GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION 231 
the distribution of the Tertiary flora of the same re- 
gions, ze. 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- 
meliacez of the New World, or the Proteacex of the 
Old World occur, which have no representatives in the 
other hemisphere. On the other hand, some orders, 
like the Composite and Leguminose, 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- 
