COLORATION OF THE EYE ITSELF 543 



tered cells on the under surface of young specimens expands on black 

 backgrounds and contracts on white ones. Pituitary and adrenal extracts 

 respectively produce the same changes in the cells. It does not appear, 

 however, that these phenomena can have any biological value — they do 

 not occur on the visible parts of the animal, and may be absent in older 

 individuals. The long-standing noctumality of the crocodilians seems to 

 account well enough for the vestigial condition of their color-changes. 

 It may seem more surprising that the snakes, with such close taxonomic 

 and ecological affinities with the lizards, should show so little evidence 

 of dynamic adaptation to their surroundings. But this explains itself 

 when it is considered as a part of the evidence for a lengthy subterranean 

 sojourn of the earliest ophidians* (see Chapter 16, section D). 



(C) Coloration of the Eye 



Basis of Iris Colors — The color of the eye itself presents some interest- 

 ing problems — histological, optical, and ecological. Ordinarily, only the 

 iris is involved. The pigment of the iris epithelium may be the only color- 

 ing matter present (p. 16), but nearly always there are stromal pigment 

 cells containing various amounts of melanin, colored oils (iridocytes) , or 

 guanin and related substances which yield metallic appearances of silver, 

 gold, or colors. As often as not, the coloration of an iris is the resultant 

 of both pigmentary factors and such optical phenomena as interference. 

 A vivid color may result from the absorption, by superficial layers, of 

 some wavelengths and the differential reflection, by underlying tissues, 

 of only certain ones of the remaining wavelengths. In this way, the green 

 spots of a frog's skin and the blue iris of a Siamese cat are produced, 

 without respective green and blue pigments being present at all. 



Possible Significance — In our thinking about the possible meanings of 

 eye colors, it is important to distinguish between pigmentation and color- 

 ation — these terms having respectively quantitative and qualitative con- 

 notations. Clearly, it is necessary for the pigments of an iris to absorb or 

 reflect the greater part of the total illumination striking it. Bright pig- 

 ments may contribute as much (or more) to the opacity of an iris, by 

 reflection, as dark pigments do by absorption. But the pattern of colors 

 the iris presents to the outside world is largely independent of the reflec- 

 tion-coefficients, or the amounts, of the pigments present. In short, a 

 given blue iris may reflect the same amount of light as a given red one. 

 Why, then, should one animal have the blue, and another, the red? 



