190 ADAPTATIONS TO DIURNAL ACTIVITY 



as acutely deep as that of any lizard. Its pure-rod retina was once a pure- 

 cone one, so that Sphenodon, having retained the fovea despite the trans- 

 mutation of its cones into rods, now enjoys the only pure-rod fovea 

 which is known to us, except for the very mysterious case of a reputed 

 fovea in one deep-sea fish (Bathytroctes). Similarly, the foveae were cer- 

 tainly much better developed in some of the owls' diurnal ancestors. 

 The shallowness and variability of the pigeon's fovea has long been con- 

 sidered the consequence of semi-domestication, for in the fully domesti- 

 cated fowls the fovea is gone completely. On the other hand, the con- 

 caviclivate foveae of the few foveate teleosts, and that of the only known 

 foveate turtle (Amy da) have probably never been any deeper — they seem 

 merely intended to counteract the convexity of the area centralis. And, 

 by the way, some pure-cone animals with extremely good vision — the 

 ground-squirrels, particularly — have never produced a fovea simply be- 

 cause their entire retina is built as well for acuity as is the macula of man. 



If the variable, shallow, and gradually-curved human fovea has not 

 degenerated from a deeper and much more abrupt depression, it is diffi- 

 cult to see what could have called it into being. Its magnifying action on 

 the image is probably negligible compared with that of a convexiclivate 

 fovea. Nothing much seemis to be known as to the shape of the foveal 

 depression in some of the monkeys and apes which are more strongly 

 diurnal than man himself. In the marmoset (Hapale jacchus) however, 

 the fovea has a very steep clivus and a small flat floor. The sooty manga- 

 bey (Cercocebus torquatus) probably has the most cone-rich retina of 

 any primate, and its foveal cones are the longest and slenderest in mam- 

 mals; but the shape of its fovea is unfortunately in dispute. One or two 

 divisions of mankind — the Hottentots, certain natives of India, and the 

 Tierra del Fuegans — are known to have phenomenal visual acuity; but 

 the profiles of their foveae are not accurately known. Their sharpness of 

 sight has always been attributed to an unusual slenderness of the foveal 

 cones. 



The distribution of areae and foveae, and particularly their topograph- 

 ical locations in various retinae, are discussed further in section C of 

 Chapter 10. As we have seen, the modifications themselves are devoted 

 entirely to the raising of local visual acuity, but their locations are of 

 such importance in connection with eye movements and space-perception 

 that their full significance can be gathered only from a later consider- 

 ation of these matters. 



