GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION 231 



the distribution of the Tertiary flora of the same re- 

 gions, i.e. similar climate and nearly continuous land 

 communication. 



The conditions in the Antarctic regions are very 

 different from those in the northern hemisphere. The 

 southern extensions of Africa and South America are 

 widely separated, and the little explored land area sur- 

 rounding the pole is totally shut off from both conti- 

 nents, and so far as known possesses a very scanty 

 flora, both on account of its isolation and the excessive 

 severity of the climate. 



While, as we have seen, the flora of the high north- 

 ern latitudes is very similar in both the eastern and 

 western hemispheres, as we go south, more and more 

 new types appear, and as a rule these are quite differ- 

 ent in the Old and New Worlds. These differences 

 become more pronounced as the tropics are approached, 

 when whole orders of plants, like the Cacti and Bro- 

 meliaceiB of the New World, or the Proteacese of the 

 Old World occur, which have no representatives in the 

 other hemisphere. On the other hand, some orders, 

 like the Compositae and Leguminosse, are cosmopolitan, 

 as are certain genera, but very few species are thus 

 widespread except as they may have been distributed 

 through human agencies, so that, in the tropics espe- 

 cially, it is exceedingly rare to find identical species in 

 the Old and New Worlds, except as they have thus 

 been introduced. 



The alpine flora of high mountains usually contains 

 a number of plants often identical with, or closely re- 

 lated to, species growing much further north, but not 

 occurring at all in the adjacent lowlands. This is es- 



