ZOOGEOGRAPHICAL REGIONS 209 
regions, kangaroos are abundant, and are for the most 
part savana animals. But where tropical forests occur, 
as in North Queensland and New Guinea, there certain 
forms have taken to the arboreal life, and have thus 
protected themselves from the competition of ground- 
living forms no less than from the attacks of some 
possible enemies. 
The same statements might be made of desert 
animals, of savana animals, or, generally, of any 
forms inhabiting a natural region. We may say, then, 
that when any stock of terrestrial animals reaches a 
new region, and is there isolated from any cause, that 
stock will tend to give rise to specialized groups, more 
or less perfectly adapted to all the possible varieties 
of habitat which the region affords. In order that 
a particular stock may give rise by specialization and 
differentiation to many such groups, some degree of 
isolation seems to be necessary, for otherwise new 
incoming stocks may colonize the vacant places before 
the first stock has had time to become differentiated. 
For example, in Australia almost all habitats suited 
to terrestrial mammalia are occupied by marsupial 
animals. Living marsupials also occur in South 
America, but there they are represented by a very 
limited number of types, because they have had to 
face the competition of placentals, practically absent 
in Australia. Similarly, in Madagascar the lemurs are 
very numerous and very diverse, being adapted for 
different modes of life. Lemurs also occur in Africa 
and India, but there, where they have to face the com- 
petition of the more intelligent monkeys, and the 
attacks of more powerful carnivores than any which 
are found in Madagascar, they are few in number, 
small in size, and less diverse. 
1404 fe) 
