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 aU the possible varieties 

 of habitat which the region affords. In order that 

 a particular stock may give rise by speciahzation 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 mammaha 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 Austraha. 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 th« 

 attacks of more powerful carnivores than any which 

 are found in Madagascar, they are few in number, 

 small in size, and less diverse. 



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