50 OSBORN. 



Under the operation of these laws a most interesting general- 

 ization or hypothesis can be made as to the three realms : geo- 

 graphical isolation has been so continuous and prolonged that 

 great orders of mammals have been evolved (Fig. Ill) in each. 

 Thus ArctogcBa, containing the broadest and most highly diversi- 

 fied land area, appears hypothetically as the center in which 

 fourteen primitive and specialized orders radiated from each other. 

 In the southern portion of Ncogcea at least four orders sprang 

 from primitive members of the above orders, and the Hystri- 

 comorph rodents enjoyed their chief radiation. In Xotogcea two 

 orders were cut off by the sea ; one of them a rapidly declining 

 type, the Monotremes, the other, the Marsupials, enjoying a veiy 

 highly diversified radiation. This hypothesis is expressed in 

 Fig. III. Two other orders of mammals, the Sirenia (prob- 

 ably a branch of the hoofed tribe) took the rivers and coasts of 

 America, Europe and probably Africa as their radiating center, 

 while the Cetacea occupied the fourth or oceanic realm. 



We mean to express by this hypothesis that Realms ivcrc tJic 

 main coitcrs of adaptive radiation of orders of uianiuiah, but by 

 no means the exclusive areas of distribution, for during the 

 periods of land contact certain members of these orders found 

 their way into adjacent realms. Each realm, therefore, contains 

 its pure autocthonous types and its migrant or derived types. 

 Regions, on the other hand, may be distinguished from realms 

 as geographical and zoological areas, which have been isolated 

 from each other for shorter periods, either by climatic barriers, 

 as in the case of the Arctic or circumpolar region or by great 

 physical barriers, such as masses of water and of desert sands. 

 In certain cases these regions, such as Africa, appear to have 

 been so large, distinct and isolated as to have become important 

 centers of the radiation of certain orders of mammals and almost 

 attain the rank of realms, but regions in general are chiefly and 

 permanently distinguished by the adaptive radiation of families 

 of mammals. 



Arctogaea may thus be still divided on the old lines into five 

 or six regions, the Arctic or Circumpolar ; the Ethiopian or 

 African, south of the Sahara ; the Indo-Malayan or Oriental, in- 



