BROAD CORNER; ROD-TO-CONE RATIOS 215 



(B) The Nocturnal Retina 



Rod'.Cone Ratios — We expect to find rods greatly predominating in 

 nocturnal retinae; and we are never disappointed. However, pure-rod 

 retinas are not as common among strictly nocturnal animals as pure-cone 

 retinae are among strictly diurnal ones. Fabulous though the cat's ability 

 may be for "seeing in the dark," she has a very respectable number of 

 cones — about a third as many as we ourselves, who are marooned among 

 the strongly diurnal animals when our artificial lights are taken away 

 from us. 



This persistence of cones in nocturnal retinae calls for a Uttle special 

 explanation, for it has served some people as a sufficient excuse for 

 rejecting the Duplicity Theory entirely. The first prominent opponent 

 of the theory — Wilhelm Krause, a contemporary of its formulator. Max 

 Schultze — saw more cones than there really were in many nocturnal 

 forms, and drew incorrect conclusions from other animals through im- 

 perfect knowledge of their habits. Several modern investigators (par- 

 ticularly Mile. Verrier) have apparently thought that if there is any- 

 thing to the Duplicity Theory, then cats and owls should have no cones 

 whatever. 



This view fails to take accoimt of the fact that whereas a diurnal 

 lizard never gets out of bed for a midnight snack, a cat may appreciate 

 a sun-bath at high noon. The nocturnal animal which wishes (as most 

 do) to be able to come out sometimes in daylight, is wise to retain some 

 cones for the improvement of form-sense, for he is otherwise at a great 

 disadvantage if taken by surprise by a diurnal enemy. 



If this interpretation seems weak, we can surrender any positive argu- 

 ment in favor of a nocturnal animal's keeping cones, and still believe the 

 Duplicity Theory to be well founded. The only pure-rod retinae are in 

 nocturnal animals, and the proportion of cones in such animals is never 

 very high. Where there are so few as to seem utterly useless, as in the 

 opossum or the rat, it may be pointed out that unneeded cones are 

 probably harder to get rid of than are unwanted rods. The vertebrate 

 eye, like the brain, is so delicately-balanced an organ that it very rarely 

 contains anything useless. The eye is comparable to a machinery-crammed 

 submarine — if there is no proper niche for a thing, it is almost certain 

 to be in the way. In a strictly diurnal eye, even a few rods can detract 

 very immediately from resolving power, and they are completely elimi- 

 nated from every good area centralis. But cones, as we have learned, keep 



