No. 407.1 THE CREST OF DAPHNIA HYALINA. 889 
character of the environment, such as depth and size of lake, 
temperature, etc., but thus far the attempt has been unsuccess- 
ful. Lakes widely different in character, such as Mendota and 
Winnebago, possess very similar forms, 
while some small shallow lakes produce 
such extremes as those of Lakes Laura 
and Henrietta (Fig. 6.) Again, the occur- 
rence of a particular variation appears to 
bear some relation to the forms with which 
it is called upon to compete. Birge (97), 
considering the occurrence of D. hyalina 
and D. retrocurva in Lake Mendota, states 
that one declined in numbers according as 
the other increased, and concludes this to 
be the result of active competition between 
the two species. My own much more 
limited observation indicates a competition 
in which certain variations are favored to 
the detriment of others. A more exact 
study of the quantitative differences, as 
well as a careful investigation of the 
environmental influences, will be made 
before we attempt to show the significance Fre. 6.—1, Form from Lake 
of these variations; but I think the facts "eua; T Lake Tau 
as here given may serve to indicate the range of interesting 
problems offered by a generally accessible but little known 
form, and thus, perhaps, invite a more extended inquiry into 
the condition of the species in other localities. 
BIBLIOGRAPHY. 
'78 BIRGE, E. A. Notes on Cladocera. Trans. Wis. Acad. Vol. iv. 
'91 BiRGE, E. A. List of Crustacea Cladocera from Madison, Wis. 
Trans. Wis. Acad. Vol. v 
'97 BIRGE, E. A. Plankton Malin in Lake Mendota, II. Wis. Acad. 
Sci. Vol. xi. 
