48 



and temperature limit the sea forms. The sea and 

 water falls are among the barriers to fresh water 

 forms. 



The way the brown rat, introduced into this coun- 

 try about 1775, crowded out the black rat, which in 

 turn had, when introduced in 1544, crowded out the 

 native rats gives a good illustration of species unable 

 to hold their ground. The English sparrow is fur- 

 nishing us other examples. 



Many islands furnish examples of the operation of 

 the third law. 



As a result of the operation of these laws through 

 past time we find the present distribution of animals 

 such that it is possible to divide the earths surface 

 up into three realms: The Australian ? comprising 

 Australia ? Tasmania, New Zealand. New Guinea, 

 and the neighboring islands; (2) the South American 

 realm, comprising South America, the West Indies, 

 and North America to the frost line; (3) the North- 

 ern realm, divisible into four provinces: North Amer- 

 ican, including the rest of North America; African, 

 including Africa south of the Desert of Sahara, south- 

 ern Arabia, and Madagascar, though the last is some- 

 times set off by itself ; the Oriental, comprising Asia 

 south of the Himalaya and Sulaiman mountains, to- 

 gether with the neighboring islands; and the Eura- 

 sian, including all Europe ? and the rest of Asia and 

 Africa. 



