THE SPECIES PROBLEM IN 

 FRESHWATER ANIMALS 



JOHN LANGDON BROOKS: yale university, new haven, 



CONNECTICUT 



Most of the problems encountered in the systematics of fresh- 

 water animals are of the same nature as those met in the system- 

 atics of terrestrial or marine animals. The only justification for 

 considering these problems in freshwater animals is the possibil- 

 ity that some of the complicating factors may operate with a 

 peculiar intensity or in unusual combinations in the freshwater 

 fauna. Whereas a comprehensive survey of the systematics of 

 the diverse animal stocks that comprise this freshwater fauna 

 might reveal the information we seek, such a survey would be 

 lengthy and our labors might not find a proportionate reward. 

 What I propose to do in this paper is to select from biologically 

 different, but equally successful, freshwater stocks two genera 

 the systematics of which have proved difficult to comprehend. 

 By analyzing the "species problem" in each, it will be possible 

 to see whether the sources of the systematic difficulty in the two 

 groups are at all similar. If any similarities are evident, it can 

 tentatively be concluded that these represent evolutionary 

 processes working with peculiar intensity in fresh waters. 



Generalizing from selected, extreme examples must be done 

 with proper caution both in the selection of the cases and in the 

 force and extent of the generalizations. The two genera selected 

 are Daphnia, small planktonic crustaceans, and Coregonus, the 

 true whitefishes. These are two of the roughly half-dozen sys- 

 tematically most troublesome genera of common freshwater 

 animals. Daphnia was chosen because that is the group with 

 which I am most familiar, and for this reason the greater part 

 of our attention will be devoted to it. Much of the information 



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