EQUATORIAL VEGETATION 



tinuous band in the vicinity of the two tropics. On the line 

 of the tropic of Cancer we have, in America, the deserts and 

 dry plains of New Mexico; in Africa the Sahara; and in 

 Asia, the Arabian deserts, those of Beloochistan and Western 

 India, and farther east the dry plains of North China and 

 Mongolia. On the tropic of Capricorn we have, in America, 

 the Grand Chaco desert and the Pampas; in Africa, the 

 Kalahari desert and the dry plains north of the Limpopo ; 

 while the deserts and waterless plains of Central Australia 

 complete the arid zone. These great contrasts of verdure and 

 barrenness occurring in parallel bands all round the globe, 

 must evidently depend on the general laws which determine 

 the distribution of moisture over the earth, more or less 

 modified by local causes. Without going into meteorological 

 details, some of which have been given in the preceding 

 chapter, the main facts may be explained by the mode in 

 which the great aerial currents are distributed. The trade 

 winds passing over the ocean from north-east to south-west, 

 and from south-east to north-west, with an oblique tendency 

 towards the equator, become saturated with vapour, and are 

 ready to give out moisture whenever they are forced upwards 

 or in any other way have their temperature lowered. The 

 entire equatorial zone becomes thus charged with vapour- 

 laden air, which is the primary necessity of a luxuriant 

 vegetation. The surplus air (produced by the meeting of the 

 two trade winds) which is ever rising in the equatorial belt 

 and giving up its store of vapour, flows off north and south as 

 dry, cool air, and descends to the earth in the vicinity of the 

 tropics. Here it sucks up whatever moisture it meets with 

 and thus tends to keep this zone in an arid condition. The 

 trades themselves are believed to be supplied by descending 

 currents from the temperate zones, and these are at first 

 equally dry and only become vapour-laden when they have 

 passed over some extent of moist surface. At the solstices 

 the sun passes vertically over the vicinity of the tropics for 

 several weeks, and this further aggravates the aridity ; and 

 wherever the soil is sandy and there are no lofty mountain 

 chains to supply ample irrigation, the result is a more^or less 

 perfect desert. Analogous causes, which a study of aerial 

 currents will render intelligible, have produced other great 



