THE FRESH WATERS 139 



The first and most important of these was 

 the power to endure slight changes in the 

 degree of saltness. This power would be 

 found most frequently in animals that lived 

 'in the shore area, for there such changes 

 occur very often. Heavy rain falling into the 

 smaller pools may make them comparatively 

 fresh, and will also affect the shallow water 

 of the sea itself, though not to the same de- 

 gree. About the mouths of streams and 

 rivers, too, the water is fresher than else- 

 where, and the tides carry up so much salt 

 water that the estuaries are salt, or at least 

 brackish, for a long way up, and only very 

 gradually become quite fresh. 



It was, therefore, probably by this route 

 that the rivers and lakes got a great part of 

 their inhabitants. We can easily picture some 

 of the more adventurous of the shore animals 

 making their way slowly up the river mouths 

 until not in a single lifetime, let us remem- 

 ber, but in the course of many generations 

 they got beyond the influence of the tide alto- 

 gether, and settled down in fresh water. 



The move seems to have been so successful, 

 in some cases at least, that the enterprising 

 colonists increased abundantly, and some of 



