CHAPTER IV 



TJIE TEOPICAL FLORAS OF THE WORLD 



Although the idea of the tropics is always associated with 

 that of a grand develojiment of luxuriant vegetation, yet this 

 characteristic by no means applies to the whole of it, and 

 the inter-tropical zone presents almost as much diversity in 

 this respect as the temperate or even the frigid zones. This 

 diversity is due almost wholly to the unequal and even er- 

 ratic distribution of rainfall, and this again is dependent on 

 the winds, the ocean currents, and the distribution and ele- 

 vation of the great land masses of the earth. 



Once a year at each tropic the sun at noon is vertical for 

 a longer period continuously tlian in any other latitude, and 

 this, combined with the more complex causes above referred 

 to, seems to have produced that more or less continuous belt of 

 deserts that occurs all round the globe in the vicinity of those 

 two lines, but often extending as far into the tropics as into 

 the temperate zone. In a few cases similar conditions occur 

 so near the equator as to be very difficult of explanation. It 

 will be instructive to review briefly these arid regions, since 

 they must have had considerable influence in determining 

 the character of the tropical vegetation in their vicinity. 

 Beginning with the Sahara, pre-eminently the great desert 

 of our globe, if we take it with its extension across Arabia, 

 we find that it occupies an area nearly equal to the whole 

 of Europe, and that the African portion extends as far to 

 the south as to the north of tlie tropic of Cancer. It thus 

 eats away, as it were, a great slice of what in other continents 

 is covered with tropical vegetation, and forms a vast barrier 

 separating the tropical and temperate floras, such as exists 



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