CHAPTER XXIII 

 ANIMAL LIFE OF SWAMPS AND OF SHORES 



Swamps and marshes, the great northern tundra in summer, river 

 banks, bottom land, and the borders of lakes and seas, contrast radically 

 in their environmental conditions for animal life with those of the dry 

 steppe. Of all other types of environment, only the tundra, known some- 

 times as the "moss steppe," extends over areas comparable with those 

 where grassland or savanna conditions prevail. The borders of streams 

 and bodies of standing water, though also of great extent, are essen- 

 tially linear. Tree growth may be present, as in the cedar-bordered 

 lakes of the northern United States or the mangrove formations of 

 the tropics; forest may be replaced by bushy vegetation, but is more 

 frequently absent. Even where forest growth is present, free space 

 both within the water and above it is available to the inhabitants of 

 the shores. 



The presence of water governs the composition of the swamp and 

 shore fauna, for its constant abundance of life affords a sure food 

 supply, which attracts a great number of air-breathing animals to these 

 situations. 



These air-breathers must be to some extent aquatic, and usually are 

 able to swim. They exhibit a transitional stage between the complete 

 return to the water of the secondarily aquatic forms, like whales, 

 ichthyosaurs, or sea snakes, and land animals proper. This amphibious 

 fauna is dominated with respect to both quantity and composition by 

 the seasonal periodicity of its habitat. In the polar and temperate zones 

 the water freezes for a varying period in winter, and in the tropics and 

 subtropics, swamps and even rives may dry out completely. This con- 

 dition forces a periodicity on the fauna as well, and it is composed of 

 wandering or migratory forms or of forms capable of hibernation or 

 aestivation. 



Amphibious habits among animals. — All the phyla of land ani- 

 mals include amphibious forms. They are fewest among the snails, 

 though not entirely wanting. The species of the genus Succinea live in 

 both fresh water and on land, and Assiminea is found in salt marshes 

 along the seacoast both above and below the water surface. 



Amphibious forms are numerous among insects. Many orders have 



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