INTRODUCTION 



37 



migrant, or derived types. Thus Ardogwa, containing the broadest and 

 most highly cUversified land areas of North America, Asia, Europe, and 

 Africa, appears as the center in which twenty-one primitive and special- 

 ized orders of mammals radiated from each other. In th(» southern Realm 

 of Xeogcca, more restricted geographically, four or five orders of mammals 

 enjoyed their chief radiation. During the larger part of the Age of Mam- 

 mals Notogaea (Australia, New Zealand, and Tasmania), shut off by the 

 sea from ArctogiBa, witnessed the highly diversified radiation of the Mar- 

 supials and of the declining group of Monotremes. 



Regions in the zoogeographical sense may be thought of as more 

 restricted areas of adaptive radiation of mammals which have been isolatcnl 



ANTA^RC 



Fig. 9. 



Chief zoogeographic Realms and Regions of the world on the Sclater- 

 Huxley system. 



from each other for shorter periods either by climatic barriers, as in the 

 case of the arctic conditions of the north, or by great physical barriers, 

 such as masses of water or desert sands. Whereas the Realms are the 

 centers in which orders originate. Regions in general are chiefly distin- 

 guished by the adaptive radiation of families of mammals. This, while 

 broadly true, is not universally true, for the Ethiopian Region (Africa 

 south of the Sahara) appears to have given birth during early Tertiary 

 times to several orders of mammals, namely: Barytheria, Embrithopoda, 

 Hyracoidea, Proboscidea, and perhaps the Sirenia and Cetacea. The two 

 marine orders of mammals, last named, are also attributed to the hypothet- 

 ical 'Atlantis,' or atlantic archipelago connecting Africa and South America 

 (see J. W. Gregory, Eigenmann, and others). 



It must be very clearly understood, therefore, that all the modern 

 zoogeographic divisions, Realms, Regions, sub-Regions, etc., are chiefly 



