736 GENETICS AND EVOLUTION 



been possible, during most geologic eras, for organisms to pass more or 

 less Ireely from one part to another. In contrast, the six realms are sep- 

 arated from each other by major barriers of sea, desert or mountains 



(Fig. 35.7). 



The Palaearctic realm includes Europe, Africa north of the Sahara 

 desert, and Asia north of the Himalaya and Nan-Ling mountains, plus 

 Japan, Iceland and the Azores and Cape Verde Islands. The animals 

 indigenous to the Palaearctic are moles, deer, oxen, sheep, goats, robins 

 and magpies. 



The Nearctic realm includes Greenland and North America north 

 of the northern plateau of Mexico. This contains many of the same 

 animals as the Palaearctic, plus species of mountain goats, prairie dogs, 

 opossums, skunks, raccoons, bluejays, turkey buzzards and wren-tits found 

 nowhere else. The land bridge connecting North America and Asia at 

 Bering Strait in former geologic times permitted the migration back and 

 forth of many kinds of animals and plants. The flora and fauna of the 

 Palearctic and Nearctic realms are similar in many respects and the two 

 are sometimes combined as the Holarctic region. 



The Neotropical realm consists of South America, Central America, 

 southern Mexico and the islands of the West Indies. Its fauna is quite 

 distinctive, including alpacas, llamas, prehensile-tailed monkeys, blood- 

 sucking bats, sloths, tapirs, anteaters, and a host of bird species— toucans, 

 puff birds, tinamous and others— found nowhere else in the world. 



The part of Africa south of the Sahara, plus the island of Mada- 

 gascar, comprises the Ethiopian realm. The gorilla, chimpanzee, zebra, 

 rhinoceros, hippopotamus, giraffe, aardvark, and many birds, reptiles 

 and fishes live only in this realm. 



The Oriental realm includes India, Ceylon, Indo-China, southern 

 China, the Malay peninsula and some of the islands of the East Indies— 

 the Philippines, Borneo, Java and Bali. Some of the animals peculiar to 

 it are the orang-utan, black panther, Indian elephant, gibbon and 

 tarsier. 



Australia, New Zealand, New Guinea, and the remaining islands of 

 the East Indies, those east of Celebes and Lombok, make up the Aus- 

 tralian realm. The line separating the Oriental and Australian realms, 

 known as Wallace's Line, separates Bali and Lombok, goes through the 

 straits of Macassar between Borneo and Celebes, and passes east of 

 the Philippines. Although the islands of Bali and Lombok are separated 

 by a channel only 20 miles wide, their respective animals and plants are 

 more unlike than are those of England and Japan, almost on the op- 

 posite sides of the world from each other. Native to the Australian realm 

 are the duck-billed platypus, echidna, kangaroo, wombat, koala bear, 

 and other marsupials. Its assortment of curious birds includes the cas- 

 sowary and emu, the lyre-bird, cockatoo and bird-of-paradise. 



Why certain animals appear in one region yet are excluded from 

 another in which they are well adapted to survive (and in which they 

 flourish when introduced by man) can be explained only by their evolu- 

 tionary history. 



