VARIATION IN THE DESMIDIEE. 909 
That variation in a species has a tendency to be reproduced 
there cau be no doubt, especially wheu the reproduction takes 
place, as iu these plants, by simple cell-division ; yet extreme 
modifications, which are obvious abnormalities * (Pl. 10. fig. 6, 
and Pl. 11. fig. 27), are never (or very rarely) repeated iu 
succeeding generations aud may be regarded merely as acci- 
dental oceurrences in the history of the species. Again, it must 
not be supposed that the variability of some diagnostie wart or 
spiue, or some other character, makes that character of no use as 
a specific distinction, for such a feature may be always present 
to a greater or less degree and hence distinctive, and even in 
those specimens in which it is absent it is often produced at its 
maximum on the completion of the development of the younger 
semicells of a succeeding generation. This fact is well illus- 
trated by certain variations, which I have described in Arthro- 
desmus convergens (see p. 398), which show the lateral spines 
reappearing on the semicells of a later generation, the few 
previous generations having been destitute of such structures. 
It is also exhibited by some specimens of Cosmarium Regnesü 
from Puttenham Common, Surrey (Pl. 10. tigs. 16 & 17), these 
examples being selected from a gathering of a large quantity of 
the species preserved while in active growth. 
It generally happens that the variation is identical on the two 
sides of a semicell, or even of a cell; this is Bilateral Symmetry 
of Variation. Thus ia the symmetrical Desmidiez (and most 
of them are perfectly symmetrical) the majority of the variations 
observed consist of “similar and simultaneous variation of 
repeated parts ” f. 
Ihe symmetry aud pattern exhibited by the Desmidiee are 
* Cfr. W. Archer, “ Notice of some cases of abnormal growth in the Desmi- 
diacez," Dub. Nat. Hist. Soc. Proc. vol. iii. t. 1, ex part.; Journ. Mier, Soc. 
1550, t. 7, ex part. Reinsch, Contrib. Alg. et Fung. t. 18, ff. 9-15. Jacob- 
sen in Botanisk ‘Tidsskrift, vol. viii, 1875, t. 8. ff. 30, 31. West in Journ. 
Bot. 1989, t. 291. f.ö. West & G. S. West in Journ. Roy. Mier. Soc. 1896, 
t. 4. ff. 55, 56; Ann. Dot. vol. xii. 1898, t. 4. ff. 39, 40. Nordstedt in Kongl. 
Sv. Vet.-Akad. Handl., Bd. xxii. no. 8, 1888, t. 7. ff. 8-11. Raciborski in 
Pamietnik. Akad. Umiej. w Krakow. Wydz. matem.-przyr. vol. x. 1884, t. 14. 
f. 3. Gutwinski in Rosprawy Akad. Uiniej. Krakow. Wydz. mat,-przyr. ser. 2, 
tom, xxxii. 1896, t. 7. f. 60. Schroder in Forschungsberichten a. d. Biol. 
Stat. zu Plon, Bd. vi. t. 2. f. 3; and many other examples which might be 
quoted. 
t C/r. Bateson, ‘Study of Variation, 1894, p. 569. 
