THE CRAYFISH 129 



a collection of strange animals. The American leaves behind 

 him such wild animals of the woodland as the Virginia tleer, 

 the skunk, the woodchuck, porcupine, and opossum, and, if he 

 goes to Europe, may see instead the roebuck, the polecat, 

 and the hedgehog, but no representative of the group to which 

 the opossum belongs. If he goes to Africa, his familiar forms 

 will be replaced by still stranger ones. Instead of the deer he 

 will meet with antelopes and a lot of new groups represented 

 by the elephant, rhinoceros, and hippopotamus. If he journeys 

 to Australia, he will find that practicallj' all the wild animals 

 belong to the same group as does our opossum. The deer are 

 there represented Ijjr kangaroos and the porcupine by the 

 wombats. This dissimilarity even of the land quadrupeds in 

 different parts of the globe has made it possible to divide the 

 land into certain life-regions. Thus we have the North Amer- 

 ican ; the Eurasian (i.e. Europe + northern and central Asia) ; 

 the Oriental, south of the Himalayas ; the Ethiopian, including 

 that part of Africa south of the Sahara ; the South American 

 which stretches north into Mexico, and, finally, the Austra- 

 lian (Fig. 129). 



These different hfe-regions are land areas more or less com- 

 pletely separated from each other by barriers of some kind, 

 such as water, mountain chains, vast desert tracts, or sudden 

 changes of temperature. In each of these isolated regions 

 a peculiar and characteristic collection of animals, known as its 

 fauna, has come to develop. The faunas of some of these re- 

 gions are more closely interrelated than those of others, and this 

 interrelation often gives an important clew to land connections 

 in former geologic times. Thus the North American animals 

 are much more closely related to those of Eurasia than are the 

 African. This similaritj' of animal forms speaks for a former 



