420 



THE EYE IN EVOLUTION 



Kite, Milvus 



Flicker, Coluptes 



the 23eriphery. The cones, which in diurnal varieties greatly outnumber 

 the rods, may be single or double. As in Chelonians, the single cones 

 and the chief element in the double cones contain an oil-droplet, a 

 prominent feature of the avian retina known to the early anatomists 

 such as Treviranus (1837) and Hannover (1840). They are of various 

 colours — red, orange, yellow — and colourless ; they tend to be brightly 

 coloured in diurnal types, particularly in small song-birds, but pallid 



Fig. 5:i5. — The Fovea of the Albatkuss, Diomedea (O'Day). 



and almost colourless in nocturnal types, 

 have been described in a few species.^ 



Green droplets are rare but 



stormy petrel, 

 ■ Procellaria 



At first supposed to be associated with colour vision (Krause, 1863), these 

 oil droplets are now more generally considered to have a pvirely absorptive func- 

 tion, eliminating light-rays which are inconvenient qualitatively or quantitively 

 and aiding the acuity of vision.- 



The fovea of Birds, particularly the central fovea, is remarkably 

 deep with liighly convex sides, resembling in its general shape the deep 



1 The domestic cock, Gallus doynesticus (Waelchli, 1883), the kite, Milvus, and the 

 Ti parrot, Chrysoiis (Kiihne, 1882), the flicker, Colaptes auralus (Walls and Judd, 

 ') and the stormy petrel, Procellaria pelagica (Rochon-Duvigneaud, 1943). 

 p. 631. 



