602 THE EYE IN EVOLUTION 



as it does in Birds ; and with eyes of relatively simple construction as 

 befits their direct descent from primitive Insectivores, the excellence 

 of their visual performance depends more on the development of the 

 central nervous mechanism of coordination and apperception than upon 

 the end-organ itself. 



It is significant that only among the anthropoid apes does there exist the 

 abiUty of actively exploring the potentialities of vision in an experimental 

 fashion. The chimpanzee, foi* example, will amuse himself by looking at the 

 world in different ways — by standing upside-down, by bending down and looking 

 through his legs, by punching a hole in a leaf and peering through it, or by making 

 a pool of urine and regarding his reflection therein. Vision has become elevated 

 from the reflex level of biological usefulness to that of sestheticism. 



THE PERCEPTION OF LIGHT 



THE LIGHT SENSE, by wJiicli light is perceived as such and gradations 

 in its intensity ajjjpreciated, is the most fundamental of the visual senses, 

 a direct development of the crude phototropic activity of the lower 

 invertebrate organisms ; in many Vertebrates it is highly developed, 

 more, indeed, than in man. The attainment of a high standard of 

 sensitivity involves certain structural specializations in the eye which 

 in their purest form are mutually exclusive of excellence of colour 

 and form vision ; the eyes of those animals, therefore, to which an 

 acute perception of light is a biological necessity can be differentiated 

 from those which find greater use in keen visual acuity. This 

 differentiation is of fundamental importance in the understanding of 

 the visual function of Vertebrates. 



From this point of view, Vertebrates can be divided into three 

 main classes ; at each of two extremes there is a high degree of ocular 

 specialization and a consequent loss in plasticity, and between the 

 extremes a combination of both faculties is attained by modifications 

 which, while lacking the efficiency of the specialized organ found at 

 either end of the scale, ensure sufficient plasticity to allow a considerable 

 degree of adaptability to most conditions. 



(1) DIURNAL ANIMALS, the cyes of which are primarily adapted 

 to bright light. A high degree of diurnality is seen in the passerine 

 birds which rise and go to bed with the sun ; an extreme degree in 

 the turtle or the marmot, an animal which never conies out by night. 

 These are essentially visual animals in the sense that their activities 

 are dominated by their eyes ; living in an environment flooded with 

 light, the perception of minute amounts or fine differences of illumina- 

 tion is comparatively unimportant, and vision is used for the appre- 

 ciation of form and perhaps colour. 



(2) NOCTURNAL ANIMALS, the eyes of which are adapted to the 

 near-darkness of night. In the less extreme degrees, a crepuscular 

 anima; finds its optimum environment in the twilight of morning or 



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