188 ADAPTATIONS TO DIURNAL ACTIVITY 



In lizards the area is central, and is circular or oval; but in birds it is 

 often a long horizontal band, as in Figure 80a (minimizing the need for 

 eye movements) and has in it a central circular or oval fovea. In a num- 

 ber of birds a second fovea, seldom as well-developed as the central one, 

 lies temporally from the latter (Fig. 80b). Such a temporal fovea is 

 comparable with the single fovea of Dryopbis or a teleost, in that it and 

 its mate in the other eye can both be brought to bear upon the same 

 point in space ahead of the bird. The central or nasal fovea is useful 

 only for monocular vision sidewise from the head; and in most birds, 

 whose eyes aim much more sidewise than forward, it is the only fovea. 

 In the owls, only a fovea temporalis is ever present, and it may be very 



Fig. 80 — Ophthalmoscopic appearance of bird eyes, showing pecten (ventrally), 

 arecE, and fovea. After Wood. 



a, right eye of pigeon guillemot, Cepphus columba, showing horizontal linear area centralis 

 and single central fovea, x 3. b, right eye of Anna's hummingbird, Calypte anna, showing 

 central foveate area, and temporal fovea (in cutaway; cf. Figs. 114-5, pp. 308-9). x 10. 



shallow or even lacking. One swift, Apiis apus, approaches the owls 

 in that its central fovea is barely visible though the temporal one is 

 well developed. Only birds ever have two foveae per eye, but George 

 Moore has recently found that some of the killifishes {Fundulus spp.) 

 have two horizontal, ventro-temporal, ridge-like areae. 



In diurnal birds and in most lizards, excepting the monitors and the 

 more chunky and sluggish of the skinks, the fovea is deep and its slope 

 Cclivus') is convex. This convexiclivate type of fovea (Fig. 81) occurs 

 only in the very best-constructed of areae centrales. The less perfect areae 

 of fishes, Sphenodon, owls, domestic birds, and man all have shallow 

 and concave Cconcaviclivate') foveae (Figs. 75b, 82). It is safe to say 

 that most of these (the fishes excepted) are degenerate and formerly, 



