44 THE LIFE OF DEVILS LAKE 



In no two lakes, however, are the environmental conditions, 

 or the fauna and flora exactly identical; it would therefore be 

 too much to expect that the behavior of the plancton should ba 

 the same in both. The behavior of any organism at any time, and 

 in any situation, constitutes a problem in itaelf, and a very complex 

 problem at that. 



3. "While, in a general way, the forms of the curves of any 

 species are similar in two successive years, nevertheless their size 

 (i. e. the number of individuals present) varies greatly. This is 

 at once obvious on inspection of the plancton curves for two or 

 more successive years for any lake. Compare, for example, those 

 given by Birge (1. c. pi. XXV) for Diaptomus in Lake Mendota 

 in 1894-95 and '96. 



"The feature of the annual distribution of the Crustacea which 

 surprised me most in the progress of my work is the great differ- 

 ence between the numbers of the same species of Crustacea present 

 in successive years. I do not refer so much to the larger or smaller 

 numbers of forms like Cyclops, for whose variations causes can be 

 assigned, at least in part, but rather to such facts as those shown 

 by Daphnia retrocurva and by Diaphanosoma, which are either 

 absent, or present in very small numbers in one season and appear 

 in great numbers in another year. For such variations it is very 

 difficult to assign even conjectural causes. 



"A similar fact has appeared in the succession of the algae. 

 It is not true for lake Mendota that the forms of algae succeed 

 one another in a definite order in successive seasons, so that one 

 can be sure of finding certain forms at certain times of year, as 

 would be the case with plants of woodland or prairie." (Birge, 

 1. c, p. 317.) 



Kobert (1919-20), after reviewing the work of several authoriii, 

 together with his own, reaches the conclusion (p. 41) that "Nous 

 ne pensons pas qu'un seul des facteurs generalement invoques: 

 temperature, circulation ou stratifications des eaux, puisse a lui 

 seul expliquer la date d' apparition des maxima et minima du 

 plancton. Ceux-ci dependent sans doute de facteurs fort complexes 

 et diflficiles a isoler, parmi lesquels ceux que nous avons etudies 

 jouent probablemciit un certain role." 



Only the principal plancton species are represented in the 

 charts. Many otliers are of sporadic* occurrence, probably being 

 carried from the Ruppia into the pelagic zone by the action of 

 the wind. A few forms, such as Characium, which grows epiphy- 

 tically on the Crustacea and elsewhere, are of frequent but incon- 

 stant occurrence in the collections; while others, such as Cothumia 

 and Pediastrum occur in too small numbers to have much signi- 



