356 Birge—The Crustacea of the Plankton. 
dant as in lake Mendota, the quantity of algae is ordinarily in ex¬ 
cess of the demands of the Crustacea, and any scarcity of food 
is wont to be brought about rather by changes in the quality of 
the algae than by an inadequacy in the total supply of vegeta¬ 
ble material. There is, however, one line of facts regarding 
the quantity of food to which sufficient attention has not as yet 
been given, namely, the correspondence of the relation of the 
rhythm of development of the algae with that of the Crustacea. 
As is well known, the successive species of plankton algae come 
on in waves of development, and between the periods when 
given species are plentiful, there are intervals, longer or shorter, 
when the food supply may be small. This relation may be best 
seen in lake Mendota at the time of the spring maximum. The 
Crustacea, during the spring, increase more rapidly than the 
algae, and when the Crustacea are at their maximum, the mass of 
plankton appears to the eye to consist of little except Crustacea. 
Under these circumstances the food supply must be inadequate, 
the number of Crustacea must fall off, and, especially, their re¬ 
productive power must decline. If the rate of increase of the algae 
coincided with that of the Crustacea, so that the time of maxi¬ 
mum amount of food agreed with the time of maximum needs 
on the part of the Crustacea, this quantitative oscillation would 
be of little importance; but, if at any time the decline of the 
dominant algae coincides with the reproductive period of a 
species of Crustacea, it may be long before the species recovers 
from the injury thus caused. This relation between food and 
Crustacea is one of the most important, and at the same time 
one of the most difficult to investigate, and one to which as yet 
but little study has been given. It is plain, however, that the 
number of a species of Crustacea must be determined—so far as 
determined at all by food—by food relations when most unfavora¬ 
ble, and that the quantitative relations of food and Crustacea 
must be followed from day to day, if this relation is to be un¬ 
derstood. 
Zacharias (’96, p. 60) expresses his surprise that the small 
Crustacea do not increase beyond a certain number when they 
are provided with so abundant food throughout the year. To 
this question he states that there is at present no answer. I 
