550 ADAPTATIONS TO PHOTIC QUALITY 



of the fishes (where it is not uncommon, as for instance in the cen- 

 trarchids). In our common box turtle (Testudo Carolina) also, the male 

 usually has a red iris and the female a yellowish or brownish one. These 

 reptiles are such splendid examples of 'disruptive coloration' — as to their 

 bodies — that they force one to believe not only that their eye colors are 

 meaningless (in view of the colors themselves and the sexual difference) 

 but that if they could have camouflaged their glistening eyes they would 

 probably have done so. 



There may even be great sexual differences in the apparent size, and 

 hence conspicuousness, of the pupil — as in the boobies (see p. 226). 

 Certain subspecific differences, like sexual ones, likewise suggest that eye 

 colors mean little or nothing. For example, one subspecies ikohnii) of a 

 certain terrapin {Graptemys pseudogeographica) has a most startling 

 snow-white iris. 



Most remarkable — and meaningless — of all differences are the tem- 

 poral ones. The iris of a newborn human baby lacks stromal pigment 

 and is consequently blue (p. 16) — even in a negro. Other primates show 

 similar changes with age — the young Indri, for example, has greenish 

 eyes while the adult has light brown ones. Deepening with age is partic- 

 ularly noticeable in the pigmentation of the iris of the domestic cat, 

 which, like man, is always bom with blue eyes. In some species of birds, 

 the color of the iris changes markedly at different periods in the life 

 cycle, while the changes in the plumage show no sort of correspondence. 

 Charles Walker has noted that in young grackles the eyes are brown, 

 becoming lighter with age — the reverse of what happens in cats and 

 humans. In Brewer's blackbird {Euphagus cyanocephalus) the breeding 

 male has a pale yellow iris, the breeding female a light brown one. In the 

 rockhopper penguin, Eudyptes cristatus, the iris and the beak both vary 

 from yellow to red and back again with the seasons. One change is as 

 meaningless as the other, though both are doubtless expressions of the 

 ebb and flow of sex hormones in the blood stream. 



Even more rapid color-changes of the eye may occur, presumably medi- 

 ated by dynamic changes in the iridocytes or perhaps by changes in the 

 optical properties of the iris stroma, induced in turn by changes in the 

 state of the iris muscles. Changes in the gross color of the iris have been 

 reported to occur, in emotional states and in illness, in cats and in an 

 occasional human. The eagle-owl of Europe, Bubo bubo, normally has 

 the usual strigine lemon-yellow iris; but when the bird is angry, accord- 

 ing to Arthur Thompson, the iris turns red and "seems to flash fire." 



