THE VERTEBRATE EYE 15 



the aberrations of horizontal contours are greater than those 

 belonging to vertical contours. The contours of trees and 

 their branches are principally vertical ; therefore, if the image 

 formed on the retina could be increased by sacrificing the 

 definition of horizontal contours it would be an advantage. 

 Interesting as this is, it seems doubtful whether the theory 

 can be successfully applied. Certainly at night or in dim 

 light the pupil of the cat is wide open and round. Further- 

 more, in daytime when the pupil is a vertical slit or oval, 

 cats hunt and catch on the ground — in other words along 

 horizontal contours. The alligator also hunts along hori- 

 zontal contours, yet it has a vertical slit pupil. Whereas 

 the nocturnal tree-dwelling lemurs have a vertical oval 

 pupil, the nocturnal Tarsius which is also a tree dweller 

 possesses a horizontal oval pupil. 



Rochon-Duvigneaud (1917) in listing the characteristics 

 of the eye of the geckos which adapt them to nocturnal 

 vision, includes the form of the pupil — a vertical slit — 

 which shows a rapidity of movement surpassing that of the 

 human eye and approaching that of birds. In dim light the 

 pupil is a large oval, or even round, as in the cat. In bright 

 light it is closed completely. According to him a round pupil 

 can dilate as well as an oval, but it cannot be entirely closed. 

 He believes that the chief function of an oval pupil, in an 

 animal adapted to twilight vision, is to protect the eye 

 against an excess of light. Presumably this is a perfectly 

 adequate explanation for the presence of an oval pupil be it 

 vertical or horizontal. My own experience with the gecko 

 pupil, however, was that it never completely closed in bright 

 illumination, but became reduced to several very narrow 

 slit-like apertures. 



