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? 
