PLANT MIGRATION. 499 



land area of the southern hemisphere is probably greater than 

 appears from the map, since it is believed that a great antarctic 

 continent underlies the ice area about the south pole, while it is 

 believed that an ice-covered polar sea lies at the north pole. 



964. Land hemisphere, or continental hemisphere. The 

 northern and southern hemispheres have their centres at the 

 poles. What is called the land, or continental, hemisphere is 

 that half of the globe which contains the largest possible area of 



Water Hemisphere. Land Hemisphere. 



Fig. 491- 



Water hemisphere and land hemisphere showing proximity of continents and 

 islands in the North Polar Zone. 



land, and has its centre in western Europe. It includes Europe, 

 Asia, Africa, North America, the northern portion of South 

 America, and the entire arctic region. It is interesting to note 

 this grouping of the continents over one half of the earth, as it 

 shows how near the areas for plant distribution are. 



965. The water hemisphere, or oceanic hemisphere. This 

 includes the southern part of South America, Australia, and the 

 East Indies, with numerous smaller islands. The land area is 

 small and the variations in climate, topography, etc., which 

 encourage variation and increase species are not so great as on 

 the continental hemisphere. The migration of species from one 

 island to another is not so easily accomplished because of the 

 greater distance, though the distance from one land area to an- 

 other is not more than about one hundred miles at any one point. 



