AQUATIC MAMMALS 



In the Sirenia the dorsally directed axis of the eye is also quite ap- 

 parent, but the size of the eyeball is very much reduced and in conse- 

 quence it is inferred that visual efficiency has become much impaired. 



The beaver probably has a greater dorsal inclination of the eye than 

 any other rodent, as might be expected, and the muskrat, capybara and 

 other aquatic sorts exhibit this character to some degree, but in rodents 

 this may be without much significance from the present standpoint, for 

 some species that are strictly terrestrial have this character considerably 

 developed. Among these are the Microtinae or meadow mice, most 

 of which follow runways through grass and must watch above for ap- 

 proaching danger. 



There may be marked visual differences, however, in eyes that have 

 moved dorsad. One extreme is represented by Hippopotamus in which 

 the protruding bony orbits are directed chiefly laterally with a slight for- 

 ward trend, thus indicating vision that is largely monocular. The well 

 developed supraorbital prominences of the platypus also obliges vision 

 in this animal to be completely monocular. The tendency of some ro- 

 dents and the hair seals, however, seems to be for the acquirement of 

 dorsal binocular vision, although it is probable that none of these 

 mammals has the power of true stereoscopic vision, in which the image 

 of one eye is exactly superimposed upon that of the other and the two 

 function together perfectly as a unit. 



Such binocular visual powers as the seal may have are doubtless of 

 a character somewhat intermediate between monocular and stereoscopic 

 vision. In order that the visual sense shall function perfectly in a reflex 

 manner the physiology must be quite complicated, and our knowledge of 

 the mechanics of the sort of monocular vision which must be employed 

 by an animal having the eyes upon opposite sides of the head is still in- 

 complete. Thus it is not certain whether the animal must first give 

 its attention to an object upon the right and then to another upon the 

 left, or whether both eyes can function as separate units with equal effi- 

 ciency simultaneously, in which case the optic colliculus of the brain 

 would have to function in a manner differing in some unknown degree 

 from that of man. Also it is unknown whether the apparatus for ac- 

 commodation of each eye can operate entirely independently of the 

 other, so that the animal can simultaneously focus one eye on a near, 

 and the other on a distant, object with an equal degree of perfection. 



It may be presumed that a tendency for the elevation of the optic 

 axis of the character encountered in the seal is useful for the purpose 

 of detecting enemies that are prone to pounce down from above, while 



[62} 



