16 INTRODUCTION. 



over its surface and the laws that regulate it, especially as con- 

 nected with the actual distribution of those natural agents which 

 chiefly influence vegetation, such as heat, light, water, &c., (in 

 other words, with climate,) give rise to GEOGRAPHICAL BOTANY, a 

 subject which connects Botany with Physical Geography. Under 

 the same general department naturally falls the consideration of 

 the changes which the vegetable kingdom has undergone in times 

 anterior to the present state of things, as studied in their fossil re- 

 mains, (a contribution which Botany offers to Geology,) as well as 

 of those changes which man has effected in the natural distribution 

 of plants, and the alterations in their properties or products which 

 have been developed by culture. 



8. Of these three great departments of the science, that of 

 Physiological Botany, forming as it does the basis of all the rest, 

 first demands the student's attention. 



