EVOLUTION AND SIGNIFICANCE OF FOVEA 119 



Subsequent to the postulation of this theory, Walls (1940) 

 found and presented a record of measurements of refractive 

 indices of a number of vertebrates made by Valentin (1879). 

 The most important figures are those of a foveate parrot 

 (Chrysotus aestiva). Valentin gave the figure 1.3475 for the 

 refractive index of the retina and 1.3391 for the vitreous. 

 By substituting these actual numbers for his own hypothetical 

 ones, Walls (op. cit.) says that angle BAC (Figure 81) proves 

 to be 1.64°, which affords an expansion of the image in the 

 middle of the hawk's fovea of approximately 30 per cent. 

 Discussing matters further from his findings, he says: '^In 

 the concaviclivate human fovea, however, the much less 

 precipitous clival slope and the presumptive lower refractive 

 index would appear to make for a degree of image expansion 

 too low to account for the evolution of the human foveal 

 pit from no depression at all directly to its present gentle 

 profile. Wherever the foveas of animals approach the fovea 

 of man in shallowness, they can invariably be shown to have 

 been deeper in the ancestors. In the light of the resurrected 

 data of Valentin, it seems more likely than ever that the 

 human fovea has degenerated, like that of Sphenodon, the 

 owls and the pigeon, from a once much more deep and abrupt 

 depression. The possibility that the higher apes have less 

 gradual foveas than man seems fairly strong, and it is to 

 be hoped that studies of them may soon be made." 



If the theory of Walls proves to be the correct one, and 

 there is strong evidence in its support, then the concept of 

 the fovea as a region devised to admit light unimpeded to 

 the photoreceptors must be discarded, and this localized 

 region of specialization must be viewed as a mechanism for 

 increasing the resolving power of the retina. It has the 

 distinct merit of supporting the generally recognized fact 

 that the foveas of reptiles and birds are more efficient mechan- 

 isms for increased visual acuity than those of anthropoids 

 and man. It remains, of course, to see whether this concept 

 will stand the test of examination from physiological and 

 purely optical viewpoints* 



