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 (T'ropidonotus 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 live in the water and breathe by gills, 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 
