POSSIBLE VALUE OF EYE COLORATION 549 



dark irides have these flecked with metaUic pigments, making a quite 

 gratuitous contrast with the black of the pupil. The common frogs, and 

 many fishes and birds have the pupil outlined by a thin gold or silver 

 line, the rest of the iris being so dark that the pupil would be beautifully 

 concealed in it were it not for this metallic frame. To the adaptive color- 

 ationists, putting this ring around the pupil must seem about as mean a 

 trick as hanging a bell on a cat. 



In many diurnal snakes, particularly those of the racer type, a black 

 blotch on the nasad part of the iris comes right to the edge of the pupil, 

 which is otherwise bordered by a C-shaped metallic line. Thus the pupil 

 appears egg-shaped, and nearly double its true area. If the gap in the C 

 (which occurs just where the important forward-looking line of sight 

 passes the pupil margin) exists to prevent distortion of the retinal image 

 through diffraction at the border of the metallic pigment, then we have 

 here an instance in which a very minor improvement in vision takes pre- 

 cedence over all considerations of iris-decoration for pupil-concealment. 



One can only conclude that few animals have even apparently made 

 any effort to conceal the pupil; and that great numbers, which could 

 easily have made the pupil to blend with the iris, have 'spoiled it' by giv- 

 ing the pupil a false size, or a conspicuous outline, which serves no dis- 

 cernible purpose. Here, as with the eye as a whole, it is likely that the 

 conspicuousness produced by glisten is so great that the animals have 

 found it quite impossible to counteract the shininess by any sort of cam- 

 ouflage. 



Sexual and Temporal Differences — Further indications of the gen- 

 eral meaninglessness of eye colorations are seen in the species showing 

 sexual dimorphism, and in those which have a capacity for changing the 

 color of the iris from time to time. If eye colors are concealing, we should 

 expect that if a few animals can change those colors, great numbers of 

 others could and would do so. We might expect to find animals, even 

 furred and feathered ones, blending their eyes into various backgrounds 

 just as a flounder, by dermal color-changes, suits its whole body (except 

 the eyes!) to the substrate. The chromatophores of the iris look enough 

 like those which alter the skin pattern so that one wonders why they 

 should not, as readily, alter the coloration of the iris. 



Outside of the birds, there are but few animals which show a sexual 

 difference in eye color. In the common adder of Europe, Vipera berus, 

 the brown-and-black female has a light brown iris, while that of the gray- 

 and-black male is red — a most unusual color for any iris to have, outside 



