586 KARL T. WAUGH 



If there were no mingling at all of optic fibres, then we should 

 be urged to the absurd conclusion that a single object which casts 

 double images in the eyes is not interpreted as one, or else that 

 there is an alternation of attention, a sort of psychical rivalry 

 in which the sensation from one eye intermittently inhibits that 

 from the other, a view which is not in accordance with the law of 

 parsimony and is most improbable. As a third possibility it 

 might be claimed that one of the images becomes the dominating 

 stimulus while the sensation from the other is entirely inhibited. 

 We find such a possibility proposed as a theory by Morgan. 4 He 

 supposes, in the case of animals like the rabbit, where the eyes 

 are so situated that they cannot combine in binocular vision, 

 that "the image that falls on the most sensitive area or yellow 

 spot of one eye suggests the focal impression, while that which 

 falls on a similar spot in the other eye is marginal to its conscious 

 consentience." The existence of such a yellow spot Morgan as- 

 sumes. The need of a theory of this sort might be vindicated, 

 were it shown that animals whose eyes diverge at so great an 

 angle possess a fovea. Schafer 6 states that only man and some 

 primates have optic axes capable of convergence and a single 

 central fovea. 



The theory proposed above would be adequate for the explana- 

 tion of the conditions which obtain in those animals, which 

 in attending to an object turn one side of the head toward it, 

 thus inhibiting any sensation from it by way of the other eye by 

 practically excluding it from the other visual field, as do most 

 birds. The mouse, however, turns toward an object enough for 

 it to be clearly perceivable that lines from the object strike both 

 eyes with a generous margin. 



In reasoning from binocular vision in man to that possible in 

 the mouse extreme caution is necessary, because in man many of 

 the phenomena of binocular vision are closely connected with the 

 central point of most acute vision in each retina. ( Convergence is 

 meaningless unless we have reference to some definite point in 



4 Morgan, C. Lloyd. Introduction t<> Comparative Psychology, chap. 10, 

 p. 160, London, 1902. 



5 Schafer, A. E. Text-book of Physiology, vol. 2, p. 1148, 1900. 



