140 THE HAUNTS OF LIFE 
them have survived even though all their 
nearest relatives in the sea have disappeared. 
Others again, after long, long ages, seem 
never to have become quite at home in fresh 
water, but have to go back periodically to 
their original home in the sea to deposit their 
eggs, so that the change from salt water to 
fresh has to be made by every individual in 
its own lifetime. The eel is one of these, and 
its life-story is so interesting that we shall fol- 
low it in detail later on. 
Another qualification necessary for migrating 
up the rivers was one which nearly all fresh- 
water animals must possess—the power of en- 
during considerable changes of temperature. 
This power, too, would most frequently be 
found among the shore animals, for, as we have 
seen, those living in the open sea have only to 
sink beneath the surface to protect themselves 
from sudden changes, while in the deep sea the 
temperature remains always about the same. 
ORIGIN OF FRESH-WATER ANIMALS 
But there is a previous question: Why do 
we think that fresh-water animals must have 
come from the sea? May they not have begun 
