J. L. BROOKS 121 



as six species of Daphnia are known to occur together in the 

 same lake, while five species of Coregonus of the same species 

 group ( superspecies ) have been reported from Sweden (the area 

 studied by Svardson). 



4. The coexistence of several closely related species in the 

 same body of water provides the possibility for hybrid formation 

 and introgression. There is evidence for both of these phenomena 

 in both Daphnia and Coregonus, evidence consistent with the 

 different reproductive biology of the two groups. The hybrid 

 individuals in Coregonus and hybrid populations in Daphnia are 

 an additional source of taxonomic difficulty. In neither group has 

 the phenomenon of introgression been carefully investigated, 

 although in both groups it stands as the most likely explanation 

 for certain general similarities between the species of a given 

 region. 



5. These factors, extreme phenotypic variation, coexistence of 

 several closely related species, and occasional hybridization ( and 

 introgression?), although by no means peculiar to freshwater 

 organisms, do appear to be largely responsible for the species 

 problem in two very different groups of freshwater animals. 

 Whether the sources of difficulty common to the systematics of 

 Daphnia and Coregonus will prove to be common to other "diffi- 

 cult" groups of freshwater animals will only be revealed by 

 further investigation. 



REFERENCES 



Anderson, E. 1949. Introgressive Hybridization. John Wiley & Sons, 



New York, N.Y. 

 Banta, A. M. 1939. Studies on the physiology, genetics, and evolution 



of some Cladocera. Paper No. 39. Dept. of Genetics. Carnegie Inst. 



Wash. Publ. No. 513. 

 Berg, L. S. 1940. Classification of fishes, both recent and fossil. [In 



Russian.] Trav. inst. zool. acad. sci. U.R.S.S., 5. No. 2. Reproduced, 



with English translation by Edwards Bros, Inc., Ann Arbor, Mich., 



1947. 



