EVIDENCE FROM DISTRIBUTION 127 



bilities of migration had been greatly changed. For 

 the mammals a very satisfactory working arrange- 

 ment has been gradually reached, which is well 

 justified by the data of geology and palaeontology. 

 This plan first of all divides the continents into two 

 groups of very unequal size, (1) Australia and its 

 adjoining islands, Tasmania, New Guinea, etc., 

 (2) the rest of the world. This division recognizes 

 the fact that Australia is, zoologically speaking, by 

 far the most isolated region of the earth; aside from 

 the bats and a few mice, its mammals are Mono- 

 tremes and Marsupials. Of the monotreme egg- 

 laying mammals Australia has all existing species 

 and of the marsupials all except the opossums of the 

 western hemisphere, with the addition in South 

 America of one interesting genus, the survivor of a 

 group that was abundant in the Tertiary of that 

 continent. 



Neglecting Australia, the other continents likewise 

 fall into two very unequal groups, (1) South America, 

 including Central America and the West Indies, 

 and (2) the other continents. South America is, 

 after Australia, much the most peculiar part of the 

 globe, a fact which is explained by its geological 

 history. The remaining continents, North America, 

 Europe, Asia and Africa, with great local differences 

 among themselves, have a certain unmistakable 

 similarity in their mammals. The local differences 

 are expressed by dividing this vast land-area into a 

 number of regions, with the curious and unexpected 



