14 
described, though a magnificent opportunity awaits the naturalist 
who has the fortitude to analyze the exceedingly variable forms 
which compose the plankton, and to determine by modern methods 
which of these variants are entitled to specific rank. It has seemed 
to the writer that the only satisfactory basis upon which species, 
and pre-eminently those of the fresh-water plankton, can rest, lies in 
a careful determination of the limits of seasonal and local variation 
within the area of distribution. This means breeding under con- 
trol,and the study of variation by modern statistical methods. 
Both of these lines of inquiry lie beyond the purpose of the present 
paper, and plainly beyond the possibilities of accomplishment by 
any one investigator, when the great number of species and the pres- 
ent state of the literature of the subject is considered. It is becom- 
ing constantly more evident that the species of the plankton are in 
the main cosmopolites, and the world literature of the subject must 
be taken into consideration in any thorough attempt to handle the 
systematic side of the subject. During the progress of this work, 
which was begun in 1894, every effort was made to secure all perti- 
nent literature bearing on the genera of plants and animals repre- 
sented in the plankton, and so far as possible in the enumeration of 
the collections the individuals were referred to “species” already 
described, or, in default of this, recorded as “unidentified.’’ In 
some groups—notably the desmids, diatoms, and_ unicellular 
algee—it was not possible under the conditions of plankton enumer- 
ation to apply to all the individuals enumerated the fine distinc- 
tions which specialists in these groups have made. They have been 
thrown under certain of the better-defined species, which thus stand 
in our records as representatives of closely related variants as well 
as of the types of the species named. Examples of this appear in 
Clostertum, where two species only were listed. Probably a num- 
ber of so-called species among the scores described in this genus 
will be found among the individuals in our plankton here referred 
to the two species C. acerosum and C. lunula. So, also, in the case 
of Melosira; two principal types were listed, M. varians and 
M. granulata,—though even these two seem at times to intergrade. 
Other described species will be found among the individuals thus 
distributed. In the case of Dzfflugia globulosa and D. lobostoma a 
large number of intergrading and variable forms are included. It 
would be possible to find among these, representatives of many 
