888 THE AMERICAN NATURALIST.  [Vor. XXXIV. 
distinct type of crest. Lake Laura, in the immediate vicinity 
of Okauchee and Oconomowoc Lakes, and Lowe’s Lake, which 
lies in the same system with these, show a uniform low-crested 
variety, while Lakes Five and Mouse exhibit the high-rounded 
type of crest, with marked subocular prominence. Theaccom- 
panying map (Fig. 4) shows the greater part of the area in 
which these forms are distributed. The lakes of the Bark 
River system were very poor in Crustacea and furnished 
almost no Daphnie. 
Seasonal Variation. — More striking still is the fact that 
this range of variation is confined to the summer forms. Of 
course, owing to the difficulty of obtaining material in the 
winter, no very extended study of the winter forms has been 
made, but Professor Birge has for years made collections of 
winter forms from Lake Mendota, and I have his permission 
to state that they are uniformly low crested, while my own 
study of the winter forms of Lake Oconomowoc and Okauchee 
gives identical results. When it is remembered that the summer 
broods of Daphnia are produced parthenogenetically, the deter- 
mination of the kind and 
conditions of variation 
must have an important 
bearing on questions of 
heredity and the origin 
of specific differences. 
Correlations in Vari- 
ability. — Although no 
quantitative study has 
yet been undertaken, 
careful com parison 
shows a direct relation 
between the length and 
curvature of the crest 
Fic. 5. —1, Lake Five; II, Mouse; III, Lowe's and the length and cur- 
Lake form. ; 
vature of the terminal 
spine. The basal joint of the antennz also varies, in size and 
strength, with the form and size of the shell. An attempt has 
also been made to relate the form of the crest with some 
