J. L. BROOKS 83 



America, 7; Eurasia, 25; Africa, 15; Australia, 3. Although the 

 subsequent discussion of the systematic problems in Daphnia is 

 based primarily on a study of the genus as it occurs in North 

 America, it is almost certain that Eurasia and Africa present 

 problems of equal or greater magnitude, and probably of a sim- 

 ilar nature. 



An indication of the extent of the difficulties of Daphnia sys- 

 tematics can be found in the varied taxonomic treatment that 

 the members of this genus have received. During the past seventy 

 years several hundred names have been proposed for Daphnia 

 populations, and these have been trinomial and quadrinomial 

 combinations, as well as the usual binomials. For example, in 

 Birge's widely used key to North American Cladocera (in Ward 

 and Whipple, 1918), six binomials, seven trinomials, and four 

 quadrinomials are used to indicate the fourteen entities that he 

 recognized. However, these polynomials, although usually based 

 on collections from one or a few localities, were usually consid- 

 ered to designate "morphological variants" which might occur 

 anywhere, rather than geographically limited variants. Thus the 

 trinomials, which are common in the literature on Daphnia 

 systematics, seldom referred to a geographical subspecies, in the 

 maimer now common in the systematics of, for example, birds. 



When Wagler, about 1936, attempted a revision of the genus 

 he was faced with this accumulated taxonomic chaos. His solu- 

 tion, seemingly a reaction against the prevailing system, was to 

 classify all Daphnia into eleven species all indicated by bino- 

 mials. The simplicity of this system is in large part due to the 

 fact that several of these "species" are aggregates of what, in 

 reality, are clearly distinct species. This, however, was not an 

 example of the lumping as opposed to the splitting of groups of 

 closely related species, because many of the aggregated species 

 are quite dissimilar. 



Two of these aggregates are Daphnia longispina and Daphnia 

 pulex. Since many phenotypic characteristics of each are var- 

 iable, both seasonally and geographically, all the species could 

 be put into "present" or "absent" categories on the basis of only 

 one relatively constant morphological feature. This was the mid- 



