THE FRESH WATERS 137 



atmosphere of pressure for every 5 fathoms, and 

 there are seals in its waters. There are shallow 

 ponds of all sizes which vary greatly in tem- 

 perature 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 

 animals is that they are often the same or 



