EVOLUTION AND SIGNIFICANCE OF FOVEA 113 



visual impressions from both eyes are recorded on the same 

 side of the brain, by reason of partial optic decussation. 



In discussing the great variations in eye positions among 

 birds in relation to binocular vision Harris (op. cit.) says: 

 ''Although the vision of a pigeon and of an owl is so different 

 in character, the one having laterally placed eyes, an extended 

 range of vision on both sides, and no binocular vision, while 

 the latter has the eyes placed right forward, with accurate 

 binocular vision, but with a less extended range, yet the 

 decussation of the optic nerves is complete in both cases." 



Among mammals, there is also considerable variation in 

 the position of the eyes. In the herbivora and rodents they 

 are laterally placed; in the carnivora and in primates they 

 are forward, and these animals possess a high degree of 

 binocular vision. The rabbit, for example, with its eyes 

 placed quite laterally, and with practically no binocular 

 vision, does exhibit some uncrossed fibers (Singer and 

 Miinzer, 1889), a condition which stands in marked contrast 

 with the laterally placed eyes and periscopic vision of many 

 lower vertebrates wherein the optic decussation is complete. 

 Apparently the decussation is complete in mice and rats 

 (Singer and Miinzer, op. cit.). 



That the development of a wide range and a greater 

 exactitude in conjugate movements is intimately linked 

 with macular and foveal development, as implied by Elliott- 

 Smith, also does not necessarily seem to follow. The chame- 

 leon, although he may enjoy temporary conjugate eye move- 

 ments and binocular vision, exhibits strikingly independent 

 movements of the eyes and probably for the most part has 

 only monocular vision. This is certainly well illustrated in 

 slow motion pictures of the chameleon's activities which are 

 at the author's disposal. 



It would seem from the studies of Kahmann (1936) that 

 marked motility of the eyes is associated with foveal develop- 

 ment, but this does not necessarily imply the existence of 

 binocular vision. In 21 marine teleosts (representing 10 

 families) which possess well-developed eye movements, he 



