8 Preface 



Since 1906, when a course in general limnology was first estab- 

 lished at Cornell University, we have been associated in develop- 

 ing an outline of stud}^ for general students and a program of 

 practical exercises. The text-book is presented herewith : the prac- 

 tical exercises are incorporated in a small brochure by J. G. and 

 P. R. Needham entitled Guide to the Study of Fresh-Water Biology. 



The limitations of space have been keenly felt in every chapter; 

 especially in the chapter on aquatic organisms. These are so 

 numerous and so varied that we have had to limit our discussion 

 of them to groups of considerable size. These we have illustrated 

 in the main with photographs of those representatives most 

 commonly met with in the course of our own work. Important 

 groups are, in some cases, hardly more than mentioned; the stu- 

 dent will have to go to the reference books cited for further infor- 

 mation concerning them. The best single work to be consulted 

 in this connection is the American Fresh-Water Biology edited by 

 Ward and Whipple and published by John Wiley and Sons. 

 Our bibliography, necessarily brief, includes chiefly American 

 papers. We have cited but a few comprehensive foreign works; 

 the reference lists in these will give the clue to all the others. 



It is the ecologic side of the subject rather than the sys- 

 tematic or morphologic, that we have emphasized. Nowadays 

 there is being put forward a deal of new ecologic terminology 

 for which we have not discovered any good use; hence we have 

 omitted it. 



Limnology in America today is in its infancy. The value of 

 its past achievements is just beginning to be appreciated. The 

 benefits to come from a more intensive study of water life are 

 just beginning to be disclosed. That there is widespread interest 

 is already manifest in the large number of biological stations at 

 which limnological work is being done. From these and other 

 kindred laboratories much good will come; much new knowledge 

 of water life, and better application of that knowledge to human 

 welfare. 



James G. Needham 

 J. T. Lloyd. 



