176 GENERAL PRINCIPLES OF ZOOLOGY. 



the formation of glaciered mountain-chains; sinking, when arms 

 of the sea are formed, which, even if only narrow, interpose 

 between two hitherto connected land areas straits which are 

 impassable for most mammals. Birds and insects which fly well 

 are less affected by all such changes of the earth's surface; the 

 majority of them can fly over arms of the sea and mountain-chains, 

 for there are birds which can even cross the Atlantic Ocean. 



The Six Primary Regions. Of the systems of animal geography 

 proposed up to the present time, the divisions advocated by 

 Sclater and Wallace finds most favor. These English scholars 

 distinguish the six following primary regions: (1) the palcearctic, 

 comprising all Europe, northern Africa as far as the Sahara, and 

 northern Asia as far as the Himalayas; (2) the Ethiopian, all of 

 Africa south of the Sahara; (3) the oriental, including upper and 

 farther India, southern China, and the western Malay Islands; 

 (4) and (5) the nearctic and the neotropical regions, which make 

 up the American continent and are divided by a line drawn at 

 about the northern border of Mexico; (6) the Australian, in 

 which, besides Australia itself, are included the larger and smaller 

 islands of the Pacific Ocean and the eastern Malay Islands, east of 

 Celebes and Lombok. 



(1) The Australian region is most sharply distinguished from 

 all the others and by many is set apart as a distinct division called 

 * Notogaea. ' Its isolated geographical position together with the 

 fact that it has long been separated from other countries 

 (apparently since the beginning of the tertiary) explains the fact 

 that only the oldest mammals, the monotremes and marsupials, 

 have entered the region, while the placental mammals have not 

 been able to follow. While the marsupials, which in the secondary 

 period also inhabited the northern hemisphere, were replaced there 

 in tertiary times by the placenta! s, they were able to develop 

 farther in the Australian region. Australia and the adjacent 

 islands are thus the land of marsupials, which have persisted else- 

 where only in South America (Cmnolestes, Didelphidaa), the 

 opossum ranging north into the United States. On. the other 

 hand, at the time of discovery Australia lacked all placental 

 mammals except those (whales, dugong, seals, bats) which were 

 not restricted by water and the Muridae, easily transported on 

 floating wood. Two larger mammals, the wild dog or dingo 

 (Canis dingo) and the pig of New Guinea (Sus papuanus), may 

 have accompanied man, this being the most probable for the dingo 

 in. spite of the fact that his remains occur in the pleistocene along 



