J. L. BROOKS 103 



Waskesiu in this and other years, has been extensively studied 

 by Rawson ( 1936 ) . The diagrammatic representation is based 

 upon his data and the specimens examined were collected by 

 him at this time. Waskesiu is a large, relatively shallow lake 

 which usually becomes stratified during midsummer, although 

 the bottom temperatures are usually higher than those consid- 

 ered typical in the diagram at the left. In 1932, however, excep- 

 tionally strong winds caused the stratification to break down 

 and mixed the surface waters to greater and greater depths, until 

 finally the water temperature at all depths was very nearly the 

 same (around 18° C). Under these conditions longiremis could 

 live throughout the entire depth of the lake. Specimens were 

 found living in the upper five meters! They bore retrocurved 

 helmets, as drawn. 



Examination of these two instances of helmeted longiremis 

 indicates that this extreme body shape was evoked when the 

 populations were forced to live in warm (and turbulent) water. 

 It also makes it seem unlikely that the genetic ability to develop 

 this phenotype under rare and abnormal circumstances is pre- 

 served in the genotype of a widespread population because of 

 any advantage conferred by this extreme phenotype during these 

 exceptional circumstances. 



It seems rather more likely that the variations in body shape 

 in D. longiremis, as in the rest of the genus, are a consequence 

 of maintaining a genetic constitution that will allow any individ- 

 ual to develop and function under any of the large range of 

 environmental conditions which the species can tolerate. The 

 morphogenetic processes must permit development at tempera- 

 tures slightly above 0° C. Morphogenesis controlled by the same 

 genotype must also produce an effective organism at a tempera- 

 ture twenty degrees higher. That there are differences in the 

 shape of the organisms produced by these same processes func- 

 tioning at such different temperatures is not surprising. 



Although the morphological changes in Daphnia are especially 

 pronounced and have considerably complicated the problem of 

 species determination, such extreme variation occurs in many 

 freshwater organisms. From our investigation of the nature of 



