J. L. BROOKS 107 



affect a greater percentage of the population. Thus the chances 

 for forming hybrids is considerably greater. 



2. The hybrid populations occur in regions where both puta- 

 tive parent species live. 



3. The hybrids themselves are usually chimaera-like in that 

 in some features they are quite like one parent, in some quite like 

 the other. In this they are very like the only experimentally pro- 

 duced hybrids between species of Daphnia, those obtained from 



MIDDENOORFIANA 



PULEX 



SCH0DLERI 



ABREPTOR, 

 FEMALE 



ABREPTOR, 

 MALE 



ANTENNULE, 

 MALE 



Fig. 9. Three similar species of pond-dwelling Daphnia common in 

 northern and western North America. Although the postabdominal claws 

 (on abreptor) of females are similar (all of pulex type), other morpho- 

 logical differences in female are apparent (head shape, length of shell spine, 

 shape of carapace). Structure of males indicates distinctiveness of each 

 species more clearly than that of female. Male of middendorffiana very rare. 

 Many Daphnia populations in regions where these species coexist appear to 

 be hybrids between various pairs of these species. (See Fig. 10.) 



