472 LAND ANIMALS 



Among reptiles numerous forms are semi-aquatic, notably all the 

 crocodilians and a majority of turtles, which reverse the life history of 

 frogs in that they spend their active life in the water but repair to the 

 land for egg-laying. They hibernate or aestivate in muddy ponds or 

 swamps. Among the few lizards which have adopted a riparian life, 

 the monitors of the East Indies and Africa, and the aquatic agamids 

 such as Hydrosaurus, with their close parallel Basiliscus among the 

 American iguanids, must be mentioned. Numerous snakes have become 

 closely confined to the neighborhood of water, notably the anaconda of 

 tropical America, the common water snakes (Natrix) of the North 

 Temperate Zone, and a great variety of harmless and back-fanged 

 snakes in the tropics. There are only a few aquatic water snakes among 

 the poisonous Elapidae (Limnonaja and Boulengerina in Africa), and 

 only the American moccasin (Agkistrodon piscivorous) among the 

 Crotalidae. The sea snakes {Hydrophiidae) have become completely 

 aquatic, and only the more primitive members of the family come 

 ashore at all. 



The semi-aquatic environment is favorable for birds. Their powers 

 of motion make them independent of seasonal variations in the water. 

 Whole orders of birds have arisen by adaptation to this habitat, and 

 these, together with isolated groups of more terrestrial orders, compose 

 the two ecological groups distinguished as swimming and wading birds. 



Birds find both food and protection in the water. All the animal 

 life of both marine and fresh water, except the largest forms, is sub- 

 jected to their toll. This food supply is for the most part abundant, 

 and the birds furthermore adapt themselves to its seasonal variation. 

 Their numbers stand in direct proportion to the available food supply. 

 Thousands upon thousands of birds are seen at such lakes as Lindu 1 

 in Celebes, Lake Chad 2 in Africa, or in the marsh areas of great rivers 

 such as the Paraguay or Nile, all of which are rich in invertebrates and 

 fishes. The water itself, the thick vegetation of marshes, and islands 

 and floating vegetation afford protection to birds, primarily from mam- 

 malian enemies, but also from birds of prey. Predators have, to be sure, 

 adapted themselves to the same habitat in pursuit of the food supply 

 offered, in their turn, by the birds. The shores of the Dead Sea, 3 by 

 contrast, are birdless wastes, while birds are abundant on the islands 

 and marshes of Great Salt Lake. 



The swimming birds characterized by some form of webbing of the 

 feet comprise eight orders. These are the penguins, loons, grebes, al- 

 batrosses and petrels, pelicans and their relatives, ducks and geese, 



