THE PLACENTALIAN EYE 677 



radiation paralleling that of the group itself. The placental-mammalian 

 eye has been carried along the ground — rapidly or slowly — and into 

 trees, into the free air, into the fresh waters, and a mile below the surface 

 of the ocean. It has been required to work in brightest sunlight and 

 faintest starlight. It has been asked to inform its owner of an enemy 

 miles away, and to analyze a tiny object held close before the face. The 

 placental eye has been able to cope with all of these situations. Only in 

 complete and permanent lightlessness has it given up, and shrivelled to 

 a pin-head hidden beneath the skin. This sort of degeneration has oc- 

 curred several times — in two distinct families of lipotyphlous insecti- 

 vores, the true, talpid moles {Talpa, Scalopus, etc.) and the golden 

 moles (Chrysochloris spp.) ; in two families of rodent 'moles', the Spa- 

 lacidae and the Bathyergidae; and in one additional genus of rodent 

 (Ellobius) which belongs to the hamster branch of the mouse family. 



The adnexa have been discussed on pp. 36-41 (man) and pp. 425-8 

 (mammals in general) ; and the special features of the sirenians, whales, 

 and seals have been previously treated (pp. 407-17, 444-8). There re- 

 mains a great deal which could be said about placentalian eyes, not 

 much of which can be squeezed into the space allotted here. For detailed 

 anatomical information the reader will have to turn to such compendia 

 as that of Franz (1934) , and to the works cited therein. 



Functional, harmonious, placentalian eyes range in size from about 

 a millimeter in the shrews and the smallest bats to that of the great 

 blue whale, Balcenoptera musculus (145 x 129 x 107mm.). Carnivores, 

 diurnal primates, and ungulates have the largest eyes relative to body 

 size. In the lowest orders (Insectivora, Chiroptera, Edentata, Rodentia) 

 the eye is both relatively and absolutely small, in sympathy with the 

 nocturnality of these animals and the unimportance of vision in their 

 lives. Orycteropus however has a large eye (22 x 22mm., 20.5mm. axis) , 

 which aligns this form with its ungulate relatives. 



The basic shape of the eyeball is the sphere; but a horizontal ellipsoid- 

 ality, at maximum about as great as it ever is in birds, occurs in some 

 ungulates and in many large-eyed aquatic forms. The cornea may pro- 

 trude from the sphere formed by the rest of the globe when it is small 

 and sharply curved throughout (e.g., man), or its apex may be acutely 

 curved even though the rest of the cornea blends with the curvature of 

 the sclera (carnivores) . The axis is somewhat shortened in many ungu- 

 lates, in which the lens has been moved forward (see Fig. 71, p. 173), 

 and also in the more fish-like aquatic eyeballs. In Galago and Tarsius, 



