GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION 541 



dant. Although many of the climbers and epiphytes, especially the 

 Orchids, may possess flowers of great beauty, these do not, as a rule, 

 occur in numbers sufficient to balance the great mass of verdure, or 

 they are borne high up above the tops of the trees. 



Of course where the rainfall is deficient, as in the Sahara and other 

 deserts, a luxuriant vegetation is impossible, and many tropical dis- 

 tricts, like certain parts of Northern South America and India, which 

 have a marked dry season, show a very different type of flora from 

 the constantly rainy equatorial forests. Depending upon the amount 

 of rainfall, these regions are either covered with heavy forests, which 

 may shed their leaves during the dry season, or the country is an 

 open savannah diversified by scattered trees and shrubs, and covered 

 with a rapid growth of plants after the rains. The veldts of South 

 Africa and the elevated plateaus of Mexico are of this type. 



The keen struggle for existence within the Tropics has produced a 

 very much greater number of plant-types than exist elsewhere, and 

 these are much more specialized. Hence we find very few forms com- 

 mon to the Tropics of the Old and New Worlds, and the floras of these 

 regions have a very marked character of their own. While certain 

 families, like the Leguminosee, Palms, Orchids, and Compositse, are 

 cosmopolitan, they are usually represented, not only by distinct spe- 

 cies, but also by distinct genera in the Tropics of the Old and New 

 Worlds. Thus among the Palms, the Date-palms (Phcenix) are Old- 

 World types, the Royal-palms (Oreodoxa) and Palmettoes (Sabal) are 

 American. Of the Orchids, the showy Cattleyas and Oncidiums of 

 our conservatories are American, the Vandas and Dendrobiums, 

 Asiatic, 



Where plants are distributed throughout the Tropics, like the Ba- 

 nana, Breadfruit, and Cocoanut, as well as certain weeds like the Sen- 

 sitive-plant, this is due, directly or indirectly, to the agency of man. 



Floras of the Southern Hemisphere. The temperate zones of the 

 southern hemisphere are much more restricted than those of the 

 north. So far as it is known, the flora of the Antarctic Continent 

 is exceedingly limited, and it is completely shut off from communica- 

 tion with the land of the north. Moreover, the wide water-areas 

 lying between the southern extensions of South America and Africa 

 prevent any such communication as exists between Asia and North 

 America. Australia, also, is completely shut off from the other lands 

 of the southern hemisphere. It is not surprising, then, to find very 

 much less similarity in the floras of the southern hemisphere, than 

 between those of the north. 



Isolated Floras. Where a region is completely shut off from com- 

 munication with other land, as happens especially in remote oceanic 

 islands like the Hawaiian Islands and New Zealand, new accessions 

 to the flora are necessarily almost excluded ; and where the isolation 



