals may be of interest here. Years 

 ago, Berlin published the following figures (among others) : young African elephant, 49 cm.; 

 horse (average of 20), 19.6 cm.; cow, 18 cm.; axis deer, 14 cm.; llama, 12 cm.; chamois, 

 10 cm.; goat, 9 cm.; sheep, 8 cm.; man, 6 cm. 



