CHAP. VII. BOTANICAL GEOGRAPHY. 307 



41. Madagascar, with the Mauritius and Isle of 

 Bourbon. 



42. Congo. 



43. Guinea. 



44. Senegambia. 



45. Canaries, Madeira and Azores. 



The centres of Africa, Asia, and other unexplored 

 districts probably afford several more regions. 



Twelve of the regions enumerated belong to the 

 northern hemisphere, between the pole and tropic of 

 Cancer ; twenty-six are intra-tropical ; and seven are 

 extra-tropical, in the southern hemisphere. The first 

 are the largest, and approach each other the nearest ; 

 the second are less extended, and more frequently se- 

 parated by the ocean and deserts ; the last are very un- 

 equal in extent, and above all more dispersed, many of 

 them being small islands in the midst of an immense 

 ocean. 



(313.) Relative Number of Individuals and Groups 

 in each Region. In contrasting one botanical region 

 with another, inquiry may be made as to the number 

 of individuals which each may be supposed to contain, 

 and also as to the number of species, genera, and fami- 

 lies. The result of the first of these inquiries must 

 depend upon the actual extent of country included in 

 the region, and upon the character of its climate. The 

 nature of the plants which grow in the region will also 

 form an important element in this inquiry, since a space 

 occupied by a single tree may contain many hundreds 

 of smaller plants, and those regions in which large 

 species prevail will not contain so many individuals as 

 those which abound in small ones. The greater or less 

 prevalence of particular species in a given region, may 

 be observed by noting the number of places in which they 

 occur ; and then representing by ciphers the relative 

 abundance in which they appear to exist in each spot, 

 the sums of these ciphers will afford some approximation 

 to the relative abundance of each species. Those regions 

 x 2 



