OF MAMMALIA. 595 



Allotheria can be traced through the Jurassic and possibly 

 into the Eocene (Tertiary). The Polyproiodontia can also 

 be traced through Jurassic and Cretaceous into the Ter- 

 tiary. Through the Tertiaries can be traced certain of the 

 Metatheria (Didelphidae) to the present day and the 

 eutherian types first occur in the Eocene. In the Eocene 

 strata there are abundant remains of many Eutheria, in 

 marked contrast to their absence in the Cretaceous. 



Let us now glance at the present distribution of mam- 

 mals. The geographical world is usually divided up into 

 three zoological realms : — 



T. NoTOGCEA — comprising Australia, New Guinea, Poly- 

 nesia and New Zealand and certain of the Malay 

 Islands. 



2. Neogcea — comprising South America, West Indies 



and part of Central America, 



3. Arctogcea — North America, Eurasia and Africa. 



This is a very unequal division of the world's surface, 

 but is justified by the quality of the faunistic differences in 

 each region. 



I. NoTOGCEA. — In this realm we may, from a mammalian 

 point of view, leave out of consideration New Zealand and 

 Polynesia, for, with the exception of bats and a rodent or 

 two, they have no mammals. The realm has an entire 

 monopoly of one sub-class of mammals, the Prototheria. Of 

 the Metatheria., it contains all the order Diprotodontia (with 

 one exceptional family in South America) and four out of 

 the five families of Polyprotodontia. Of the third sub-class, 

 Eutheria^ there are extremely few representatives. There 

 are seven genera of Rodentia (Muridae) and the dingo or 

 native dog, together with many bats, and a pig in New 

 Guinea. 



Notogoea is essentially a Prototherian and Metatherian 

 world. Here the Metatheria reach an extraordinary diversity 

 in structure and show adaptations closely resembling those 

 met with in the Eutheria elsewhere. The realm gradates to 

 the south-east of Asia by a series of islands of the Austro- 

 Malay region, and here the characters of Notogoea and 

 Arctogcea merge more or less sharply into each other. The 



