246 MORRIS M. WELLS. 



themselves point out many contradictions. According to their 

 idea, the plankton forms must in many instances be reacting 

 positively to concentrations of oxygen which are as small as 

 .1 c.c. per liter, or even less. This supposition is contrary to 

 all the experimental evidence regarding the reactions of aerobic 

 fresh-water organisms to this gas. In an attempt to correlate 

 the distribution of the zooplankton with the chemical reaction 

 of the water, I have gone over Birge and Juday's tables and 

 figures, and have come to the conclusion that such correlation 

 exists. Their data indicate in practically all of the lakes (in 

 the summer condition) a point at some depth below the surface 

 of the lake, where the organisms are more numerous than at 

 any other depth. In many cases this rise is proportionately 

 very high and is usually of small amplitude. Thus the large 

 number of forms occurs in a rather limited region vertically. 

 After the rise, there is a marked diminution in the number of 

 forms and then again at a little greater depth there is another 

 increase, smaller than the first, but still very noticeable in their 

 curves. This increase is followed by a second diminution. 

 The first diminution usually occurs in or near the thermocline 

 where the temperature often shows a very sudden lowering. 

 The oxygen supply sometimes falls off here also, but not always, 

 and in the lakes to which I refer particularly, the oxygen supply 

 is practically the same at all depths. A very important fact, 

 however, is that the water in the region of the thermocline, i. e., 

 at the region of smallest numbers of plankton, is often neutral 

 or very nearly so (summer condition). Above this region the 

 water is alkaline, and below acid. From the data given in 

 Birge and Juday's Tables XVIII. and XIX. ('12, pp. 602-608), 

 I have compiled the following table (Table IV.) to show the re- 

 lation of the zooplankton to this neutral region. Birge and 

 Juday's Table XVIII. is a record of temperatures and gas con- 

 tents at the different depths; Table XIX. is an analysis of the 

 plankton catches made in ten lakes. The records for a given 

 lake were all made on the same day. Table XVI 1 1 . gives titration 

 records which show that in three of the lakes at a definite depth, 

 the water was neutral. Table XIX. gives the plankton col- 

 lections at different depths in these three lakes, on the same 



