CHAPTER XXX 

 THE AQUATIC VERTEBRATES 



By C. H. EIGENMANN 



Professor of Zoology in Indiana University, and Curator of Fishes in the Carnegie Museum, Piiisburgh, Pa. 



INTRODUCTORY 



The chief object in the life of any animal is to leave another 

 like it in its place when it dies. To accomplish this object it must 

 find a range in which it may secure its food and itself escape be- 

 coming food; it must secure a mate and a home in which its young 

 may be reared to the point of self-dependence. The world con- 

 tains a great variety of animals adapted to all possible environ- 

 ments. Either the greatly diverse characters of these animals 

 have arisen to adapt them to their different ranges and homes, or 

 the greatly diverse environments have been selected because they 

 were adapted to the otherwise and elsewhere acquired characters 

 of different animals. Certainly when new water or land areas arise 

 the latter will be the origin of its adopted fauna. 



The vast territory containing the majority of the innumerable 

 lakes and streams with whose fauna and flora we are concerned, 

 extending from the Arctic regions south to the region of the Ohio 

 River, was a few thousand years ago covered with a continuous 

 sheet of ice. The fauna and flora of this area are composed 

 of immigrants, of animals and plants that moved in as the ice 

 moved out and selected the places adapted to their requirements. 

 While no doubt many of them have become modified since their 

 advent into this area, there can be no doubt that their fundamental 

 adaptations were elsewhere acquired and that in their case it has 

 been a selection of environments to suit these adaptations. 



Fresh waters may be and are used first, for ranges; second, for 

 homes; or third, for both purposes by various animals. One finds 

 animals which breed on land but are adapted to utilize fresh waters 



