THE FRESH WATERS 145 



flies, whose larvae live in the fresh waters. But 

 no one can suppose that these insects, or any 

 insects, had their original home in water. The 

 explanation 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 

 larvae 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 duck- 

 weed, with roots, and bladderwort, without 



