THE PERCEPTION OF SPACE 



701 



in this way from a succession of angles in rapid succession, parallactic 

 observations must be made providing a basis for the estimation of 

 distance and relief ; in this rapid process the simultaneous parallactic 

 clues of binocular vision are replaced by the successive clues of uniocular 

 vision. It was stressed by Grinnell (1921) that before pecking their 

 food birds adopt the similar habit of " rapid peering " — cocking their 

 heads now to one side and now to the other to view the grain or the 

 berry from different aspects and localizing it against the background 

 from different angles. The pendular head movements of snakes and 

 the nodding of many types of lizards probably come into the same 

 category. It was found by Bemier (1938), for example, that one-eyed 

 chicks peck as accurately as two-eyed specimens, relying (presumably) 

 largely on uniocular parallax for the accuracy of their judgments of 

 distances. The importance of shadow-effects was also brought out by 

 Bemier ; if the seed-grains were illuminated in such a way that their 

 shadows were eliminated, his chicks neglected them, while painted 

 representations of shadowed grains deceived them. The judgments of 

 distances possible by uniocular vision may, indeed, be of extreme 

 accuracy. We have already commented on the deft way in which the 

 chameleon, suddenly converging both eyes upon an insect, captures it 

 without fail with its long tongue ; while ordinarily both foveae seem 

 to be employed in this action, nevertheless Canella (1936) found that 

 after the loss of an eye it could catch its prey with the same infallibility, 

 retaining while so handicapped its accurate evaluation of three- 

 dimensional space. 



Binocularity, however, with the possibility of stereopsis must add 

 considerably to the animal's appreciation of space and its judgment of 

 distances, particularly near at hand. That such judgments are often 

 good and occasionally superb is obvious from the many instances of 

 behaviour that could be cited. The extraordinary agility of small 

 FISHES darting rapidly up a shallow stream so quickly as almost to 

 escape human observation and at the same time avoiding all obstacles, 

 provides a good example of the excellent judgment of distances possible 

 in some species. The schooling behaviour of many species induces fine 

 visual judgments : vast aggregations of fish, both fresh-water and 

 marine, wherein each individual maintains its position alongside its 

 neighbours retaining a constant distance between each other like ranks 

 of soldiers on parade can only be based on extremely precise visual 

 orientations (Morrow, 1948 ; Gudger, 1949) (Fig. 847). 



The judgments of size by certain fishes is exemplified by their 

 response to visual illusions. Herter (1930) found that the response to 

 such illusions was the same as in human beings ; fish trained to 

 feed from the larger of two black circles chose the left-hand circle in 

 Fig. 848. The astonishing visual accuracy of the archer-fish, Toxotes 



Toxotes jaculator 



