which he has little defence 
     
     when out of the water. Even in 
     
     theabsence of predators, 
     
     beavers require water, for 
     
     their whole system of transport 
     
     and storage of winter food 
     
     depends upon it. Beavers are 
     
     rodents, but they have evolved 
     
     distinctly aquatic features 
     
     such as webbed feet and a 
     
     rudder-like tail. 
     
    
   
   
    
     Polar bears show little 
     
     exterior difference from 
     
    

    
     other bears, yet they are 
     
     almost completely marine in 
     
     their habits. Seals are their 
     
     major prey, and they spend 
     
     most of the year hunting them 
     
     on and among ice floes. Polar 
     
     bears are even able to repro- 
     
     duce away from land; females 
     
     require an encloSed den in 
     
     which to bear their cubs, and 
     
     this need is often met by 
     
     cavities among ice pressure 
     
     ridges. 
     
    
   
   
    
     Seals represent the next advance 
     
     in aquatic evolution, since 
     
     their locomotion is basically 
     
     adapted to swimming rather than 
     
     walking. True seals, including 
     
     our own gray and harbour seals, 
     
     use only their hind flippers 
     
    

    
     for propulsion, while steering 
     
     and braking with their front 
     
     flippers. DeSpite the extensive 
     
     modifications to their limbs, 
     
    
   
   
    
     seals are able to hump along on dry 
     
     land with surprising agility. Being 
     
     able to move on a solid surface is 
     
     necessary for seals, since whelping 
     
     takes place either on ice floes or 
     
     on land. 
     
    
   
   
    
     The final step in the march to the 
     
     sea has been taken by the whales, 
     
     which are not only completely at 
     
     home in the water, but absolutely 
     
     helpless outside it. The front 
     
     limbs of whales are modified as 
     
     flippers, and the hind limbs have 
     
     degenerated into a few bone 
     
     fragments that are not even 
     
     attached to the spinal column. 
     
     Propulsion is provided by the 
     
     long tail with its horizontal 
     
    
   
   
    
     flukes, which moves the fish- 
     
     shaped body with up-and-down 
     
     strokes. Whales keep warm in 
     
     cold water not by fur, but 
     
     rather by their thick blubber 
     
     layer. Being unable to leave 
     
     the water, whales of course bear 
     
     their young directly into the 
     
     water. 
     
    
   
   
    
     None of the animals considered 
     
     above can be considered to have 
     
     “descended” from the previous 
     
     group with less-developed 
     
     aquatic adaptations, but rather 
     
     each represents the culmination 
     
     of an independent line of evol- 
     
     ution. 
     
    
   
   
    
     Although the great sea monsters of 
     
     the age of dinosaurs have long 
     
     since disappeared, sea turtles 
     
    

    
     and diving iguanas still remind 
     
    

    
     us that reptiles too have 
     
    

    
     followed the call back to the 
     
    

    
     sea. But present day marine 
     
     reptiles still have one terrestrial 
     
     tie that is shared by marine birds 
     
     as well: they must return to the 
     
     land to reproduce. Although many 
     
     seabirds spend the majority of 
     
     their lives at sea, none has yet 
     
     learned to incubate an egg on the 
     
     water. Only those Antarctic 
     
     penguins which breed on ice 
     
     shelves can claim to complete 
     
     their life cycle without ever 
     
     touching dry land. 
     
    
   
   
    
     David Cairns 
     
     60 Newland Crescent 
     
     Charlottetown, P.E.I. 
     
    
   
   
    
       
     
    
   
   
    
     Conserve water 
     
    
   
   
    
     Beavers make many small dams, each based 
     
     on hydrological principles and maintained 
     
     regularly so that they hold back water 
     
     while allowing excess to seep through or 
     
     flow over. Water impounded behind their 
     
     dams in rainy times is held in reserve for 
     
     times when there is no rain.