296 NATURAL SCIENCE [May 



carried out the very praiseworthy but arduous work of systematically 

 watching the Crustacea of the plankton of Lake Mendota. The 

 results, which have now been published in the Transactions of the 

 Wisconsin Academy of Sciences,, etc. (Vol. xi. pp. 274-448), form 

 extremely instructive reading to those who, like ourselves, are in- 

 terested in the progress of fresh-water biology. 



Briefly, the method adopted in the work was as follows : Vertical 

 hauls were made with a plankton net, at very frequent intervals, 

 at a spot about half a mile from the southern shore of the lake. A 

 separate haul was made for every three metres of the total depth 

 (18 m.), the mouth of the net being closed at the proper moment 

 by a messenger sent down the line. The Crustacea contained in a 

 definite fraction of each collection were then counted and the 

 figures so obtained formed the basis of all the subsequent calcula- 

 tions as to the periodicity, vertical distribution, etc., of the different 

 species. 



It appears that Lake Mendota has altogether eleven or twelve 

 species of limnetic Crustacea (Entomostraca). Of these, only eight 

 are of real importance as constituents of the plankton. As regards 

 the seasonal distribution of the Crustacea as a whole, Prof. Birge 

 finds that there are, during the course of each year, three maximum 

 periods of development, occurring in May, July, and September- 

 October respectively. During the spring maximum, which is the 

 greatest, and is produced almost entirely by the marvellous increase 

 of a single species of Cyclops, as many as 3,000,000 individuals 

 may be present under each square metre of the surface of the lake. 

 As regards the occurrence of the individual species a very valuable 

 collection of facts has been obtained, but it is impossible to refer to 

 these in detail. It may be specially mentioned, however, that in 

 one case, Dajjhnia pidicaria, the evidence seems to show that 

 biennial periodicity may occur among plankton organisms as well as 

 annual. 



The vexed question of the uniformity or otherwise of the 

 horizontal distribution of the plankton is answered by Prof. Birge 

 as follows : " In general, there is no evidence of swarms in my 

 observations, either of all the Crustacea or of single species," 

 but " the variation of the numbers of the Crustacea in Lake Mendota 

 does not support extreme views either on the side of uniformity of 

 distribution or the opposing theory of swarms." 



The " Spkungschicht " or Thermocline 



One of the most characteristic features of the work carried out on 

 Lake Mendota is the attention which has been paid to what is 

 known to German authors as the '■' Sprungschicht," or, as Prof. 

 Birge calls it, the Thermocline. It was first announced by E. 



