100 



FRESHWATER ANIMALS 



Fig. 6. Phenotypic variation in Daphnia longiremis Sars. D. longiremis 

 is distributed throughout northern North America and northern Eurasia. 

 Throughout this range most populations are composed of individuals re- 

 sembling the topmost specimen. However, a few scattered North Ameri- 

 can populations are known in which individuals with large helmets have 

 appeared. These helmets are of the two forms indicated. The environ- 

 mental circumstances in which the two populations (one in Wisconsin, one 

 in Saskatchewan) produced retrocurved helmets arc indicated diagram- 

 matically in Fig. 7. 



cally determined. These extreme helmets occur in widely scat- 

 tered localities, yet the broad type from northern Alaska is very 

 like the broad type from Wisconsin. Furthermore, we know that 

 the population in Saskatchewan, which during one particular 

 season contained individuals with retrocurved helmets, in other 

 years consisted of individuals with not! dug but a small crest 



