546 ADAPTATIONS TO PHOTIC QUALITY 



skin. In birds and lizards, particularly, there seems to be rather a ten- 

 dency to make the eye as contrasty as possible, and to employ it as a 

 decoration! But some representatives of nearly every class of vertebrates 

 bear markings which are supposed to be intended to conceal the eye : 



These are such things as 'masks' and stripes of the head pattern which 

 continue unbroken across the conjunctiva and iris (Fig. 159). The eye- 

 masks of some fishes, frogs, and snakes are wide dark stripes passing 

 horizontally (or, in fishes, about as often vertically) across the eye, which 

 then loses its roundness since it is wholly 'absorbed' into the stripe. The 

 mask may not include all of the eye; but if the pupil is included within 

 its border, it is believed to serve just as well. Some masks are certainly 

 fortuitous — for instance, Cott figures an antelope which has a black 

 cheek-stripe sweeping up through the eye. Since a similar stripe develops 

 from the eye through the crumen to the jaw in an adult Hampshire 

 sheep (where it cannot possibly have any adaptive significance) such 

 markings in mammals must be viewed with suspicion from our present 

 standpoint. 



More convincing, by far, are the instances where several fine stripes in 

 the head coloration pass unbroken over the conjunctiva and iris. Unfor- 

 tunately for any general acceptance of eye-concealment, such cases of 

 so-called coincident disruptive coloration of the eye are excessively rare. 

 Really good ones are such teleosts as Pterois volitans, Labrisomus nuchi- 

 pinnis, Ogcocephalus cubifrons, and Scorpcena plumieri; and young 

 specimens of our common painted turtles (genus Chrysemys), in which 

 several black and yellow lines cross the eye (Fig. 160), The pattern in 

 Chrysemys is closely imitated by that in the teleost Apogon maculatus. 

 Ida Mann noted a similar situation in one other turtle (Clemmys cos- 

 pica) and in one newt, Triturus torosus; but both of these examples are 

 inferior to Chrysemys. 



It is claimed that the goatsuckers and frogmouths (and a couple of 

 lizards) close the eyes almost completely, as a means of concealing them, 

 when danger threatens. This action might also be expected in their close 

 relatives, the owls — who have so much more to conceal, since their irides, 

 though sometimes black, are most often a vivid yellow or orange.* But 



*In one — Athene noctua — such a performance would only make the eye more conspicuous; 

 for, though this owl has yellow irides, it has white lids. In the daytime, owls have their 

 eyes closed (to conceal them? — or in sleep?) particularly when danger is not threatening. 

 The usual daytime photograph of an owl shows the eyes wide open, but this is because the 

 photographer has disturbed the bird. A truly natural picture of an owl huddled against a 

 tree-trunk (in its 'hiding' posture) always shows the eyes closed. 



