CHAPTER XXII 

 THE ANIMAL POPULATION OF DRY, OPEN LANDS 



Open country constrasts sharply with the forest. The extensive, 

 sunny, and windy open lands varying from savanna to desert, either 

 level or rolling, often with sharp and wide fluctuations of temperature 

 and humidity, compose one of the major types of terrestrial environ- 

 ment. Air and soil moisture show various gradations in open countries 

 from relatively moist regions such as the savannas of the Congo or 

 the Guianas, the campos of Brazil, and the grassy openings of the 

 Sunda Islands, to steppes of all kinds, and these in turn pass by gradual 

 stages into semi-deserts and finally into deserts proper. Though the 

 zone above the tree limit on high mountains belongs to the category of 

 open land, it offers such an abundance of climatic and topographic 

 peculiarities due to its elevation above sea level that it requires special 

 treatment (Chapter XXIV). Swamps and marshes, including the 

 tundra, and also river, lake, and ocean shores, are characterized by 

 their large amount of moisture, and by the proximity of more or less 

 wide expanses of water surface, and will be treated as distinct types of 

 environment (Chapter XXIII). The icy barren ground of the polar 

 zones will also be treated in a special chapter. But, although we com- 

 bine in the present chapter the consideration of such dissimilar en- 

 vironments as tropical savanna and barren desert, their animal life 

 exhibits similarities and adaptations which are explained by the com- 

 mon environmental factors. 



Water relations. — The forest requires a certain minimum moisture 

 content in soil and air for its maintenance. Where this is not reached, 

 no large areas of forest exist. More or less scattered groups of trees or 

 individual trees may occur in transition areas, especially along water- 

 courses; but the ecological peculiarities of the forest depend upon its 

 continuity over wide areas. When sunlight and air currents are no 

 longer excluded, all the phenomena appear that depend upon their 

 fluctuations. The reduced moisture in the grasslands, which must be 

 regarded as one of the main causes of the disappearance of the forest, 

 and the irregular distribution of rainfall during the course of the year, 

 are of importance to the associated animal life. Relatively slight 

 precipitation is especially characteristic of extensive plains. These 



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