446 THE EYE IN EVOLUTION 



(6) the TYLOPODA (camel and dromedary of Africa and Asia, and the 

 llama of S. America) ; 



(c) the PECORA (Ruminants) (deer and giraffe, and the Bovidee — ox, 

 bison, sheep, goat, antelope, gazelle) ; 



(d) the TRAGULiNA, Small chevrotains of the East and Africa. 



(11) PERissoDACTYLA. Odd-toed hoofed animals wherein the foot is 

 essentially formed by the enlarged third digit — Equidse (horse, ass, zebra), 

 Rhinocerotidse (rhinoceros), Tapiridse (tap ii"). 



(12) HYRACOIDEA, the Small rodent-like hyraxes (" coneys ") of Africa and 

 Syria of arboreal habits. 



(13) PROBOSCIDEA, the vegetarian elephants of Africa {Loxodonta africaria) 

 and the Orient {Elephas inaximus). 



The Artiodactyla, Perissodactyla, Hyracoidea, and the Proboscidea used 

 conveniently to be classed in one heterogeneous group of ungulata (hoofed 

 animals). 



(14) siRENiA.i The sluggish, vegetarian, and fully acjuatic fish-like sea- 

 cows, which crop grasses in shallow littoral waters — the manatee {Manatus ; 

 Trichechus) of S. America and S. Africa, and the dugong (Halicore) of Oriental 

 and Australian coasts. 



(15) CETACEA. The carnivorous fish-like whales and dolphins, fully adapted 

 for marine life. There are two distinct orders : 



(a) the baleen whales (mystacoceti) with baleen (or whale-bone) plates 

 instead of teeth, which sound to greal depths and feed blindly by trawling for 

 plankton which they strain through the frayed margins of their plates (the 

 right-whale, Balcena ; the hump-back, Megaptera ; the blue whale or rorqual, 

 Balcenoptera, etc.) ; the great rorquals (particularly the blue whale) are the 

 largest animals in existence, over 100 feet in length and well over 100 tons in 

 weight ; 



(6) the toothed whales (odontoceti), squid- and fish-eating animals which 

 use their vision to catch their prey and are therefore adapted with more perfect 

 eyes, some of them swimming in packs like wolves attacking the unwieldy 

 whale-bone whales (the sperm-whale, Physeter; the killer whale, Orca; the narwhal, 

 Monodon ; the porpoise, Plioccena ; the dolphin, Delphinus). There is a small 

 family of fresh-water dolphins (the susu, Platanista) with rudimentary eyes. 



Within the many orders of Placentals a considerable range of 

 variations in the structure of the eye occurs, but throughout the entire 

 class the similarity is great. It seems likely that the first representatives 

 (Insectivora) were nocturnal in habit, and that, as occurs in snakes, 

 the eye has evolved from this as a basis showing innumerable adaptive 

 changes to suit the many environments (diurnal, arboreal, aquatic, etc.) 

 to wliich the prolific class has suited itself. Only in a few instances 

 among the Insectivores (moles) and Rodents has the burrowing habit 

 led to the degeneration of the eyes.^ The general characteristics of 

 the jDlacental eye may be summarized as follows (Figs. 554 to 563). 



' The legend of the mermaid is said to derive from sailors' fanciful descriptions 

 of the manatee sitting on the rocks nursing its baby in its arms ; hence the generic 

 name, Sirenia. It is to be remembered that a third species, Rhytina (Steller's sea- 

 cow), growing to enormous dimensions (25 feet or more), was found in great herds by 

 Bering in 1741 near the Asiatic coasts of the Bering sea. Sluggish and docile in habit 

 itbecai. ■ extinct at the end of the 1 8th century owing to its wholesale massacre for food. 



" p. 73.3. 



