4. A fourth haunt of life is that of the freshwaters, including river
and lake, pond and pool, swamp and marsh. It may have been colonised by
gradual migration up estuaries and rivers, or by more direct passage
from the seashore into the brackish swamp. Or it may have been in some
cases that partially landlocked corners of ancient seas became gradually
turned into freshwater basins. The animal population of the freshwaters
is very representative, and is diversely adapted to meet the
characteristic contingencies--the risk of being dried up, the risk of
being frozen hard in winter, and the risk of being left high and dry
after floods or of being swept down to the sea.