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 



