OF MAMMALIA. 505 
Allotheria can be traced through the Jurassic and possibly 
into the Eocene (Tertiary). The Polyprotodontia can also 
be traced through Jurassic and Cretaceous into the Ter- 
tiary. Through the Tertiaries can be traced certain of the 
Metatheria (Didelphide) to the present day and the 
eutherian types first occur in the Eocene. In the Eocene 
strata there are abundant remains of many £utheria, 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. Notoca@a—comprising Australia, New Guinea, Poly- 
nesia and New Zealand and certain of the Malay 
Islands. 
2. NEoGa@Aa—comprising South America, West: Indies 
and part of Central America, 
3. ARcroc@Ha—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. 
1. Noroca@a.—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 AZefatheria, 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, 
Liutheria, there are extremely few representatives. There 
are seven genera of Kodentia (Muridz) and the dingo or 
native dog, together with many bats, and a pig in New 
Guinea. 7 
Notogcea is essentially a Prototherian and Metatherian 
world. Here the Me¢atheria reach an extraordinary diversity 
in structure and show adaptations closely resembling those 
met with in the Lutheria 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 Notogcea and 
Arctogcea merge more or less sharply into each other. The 
