110 FRESHWATER ANIMALS 



females of which are sometimes difficult to distinguish, together 

 with scattered populations combining characteristics of any two 

 in somewhat chimaera-like fashion, has posed a considerable 

 systematic problem, probably the most difficult one encountered 

 in the North American assemblage. 



Populations intermediate between other species also occur, but 

 they are largely confined to regions such as the Pacific North- 

 west where northern and southern species assemblages have 

 come into contact (Brooks, 1956). The distribution and proper- 

 ties of these populations bear the same resemblance to the puta- 

 tive parents as did those in the case just discussed. 



On the basis of this kind of evidence it seems justified to 

 propose that introgressive hybridization is a factor that has com- 

 plicated the systematics of Daphnia. As yet, however, neither 

 experimental crosses nor analyses of the sort that Anderson 

 (1949) has developed for plants have been done with these 

 Daphnia populations. Because hybridization has been viewed 

 with much skepticism, often justified, comparable data on its 

 importance in the systematics of most other groups of animals 

 are wanting. Hubbs (1955) has provided an excellent summary 

 of hybridization in North American fishes, which are the most 

 adequately investigated group of animals from that point of 

 view. We shall again mention the possibility of introgressive 

 hybridization in the discussion of the species problem in Core- 

 gonus. 



The Species Problem in Coregonus 



Coregonus, the genus of the true whitefishes, is one of the three 

 genera that comprise the whitefishes. These genera are con- 

 sidered by Berg (1940) to constitute one of the two subfamilies 

 of the Salmonidae. Members of the genus Coregonus live in the 

 rivers (many species tolerate brackish water) and lakes of north- 

 ern Eurasia and North America. These whitefish are exceedingly 

 numerous in the fresh waters of the very northern parts of these 

 two continents. 



The classification of these fish has aroused interest, possibly 

 because some provide excellent food. Aside from the inherent 



