LAKES AND TARNS 137 



to the picture. A lake often seems very empty of 

 life, but we have to remember that besides those we 

 look for often in vain in the shallows, there is a 

 population in the open water and another on the dark 

 or vaguely lighted floor. 



Where did these lake animals come from? The 

 answer is threefold, (i) Some are the descendants of 

 forms that migrated gradually from the sea or were 

 transported step by step. This is doubtless true of 

 the freshwater sponges and the freshwater polyps, for 

 there are only a very few different kinds in freshwater 

 and hundreds of different kinds in the sea. The 

 Common Eel comes from the sea and returns to the 

 sea in its individual lifetime, but it does not come back 

 to the freshwaters a second time. (2) It may be that 

 some lakes are dwindling remnants of ancient seas 

 relict seas in short or are connected with relict seas 

 by changes of land-level; and that some lake animals 

 are the transformed descendants of original marine 

 animals. (3) Of many lacustrine animals it may be 

 safely said that they are terrestrial creatures which 

 have taken secondarily to aquatic life. This must be 

 true of the Water Beetles and of the Water Spiders. 

 Finally, when a freshwater population is established 

 in any basin the minute eggs or embryos may be 

 spread to others on the feet of water birds, by the 

 wind, and by slow changes in surface relief. 



A tarn is a small upland loch, deeper than a mere 

 pond. It is often surrounded by peat and fringed 

 with rushes; its water is often very brown, which 

 means that it is rich in humic acid dissolved out from 

 decaying plant remains. 



There are some very interesting plants in the water 



