64 GENTlnAL HISTORY OF 



the forms characteristic of Oregon and California; whilst, on the 

 other hand, the peculiar forms of these latter countries are met with 

 in Siberia. All this is remarkably confinned in this, that the gold 

 region of the Sacramento, in the extent and abimdance of its 

 Infusorial products, finds its parallel only in Siberia." (Monatsbericht, 

 Berlin Acad. Feb. 1849.) 



Most of the preceding remarks apply with greater force to those 

 siliceous shelled microscopic organisms included in the great family 

 Bacillaria. It is these beings which at previous eras have built up 

 rocks, raising land from the sea bottom, and which now, by their 

 indestructibility, admit the extensive review of their distribution in 

 time and space made by Ehrenberg. "Whereas the illoricated, soft, 

 and evidently animal Infusoria, have, by their destructibility, been 

 removed from the sphere of our investigations into their diffusion 

 in past time. Yet sufficient has been made out to indicate the pre- 

 valence of a law of distribution in space even in their case. 



The determination of species characteristic of certain climes, has 

 enabled Ehi-enberg to arrive, in many cases, at the probable source 

 of those meteoric showers of dust which occasionally occur. For 

 instance, those which fall in the Atlantic about the Cape de Verd 

 Islands, and as far eastward as Genoa, Malta, and other districts of 

 Southern Europe, are found to be made up from l-3rd to l-6th of 

 organic matters, chiefly of Polygastrica ; and although most of the 

 species alike occur in the most widely separated places mentioned, 

 and are of fresh water habit, yet there are others of limited and 

 special distribution. Thus the Sirocco dust, which fell in Genoa in 

 May, 1846, contained Synedra Entomon, a characteristic South 

 American form, along with African species ; the latter, however, 

 being in no greater quantity than in other dusts falling within the 

 limits above spoken of. Erom this, Ehrenberg surmises that there 

 is a curi'ent of air uniting Africa and America, in the region of the 

 trade-winds, and occasionally directed towards Europe. 



In the various kinds of meteoric dust, Ehrenberg has determined 

 no less than 320 specific fonns of Polygastrica, Phytolifharia, Poly- 

 tJialamia, and the soft parts of plants. The predominating genera of 

 Polygastrica (marine and freshwater,) are Coscinodiscus, Biploneis, 

 Goniothecium, Grammatophora, and Biddulphia. In Ehi'enberg's 



