There is little need that we should give any extended 

 account of the groups of back-boned animals fishes, 

 amphibians, reptiles, birds and mammals. In water as 

 on land they are the largest of animals, and are all 

 familiar. The water- dwellers among them, excepting 

 the fishes and a very few others, are air-breath- 

 ing forms that are mainly descended from a terrestrial 

 ancestry. They haunt the water-side and enter the 

 shoals to forage or to escape enemies, but they cannot 

 remain submerged, for they have need of air to breathe. 



The fishes have remained strictly aquatic. They 

 dominate the open waters of the larger lakes and streams. 

 They have multiplied and differentiated and become 

 adapted to every sort of situation where there is water 

 of depth and permanence sufficient for their mainten- 

 ance. They outnumber in species every other verte- 

 brate group. 



Within the water the worst enemies of fishes are 

 other fishes; for the group is mainly carnivorous, and 

 big fishes are given to eating little ones. Hence, tho 

 all can swim, few of them do swim in the open waters,. 

 and these only when well grown. Those that so expose 

 themselves must be fleet enough to escape enemies, or 

 powerful enough to fight them. Little fishes and the 

 greater number of mature fishes keep more or less 

 closely to the shelter of shores and vegetation. The 

 accompanying diagram, based on Hankinson's (08) 

 studies at Walnut Lake, Michigan, represents the 

 distribution of fishes in a rather simple case. The 

 thirty-one species here present range in adult size 

 from the pike which attains a length above three feet, 

 to the least darter which reaches a length of scarcely an 

 inch and a half. One species only, the whitefish, 



231 



