CHAPTER VI 



AQUATIC SOCIETIE 



LIMNETI 

 SOCIETIE 



REAT bodies of water 

 furnish opportunity for 

 all the different lines 

 of adaptation discussed 

 in the preceding chap- 

 ter. The sun shines 

 full upon them in all its 

 life-giving power. The 

 rivers carry into them 

 the dissolved food sub- 

 stances from the land. 

 Wind and waves and 

 convection currents dis- 

 tribute these substances throughout their waters. 

 Both the energy and the food needed for the main- 

 tenance of life are everywhere present. Here are 

 expanses of open water for such organisms as can float 

 or swim. Here are shores for such as must find shelter 

 and resting places; shores bare and rocky; shores low 

 and sandy ; shores sheltered and muddy, with bordering 

 marshes and with inflowing streams. The character 

 of the population in any place is determined primarily 

 by the fitness of the organisms for the conditions they 

 have to meet in it. 



293 



