228 ADAPTATIONS TO NOCTURNAL ACTIVITY 



of those in Scylliorhinus. Seals have vertical slit pupils, excepting in one 

 species (Phoca barbata) whose slit is diagonal — indeed, almost horizontal 

 as is the slit in Paradoxums and in the hippopotamus. But the seal's 

 pupil, as will be made clear later, needs its slit form for a reason quite 

 different from the one which accounts for probably every other slit pupil 

 in the vertebrates. 



The history of the primate group has been one of increasing diumality 

 from strictly nocturnal beginnings, with 'successfulness' increasing along 

 with the tendency toward diurnality. The range in size from the timid, 

 nocturnal, three-inch mouse galago to the monstrous, diurnal gorilla is 

 most striking. All but one of the lowest prosimians (the bush-babies, 

 lorises, etc.) have vertical slit pupils. The true lemurs (genus Lemur) 

 have vertical pupils which are not at all slit-like, but only slightly oval. 

 They and their closest relatives {e.g., Indr'is) do all of their sleeping at 

 night. All of the simians (monkeys, apes and man) except Actus are 

 diurnal, with the great apes most strongly so. The eyes of some pro- 

 simians are so sensitive that, despite the protection afforded by the slit 

 pupil, they are prone to undergo retinal degeneration and to become 

 blind when, in zoos, they are kept in too strong light. Similar changes are 

 said to occur, by the way, in nocturnal birds, fruit-bats, and some bears. 



Tarsius is the one primitive lemuroid which does not have the slit; but 

 the pupil in this genus has an enormous excursion from a large circle to a 

 broadly horizontal oval only half-a-millimeter in diameter. In its range 

 of sphincter-length, the tarsier's iris has a very few close rivals : those of 

 the two-toed sloth, the African jumping hare (Pedetes) , the sea-snakes, 

 and the pearl-fish (Encheliophis) . One suspects that in such animals the 

 sphincter must have some special organization; but the details are as yet 

 unknown. They have somehow found the secret of obtaining an ex- 

 tremely small pupil-area without resorting to the slit form, or to the even 

 more elaborate device of an expansible operculum. 



(D) The Tapetum Lucidum 



The standard condition of the chorioid coat is one of heavy pigmen- 

 tation. The pigment epithelium may or may not contain much pigment 

 also, depending chiefly upon whether this pigment is migratory or not. 

 It is the pigment of the chorioid, alone, which has the real responsibility 

 of preventing reflections within the eyeball which might blur or even 

 multiply the image. 



