INTRODUCTION II 



other fresh-water forms and closely related to marine animals. 

 This fauna is often regarded as the survival from a period when 

 connections with the ocean were more immediate, or when climatic 

 conditions were different as during a glacial epoch. 



The poverty of fresh-water life in variety and number of types 

 in comparison with that of the sea has often been emphasized. 

 Experimental data show it can hardly be due to lack of opportu- 

 nity for marine organisms to adapt themselves to fresh water for 

 in some geologic periods conditions have been very favorable 

 though in others distinctly the opposite. The severity of the 

 fresh-water climate, the obstacle of an ever outflowing current 

 and the relative newness of fresh-water bodies are evident difficul- 

 ties. Furthermore marine animals have generally free-swimming 

 embryos, distributed by water movements and sure therefore to be 

 eliminated gradually from the fresh-water environment even if 

 the adults were introduced successfully. Fresh-water animals 

 rarely have free-swimming larval stages and manifest what is 

 known as an accelerated or abbreviated development in which the 

 young on emerging from the egg is at a well-advanced stage. 



Man has been a powerful agent in modifying fresh-water life. 

 By hunting and fishing he has exterminated many forms directly. 

 Through modifications of streams or shore for commercial pur- 

 poses he has indirectly eliminated many more and finally by pol- 

 luting the waters with sewage and waste he has rendered extensive 

 water areas almost devoid of aquatic life except bacteria and even 

 incapable of supporting any other forms. Streams below great 

 cities and in mining and manufacturing districts are aquatic 

 deserts. 



Fresh-water biology is relatively a new field of study. Its 

 earliest records on this continent are hardly more than half a cen- 

 tury old. Among individual investigators in this field mention 

 should first be made of S. A. Forbes, whose pioneer work on the 

 Great Lakes has been followed by important work on the Illinois 

 river system. The work of Birge on Wisconsin lakes, of Reighard 

 on Lakes Erie and St. Clair, and of Kofoid on the Illinois river, 

 warrant also especial notice. Many others whose names and work 

 are recorded in the following chapters have made valuable con- 



