126 



ANALYSIS OF THE ENVIRONMENT 



ing forms, but as the upper limit of the 

 lightless zone is approached, the eyes in- 

 crease in size and diminish again in the 

 deeper sunless water. The deep-sea con- 

 tains many species of fish with vestigial 

 eyes; benthal fishes with eyes, apparently 

 using animal fight, are also present. Some 

 pelagic cephalopods have large eyes, but 

 their habits of fife are not sufficiently 

 known for us to be certain of the sug- 

 gested correlation. In the twifight stratum 

 of the sea, the increase in size of eyes 

 suggests comparison with the similarly 

 large eyes found in terrestrial geckos, owls, 

 and tarsiers, for example, that are among 

 the twifight or nocturnal animals of the 

 land. 



The enlargement of the eyes in the twi- 

 fight zone of the sea may involve the whole 

 organ or may be fimited to an overgrowth 

 of the lens to form the so-called telescopic 

 eyes of small fishes; such telescopic eyes 

 have evolved in five difiEerent orders and in 

 eight different suborders. Further, the ret- 

 inas of deep-sea fishes contain only the 

 more fight-sensitive rods; the cones are 

 largely or entirely absent, and the sensitivity 

 to fight is much increased. 



Vision 



Light is important in photosynthesis, in 

 growth and differentiation, in toleration, 

 and, for many animals, in the initiation of 

 annual breeding activities that may include 

 migrations on a geographical scale. Even 

 so, the most significant aspect of fight for 

 men and for other animals that, fike man, 

 five primarily in a world of visual shapes, 

 lies in the fact of its visibifity. The limits 

 of visibifity vary with different animals. 

 Many vertebrates react as though their 

 visual fimits closely approach those of man. 

 The fimits of sensitivity of insects to fight 

 may differ widely from the human stand- 

 ard; some react to red fight as they do to 

 darkness, and others respond to ultraviolet 

 patterns that are invisible to man. 



With this approach we have another op- 

 portunity to appraise the biological signifi- 

 cance of the penetration of fight into water. 

 Many fish depend on eyesight to locate 

 their food. Consideration of numerous ex- 

 ceptions belongs elsewhere. As a result of 

 the work of physiologists and experimental 

 biophysicists, we are able to make an ap- 

 proximation of the relations of certain fish 

 to visible fight. Analysis of the visual power 



of a sunfish, Lepomis, show that its maximal 

 visual sensitivity has a sharp peak in the 

 yellowish-green (5300 to 5500 A), which 

 is the part of the spectrum of dayfight that 

 penetrates most effectively into the waters 

 of lakes and coastal regions where there is 

 medium transparency. These are the waters 

 in which fishes are most numerous. By ex- 

 trapolation from the ascertained minimum 

 of effective illumination for Lepomis, it fol- 

 lows that fishes with similar visual sensitiv- 

 ity would be able to see objects at the 

 bottom of such waters and down to about 

 430 meters in the Sargasso Sea (Clarke. 

 1936; Bigelow and Welsh, 1924). 



For such fish the effect of fight on photo- 

 synthesis of plant plankters, while funda- 

 mental for their existence, lacks the imme- 

 diate importance of the visible quafities of 

 fight. The reflection of much incident fight 

 from the surface of water and its rapid 

 differential absorption with depth are vital 

 to the fish because of the effect on the 

 depth at which food can be seen. Similarly, 

 the long days of the Arctic summer permit 

 birds to feed for long hours during the sea- 

 son of rapid growth of their helpless young. 

 On the other hand, the short winter days 

 may not give birds time enough to find 

 sufficient food to maintain their high inter- 

 nal temperatures, despite their effective 

 insulation by feather-bound air and fat. 



Many animals can change their color pat- 

 terns, and for these the stimulus for 

 chromophoral changes often is picked up 

 by the eye. Frequently, the ratio between 

 the intensity of direct fight from above, as 

 contrasted with that reflected from the 

 bottom, determines the shade that will be 

 assumed. Some of these animals— the 

 flounder, for example— when disturbed from 

 a background to which they have been long 

 adapted, will come to rest, if convenient, on 

 a background with a color pattern fike that 

 to which they are akeady adjusted. 



The response may be more compficated; 

 apparently, toads, like men, can react to a 

 patch of shade as a sign of associated fac- 

 tors, coolness or increased humidity, per- 

 haps. Responses to what are essentially 

 patterns of fight and shade, especially when 

 the patterns are in motion, bring visually 

 motivated animals to their food and help 

 them escape their enemies. In many instan- 

 ces these also involve the reaction to a sign, 

 such as when waving grass discloses the 

 presence of a mouse to the hunting hawk or 



