SOME CONSIDERATIONS ON SIGHT IN BIRDS.^ 



By Dr. J. C. Lewis. 

 R. A. 0. U., Melbourne. 



[With 5 plates.] 



That continual adjustment, so necessary for life, between internal 

 relations of an organism and the external world would be impossible 

 were it not for the communion of the sense organs. The}'^ stand, as 

 it were, midway between the organism and its surroundings, keeping 

 the internal relations aware of and alive to the external happenings 

 and conditions. These fimctions probably arose with the necessity 

 for adaptation to environment and its ever-changing demands, and 

 in the struggle for existence they are necessary factors for the 

 survival of the race. 



Of the different special senses, hearing and sight stand apart in 

 the degree of specialization, and this specialization, again, varies 

 greatly in the divisions of the animal kingdom. In the animal 

 world, for example, we find all stages from blindness to acute vision. 

 Where the sight is poor, smell and hearing are, in compensation, 

 extremely acute. The vision of the rhinoceros is limited to some 

 50 yards or so and is poor even for that short range, but the acute- 

 ness of the sense of smell makes good the sight deficiency. In birds 

 specialization of sight reaches its highest degree of development; 

 and though hearing is fairly acute, the sense of smell is certainly 

 vestigial. One feature of the functions of hearing and sight is the 

 projection of their sensory impulses. Taking sight, we find that 

 light reflected from a distant object is picked up by the cornea and 

 lens and brought into focus at a point on the retina. The stimulation 

 of the numerous endings of the optic nerve sets up an activity which, 

 after passing through many systems of relays, reaches the sight 

 centers in the brain, giving rise to a complex chemical action in the 

 cells, where the myriad impulses are figured out into a light pattern 

 in the image of the original object. Though the action setting up 

 these impulses originates in the brain, where the image is really 



1 Reprinted from the Emu, Vol. 15, Pt. 4, April, 1916. 



337 



