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 degree. About 

 the mouths of streams and rivers, too, the 

 water is fresher than elsewhere, 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 remember, 

 but in the course of many generations they 

 got beyond the influence of the tide altogether, 

 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 



