Desmidiacece 147 



mind that the transference by natural means of living specimens 

 of any Desmid from some of these countries to any of the others 

 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 1 . Moreover, zygospores, which might 

 possibly withstand the entailed vicissitudes if circumstances arose 

 by which they could be transferred from one country to the other 

 (such as by the long flight of a wacling-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, namely, that such a species has 

 been perpetuated by isolated communities which were derived 

 originally from a common stock, and that the individuals of these 

 communities have retained their original characters in an extra- 

 ordinarily constant manner. Thus, it seems probable that a sound 

 knowledge of the distribution of Desmids would shed much light 

 on the subject of previous land-connections, and might perhaps 

 afford more reliable evidence on this point than that afforded by 

 the known distribution of any other plants or animals. 



As a significant instance, it may be mentioned that several 

 remarkably fine Desmids, such as Staurastrum Ophiura Lund., St. 

 Arctiscon (Ehrenb.) Lund., St. jaculiferum West, St. longispinum 

 (Bail.) Arch., St. Cerastes Lund., St. Brasiliense Nordst. var. Lundellii 

 W. & G. S. West, Micrasterias furcata Ag., M. conferta Lund., 

 Pleurotcanium nodosum (Bail.) Lund., and others, are not uncommon 

 in the eastern parts of N. America and that in Europe they are 

 only abundant on the north-western shore districts of Ireland, 

 Wales, Scotland, the Outer Hebrides, Scandinavia and Lapland. 

 One of these species Staurastrum jaculiferum West is also 

 known from the Shetland Is., the Faeroe Is. and Iceland. 



Borge has examined a number of subfossil Desmids from the 

 glacial clays of the Isle of Gotland, and a few subfossil forms of 

 existing species have also been noted from an ancient peat deposit 

 near Filey in E. Yorkshire. 



Desmids thrive best in soft water, and they are most numerous 

 in peaty water which has a trace of acidity. With few exceptions 

 they do not flourish in water containing carbonate of lime in 

 solution, and no good collection of Desmids can be made in a 

 purely limestone district in which the water is hard. 



1 One Desmid has been described as inhabiting brackish water, viz.: Cosmarium 

 saliniun Hansg. in Oesterr. bot. Zeitschr. 1886, p. 335. 



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