138 THE HAUNTS OF LIFE 



the same in widely separated basins. A lake 

 in the Scottish Highlands, one of the thou- 

 sand lakes of Finland, a lake in Japan, may 

 have similar tenants. Why is this? It is partly 

 because water-birds carry the same small ani- 

 mals on their feet, or in clodlets on their feet, 

 from one lakeside to another, because the 

 wind sometimes does the same, and because 

 changes in the surface-relief of the earth's 

 crust not only make valleys separate from one 

 another, but bring them together again. But 

 the most important reason is probably that the 

 animals which colonised the fresh waters came 

 for the most part from the shore, and that only 

 certain kinds of constitution could stand the 

 change. Let us think for a little what the 

 change from the shore to the fresh waters 

 would mean, always bearing in mind that it 

 would be a very slow and not a sudden 

 change, for most salt-water animals die im- 

 mediately if they are put into fresh water. 



FROM SALT WATER TO FRESH 



What characters or qualifications were 

 necessary before the transition from salt 

 water to fresh water could be even attempted? 



