PART IV. 



GEOGRAPHICAL 



AND 



GEOLOGICAL BOTANY. 



CHAPTER I. 

 GENERAL CONSIDERATIONS. 



The object of this department of Botany is to determine the laws 

 which rule over the Distribution of Plants in Time and Space. 

 This abstract study is founded upon the facts furnished by 

 Botanical Geography and Geology, or the description of the 

 present and past conditions of the Vegetable inhabitants of the 

 globe, and, in return, supplies those branches of practical inquiry 

 with principles by means of which they may be systematized. 



Botanical Geography, which undertakes the description of the pecu- 

 liarities of the vegetation occurring at present upon the earth's surface, 

 and Botanical Geology, which pursues the investigation of the conditions 

 of vegetation which have successively existed in different Geological 

 epochs, deal with facts so far distinct in their character that it is most 

 convenient to treat of them separately ; but the principles are common 

 to both, and may be taken as one subject. 



Explanations of the facts which are obtained by geological inquiries 

 can only be securely founded, either directly or indirectly, upon laws de- 

 rived from facts furnished by experience of existing phenomena ; hence 

 the principles laid down in the present chapter are for the most part 

 only actually valid in relation to existing conditions, and, the subject yet 

 being in its infancy, are mostly only speculatively applied to the elucida- 

 tion of geological phenomena. 



Sect. 1. INFLUENCE OF EXTERNAL AGENTS UPON VEGETATION. 



Climate. Plants are endowed with means of diffusing them- 

 selves, more or less efficiently in different cases, over the surface 

 of the globe ; but in most cases their existence is limited to certain 

 regions or countries. Geographical obstacles, such as seas, 



