J. L. BROOKS 109 



a cross between European Daphnia pulex and Daphnia obtusa 

 performed by Agar and reported by Scorn field (1940). 



The species most frequently involved in these presumably 

 hybrid populations, called, for the sake of brevity, simply "hy- 

 brid," are three pond-dwelling species of the northern and west- 

 ern parts of North America. These are Daphnia middendorffiana 

 Fischer, Daphnia pulex Leydig, and Daphnia sch0dleri Sars. Al- 

 though each of these is variable, the females drawn in Fig. 9 can 

 be considered characteristic. The postabdominal claw in each 

 is similar and variable, so that it is not difficult to understand 

 that all three have in the past usually been assigned to Daphnia 

 pulex when the possession of such a claw was taken as the sole 

 species criterion. The abreptor and antennule of the male 

 are distinct in all three, and when males occur, the populations 

 can be readily determined. Both pulex and sch0dleri range widely 

 over the western parts of this continent (pulex actually over most 

 of it), and hybrid populations between them occur scattered 

 throughout the large area of overlap. However, the best illustra- 

 tion of such hybrid populations concerns those between pulex 

 and middendoiffiiana. As can be seen in Fig. 10, Daphnia mid- 

 dendorffiana is most common in the very northern parts of North 

 America, and is one of the chief zooplankters in the numerous 

 tundra ponds and lakes of that region. Daphnia pulex extends 

 fairly far to the north but apparently cannot compete success- 

 fully with middendorffiana in the extreme north. Between the 

 areas in which these two species are common there is a broad 

 belt, from central Alaska eastward at least to Hudson Bay, where 

 most of the pulex-like Daphnia exhibit a combination of pulex 

 and middendorffiana characteristics. These have been labeled 

 "pulex with middendorffiana introgressed" in Fig. 10, merely to 

 indicate a guess as to their probable nature and origin. Although 

 it is not indicated on Fig. 10, Daphnia middendorffiana occurs in 

 scattered populations south as far as central California and 

 Wyoming. Scattered populations in this region of western United 

 States resemble the hybrids common to the region indicated 

 on the map. 



This complex of middendorffiana, pulex, and sch0dleri, the 



