THE VERTEBRATE EYE 255 



The requirements of aerial vision when Amphibians left the water 

 for the dry land were met by an optical reorientation of the primitive 

 aquatic eye to suit the new medium and the provision of lids equipped 

 with elaborate glandular structures as a protection against drying ; a 

 return to water (as in the whale or the dolphin) has led to a reversion 

 of this process. The requirements of an amphibious life have resulted in 

 the adoption of a host of ingenious devices to allow an easy transition 

 from one medium to the other and to maintain adequate vision in each. 

 The dangers of a burrowing habit or a sandy environment have led to 

 the acquirement of protective " spectacles " (in lizards and snakes). 

 The vagaries of nocturnal, crepuscular or diurnal vision are met by 

 several expedients — variations in the size of the eye and the lens, in 

 the relative proportions of the percipient elements in the duplex retina, 

 in the size, shape and motility of the pupil, and the provision of a 

 tapetum or argentea, choroidal or retinal in site, fibrous, cellular or 

 crystalline in nature, which augments a scanty supply of light by its 

 mirror-like effect. The requirements of acuity of vision are met by the 

 development of an area centralis and a fovea, the receptor elements of 

 which are provided with individual nervous connections ; stereopsis 

 by the provision of more than one fovea or by a swinging forward of 

 the visual axes ; focusing at varying distances by a host of accommoda- 

 tive devices — the development of accessory retinae close to the dioptric 

 apparatus (as in the tubular eyes of deep-sea fishes), variations in the 

 position of the visual cells relative to the lens (as in some bats or in the 

 horse), the use of a stenopoeic pupil (as in the gecko or the cat), the 

 deformation of the eye by muscular action from outside (as in the 

 lamprey), the pushing or pulling of the lens backwards or forwards (as 

 in some Fishes, Amphibians and snakes), or a change in its shape by 

 squeezing it (as in Reptiles and Birds) or relaxing it (as in Mammals). 

 These serve to illustrate the multitude of expedients adopted by an 

 organ of unique plasticity to meet the requirements of environments 

 so completely different as the abyss of the ocean and the upper air, or 

 habits so diverse as the sluggishness of a parasite and the activity of 

 a bird-of-prey. 



The general scheme of phylogenetic development of the vertebrate 

 eye is therefore interesting in that it does not show a steady and 

 gradual increase in efficiency, but illustrates the elaboration of more 

 than one type from a common beginning along different lines to reach 

 more than one culminating point. The common beginning may be 

 found in Cyclostomes, the eyes of which are primitive and show no 

 specializations. From this starting point three peak-points have 

 evolved in types which in their habits of life are peculiarly visually 

 conscious — in teleostean Fishes, Sauropsida (lizards and Birds) 

 reaching its highest development in Avians, and among Mammals in 



