548 ADAPTATIONS TO PHOTIC QUALITY 



side of that. The proponents of adaptive coloration do not tell us why 

 so many species of vertebrates — fishes, lizards, and birds with bright- 

 colored irides; anurans, lizards, and cats with metallic ones — should 

 advertise their eyes, particularly when so many of these very same ani- 

 mals have their bodies 'concealingly' colored. The nocturnal animals on 

 which cats and owls prey do not, of course, see their enemies' irides as 

 colored ; but even so, the green of the cat's eye and the lemon iris of the 

 owl would assuredly be seen as light-toned spots, even by an animal 

 whose own vision was completely achromatic. 



The difficulty of concealing the little black eyes in transparent fish 

 larvae has been discussed previously (see pp. 237-8). Partial success may 

 be attained by a precocious development of the silvery argentea layer of 

 the chorioid, just within the transparent sclerotic envelope. A situation 

 in one genus of batfishes, Lophiomus, to which Dr. Hubbs has called 

 the writer's attention, serves to emphasize most strikingly the fact that 

 the eyes of baby fishes often serve as a label, saying all too plainly: 

 "Here is food." The batfishes are related to the anglers, and like the 

 latter they are flattened dorsoventrally, with cavernous mouths over 

 which, in the various species, there are suspended various sorts and sizes 

 of 'illicia', or baits. The illicium dangles from a fishpole, rooted on the 

 animal's back, and serves to lure small fishes within reach of the maw 

 beneath it. In Lophiomus, the illicium takes the form of a translucent 

 fish larva — complete with a pair of beady black 'eyes' at the 'head' end. 



Concealment of the Pupil? — The roundness and blackness of the 

 pupil are concealed well enough when the iris as a whole is dark in color 

 — though hiding the pupil in the iris only means that the whole iris is 

 now as hard to hide as a pupil of the same size. In many fishes a thin 

 black stripe, no wider than the pupil, may contain and absorb the latter. 

 Such cases are enormously outnumbered, however, by those in which the 

 pupil is rendered conspicuous or made to appear larger than it really is : 

 No more conspicuous pupils exist than those of most fishes, since the 

 irides of most fishes are silvery. In birds, the iris may be dark brown 

 (most passerines) ; but it may also be yellow, blue, green, etc. and these 

 colors may contrast vividly with those of the feathers. Where the iris is 

 brightly colored, the eye itself is rendered conspicuous and at the same 

 time the pupil is rendered doubly so. This latter point may be dismissed 

 as accidental; but not so the many instances among lizards, where not 

 only is the range of iris colors greater than in birds, but even forms with 



