THE FRESH WATERS 157 



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 become 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 

 animal, ageing animal, and to think of it as a 



