VARIATION IN THE DESMIDIEE. 405 
Many of the so-called varieties that have been described for 
certain species are shown, by a study of the variation of the 
species, to be merely transitory forms founded upon accidental 
differences of a temporary character. (Cfr. Cosmarium biretum 
var. intermedium, C. biretum forma grenlandica, and C. biretum 
forma subconspersa, p. 390, supra.) In a few cases, however, it 
ean be shown by the same means that what was at one time 
regarded as a variety of a particular species, is in truth as much 
a distinet species (so far as we can comprehend the word 
‘species’) as the plant to which it was formerly referred. In 
fact, the true affinity existing between many species of Desmids 
can be ascertained only by a careful study of their various forms, 
as exhibited in large numbers of specimens from divers districts ; 
and in the following pages 1 have endeavoured to show clearly 
the particular relationship which exists between certain species 
about the position of which there has always been much diseus- 
sion. It will also be seen that this study proves in some 
instances that species which have been imagined to possess a 
close relationship have only an apparent affinity, and have really 
arisen at some carlier or later stage in a particular line of descent, 
or else along totally different lines of evolutionary development. 
I will first call attention to three species which, although very 
local, are widely distributed in Europe and N. America, viz.:— 
Staurastrum vestitum, Ralfs, S. aculeatum, Menegh., and S. con- 
troversum, Bréb. SS. vestitum is a species which cannot readily 
be mistaken, and yet at the same time is one which exhibits a 
wide range of variation (cfr. page 395, supra). On the other 
hand, S. aculeatum is a species which has frequently been mis- 
understood, and, although closely related to 8. vestitum, it 
exhibits a less range of variation than the latter. 8. contro- 
versum appears to me to be more nearly related to S. restitum, 
var. semivestitum than to S. aculeatum ; and I think it highly 
probable that both 8. controversum and S. aculeatum were 
originally evolved from S. vestitum along different lines, the 
former through such forms as that mentioned by Schmidle in 
‘ Hedwigia, 1895, t.i. f. 22. 8. vestitum itselt probably arose 
alonga series terminating with S. Pseudosebaldi, Wille, S. Sebaldi, 
Reinsch, 8. Sebaldi var. altum, West & G. S. West, and 
S. anatinum, Cooke & Wills, its earlier forms being long-armed 
like 8. anatinum, and its later forms, which gave origin to 
S. aculeatum, being short-armed. 
