162 GENERAL PRINCIPLES OF ZOOLOGY 



surface often opposes impassable barriers to most mammals: rising, if it lead to 

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

 formed, which interpose straits or broader bodies, impassable to most mammals, 

 between two land areas. Birds and strong-flying insects are less affected by 

 all such changes; the majority of them can fly over arms of the sea and moun- 

 tain-chains; there are birds which can even cross the Atlantic. 



The Six Primary Regions. Of the systems of animal geography proposed 

 that advocated by Sclater and Wallace finds most favor. They distinguish six 

 primary regions: (i) the pakcarctic, comprising all Europe, northern Africa as 

 far as the Sahara, and northern Asia to the Himalayas; (2) the Ethiopian, 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 neo- 

 tropical regions, which make up the American continent and are divided at about 

 the northern border of Mexico; (6) the Australian, with, besides Australia itself, 

 the larger and smaller islands of the Pacific Ocean and the Malay Islands, east 

 of Celebes and Lombok. 



(1) The Australian region is most sharply distinguished 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 why only the oldest 

 mammals, the monotremes and marsupials, have entered the region, while the 

 placental mammals have not been able to follow. The marsupials, which in 

 the secondary period also inhabited the northern hemisphere, and were replaced 

 there in tertiary times by the placentals, were able to develop farther in the 

 Australian region. Australia and the adjacent islands are thus the land of 

 marsupials, which have persisted elsewhere only in South America, the opossum 

 ranging north into the United States. On the other hand, at the time of dis- 

 covery Australia lacked all terrestrial placental mammals except those (bats) 

 which were not restricted by water and the Muridae, easily transported on floating 

 wood. Two larger mammals, the wild dog (Canis dingo} and the pig of New 

 Guinea (Sns papitanus), may have accompanied man, this being probable for the 

 dingo, although his remains occur in the pleistocene along with those of the 

 giant marsupials. Further peculiarities of the Australian region are the birds- 

 of-paradise in New Guinea, the egg-laying mammals (monotremes), and the 

 cassowaries and the Australian ostrich (DromcBus novcchollanditf}. 



It is easily understood that the island groups of the South Sea (Polynesia) 

 have developed many faunistic peculiarities, as well as that an exchange of forms 

 may have taken place between the islands of the oriental province and those 

 faunally related to Australia, and that 'Wallace's Line' (p. 34) is not so sharp 

 a boundary as was once thought (extension of marsupials into Celebes, of pla- 

 centals into the Moluccas). On the other hand the distinctness of New Zealand 

 needs mention. It is distinguished from Australia by a large number of peculiar 

 birds (Apteryx and the extinct Dinornithidas), reptiles (the ancient Sphenodcni), 

 and molluscs. If the bats and mice be excepted, New Zealand lacks all native 

 mammals, even marsupials. 



(2) The Neotropical province (South and Central America) is, next to 

 Australia, the most sharply characterized, and has also been set aside as a 

 special division 'Neogaea,' partly in view of its geological history; during the 

 cretaceous and early tertiary time it was separated from North America by the 

 sea and had developed a peculiar fauna (e.g., gigantic edentates, no carnivores). 

 These peculiarities disappeared towards the end of the tertiary by the entrance 

 of carnivores and ungulates from the north and an extension of the edentates, 

 marsupials, humming birds, etc., to the northern hemisphere. To Neogaea 

 belong the platyrhine apes, the catarrhine to the Old World. Characteristic 

 edentates are trie armadillos, sloths, and ant-eaters; the marsupials are repre- 



