214 ZOOGEOGRAPHICAL REGIONS 
and fast line must necessarily be of a more or less 
artificial character. The difficulty 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 line 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 soon. 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 Australia 
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. 
