450 THE EYE IN EVOLUTION 



much larger than that of the whale, which, in fact, measures only 

 1/250 to l/600th of its gigantic body (Figs. 555, 564) ; that of the 

 elejjhant (axis 35 mm.) or rhinoceros (axis 23 mm.) is correspondingly 

 small (Fig. 560), and the minute eye of the vole (axis 175 mm.) is rela- 

 tively greater in comparison with the length of its body (10 cm.) 

 than is the eye of man. Although as a general rule Haller's ratio ^ — 

 that the size of the eye varies inversely as the size of the body- — holds 

 good, marked variations occur with the visual habits of the animal. In 

 the lower orders of nocturnal habits which depend little on vision 

 (Insectivores, Chiroptera, Edentates and some Rodents) the eyes are 

 small relatively and absolutely ; in the more liighly developed and 

 visually alive types they are larger. Among these it varies generally 

 with the visual efficiency and swiftness of movement, and is generally 

 larger in nocturnal species. Thus the eye of the nimble horse (axis 

 45 mm.) is larger than that of the lethargic elephant (axis 35 mm.) 

 (Fig. 557), while the small (usually nocturnal) Primates have com- 

 paratively larger eyes than the large diurnal species (with the con- 

 spicuous exception of the Hapalidse — marmosets and tamarins) 

 (Ashley -Montague, 1943-44) (Figs. 752 and 753). 



Measurements of the various placental eyes are found in Emniert (1886), 

 Putter (1903), Hotta (1906), Kolmer (1910), Franz (1912), Linsenmeyer (1912), 

 Guist (1923), Wolfrum (1926), Rochon-Duvigneaud (1943) and Steindorff (1947) ; 

 their weight and volume in Liebig (1874), Koschel (1883), Emmert (1886), 

 Welcker (1903), Schleich (1922), Vitello (1931), Steindorff (1947) and Henderson 

 (1950). 



The corneoscleral enveloj^e corresponds with that of man with the 

 exception of the aquatic Placentals, apart from the generalization that 

 the eye of a relatively large animal tends to have an unusually thick 

 sclera — elephant, rlmioceros, etc. The envelope is entirely fibrous 

 without any supporting skeletal structures.^ Among the Cetaceans par- 

 ticularly the sclera is enormously thick, a feature described by Bennett 

 (1836) ; indeed, the sclera at the posterior pole may be 3/4 the length 



1 p. 401. 



^ Magnitudo oculorum est fere in ratione inversa animalium. Bala?n£e, Rhino- 

 ceroti, Elephanto parvi sunt oculi. Haller, Et. Phys. IV-XVI (1768). 



* A FIBROUS SCLERA is also fouiid ill Cyclostomcs, pearl-fishes and some eels, adult 

 Urodeles (excluding Triton and Hynohius, and degenerative limicoline types), some tree 

 frogs, snakes and Marsujjials (excluding Notoryctes). 



CARTILAGE is found (rt) in the form of a posterior cup in Fishes (except Teleosteans), 

 adult Aiuirans (except some tree frogs), larval Urodeles, Keptiles (excluding snakes 

 and the chameleon). Birds and Monotremes ; (6) in the form of a ring in Teleosteans ; 

 (c) as islands in elephant fishes, Triton and Hynobius, limicoline Urodeles (enormously 

 large), the chameleon (at the fovea) and Notoryctes; {d) calcified in some Selachians 

 and some Teleosteans. 



BONE is found (a) as anterior ossicles in most Teleosts, Chondrosteans, Coelacanths, 

 Reptiles (excluding snakes and Crocodilians) and Birds ; (6) in the form of a ring in 

 Xiphic-^ snd Thunnus (anteriorly), Hypopaclius, and many Birds (posteriorly as the 

 OS opti 



