474 LAND ANIMALS 



adopted an almost exclusive fish diet and capture their prey by vertical 

 diving. The same is true of the osprey, which is so successful a fisher- 

 man that the American bald eagle robs it systematically. 5 Swallows 

 feed on the adults of aquatic insects, finding free space for their evolu- 

 tions over the water. The bank swallow (Riparia riparia) nests in 

 colonies in holes in vertical banks and cliffs along lakes or rivers. 

 Brehm reports colonies of this bird with thousands of nests on the Obi. 

 Bee eaters frequent watercourses solely on account of a similar nesting 

 habit. Numerous perching birds such as the red-winged blackbird and 

 marsh wrens frequent cattail marshes and nest in them. 



The semi-aquatic habitat has attracted a great variety of mam- 

 mals. The duckbill of Australia is notably aquatic, with completely 

 webbed feet and close underfur; it nests in burrows with an under- 

 water entrance. The marsupials have an aquatic representative in the 

 similarly web-footed water opossum (Chironectes) of South America. 

 Numerous rodents frequent streams, lakes, and marshes, notably the 

 muskrat and beaver in the north, and the coypu and capybara in South 

 America. The aquatic mammals not infrequently have the tails flattened 

 from side to side as a swimming organ, in such diverse stocks as the 

 muskrat and the African insectivore Potamogale. Such flattening is 

 ridiculously conspicuous also in the tail of the hippopotamus. A whole 

 series of mustelids illustrates various degrees of adaptation to swim- 

 ming habits — from the riparian and frog-eating Putorius putorius, to 

 the almost pelagic marine sea otter. The beaver is unique among aquatic 

 animals in the successful control of water level to suit its mode of 

 life. The seals, among Carnivora, present still more complete adapta- 

 tion to marine habits, coming ashore only for rest and reproduction. 



Swamps and stream borders attract a number of ungulates. These 

 usually have wide spreading hoofs, for support on soft ground. A partial 

 list of such animals includes the hippopotamus, tapir, various pigs, 

 African antelopes of the genera Cobus, Limnotragus, and Tragelaphus, 

 the northern reindeer and moose, and the water buffalo of the orient. 



Swamps and marshes. — Swamps and marshes, in topographic 

 succession, represent a transitional stage between open water and dry 

 land. The evanescent character of lakes in general has already been 

 emphasized (p. 86). Abundance of lakes characterizes the North Tem- 

 perate Zone, primarily by reason of the recent glaciation. These lakes 

 are frequently surrounded by a narrow ring of open sphagnum bog; 

 tamarack or spruce and various other trees rapidly encroach on the 

 bog zone as it gains in firmness. Great stretches of older swamp are 

 covered with dense stands of timber — tamarack, spruce, and white 

 cedar, with a substratum of sphagnum. These forested swamps are no 



