368 
Birge—The Crustacea of the Plankton. 
either of all the Crustacea or of single species. It will be 
seen from the tables giving the maximum and minimum catches 
for each two week period that in the more numerous species the 
maximum catch is about four times the minimum, when the 
species is neither increasing or decreasing in numbers to any 
marked degree. Where the species is present in small numbers, 
the range of variation is far greater. Thus, in July, 1895, Lep- 
todora showed a variation from 1 to 50 individuals in the 39 
catches made during that month. It varied from 1 to 19 in 
catches made on the same day, and was wholly irregular in its 
variations during the month. During the same month the catch 
of Cyclops varied from 1290 to 6100; and on no day were two 
catches made in which one was double the other. In each of 12 
days in 1895 and 1896 two catches were made at points about 
two kilometers apart, and the ratio of the predominant species 
in these 12 cases was as follows: 
Average 
ratio. 
Maximum 
ratio. 
Minimum 
ratio. 
Diaptomus ... 
A: B:: 1:1.62 
1:2.4 
1:1.1 
Cyclops . 
A:B:: 1:1.55 
1:2 
1:1.1 
D. hyalina . 
A: B:: 1:1.58 
1:2 
1:1.1 
In each case A denotes the smaller catch, which was about 
equally divided between the two stations. 
Again, if comparisons are made of catches extending over 
a period of time when the average number remains nearly con¬ 
stant, and when there is no reproduction, the distribution can 
readily be inferred. Fifty-six catches of Diaptomus were made 
between December 1st, 1894, and March 30th, 1895. Of these 
there were: 
Below 10,000 per square meter, 1 catch. Between 40,000 and 50,000,5 catches. 
Between 10,000 and 20,000, 14 catches. Between 50,000 and 60,000, 2 catches. 
Between 20,000 and 30,000, 21 catches. Over 70,000 per square meter, 1 catch. 
Between 30,000 and 40,000, 12 catches. 
The figures also show that all of the December and January 
catches were below 30,000, all of March above 20,000, and only 
about one-fourth of them below 30,000; while the February 
