8 



GEOGRAPHICAL DISTRIBUTION 



CHATTER III. 



EXPLANATION OF THE MAP OF THE GEOGRAPHICAL DISTRIBUTION 



OF PLANTS. 



Plants, being attached to the soil, have not the power of locomo- 

 tion ; but by means of their seeds, transported by winds and waves, 

 they traverse belts of ocean that are impassable to land quadrupeds. 

 The genera and species of plants are exceedingly numerous ; and the 

 individual profusion thickly invests, covers indeed the land-surface of 

 the globe, except only in comparatively limited Desert tracts. On the 

 other hand, the species are restricted by the local variations in moist- 

 iiess of soil, and are influenced by climate more directly than land 

 quadrupeds ; so that more centres of difi'usion are distinguishable, as 

 will appear by comparing the maps. 



Among land quadrupeds, five separate methods of adaptation are 

 distinguishable in as many portions of the Earth's surface ; but among 

 plants, we are unable to point out any departure from one general 

 plan, the modifications in structural organization very closely corre- 

 sponding to the main divisions of climate. Yet certain genera, and 

 even whole Families of genera, are confined within narrow geogra- 

 phical limits; and well-characterized clusters of regions may also be 

 distinguished. 



In the accompanying map, I have marked out eleven Divisions or 

 clusters of regions, corresponding in general to the principal variations 

 in climate; but some of these clusters deriving a distinctive character 

 from geographical isolation. 



I. The Polar and Alpine Adaptation. 



Where the soil is chiefly or altogether watered by melting frosts 

 and snow. Prevailing over eight distinct botanical regions. 



