I02 2 FRESH-WATER BIOLOGY 



daily or seasonally for ranges; others that range on land but visit 

 the water during breeding seasons to make therein their homes and 

 to enable their young to grow up in it. Still other animals utiHze 

 fresh waters both as a range and a home, — rarely, or never, leave 

 it or even are incapable of leaving it. Roughly speaking, mam- 

 mals, birds, and reptiles, in so far as they are aquatic, belong to 

 the first of this ecological group. Batrachians belong to the sec- 

 ond, a few batrachians and all fishes to the third. 



The first of these groups is composed of more or less perfect 

 readaptations of land animals to water. The second is composed 

 of originally aquatic animals as yet imperfectly adapted to the 

 land, while the members of the third group are, and at all times 

 have been, the aquatic animals par excellence. While the visitors 

 or inhabitants of fresh water may be sharply distinguished from 

 the non-aquatic, the relations and adaptations of aquatic animals 

 to the different regions of the water are very diverse. 



Mammals 



The aquatic mammals are but imperfectly adjusted to some 

 part of the aquatic habitat and confine themselves to shallow 

 water and the shore. None of them could five in an enclosed space 

 filled with water. The number of truly aquatic mammals is small. 

 Most mammals only visit the water to drink. Others, as the moose, 

 seek the water to browse on the marginal vegetation or to escape 

 enemies. Others less inclined to enter water secure part of their 

 food from it. The raccoon fishes along the margins of streams 

 for crayfishes. A dexterous tomcat, proverbially wary of wetting 

 his feet, one memorable night neatly cleaned out two aquaria, one 

 stocked with rare blind fishes and one with still rarer axolotls. 

 None of the above dive. 



The mink secures most of its food on land but it catches both 

 fishes and muskrats in the water into which it does not hesitate 

 to dive to escape an enemy or to secure food. 



The more distinctly aquatic mammals are the star-nosed mole, 

 the muskrat, the beaver, and the otter. All of these use the water 

 as a range, making their homes in very close proximity to the 

 water if not actually in it. Of these the otter is a carnivore, the 



