194 THE ANIMALS OF LAKES AND RIVERS 



it leaves the water, should want of food or any 

 other cause render this necessary, and is capable of 

 (awkward) progression on land, in a way that would 

 be impossible for a marine turtle. In Central and 

 Southern Europe the ponds and swamps are inhabited 

 by the pretty little pond tortoise {Emys europea), which 

 has also webbed feet. These little creatures are fond 

 of coming out of the water to bask, and migrate from 

 one pond to another if the first shows signs of drying up. 



The same tale of partial adaptation to aquatic life 

 can be told of the various kinds of crocodiles, which, 

 like the forms just mentioned, seem to seek the water 

 for the sake of the food to be obtained there. The 

 living crocodiles are still so far bound to the land that 

 they leave the water voluntarily, either to bask in the 

 sun, or in order to migrate from one region to another, 

 or to lay their eggs. 



None of the snakes show such definite adaptations 

 to life in fresh water as do the sea-snakes to life in the 

 sea, but certain forms enter the water freely in search 

 of food. This is especially true of the common grass- 

 snake {Tropidonotus natrix), which sometimes spends 

 the greater part of its life in water, in which it swims 

 easily. 



The amphibians show some very curious conditions 

 in connexion with adaptations to the aquatic life. The 

 normal condition, as already suggested, is that the 

 young Uve in the water and breathe by giUs, while the 

 adults live on land and breathe by lungs. But there 

 are some interesting exceptions. Throughout much of 

 North America, and especially in Mexico, there occurs 

 a salamander (Amblystoma tigrinum), which when adult 

 is normally terrestrial, except at the breeding-season, 

 and is of a brownish colour, with yellow spots and 



