VARIATION IN THE DESMIDIER. 375 
In other species the variation may be confined wholly, or 
partially, to the extent to which some particular character is 
developed or suppressed. This is illustrated by Arthrodesmus 
convergens, Ehrenb., in which one type of variability is the 
multiform character of the simple lateral spines (cfr. fig. 4, 
p. 398). 
A few further remarks have also been made by Schmidle * on 
variation in the genus Cosmarium, more partieularly with regard 
to C. striatum, Boldt, under which he includes forms of seven 
species. 
I may here appropriately recall a remark made by Wallich 4, 
that “the law which it is assumed governs the limits of a species 
is no law, but only a conditional direction, holding good only so 
long as the surrounding conditions continue the same," or, as 
amended by Turner ¢, “so long as the conditions of environment 
remain the same through a lengthened period of time." It has 
been already stated that specimens obtained from localities in 
which the conditions of enviroumeut are very diverse exhibit 
no marked constitutional differences adaptive to their several 
requirements. This being so, it is difficult to imagine the 
possibility of a slight change in surrounding conditions seriously 
affecting the characters of a species, unless the changed con- 
ditions continue to exist for a long period of time; and, again, 
any serious alteration in the characters of a species is doubtless 
effected by several such changed conditions, each of which eon- 
tinues for an extended epoch. That the Desmidiez have existed 
through a vast period of time in much the same forms as they 
exhibit at the present day is highly probable. 
The five statements which follow represent the result of the 
direct observations on variation in this group of plants, and may, 
owing to our insufficient knowledge of the question, be sub- 
ject to further alterations. The first statement is a modification of 
Schmidle’s first proposition; the second includes his second and 
third; in the third I disagree somewhat with the conclusions set 
down in his fourth ; and the fourth and fifth are additional. 
* W. Schmidle, “Aus der Chlorophyceen-Fl, der Torfstiche zu Virnheim,” 
Flora , 1894, Heft 1, pp. 52-56. 
t G. C. Wallich in letter to Archer; cfr. Trans. Dubl. Mie. Club, 1865. 
t W. B. Turner, * Freshw, Alg. of E. India,” Kongl. Sv. Vet.-Akad, Handl. 
Bd. xxv. no. 5, 1892, p. 6. 
