214 ZOOGEOGRAPHICAL REGIONS 



and fast line must necessarily be of a more or less 

 artificial character. The difficiilty is sometimes got over 

 by the erection of a transitional region. It is enough 

 for our purpose to know that such a transition exists. 



The other area where a Mne is difficult to draw is 

 in the American continent. There are many striking 

 differences between the faunas of North America and 

 South America, but the two regions are now in free com- 

 munication. Necessarily, therefore, a certain amount 

 of mingling occurs. That this mingling is not greater 

 than it is is doubtless due to climatic differences, to 

 the occurrence of desert regions, e.g. in Mexico and the 

 west, and so on. Here again the difficulty is usually got 

 over by the erection of a transitional region to include 

 a considerable part of the south-western United States. 



The rest of the globe is sometimes included in one 

 Realm, forming the Notogaeic Realm, or Southern 

 World, and including on the one hand Southern and 

 Central America, with Mexico and the West Indian 

 islands, and on the other the continent of Austraha 

 with Tasmania, New Guinea, the islands of New 

 Zealand, and some of the islands of the Malay Archi- 

 pelago, as well as the scattered groups of Polynesia. 

 This union of Australia and South America, despite 

 their considerable separation in space, seems to be 

 justified by the increasing proofs of the affinities of 

 their faunas, and the increasing probability that they 

 were connected, at a not very remote period, through 

 the Antarctic continent. 



The Notogaeic realm so defined is characterized by 

 the presence of monotremes (in Australia) and mar- 

 supials ; by the presence of running birds and dipnoi 

 (these being characters shared with Africa), and by the 

 absence of the highly differentiated forms named above. 



