1 40 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 



