606 



THE EYE IN EVOLUTION 



through the dioptric system is concentrated into a small image of the 

 maximum possible brightness (Figs. 740 to 743 ; 752 to 754). 



Enlargement of the eye in the interests of nocturnal vision is common but 

 is seen in its most extreme degree in certain deep-sea Fishes wherein the eyes may 

 be larger than the remainder of the head ; this tendency, in combination with 

 the evolution of a maximal size of the lens, leads to the development of a tubular 

 eye when the head is not sufficiently big to accommodate a spherical organ of the 

 necessary dimensions.^ The large lens occupying a high proportion of the globe 

 and closely approaching the retina is well seen in the eyes of the smaller bats 

 and Rodents (Figs. 741).- 



FiGS. 750 AND 751. — The Eyes of Birds. 

 To contrast the relatively small eyes of a diurnal bird and the large eyes 

 and widely open pupil of a nocturnal bird. 



Fig. 



^JO. — The crowned hawk eagle, 

 Stephanoaetus. 



Fig. 751. — Sa\igny's eagle owl, Bubo 

 ascalaphus. 



A TAPETUM LUCiDUM is an accessory to the optical system to aid 

 vision in dim illumination ; it is essentially a mirror-arrangement so 

 that light, having traversed the sentient elements of the retina, is 

 reflected backwards again and its effective intensity is thus augmented. ^ 

 Not only is the amount of light available for stimulation thus materially 

 increased but slight differences in luminosity between an object and 

 its backgroimd are proportionately accentuated so that the total 

 effectivity of vision in dim illumination is correspondingly improved. 

 It is this reflected light seen by an observer standing beyond the 



1 p. 322. 



- It is to be remembered that, for entirely different reasons spherical lenses are 

 also found in aquatic Vertebrates (except Sirenia) — Cyclostomes, practically all Fishes 

 (except amphibious tj'pes, as Periophthalmus) , aquatic amphibians, marine turtles and 

 Crocodilians, and Cetaceans and Pinnipedes. See p. 277. 



' The effective intensity would theoretically be doubled by a perfect mirror. A 

 tapetum probably ensures an increase of about half as much — 40% in the cat 

 (Weale, 1"53). 



