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-larvae, 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. It is 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, 



