THE VISION OF VERTEBRATES 599 



This is not, however, to say that vision among Fishes is useless. 

 When it can be utihzed it is of immense biological value and occasion- 

 ally it reaches a high standard, although never equal to that found in 

 Birds or Primates. Indeed, it would seem, as Herter (1953) suggested, 

 that the visual capacity of many fishes is so high that it cannot be 

 adequately utilized in their natural life — an example of a wide general- 

 ization that the sensitivity of a sensory mechanism is usually greater 

 than is justified by the apparent biological importance of the stimuli 

 concerned, a tendency which perhaps allows the fullest efficiency at the 

 normal level of stimulation. 



When Vertebrates left the water to seek life on land, the better 

 optical medium provided by air allowed a higher standard of vision. 

 Among AMPHIBIANS, all the Apoda and many of the Urodeles remained 

 in lightless surroundings, living a secretive sluggish life at a low 

 potential, burrowing in the earth or in mud or under flat stones in 

 shallow water ; these have ill-developed eyes and base their activities 

 to a negligible degree upon ^dsion. On the other hand, the more 

 active Amphibians rely largely on their eyes, and in the Anurans 

 vision is well developed ; frogs, indeed, are essentially visual animals, 

 catching their food and recognizing their mate some distance away by 

 vision (Banta. 1914). This tendency becomes greater in reptiles. 

 Even among the turtles, the most primitive Reptiles extant, vision is 

 the dominant sense ; it is less important among the Crocodilians but 

 eminently so among lizards. The visual activity and accuracy of the 

 chameleon as it catches insects with its bifid tongue is proverbial ; in 

 this otherwise sluggish animal the eyes, indeed, are the only organs to 

 show obvious activity. Yet most Reptiles rely to a large extent on 

 other senses. Thus snakes and lizards follow a trail, either of 

 prey or their mate, by smell, the flickering tips of the tongue picking 

 up odoriferous particles from the groimd and transferring them to the 

 extremely well-developed Jacobson's organ in the roof of the mouth 

 where they are smelt and tasted. The rattlesnake, Crotalus, for 

 example, readily recognizes and viciously attacks the king-snake, 

 LamprojJeltis, and will do so with equal efficiency and zest when 

 blindfolded ; deprived of his tongue, however, which removes an 

 essential part of his olfactory mechanism, he is unable to recognize his 

 enemy by visual clues alone and remains passive. Similarly nocturnal 

 snakes, which have particularly good olfactory powers, can locate and 

 strike their prey entirely without the use of vision. Apart from the 

 visually alive arboreal types, snakes are probably alerted not so much 

 by vision as by the conduction of ground vibrations to the inner ear 

 through the lower jaw with which the single bone corresponding to the 

 aural ossicles of man connects; while the sensory facial pits of some 

 species such as crotalid vipers locate warm-blooded prey by radiant 



