310 ADAPTATIONS TO SPACE AND MOTION 



trident) for perfect judgment of the distances of objects has been 

 brought out experimentally in the interesting experiments of Portier on 

 the Northern gannet, Moms bassana. This bird has only central foveae 

 and is one of the many fish-eaters which dive after their prey. Such birds, 

 plunging into yielding water with the beak open or with talons spread, 

 have only to continue in the right direction to seize their fish — they need 

 not have good judgment of distance. A falcon, however, stooping for a 

 rabbit, must know where and when to check its flight or else collide dis- 

 astrously with the ground. To study the gannet's ability to do this, 

 Portier fastened fish on top of floating bits of board and then rowed 

 away to let the birds get a good look. He found that the gannets, diving 

 upon the fish bait, could not tell where to stop and would even transfix 

 the soft wood with their beaks, thus trapping themselves. This bird — 

 often called 'booby' — may not be able to learn much; but the falcon is 

 not given a chance to learn that the hard earth will kill him. He must 

 have the equipment for distance-estimation ready-made, and use it 

 instinctively. The gannet, not being similarly equipped with the complete 

 foveal trident, could never have mastered the problem Portier set for 

 him, even if he were far more intelligent than he is. 



Clearly, the central foveae are of no value in binocular distance-judg- 

 ment, but are of use to the flying bird only for seeing and avoiding obsta- 

 cles while the temporal foveae are kept aimed straight ahead. Birds in 

 flight are commonly observed to tilt the head on one side to look down 

 to the ground monocular ly; and this is as true of those provided with the 

 visual trident as of those which have only central foveae. 



Mammals — In the matter of eye movements, the mammals are at once 

 set off from all other vertebrates by the fact that whenever voluntary 

 movements are possible at all, the two eyes are never independent but 

 are always conjugated. 



This universal conjugation is associated with the fact that mammals 

 (whales, rabbits, and some others excepted) examine things only binocu- 

 larly — even the bats, small rodents, insectivores, and other nose- or ear- 

 minded nocturnal forms whose eyes never move even reflexly. Where the 

 eyes are placed laterally as in the rabbits, there is usually no area cen- 

 tralis, let alone a fovea, and there are no spontaneous movements at all. 

 But even the rabbits have the gyroscopic reflex eye movements, including 

 the optomotor reaction. These compensatory movements in mammals are 

 always most extensive in the plane of greatest biological usefulness, which 



