I026 FRESH-WATER BIOLOGY 



fauna. Birds derive their food from the water. The few that, as 

 carrion, serve as food for other aquatic animals or that may be 

 captured by fish, otter, or alligator are a negligible quantity. 



Reptiles 



Reptiles, Kke mammals, are shallow-water and littoral forms, 

 largely in summer. As with mammals, a gradual gradation is 

 found from species Hving exclusively on land, — like the turtles and 

 snakes of the Mojave desert or the land tortoise and green snakes 

 of the Mississippi valley, — through those which do not ordinarily go 

 into the water but will enter it without hesitation if circumstances 

 demand, — like the black snake and garter snake, — to such as the 

 water snakes, leather snakes, geographic turtle, painted turtle, 

 and snapping turtle that bask on the margins of lakes and streams 

 but take to the water for food or at the sHghtest sign of danger; 

 and lastly, to the alligators, musk turtles, and soft-shelled turtles 

 which give the final gradation to adaptations for Hfe in water. Of 

 these, the soft-shelled turtle, which can utilize the oxygen dis- 

 solved in the water, has probably reached the highest adjustment 

 to aquatic existence. But no hard and fast line can be drawn. 

 The habits of different species overlap so neatly that one finds a 

 shading from reptiles with a purely aquatic range to those with an 

 entirely terrestrial range. All of them have their homes on land 

 in so far as they have homes at all. Some secure only a part, 

 others all of their food from the water. Of those that obtain it 

 from the water some feed on fishes (purely aquatic food); others 

 like the alligators, which catch water birds, utiKze the water to 

 secure terrestrial visitors in part. Others may seek both sorts 

 of food. Snakes take to the margin of water in part for fishes, 

 in part for frogs, etc. 



The water snakes give birth to living yoimg. Since the yoimg 

 may be liberated in the water these snakes, in one sense, are the 

 most aquatic of the reptiles. But since they cannot utilize the 

 oxygen in the water the soft-shelled turtles exceed them in adap- 

 tation to an aquatic existence in this respect. All the turtles, as 

 well as the alligators, are compelled to make their homes or nests on 



