109 
The question of specific limits and variation in this organism 
is one of exceeding difficulty, and I see no satisfactory solution for 
it until some one attacks the problem by a study of the variation 
by modern quantitative methods, and endeavors by breeding under 
control to establish the limits of variation within the normal range 
of seasonal changes of the environment. When this is done, some 
more satisfactory criterion for species in this group of planktonts 
will be feasible than the present condition affords, in which slight 
differences from previous descriptions are held to be valid for specific 
distinctions. Thus, jn recent years, species of plankton Dzfflugia have 
been described by Heuscher (’85) (D. urceolata var. helvetica) from 
Swiss lakes; by Zacharias (97) (D. hydrostatica) from Lake Pl6n; by 
Garbini (98) (D. cyclotellina) from Italian lakes; by Levander (’00) 
(D. lobostoma var. limnetica) from Finnish waters; and by Min- 
kiewitsch (98) (D. planktonica) from Russian waters. All of these 
forms occur in the Illinois River, and there are others equally worthy 
of specific designation in our plankton as yet undescribed. They 
occur most abundantly at the times of the pulses, especially of those 
in stable conditions. In my opinion they are all mere limnetic 
varieties of D. globulosa or D. lobostoma, the form of the shell and its 
constituent particles being modified by the habit of life in which 
these individuals of the seasonal cycle are found. They occur at 
times of abundant food, rapid multiplication, and limnetic environ- 
ment. Their shells are accordingly lighter, more chitinous and 
transparent, and the foreign particles adherent to them partake of 
the nature of those of the silt in suspension. This, however, 1s 
merely an opinion based upon an examination of the statistics of 
occurrences, and upon the work of plankton enumeration in which 
all individuals must be assigned tu some species. This is at least a 
different point of view from that of the systematist, who may, per- 
haps, lay more stress upon divergences from described types and 
less upon links connecting such variants. For the sake of genuine 
progress in the science it would seem to the writer extremely desir- 
able that more attention be given to the question of variation and 
less to the description of new species under criteria now in vogue. It 
may be desirable, indeed necessary, to distinguish such forms in the 
plankton. It would be both safe and conservative to designate 
them as forms, or, at the most, as varieties. 
