366 Birge — The Crustacea of the Plankton. 



was substantially equal, being 57,000 per square meter, in 

 1896 and 50,000 in 1895, but in 1895 the number of Daphnia 

 hyalina was very great, being 260,000 or more during the en- 

 tire month; while in 1896 Daphnia hyalina had fallen off to 

 61,000 in the latter part of August, being therefore substan- 

 tially equal with D. retrocurva. 



The autumn history of D. retrocurva was very different in 

 the two years. In 1895 it declined in the early part of Septem- 

 ber and showed only a feeble rise in October, while in 1896> 

 both species of Daphnia rose together at an equal rate, and re- 

 mained practically identical in numbers until the sexual repro- 

 ductive period of D. retrocurva was passed. I can hardly at- 

 tribute this difference in the development of the species to any- 

 thing excepting the occupation of the water by D. hyalina in 

 1895. 



Another case in which competition may possibly play a part 

 may be found in the spring development of the Crustacea. In 

 no year do Diaptomus or Daphnia hyalina begin to develop 

 their swarms in the upper water until Cyclops has begun to de- 

 cline, and its numbers in the upper water are greatly reduced. 

 It would seem as though these latter species waited until 

 Cyclops was out of the way before they began their main devel- 

 opment. But in this case the increasing temperature of the 

 lake is unquestionably a factor in the development, and the re- 

 lation of competition is accordingly more doubtful. 



HORIZONTAL DISTRIBUTION : SWARMS. 



" Ob man die Befunde als Beweise der Ungleichheit oder der 

 G-leichfQrmigkeit bezeichnen will, kann freilich so lange G-e- 

 schmacksache bleiben, als man den Ausdruck nicht pracisirt. 

 Falls man aber pracisirt und gleichmassig nennt wenn durch- 

 schnittlich die Dichte nur um das Doppelte oder Dreifache wech- 

 selt, ungleichm'assig also, wenn die Vertheilung als so unre- 

 gelmassig erweisen wird, wie etwa die Bewohnung der Erd- 

 flache durch Menschen oder Thiere, so kann eine Meinungsver- 

 -schiedenheit nicht wohl bestehen bleiben. Ich betrachte die 

 Bewohnung einer Stadt noch als ziemlich gleichmassig und wenn. 



