204 ADAPTATIONS TO DIURNAL ACTIVITY 



or nocturnal, with twenty-four-hour eyes among the ungulates and car- 

 nivores. None of these have filters. Only a few placental mammals, 

 mostly squirrels or primates, are strongly diurnal. The yellow lenses of 

 tree-shrews and squirrels, like those of diurnal geckoes and snakes, are to 

 be looked upon as substitutes for the irretrievable oil-droplets of remote 

 diurnal ancestors, which had been discarded by more immediate noc- 

 turnal ancestors. 



The retinal capillary supply likewise makes its first appearance (except 

 for the eels) among the mammals, and cannot be ignored as a yellow- 

 filtering device. However, it is the least effective of all such devices, for 

 the capillaries are in general no less abundant in nocturnal mammals 

 than in diurnal ones, indicating that they absorb so little light that they 

 do not interfere with scotopic vision. Again, in areas centrales the capil- 

 lary network is not richer but is actually diminished, as though the shad- 

 ows of the vessels caused damage to the image which was not compen- 

 sated for by any differential filtering action. In the few mammalian areae 

 which have foveal depressions the capillaries are eliminated entirely. It is 

 in such areae (in the primates) that we find yellow pigment in the inner 

 layers of the retina — filling in, as it were, the lacuna in the capillary 

 plexus, but far more efficient as a filter than any equal area of the capil- 

 lary screen. 



The nature of this pigment in the macula lutea is unknown. No studies 

 have been made of its status in sub-human primates. The amount is 

 known to vary greatly in different human individuals — being sometimes 

 so great as to render the person wholly blind to blue. Old observations, 

 now considered questionable, seemed to demonstrate more of the pig- 

 ment in brown-eyed than in blue-eyed persons. It has been claimed to be 

 soluble in alcohol and to change color in acids and alkalies. No modern 

 biochemist has given any attention to the pigment or to a resolution of 

 these apparent ambiguities of genetic and chemical behavior; but a fair 

 guess is that the substance belongs to the carotenoid family of pigments 

 and may be subject to the influence of diet. Simple experiments on the 

 perceptibility of blue stimuli would show whether the macular pigment 

 can be increased by feeding carotene or related substances to human 

 subjects; but such experiments have yet to be made. 



The effectiveness, in human vision, of the combination of macular 

 pigment and the yellow lens is difficult to evaluate. We do know that 

 for a few weeks after a person has had cataracts removed, white light 

 looks decidedly bluish to him. We can only guess how much less sharply 



