' . '-. THE DISTRIBUTION OF ANIMALS 441 



from that of Indiana or Iowa, because there are no barriers 

 between the States, and they are alike physically. On 

 the other hand the fauna of California differs much from 

 that of the Eastern States because of the great barriers 

 (the desert and the Sierra Nevada Mountains) which lie 

 between it and these States, and because of the great 

 differences in the physical and climatic conditions of the 

 two regions. 



The land-surface of the earth has been divided by 

 zoogeographers into seven great realms of animal life, 

 based on the distributional characters shown by these 

 various regions. These realms are separated by barriers 

 of which the chief are the presence of the sea and the 

 occurrence of frost. These realms are named, from their 

 geographical region, the Arctic, the North Temperate, 

 the South American, the Indo- African, the Madagascar, 

 the Patagonian, and the Australian. Of these the 

 Australian alone is sharply defined. Most of the others 

 are surrounded by a broad fringe of debatable ground 

 that forms a transition to some other zone. 



Habitat and species. The habitat of a species of 

 animal is the region in which it is found in a state of 

 nature. It is currently believed that the habitat of any 

 animal is the whole of that region for which it is best 

 adapted. But this is not necessarily true. In fact in 

 most cases it is not true. The trout naturally debarred 

 from the rivers in Yellowstone Park by the waterfalls 

 could live there well if the barrier could be passed. In 

 the case of one stream it has been passed and the trout 

 flourish above the fall. The success of the black and 

 brown rats and the English sparrow in America, of the 

 rabbit in Australia, of bumblebees and house-flies in New 

 Zealand, all of which animals had a natural habitat not 

 including these regions, but by artificial means have been 

 carried over the barriers and into die new territory, prove 



