Adaptations to Environment 125 



that it has arisen by many contributions from the various shore- 

 haunts. Following the down-drifting food, many shore-animals 

 have in the course of many generations reached the world of 

 eternal night and winter, and become adapted to its strange 

 conditions. For the animals of the deep-sea are as fit, beautiful, 

 and vigorous as those elsewhere. There are no slums in Nature. 



IV. THE FRESH WATERS 



Of the whole earth's surface the freshwaters form a very 

 small fraction, about a hundredth, but they make up for their 

 smallness by their variety. We think of deep lake and shallow 

 pond, of the great river and the purling brook, of lagoon and 

 swamp, and more besides. There is a striking resemblance in the 

 animal population of widely separated freshwater basins: and 

 this is partly because birds carry many small creatures on their 

 muddy feet from one water-shed to another ; partly because some 

 of the freshwater animals are descended from types which make 

 their way from the sea and the seashore through estuaries and 

 marshes, and only certain kinds of constitution could survive the 

 migration; and partly because some lakes are landlocked dwind- 

 ling relics of ancient seas, and similar forms again would survive 

 the change. 



A typical assemblage of freshwater animals would include 

 many Protozoa, like Amoebse and the Bell- Animalcules, a repre- 

 sentative of one family of sponges (Spongillidae), the common 

 Hydra, many unsegmented worms (notably Planarians and 

 Nematodes), many Annelids related to the earthworms, many 

 crustaceans, insects, and mites, many bivalves and snails, various 

 fishes, a newt or two, perhaps a little mud-turtle or in warm coun- 

 tries a huge Crocodilian, various interesting birds like the water- 

 ouzel or dipper, and mammals like the water-vole and the water- 

 shrew. 



Freshwater animals have to face certain difficulties, the 

 greatest of which are drought, frost, and being washed away in 



