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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^iormal 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, in recent years, species of plankton Difflugia have 

 been described by Heuscher ('85) (D. urceolata var. helvetica) from 

 Swiss lakes ; by Zacharias ('97) (D. hydrostatica) from Lake Plon ; by 

 Garbini ('98) (D. cydotellind) from Italian lakes; by Levander ('00) 

 (D. lobostoma var. limneticd) from Finnish waters; and by Min- 

 kiewitsch ('98) (D. planktonicd) 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, is 

 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 to 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. 



