THE FRESH WATERS 137 
of pressure for every 5 fathoms, and there are 
seals in its waters. There are shallow ponds 
of all sizes which vary greatly in temperature 
from day to night, and from season to season. 
They may bear a foot of ice in the depths of 
winter, and be dried up altogether in the heat 
of summer. Yet year after year these shallow 
ponds show an abundance of life. It may be 
noticed that the strict difference between a 
pond and a lake is not in size, for a pond may 
be a mile long, but in depth, for a true pond 
is always shallow. Then there are the lonely 
mountain tarns with their dark, mysterious 
waters and a rather sparse animal population; 
there are great rivers and purling brooks, 
swift torrents and sluggish streams with little 
fall; there are marshes grading into the shore, 
and others passing insensibly into dry land. 
There are also artificial fresh waters, as in 
canal and quarryhole. There is a consider- 
able fauna in the water-supply of some cities. 
SIMILAR ANIMALS IN WIDELY 
SEPARATED PLACES 
A striking feature about the fresh-water ani- 
mals is that they are often the same or nearly 
