GEOGRAPHY OF PLANTS AND ANIMALS. 345 



than doubtful, it is certainly very possible that small in- 

 fusoria may be passively carried up with ascending aqueous 

 vapour, and may float for a time in the atmosphere like the 

 pollen of the flowers of pines which falls every year from the 

 air. ( 427 ) This circumstance deserves to be especially con- 

 sidered in any renewal of the old discussion respecting 

 " spontaneous generation " ( 426 ) and the more so, because, 

 as I have before noticed, Ehrenbsrg has discovered the re- 

 mains of eighteen species of siliceous-shelled polygastric 

 infusoria, in the dust or sand which often falls on ships 

 navigating the ocean near the Cape Yerd Islands, at a 

 distance of 380 geographical miles from the African coast. 



The geography of plants and animals may be considered 

 either with reference to the differences and relative numbers 

 of typical forms, (the distribution of genera and species 

 in different localities,) or to the number of individuals 

 of each species on a given surface. Both in plants and in 

 animals it is essential to distinguish between social or grega- 

 rious, and solitary species. Those species of plants which I 

 have termed <e social" ( 427 ) spread a uniform covering over 

 large extents of country : to this class belong many kinds of 

 sea-weeds, cladonise and mosses which spread over the waste 

 plains of Northern Asia,' grasses, and cacti growing like 

 the pipes of an organ, avicennise and mangroves in the 

 tropics, and forests of coniferse and of birches in the 

 plains of the Baltic and of Siberia. It is especially by 

 this particular mode of geographical distribution, that cer- 

 tain vegetable forms constitute the leading feature in the 

 physiognomy of a country, imparting to it a character 



