Il8 ORGANIC EVOLUTION 



a most effective means of protection to these transparent 

 forms. 



Fish are commonly dark-colored above and light-colored 

 below. To any enemy, such as a sea-gull, looking down 

 upon them from above, their dark color would cause them 

 to harmonize with the dark appearance of the water, while 

 another fish looking at them from below or from the side 

 would be less likely to detect them than if they were 

 dark-colored instead of light-colored beneath. Were the 

 lower surface as dark-colored as the dorsal surface it would 

 appear to be much darker still, because of its being in shadow. 

 The light-colored sides and belly of most fish, when the 

 light comes upon the fish from above, are shaded, and 

 being in shadow appear about as dark as the dorsal sur- 

 face. If the sides and ventral surface were actually dark- 

 colored the added shadow would make them seem very 

 dark and would make the fish conspicuous. The accom- 

 panying photograph of a bluefish, taken while the fish 

 was swimming in an aquarium with the light coming from 

 above, shows the really brilliant white sides and belly ap- 

 parently as dark as the steel-blue back, because of their 

 being in shadow (Plate 48, A). The color of most fish 

 resembles that of their environment. The flatfish and others 

 which live upon or near the bottom often closely resemble 

 the bottom in color (Plate 48, It). 



Most birds are so colored as to conform to the sur- 

 roundings in which they live. Think for a moment of the 

 sparrows, streaked and speckled browns and grayish browns 

 like the grasses and bushes among which they are com- 

 monly found (Plate 49, A)\ of the whole grouse tribe, the 

 quail (Plate 49, B), the pheasants, the ruffed grouse (Plate 



