324 ADAPTATIONS TO SPACE AND MOTION 



made by having frontal eyes and wide binocularity, is it likely that ani- 

 mals would seek it if, to get it, they had also to tolerate a perpetual dip- 

 lopia? It is far more likely that the vertebrates would long since have dis- 

 carded one eye and come to have a single, frontal, cyclopean visual organ 

 like that of the ascidian tadpole. That they have not done so is evidence 

 in itself that they have always seen singly in the binocular field, that the 

 'physiologically cyclopean eye' which the psychologists like to talk about, 

 when they are stressing the singleness and straight-aheadness of human 

 vision, is not a primate (or even mammalian) invention at all. 



If each side of our owl's brain projects its image of the mouse into the 

 same part of space, will the owl not see one mouse there? Is not his dual 

 projection to the same place, which could be occupied by only one thing, 

 what we mean by fusion? Well, no, not quite; for there might be only 

 superposition of the two mental images of the mouse. This woiild not be 

 fusion — it would be more like the mess one would have if one projected 

 onto a screen, superimposed on each other, the right- and left-eyed images 

 from an ordinary stereoscopic viewing-card. Would the vision of a total- 

 decussation vertebrate have to be like that throughout his binocular field? 

 If it must, one wonders again why the animals with two eyes have not 

 thrown one away or at least religiously kept their two monocular fields 

 from overlapping. 



In ourselves, fusion is not through superposition or even a complete 

 blending of the whole of one image with the whole of the other. Rather, 

 it is a sort of mosaic process which is dynamic, with constant shiftings of 

 the conspicuousness of the parts of the images, little suppressions of one 

 part of one or the other as the gaze wanders over the object. In those of 

 us who have a strongly 'dominant' eye, the solid image is mostly the dom- 

 inant-eye image, with the image of the other eye used to paint in the 

 solidity, so to say. If binocular vision in the lower vertebrates yielded a 

 singleness whose basis was superposition rather than mosaic unification, 

 then their perception of the form and pattern of solid objects ought to be 

 far better with one eye than with two, for the superposition of disparate 

 images would be tantamount to diplopia. But blennies, and chameleons, 

 and birds with temporal fovese, and mammals all look at things binoc- 

 ularly from choice, even though, if they wanted to, they could look 

 monocularly just as lizards are forced to do by their adherence to a 

 centrally-positioned fovea. Yet, none of these animals is ever observed 

 to close one eye in order to get a better look! 



