544 ADAPTATIONS TO PHOTIC QUALITY 



Apart from the possible mirror-action of silvery fish irides (see p. 238), 

 there is no conceivable way in which the coloration of the iris can affect 

 the vision of the animal, except perhaps at the very border of the pupil 

 where the presence of brightly reflective material should theoretically 

 be detrimental (though a metallic ring at the pupil's edge is extremely 

 common in the lower vertebrates!). If iris colors in general have any 

 explanations, these must be in terms of the interpretations offered for the 

 body colorations of animals. A given iris pattern, then, might: (a) be 

 intended to conceal the eye; (^) be intended to make the eye conspicu- 

 ous; or (c) mean little or nothing. 



Conspicuousness of the Eye — Many a writer on the subject of the 

 adaptive coloration of animals has dwelt upon the conspicuousness of the 

 eye and the means employed to abolish it. Three things tend to make the 

 eye stand out on an animal, so that potential prey and enemies may dis- 

 cover the animal by noticing its eyes, even though the rest of the body 

 may be well camouflaged. These three things are: (a) its roundness; 

 (b) the blackness and roundness of the pupil; and (c) its glisten, due to 

 its wetness. 



Cott's clever drawing, reproduced here as Figure 158, shows strikingly 

 how a round object set among other, even larger (but irregular) objects, 

 takes the attention of the beholder. A species of animal may so arrange 

 its coloration that the roundness of the eye is concealed. It is even poss- 

 ible to do something about the roundness and blackness of the pupil. But 

 there is no conceivable way of eliminating the glistening of the cornea — 

 though it eliminates itself under water, of course, in aquatic and amphi- 

 bious forms. All three of these causes of conspicuousness, it is interest- 

 ing to note, are employed in the 'warning' false eye-spots seen on the 

 wings and elytra of many insects, and also in fishes (e.g., Chelmon 

 rostratus) and toads ie. g., Mantipus ocellatus). Such spots are round, 

 black-centered, and are often even high-lighted to give the appearance of 

 glistening. 



Concealment of the Eye? — The enthusiasts {e.g., Cott) say that the 

 color of the iris often matches that of the head as a whole. This is true 

 enough, and yet it is no evidence for a concealment function; for, aside 

 from the coincidence of silvery irides and silvery skins in many fishes, it 

 is true only of nocturnal and crepuscular vertebrates, and not of all of 

 them by any means. The crocodilians, for example, all have conspicuous 

 buff or yellow eyes, and yet their bodies are very dark or even black. 



