372 MR. G. S. WEST ON 
elapsed since the original distribution of Ewastrum trigibberum 
in Madagascar and the United States, yet this species has been 
perpetuated through this long period of time by two such far- 
distant assemblages, and the resultant present-day N. American 
form, far from being greatly differentiated from the Malagasy 
type, is identieal with it. It might be argued that this result 
has been obtained by a parallelism of modification in the course 
of the evolution of these forms; but this is most unlikely, as 
proved by the occurrence of many species with such a distribu- 
tion as the following :— Micrasterias foliacea in North and South 
America, India, Burmah, and Queensland ; Triploceras gracile in 
North and South America, Europe, India, China, Australia, and 
New Zealand. I may also mention that the transference by 
natural means of living specimens of any Desmid from one of 
these countries to the other is an utter impossibility, desiccation, 
or in many cases even partial drying, being quickly followed by 
death, and submergence in sea-water is equally fatal *. More- 
over, zygospores, whieh might possibly withstand the entailed 
vicissitudes if circumstances arose by which they could be trans- 
ferred from one country to another (such as by the long flight 
of a wading-bird), are so rarely found that distribution by 
their means across an expanse of ocean is almost impossible. 
There is but one conclusion to be arrived at from a consideration 
of these facts, viz.: that such a species has been perpetuated by 
two isolated communities which were derived originally from one 
assemblage, aud that vhe individuals of these communities have 
retained their original characters in an extraordinarily constant 
wanner. 
This constancy of character is not only found in certain rare 
species of restricted distribution, such as those just mentioned, 
but in many much commoner species. Take, for instance, 
Docidium Baculum, Bréb. ; this plant has a world-wide distribu- 
tion f, and yet a Malagasy specimen of the type-form could 
not be distinguished from an English specimen. From these facts 
* One Desmid has been described as inhabiting brackish water— Cosmarium 
salinum, Hansgirg in Oesterr. bot. Zeitschr. 1886, p. 335; cfr. also “ Prodrom. 
Algenil. Bolin,” Archiv der naturwissenschaft. Landesdurchf. von Böhmen, 
1888, Bd. vi, no. 6, p. 194, cum f. 115. 
T Docidium Baculum, Bréb., is found throughout the greater part of Europe 
and N. America, also S. America, Northern India, Burmah, Madagascar, and 
Australia. 
