CHAP. VII. BOTANICAL, GEOGBAPHY. 295 



and these last we are obliged in our latitudes to preserve 

 in the stove or conservatory. The term " habitation " 

 has been given to any tract of country throughout 

 which each particular species is found naturally distri- 

 buted in stations adapted to its growth. The deter- 

 mination of these stations and habitations of plants 

 leads to an inquiry into the laws and circumstances 

 which regulate the distribution of species. We must 

 suppose that there exists a mutual relation between the 

 external conditions under which each species is naturally 

 disposed, and its own peculiar organization ; and this 

 relation must be sought for by a patient comparison of 

 the various species, genera, and families peculiar to dif- 

 ferent regions, with the precise conditions under which 

 they there exist. The problem is one of a most com- 

 plicated description, and it cannot be said that any very 

 decided progress has hitherto been made towards its 

 solution. We shall mention some of the more obvious 

 conditions under which all inquiries of this description 

 must be regulated, and present the reader with some of 

 the conclusions at which botanists have already arrived. 



Influence of external Circumstances on the Geographic 

 Distribution of Plants. 



(303.) Temperature. The influence of temper- 

 ature is the most decided of all the circumstances which 

 regulate the distribution of plants on the surface of the 

 earth. It seems evident, that each species is constitu- 

 tionally adapted to thrive best between certain limits of 

 temperature, and that every excess of heat or cold 

 (beyond these) is alike injurious to it. Hence, every 

 species must naturally be restricted within those geo- 

 graphical boundaries beyond which the temperature 

 either exceeds or falls short of these limits. These 

 boundaries will not necessarily coincide with any de- 

 finite parallels of latitude ; for it is well known that 

 the climate of different places having the same latitude 

 is very different. By drawing lines through those 

 u 4 



