LIMNOLOGICAL PKOBLEMS 
415 
floating power. I shall here restrict myself to the remark, that there 
are species of which every lake may practically he said to have its own 
race. These racial characters seem invariable from year to year ; 
this is especially the case with some Daphnids {D. hyalina^ Bosmina 
coregoni). Thorough investigations have further given the following 
results. 
It has been proved for Rotifers as well as for Cladocera that, how- 
Fig. 60, — Daphnia liyalina. Upper row : summer races from different Danish lakes. 
Lower row : winter races from the same lakes. The summer races differ very much 
from each other ; the winter races are almost of the same appearance. 
ever far the local variation is carried, these races^ ivherever the research 
has been made ^ fall hack in imnter iipon one and the same common race^ 
which in each species is invariahly the same in all localities investigated 
(fig. 60). On the seasonal variations beginning in spring, the local races 
proceed from this common form. The fact that this common winter 
form has been shown to be the homogeneous element from which all 
summer races proceed and to which all return, is to my mind of no small 
importance. When first I got a clear understanding of this pheno- 
