CHAPTER IV 



THE TROPICAL FLORAS OF THE WORLD 



ALTHOUGH the idea of the tropics is always associated with 

 that of a grand development 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 

 erratic distribution of rainfall, and this again is dependent 

 on the winds, the ocean currents, and the distribution and 

 elevation 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 than 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 the 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 



40 



