VARIATION IN THE DESMIDIEÆ. 871 
of examples of Cosmarium pseudotaxichondrum and allied species 
and varieties from many parts of the world, and can truly say 
that Prof. G. v. Lagerheim did not give “too much prominence 
to simple differentiations.” Many of these differentiations are 
combinations of characters which are repeated in hundreds, and 
I may say in thousands, of individuals, and can therefore be rightly 
considered as constituting distinct varieties. Moreover, how is it 
that identical species of these minute forms of plant-life, which 
Wolle would have us believe so easily give rise to differentiations 
and vagaries, are found in such widely separated places as 
Madagascar and the United States, with precisely the same 
Fig. 1. 
N 14 
a. a' 
b b 
Semicell of Micrasterias denticulata, Bréb., from Wrynose, Lancashire. 
x 200. 
characters—-the same markings, granule for granule? Such is 
the case with Euastrum trigibberum*, Now, if “permanent 
variation " (and I have attempted to show that these varieties are 
permanent) did occur so readily, then in a very few generations 
the character of the species would be totally changed; but the 
adducible evidence is quite against such a supposition. It must 
be admitted that in all probability a vast period of time T has 
* West & G. S. West, “ Freshw. Alg. of Madag.," Trans. Linn. Soc., Bot. 
ser. IT. vol. v. p. 53, pl. vi. f, 22; “N. Amer, Desm.,” 2. c. pt. 5, p. 245, pl. xiv. 
f. 34. 
t These lowly forms of plant-life, to quote a somewhat generalized state- 
ment by Wallace (‘ Darwinism,’ p. 114), occupy a position in the vegetable 
kingdom similar to that occupied by earthworms in the animal kingdom, 
“filling places in nature which would be left vacant if only highly organized 
plants existed. There is, therefore, no motive power to destroy or seriously 
to modify them ; and they have thus probably persisted, under slightly varying 
forms, through all geological time." The facts of distribution, although at 
present very imperfectly known, all tend to confirm the persistent nature of 
these plants. 
