vi PREFACE 



for this continent up to the time at which the chapter was 

 closed. 



The first few chapters are devoted to a discussion of general bio- 

 logical factors. Evident space limits prevented extended discus- 

 sion of many most interesting biological topics, which are at best 

 only outHned here. The exact citation of sources at the close 

 of these chapters will aid the reader to pursue such topics further 

 if desired. Not all discussions on general questions have been 

 confined to the introductory chapters. The chapter on Rotifera, 

 by Jennings, presents an admirable description of Ufe processes, 

 which, altho written specifically for that group, appHes with ap- 

 propriate modifications to all groups of many-celled organisms. 

 In the chapter on Copepoda, Marsh has treated with some detail 

 the general question of distribution as illustrated by this group; 

 yet the very factors which he shows to be operative in it are 

 those that lie at the basis of the distribution of most if not all 

 other groups. The discussion of the aquatic vertebrates by Eigen- 

 mann is purely biological and the systematic outhne is omitted 

 entirely, since that of itself would demand an entire book for its 

 adequate presentation. The same is true of the chapter on Bac- 

 teria, by Jordan, and of that on the higher aquatic plants which are 

 treated by Pond in the physiological (chemico-physical) aspect 

 primarily. 



Apart from those just mentioned all chapters conform to the 

 same general plan. Each is devoted to a single group of organ- 

 isms and opens with a general account of the occurrence and his- 

 tory of the group. The description of the anatomy of the forms 

 treated is very brief and deals chiefly with such features as are of 

 special value in the key. Similarly the life history is given in 

 condensed form. More attention is devoted to the biological 

 relations which at this point are discussed with reference to the 

 entire group, whereas individual features are left for later record 

 under individual species except as they are needed for illustrations 

 of general questions. Care has been exercised to include descrip- 

 tions of special methods for collecting, preserving, and studying 

 the organisms of each particular group. 



Special details both biological and morphological regarding genera 



