318 The Realm of Nature CHAP. 



406. Deserts. In many respects Plate XVIII. gives the 

 most interesting division of the world according to its vege- 

 tation. It shows three great barren zones forming broken 

 girdles round the Earth, and covering, according to Mr. 

 Ravenstein's calculation, more thari 4,000,000 square 

 miles. Ice deserts surround the north pole, and are suc- 

 ceeded in the north of Europe, Asia, and America by a 

 belt of frozen land called the Tundra, which thaws on the 

 surface in summer and supports a thin growth of moss and 

 stunted grass. Arid deserts occur in all areas of great heat 

 and very small rainfall. A northern zone includes the vast 

 Sahara, the interior of Arabia, and Central Asia, terminating 

 in the dreary Gobi, and the Great Basin of North America. 

 Horny cactuses, the saxaul with foliage like wire, and the 

 dull-gray sage-bush, are characteristic of the scanty plant 

 life. A similar set of smaller deserts appears in the southern 

 hemisphere, near the cooler but drier western sides of the 

 continents, the Kalahari in Africa, the great Victoria Desert 

 in Australia, and the small salt desert of Atacama in South 

 America, forming links in the chain. Solar energy here falls 

 on barren land, and, not being absorbed by plants, does the 

 work of heating air and maintaining the permanent winds of 

 the globe, which carry rain to more favoured regions. Thus 

 in a sense the existence of fertile lands is a consequence of 

 deserts. Treeless plains are common in all regions of scanty 

 rainfall and great range of temperature, such as the borders 

 of deserts. They occupy about 14,000,000 square miles of 

 surface, covered with rich grass during part of the year, 

 transformed into deserts of driving dust in the dry season, 

 and flooded or covered with snow, according to the climate, 

 during the rainy season or winter. The fertile prairies of 

 North America, the llanos and pampas of South America, 

 and the steppe-lands of Russia and Central Asia, are examples 

 of such semi-deserts. 



407. Tropical Forests. When the grassy plains sur- 

 rounding the tropical deserts on the equatorial side begin 

 to receive a larger rainfall, bushes first break their mono- 

 tony, and then great forests are formed, the trees standing 

 well apart, but growing closer as the heavy rains of the 



