BINOCULAR SINGLE VISION 325 



On logical grounds alone, we can thus make out a strong case for 

 believing that the lower vertebrates have singleness of perception of 

 objects in their binocular fields, despite their independent eye movements, 

 and their lack of any system of corresponding points, and their total 

 decussation. The mammals, though ranking higher, seem at first glance 

 to have lost, not gained, something. They are unique in having in com- 

 bination just these things that the other vertebrates lack — conjugate eye 

 movements, dependence for fusibility upon corresponding points, and 

 partial decussation. We shall see that this combination of mammalian 

 peculiarities expresses a relationship of cause and effect, and that it does 

 represent a gain of something after all. 



It is not known whether lower vertebrates can make binocular color 

 mixtures (see pp. 90-1), though if they can do so it would require us to 

 believe in fusion for them. And, the matter should be susceptible of 

 experimental attack. A fish might be trained positive to purple and neg- 

 ative to red and also to blue. Provided then with a red covering over one 

 eye and a blue one over the other, and placed in white surroundings, he 

 might or might not give a positive 'purple' response; and if he did do so, 

 it would indicate fusion. But apart from strictly visual phenomena, there 

 are many indications that the two eyes are interconnected through the 

 nervous system even where total decussation of the optic nerves obtains : 



In some fishes at least, one eye can control the dermal color changes 

 of the whole body as well as the two eyes normally do (p. 532). In the 

 rays, there is a consensual pupil reflex — both pupils contract when only 

 one eye is illuminated. In the pigeon, recent work has shown that there 

 is not only a consensual pupil reflex but that usually the two eyes blink 

 when one cornea is touched; and the two nictitating membranes also act 

 consensually. Moreover, in all vertebrates the two eyes are coordinated 

 in their reflex movements, though of course this association of the eyes is 

 strictly motor and has in it nothing of the photosensory element which 

 exists in the control of the pupils and of the dermal chromatophores. 



All in all, there is considerable reason to believe that the binocular 

 vision of all vertebrates is single vision. The 'independence' of the eyes 

 due to total decussation has been much over-rated. There is such an inde- 

 pendence, on the motor side; but this does not make it inevitable that 

 there shall be sensory independence as well. After all, our two hands 

 move independently, but when they both grasp the same object its single- 

 ness is appreciated without benefit of any partial decussation of the spinal 

 sensory tracts. In the tactual modality of sensation, there is even an 



