THE FRESH WATERS 157 
the blanket of ice thickens, and this tends to 
prevent the water of the pond from becoming 
colder and colder and eventually solidifying. 
For eighty-five days in the year—the winter 
season—the warmer water of the fresh-water 
basin is at the bottom; the pool does not be- 
come solid ice, except in very rare cases; the 
fresh-water animals are able to continue year 
in, year out, and from this many consequences 
flow. 
THE DANGER OF FLOOD 
Another great risk—in streams, especially 
—is that of being washed down to the sea, or 
carried out into a flood-bed and left high and 
dry, or in stagnancy. We can understand, 
then, why many fresh-water animals, such as 
brook-leeches and insect-larve, have gripping 
organs or suckers which anchor them. 
But another method of circumventing the 
danger of being washed away is to shorten 
down the juvenile stages of the life-history, 
when the risks are greatest. Itis useful to think 
of an animal’s life-history as a whole—egg, 
embryo, larva (if there is such a stage), young 
creature, adolescent animal, full-grown ani- 
mal, ageing animal, and to think of it as a band, 
