THE PERCEPTION OF SPACE 



673 



the other hand, is determined by the greater vahie of the fine judgment 

 rendered possible by binocular vision in pursuit and attack by the pre- 

 dator, in its ordinary activities by the arboreal animal, or by the 

 Primate the eyes of which have become accurately correlated with the 

 use of its hands. In each species a compromise is reached between the 

 biological value of the reflexes of self-preservation and those of aggres- 

 sion ; the former depend on the largest possible total field of vision, the 

 latter on the visual refinements resulting from the near-coincidence of 

 the optic and visual axes when the latter intersect on the fixation point. 

 To attain this end a swing forwards of the optic axes of the primitive 



Figs. 805 and 806. — Binocular Fields. 



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Fig. 805. — The panoramic field of a 

 hunted animal (the rabbit) with a 

 small binocular segment in front (10°) 

 and behind (9^), and a large uniocular 

 area (170-5° on each side). 



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of a 



large 



Fig. 806. — The binocular field 

 predator (the cat) showing a 

 anterior binocular area (120°) a large 

 posterior blind area (80 ) with rela- 

 tively small uniocular area (80°). 



fish is necessary and since this entails the sacrifice of much of the total 

 field it can only be adopted by animals amply sure of themselves either 

 because of their strength and ferocity or their superior intelligence. 

 The wide panoramic field was undoubtedly the more primitive in 

 evolutionary sequence ; frontality for the increased efficiency of 

 binocular vision is attained first by a swivelling forwards of the eyes 

 so that by a reduction of the angle gamma the visual axes, intersecting 

 on the fixation point in front, will more nearly coincide with the optic 

 axes, and secondly, to make this mechanically possible, by a reduction 

 in the divergence of the orbital axes (see Figs. 811-3, 837). 



In addition to the biological vahie of binocular vision as an asset to predacity 

 and fine manipulation, an increase in sensitivity to light may be a third factor 

 in determining its acquirement (Weale, 1955). The binocular sensitivity to light 

 is greater than the uniocular (by 10% in man, Pirenne, 1943). This may account 

 for the parallelism of the visual axes in some strongly nocturnal types such as 



S.O. — VOL. I. 43 



