19 
that we are dealing in the case of the species of Dinobryon above 
cited with a single variable organism, whose extremes of variation 
only have been regarded as separate species. The connecting links 
are sufficiently abundant still and the union of several types in a 
single colony is sufficiently frequent to lend some weight to my con- 
clusions with regard to those forms which have been under my 
observation. In the interests of utility as well as in the interests 
of well-grounded taxonomy, it is extremely desirable that the 
establishment of new species among variable plankton organ- 
isms should be attempted with extreme caution and only after the 
fullest study of the range and conditions of variability. The insta- 
bility of the taxonomic structures which Brunnthaler and Lemmer- 
mann have recently raised, is evidenced by the differences 1n syno- 
nymic, varietal, and specific rank given to the variants of Dino- 
bryon by these two systematists, who have but recently mono- 
graphed the group, largely if not wholly from the systematic point 
of view. The changing estimate of validity which Lemmermann 
himself has put upon his own species or varieties—for example, 
schaumnslandu, pediforme, and curvatwm—gives further evidence 
that the basis upon which they rest is at the best but slight. 
It is my firm conviction that the establishment of new species 
among the organisms of the plankton of fresh water can be 
satisfactorily accomplished only after careful analysis of the limits 
of variation within the range of environmental conditions. Stand- 
ards less comprehensive than this can yield results of but temporary 
or local value and can lead to but little permanent advance in 
science, and they bring only perplexity and chaos where order 
should reign. 
Diplosiga frequentissima Zach.*—Average number, 1,736,538. 
This minute flagellate is found upon the rays of the colonial diatom 
Asterionella, often in great numbers and so thickly set as to leave 
little unoccupied space. It was found in each year at the time of 
the vernal pulse of Asterionella in April-May, and was as a rule most 
abundant immediately after the maximum growth of Astertonella 
had been attained. Beyond an isolated occurrence in January it 
was not recorded at other times than during the months of April 
and May. 
Eudorina elegans Ehrbg.—Average number, 14,362. About 
twice as abundant in 1897. The distribution of this species is 
