154 THE GEOGRAPHICAL DISTRIBUTION 



vegetable kingdom, attains its highest development, whilst 

 about the poles it manifests the same organic inferiority. 



If we begin to reflect on the probable causes which have 

 produced this diversity of vegetation and animality in the 

 different parts of the world, we shall soon perceive that the 

 whole may be referred to a few general laws. This part 

 of the sciences of Zoology and Botany, however, demands 

 new researches. The nature and number of the animals 

 and plants in all parts of the world is not yet known and 

 it is only this particular knowledge of the plants and ani- 

 mals of each country, joined to numerous and exact geo- 

 graphical and meteorological observations, which will guide 

 us to a correct knowledge of those general laws which re- 

 gulate the distribution of the plants and animals on the 

 earth's surface. 



This branch of Natural History is of very recent origin, 

 and owes its existence to the philosophical researches of 

 Humboldt, Decandolle, Robert Brown, Schow, Mirbel, and 

 other eminent naturalists of the present century. Through 

 their labors, considerable progress has been made in this 

 interesting department of the natural sciences. 



Some plants appear to be capable of adapting themselves 

 to almost any climate. Thus many ferns and mosses are 

 common to both Europe and America, and numerous Eu- 

 ropean weeds infest the fields and woods throughout the 

 United States, to the exclusion in some instances, even of the 

 native denizens of the soil. So the spores of Cryptogamous 

 plants are so light that they are easily borne on atmospheric 

 currents across mountains and oceans, and this accounts for 

 the wide spread of the same genera and species over the 

 European and American continents; but the European 

 weeds which everywhere present themselves to the eye in 

 America, are certainly the result of commercial intercourse 



