A DEFINITION OF EVOLUTION 



BIOGEOGRAPHICAL REGIONS 



One of the important aspects of biogeography is the division of the world 

 into six very distinct biogeographical regions (Figure 3), so that a bio- 

 logical explorer may well feel that he is entering an entirely different 

 world when he goes from one region to another. These were originally 

 defined on the basis of the avian faunas of the various parts of the world, 

 but their validity is general. Thus, the Holarctic Region includes all of 

 Europe, Asia north of the Himalaya and Nan Ling (mountains), Africa 

 north of the Sahara Desert, and North America north of the Mexican 

 Plateau. Typical mammals of this great region include the caribou and the 

 elk, foxes of the genus Vidpes, bears, and the marmot tribe. This Holarctic 

 Region is often broken up into the Palearctic Region (Old World) and 

 the Nearctic Region (North America), because the two do present char- 

 acteristic differences, though commonly only on the specific or generic 

 levels. 



The Ethiopian Region comprises Africa south of the Sahara Desert, and 

 is marked by such mammals as the gorilla, giraffe, lion, and hippopotamus. 

 The Oriental Region includes the portions of Asia south of the Himalayas 

 and the Nan Lings and is marked by tarsiers, the orang-utan, the Indian 

 elephant, and flying "foxes" (frugivorous bats). The Neotropical Region 

 includes South and Central America, and is marked by tapirs, sloths, pre- 

 hensile-tailed monkeys, and vampire bats. Finally, the Australian Region 

 includes Australia and (like all of the above regions) the associated is- 

 lands. As is well known, it is marked bv a predominantly marsupial mam- 

 malian fauna, together with many other relic forms, and by the complete 

 absence of any native placental mammals other than bats (and a few 

 rodents which may have been introduced by primitive man). 



All of these biogeographical regions are separated from one another by 

 very nearly impassable barriers of sea, desert, or mountain, or by climatic 

 zones, and these barriers are very ancient geologically. Thus the Palearctic 

 and Nearctic Regions are separated by the Atlantic and Pacific Oceans. 

 But because the north Pacific is quite shallow in the vicinity of Alaska, 

 the two regions have been connected in the past, and hence the desir- 

 ability, for some piu'poses, of describing them as a single region, the 

 Holarctic. The southern continents are widely separated from each other 

 by the great ocean basins of the world. The Ethiopian Region is separated 

 from the Palearctic by the Sahara Desert, a very formidable barrier indeed 

 to any organism adapted to temperate conditions, or to cold. South Amer- 

 ica is at present connected to North America by the Isthmus of Panama. 

 During great eras of the earth's history, however, this tenuous connection 

 has been submerged, so that Sontli America was completely isolated from 

 all other land masses. And even at present, climatic factors prevent most 

 plants and animals from using this connection between the regions. The 

 Oriental Region is separated from the Palearctic by the most lofty moun- 

 tain chains in the world— the Himalayas and the Nan Lings. A glance at 

 the map shows that the Malay Archipelago extends down from southern 

 Asia and approaches quite near to Australia. Thus there is a broken link 



18 



