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 
tivers, 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 
