296 ADAPTATIONS TO SPACE AND MOTION 



ing, were its eyes not positioned as they are. Another interesting peculi- 

 arity is seen in the various genera of bitterns. When alarmed, these birds 

 freeze, with the bill canted up into the air at a steep angle, making them- 

 selves as tall and slender as possible so as to blend with the rushes among 

 which they stand. Any binocular field in an anatomically anteriad direc- 

 tion — that is, along the direction of the bill — would then be aimed use- 

 lessly at the sky; but the bitterns' eyes can be turned so far ventrally that 

 they can see binocularly around and under their own chins, and thus 

 truly forward and parallel to the ground (Fig. 116, p. 309). 



The mammals are mostly large enough so that the eyes are carried 

 well above the ground. Few of them therefore have the optic axes tilted 

 at all upward as they are in most other terrestrial vertebrates. The excep- 

 tions are the platypus, some rodents (particularly the beaver), insecti- 

 vores, bats, a few 'edentates', and the seals. In the platypus, the beaver, 

 and the seals, the upward tilt is strong and constitutes a definite adap- 

 tation to keep the eyes in the air while swimming awash. In the whales 

 there is a marked downward tilt, for these forms have abandoned all 

 hope of seeing into the air. The sea turtle Chelonia mydas also shows 

 this ventrad slant of the optic axes, which diverge downward at 150 

 from each other, in line with the habit of floating at the surface and 

 keeping watch below for possible food. A similar situation in synento- 

 gnath fishes has already been mentioned (p. 293). 



The angles between the optic axes of various mammalian groups and 

 species are shown in Figure 113, which brings out graphically the differ- 

 ences, in this respect, between the pursuers and the pursued. Among the 

 most defenseless of all mammals are the rabbits, whose optic axes are 

 nearly in a straight line. The anterior binocular field in different kinds 

 of rabbits has been found to vary from 10 to 34 . Lindsay Johnson 

 estimates that a hare has monocular fields of 190°, overlapping both 

 anteriorly and posteriorly*. The European squirrel, too, is claimed to 

 see behind him with the eyes at rest. Toward the other extreme there 

 range the carnivores, with the lords of brute creation, the cats, rivalling 

 man in their degree of frontality — the axes diverging only from 4° to 9° 

 in different species. The higher primates seem anomalous in their pos- 

 session of completely parallel optic axes, for they are not predatory. A 



*Indeed, Arthur Thompson states that the brown hare {Lepus europaus) makes a habit 

 of not looking direaly ahead when running. The animal is credited with keen sight — it is 

 claimed to watch the eyes of an enemy, and to flee if looked at directly; but it may run 

 almost into a man, particularly if the latter is standing in a furrow down which the hare 

 is speeding. 



