The Annual Distribution of the Crustacea. 317 



the other two leading genera. In 1896 Cyclops held a similar 

 place, recovering rapidly from its early summer depression and 

 maintaining its numbers full throughout July and the early part 

 of August. 



The diagrams show further how all the species of Crustacea 

 increase in September, and that the rise persists to different 

 dates in the later autumn. In 1895 Diaptomus showed a max- 

 imum in late September, and that of Cyclops came in the first 

 half of October. In 1896 Daphnia hyalina and D. retrocurva 

 rose together from the latter part of August to the middle of 

 October, when the former species had a period of enormous re- 

 production, while D. retrocurva, which had produced its ephip- 

 pial eggs, rapidly declined in number. The increase of Cyclops in 

 this year also continued until late October. The diagrams show 

 further how all species rapidly decline in number in November, 

 and then more slowly during December, reaching their perma- 

 nent winter condition in December, or at latest about the first 

 of January. 



The feature of the annual distribution of the Crustacea which 

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

 ference 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 suc- 

 ceed 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. For example, in the winter of 1894-95 Aphanizomenon 

 and Clathrocystis were the predominant algae after the early part 

 of January. In the succeeding winter these plants were almost 

 entirely absent and Diatoma was the predominant form. In the 



