CHAPTER X 
ZOOGEOGRAPHICAL REGIONS 
WE have now surveyed those natural regions of the 
earth described in the Introduction, and have noted 
their more important occupants, and the adaptations 
which these show to their particular habitats. In the 
course of these descriptions it has become obvious that 
what may be called the raw material upon which 
adaptation has worked is not the same in all parts of 
the globe. We have found that animals inhabiting 
dense forest generally show certain arboreal adaptations, 
such as the prehensile tail, the opposable thumb and 
great toe, and so forth, found wherever the forest 
occurs. But in New Guinea, for example, certain 
kangaroos have taken to the trees and become arboreal ; 
in Madagascar lemurs swarm in the forests ; in South 
America particular kinds of monkeys, the opossums, 
and the sloths, as well as other animals, people the 
selvas, and so on. In other words, each isolated forest, 
or other area, has its own types of adapted animals. 
The only possible explanation of these conditions is 
the hypothesis that the specialized forms in each 
separate region have been evolved from pre-existing 
unspecialized forms inhabiting the region. In South 
America we find fossil representatives of ground sloths. 
These have long since become extinct, and the exist- 
ing sloths have apparently been saved from a like 
fate by the acquisition of marked arboreal characters, 
which protect them from many possible enemies. In 
the continent of Australia, and parts of the adjacent 
