92 EVOLUTION OF THE HUMAN EYE 



of the retina shows very striking differences in them, both 

 as regards shape and size. The foveal cones, especially 

 those in the very center, are long and slender. The extra- 

 foveal cones decrease in length gradually as they extend 

 toward the periphery, the change being most marked in 

 the outer segment, which also gradually becomes wider. 



Careful histological examination of the cones in the retina 

 of congenitally color-blind persons might, I suggest, add 

 considerably to our knowledge of color perception. I have 

 been for some years on the lookout for a specimen in which 

 to make such an examination, but so far have not been 

 successful in obtaining it. 



For the perception of colors in the peripheral parts of the 

 field a larger object, intenser light and greater saturation 

 of color is required than in the center. Given cones with 

 a hue-perceiving capacity, these requirements might be 

 largely accounted for by varying degrees of concentration 

 of the cones in different parts of the retina. The retinae 

 of Primates differ from those of lower mammals in having 

 an area, the macula, in which only cones, closely packed 

 together, are present; we should, therefore, expect that the 

 hue-perceiving faculty of Primates would be endowed with a 

 capacity of seeing small patches of color, color less brilliantly 

 illuminated, and less saturated, than in those mammals in 

 which the cones are more widely scattered. To put it in 

 another way, we should expect that the color-sense of mam- 

 mals, with widely diffused cones, would be comparable to 

 that of the peripheral portions of the retina of Primates. 



The perception of small colored objects is essential for 

 the existence of arboreal animals which live mainly on 

 -fruits, that they pick up with their fingers. It is by no 

 means essential for the existence of carnivorous or herbivo- 

 rous mammals. ? For the former even the capacity of 



