476 LAND ANIMALS 



each of these regions, but the hippopotamus in Africa, tapir, capybara, 

 and otter in South America, and the smaller but vastly abundant musk- 

 rat in the Mississippi are reminders that mammalian life is also 

 important. Reptiles, conspicuously crocodilians, are well represented 

 in this river-marsh habitat. Their enormous concentration in the lower 

 Amazon is familiar through the fine picture in Brehm. 6 On the Marajo 

 Island, in the mouths of the Amazon, regular drives are made against 

 the local crocodilians (two species of caiman), in which thousands of 

 individuals are killed. Amphibians are abundant, but except in the 

 southern United States with its remarkable salamanders, they tend to 

 be reduced in variety. 



Floating vegetation extends outward from the shores in tropical 

 rivers, much as bog mosses do in the north, but here the mats become 

 much more extensive and may choke the entire river course, as on 

 the upper Paraguay, where the floating plants include especially a 

 large species of grass, with finger-thick stems, and great amounts of 

 water hyacinth. These mats of vegetation form floating islands when 

 detached, and may remain intact until they reach the sea, One of the 

 most notable of the adaptations of air-breathing animals to floating 

 vegetation is that of the jaganas, whose elongate toes distribute their 

 weight. They are represented by the genus Jacana (Fig. 122) in South 

 America, Actophilornis in Africa, and Hydrophasianus from south Asia 

 to New Guinea. 



Bottom lands, subject to seasonal overflow, offer peculiar environ- 

 mental conditions. Many animals are excluded from such areas either 

 completely or seasonally. These overflow areas may be vast, as in the 

 Amazon basin, which floods more than 500,000 square miles. Fishes 

 and other aquatic animals are left behind in ox-bows and other depres- 

 sions with the fall of the water, and may then become so concentrated 

 as to afford an effortless source of food to their enemies. The birds and 

 mammals and snakes and turtles may then become seasonally abundant. 

 In the Amazon region, the majority of the water birds take advantage 

 of this favorable period for their breeding season. 11 



Tundra.— The lakes and marshes and bogs of the tundra with 

 their extremely characteristic faunal picture cover a vast extent in 

 both North America and Eurasia. The frozen subsoil prevents the 

 escape of water by seepage, so that the melting ice and snow of winter 

 produce innumerable lakes and ponds, while spagnum bog covers every 

 level stretch not occupied by open water. The vast bulk of the life of 

 the tundra consists of invertebrate forms which flourish only in summer, 

 and of migratory birds, among which the water birds are by far the 



