THE FRESH WATERS 145 



whose larvse live in the fresh waters. But no 

 one can suppose that these insects, or any insects, 

 had their original home in water. The explana- 

 tion is that when an animal lives in a haunt full of 

 dangers for the young, it has often circumvented 

 the difficulty by finding another haunt for the 

 juvenile stages. The aquatic larvse of insects 

 are not old fashioned ; they show new-fashioned 

 fitnesses to a haunt which is really rather foreign 

 to the insect's nature. 



CIRCULATION OF MATTER IN THE 

 FRESH WATERS 



To understand the animal life of a lake or 

 pond we must as usual start with the plants. 

 For the plants, which are able to feed upon 

 the not-living, supply food for the animals which 

 feed upon the living, or what has been living, 

 or what has been made by something living. 

 There are many fresh-water plants growing 

 round the margin, like bog-bean, mare's tail, 

 iris, and bullrush ; others, like water-lilies, are 

 rooted at a considerable depth, and send their 

 leaves and flowers on long stalks up to the 

 surface ; others, like duckweed, with roots, and 

 bladderwort, without roots, float freely. Now 



10 



