598 THE EYE IN EVOLUTION 



Among the pre-Fishes, the cyclostomes have Kttle use for vision. 

 For the greater part of their hves most of them are parasitic and, as 

 we shall see, many of them have allowed their eyes to degenerate.^ 



The activities of fishes must be dominated largely by chemo- 

 receptors and the vibratory receptors of the lateral line ; organs of 

 tactile sense and hearing of high acuity are also available in many 

 species. It is true that the vast number of pelagic and surface fishes 

 can — and do — avail themselves of vision, a fact borne out by the 

 extraordinary anatomical development and high functional attain- 

 ments of the teleostean eye, an organ capable of appreciating colour 

 and sometimes provided with a fovea. In most other types, however, 

 the high refractive error and the frequent absence of efficient accom- 

 modation entail a very defective visual acuity and the eye is geared 

 essentially for the appreciation of light rather than form. Moreover, 

 apart from a narrow belt beneath the surface, the intensity of light in 

 the sea is insufficient for the attainment of a refined degree of form 

 vision and even in the most favourable circumstances the amount of 

 light reflected laterally from objects under water is meagre. In any 

 aquatic environment vision at any great distance is impossible ; in 

 muddy or turbulent waters and in the deeps of the seas light is practi- 

 cally non-existent and in the clearest water is completely absorbed 

 below a depth of 500 metres ^ ; in the abyss darkness is absolute. 

 Here, indeed, the only light available is created by the fish themselves 

 by their luminous organs,"*' and these, presumably, are used as social 

 signals rather than visual aids. The activities of vast numbers of fish 

 must therefore dejjend of necessity largely or entirely on the sensations 

 of taste, smell, touch, hearing and vibration. Most fishes, in fact, live 

 happily and apparently fully without vision even although they may 

 be provided with excellent eyes and normally use them. 



Thus the trovit and other Teleosteans of mountain streams live and seem to 

 thrive as well when the melting snows towards the end of spring convert the water 

 to an opaque turbulence in which human vision is impossible for a distance of 

 more than a centimetre or two ; again, in the high lakes of the Alps they nourish 

 themselves as well during the 7 or 8 months when the water is covered with 

 a layer of ice and snow sufficiently thick to preclude all light, as they do in the 

 months of summer. Among Selachians vision can mean little more than the 

 perception of light and movement ; even among Teleosteans vision is usually 

 a subsidiary sense and food is recognized primarily by olfaction.* A blinded fish 

 in an aquarivim may acquire his food and conduct himself in a way indistin- 

 gviishable from a normal fish (the dog-fish, Scyllium ; the ray, Torpedo — Verrier, 

 1938). Of all classes of Vertebrates, indeed. Fishes seem the least incapacitated 

 by the deprivation of vision ; the blind cave-fishes ^ are as alert and well fed 

 as their sighted cousins. 



1 p. 734. 2 p. 722. ' p. 736. 



* See p. 660. « p. 725. 



