CENTERS OF CIVILIZATION — SWANTON 375 



are believed to have continued to reproduce in the environments to 

 which they had become adapted and to have died out if radical 

 changes in such environments took place. Under perfectly uniform 

 environmental conditions it might be assumed that new species, 

 genera, families, orders, and so on would spread wave-fashion from 

 this center, but external conditions introduce modifications in any 

 theoretical pattern so that considerable samples of these forms are 

 found from center to circumference. In numerous cases, in fact, 

 earlier and later forms persist in the same environment. In large 

 areas like continents we should expect differentiation to extend over 

 considerable areas and to bear some relation to the completeness of the 

 separation between continent and continent and the time when such 

 separating occurred. 



In volume 5 of Bartholomew's Physical Atlas the following zoo- 

 geographical provinces are indicated, based upon earlier work by a 

 number of students and valid in the main today : 



The Palaearctic region, including all of Eurasia except India, Indochina, and 

 southern China ; the Ethiopian region, including Africa south of the Saliara and 

 southern Arabia ; the Oriental region, including nearly all of India, Indochina, 

 southern China, and the nearer East Indies; the Australian region, including 

 Australia, Tasmania, New Zealand, the East Indies from the eastern end of 

 Java to the Solomon Islands, and Polynesia ; the Nearctic region, including all 

 of Canada, Alaska, the United States, and northern Mexico with a tongue of 

 land down the JMexican highlands to the Isthmus of Tehuantepec; and the 

 Neotropical region, including the rest of central and southern Mexico, Central 

 America, and South America. 



It is undoubtedly significant that the number of these regions is 

 exactly equal to the number of grand continental divisions, and in some 

 cases at least we can account for their differentiation by the isolation 

 of the continent in question at a particular period in its history. Thus 

 the Australian region owes its peculiar land fauna to the fact that it 

 was set apart from Asia by the Wallace Deep shortly before the appear- 

 ance of placental mammals. Considerably later South America was 

 separated from North America by the submergence of the Panamanian 

 section, although North American fauna intruded into it when union 

 was reestablished. Again, the faima of North America is known to 

 resemble that of the Palaearctic region owing to the existence of a land 

 bridge which persisted to a much later period than the Central Ameri- 

 can isthmus. The African region has been differentiated from the 

 Palaearctic in part by the interposition of the Mediterranean but still 

 more by the sea of sand which we call the Sahara. Farther east the 

 Palaearctic and Oriental regions have acquired their differences in 

 part owing to the interposition of the Indian Ocean which once ex- 

 tended entirely through to the Arctic and in part by the great deserts 

 of central Asia which succeeded. 



