PHYSICAL DESCRIPTION OF THE UNIVERSE. 49 



habitation, and to the differently inflected isothermal 

 curves, than according to principles of gradation in the 

 development of their internal organisation. But botany 

 and zoology, which are the two branches of the descriptive 

 natural history of organised bodies, are the fruitful sources 

 from whence we draw the materials, without which the 

 study of the relations and connection of phenomena would 

 want a solid foundation. 



We will here add an important observation. The first 

 general glance over the vegetation of an extensive portion 

 of a continent, shews us an assemblage of dissimilar forms, 

 graminese, orchideae, coniferae, and oaks : we perceive these 

 families and genera, instead of being locally associated, scat- 

 tered apparently as it were by chance : but this irregular 

 dispersion is only apparent ; and it is the province of phy- 

 sical geography to shew that vegetation every where presents 

 constant numerical relations in the development of its forms 

 and types; that, in the same climates, species which are 

 wanting in one country, are replaced in a neighbouring one 

 by other species of the same families, according to a law of 

 substitution, which seems to belong to the yet unknown 

 relations of organised beings ; and by which the numerical 

 proportion of particular great families to the whole mass of 

 the phsenogamous floras in adjoining countries is main- 

 tained. There is thus revealed in the multitude of organic 

 forms by which these regions are peopled a principle of 

 unity, a primitive plan of distribution. There is also dis- 

 covered in each zone, diversified according to the families of 

 plants, a slow but continuous action on the aerial ocean, an 

 action which depends on the influence of light that primary 

 and essential condition of all organic vitality ou the solid or 



