PDF - Wallace Online

EQUATORIAL VEGETATIONtinuous band in the vicinity of the two tropics. On the lineof the tropic of Cancer we have, in America, the deserts anddry plains of New Mexico; in Africa the Sahara; and inAsia, the Arabian deserts, those of Beloochistan and WesternIndia, and farther east the dry plains of North China andMongolia. On the tropic of Capricorn we have, in America,the Grand Chaco desert and the Pampas; in Africa, theKalahari desert and the dry plains north of the Limpopo ;while the deserts and waterless plains of Central Australiacomplete the arid zone. These great contrasts of verdure andbarrenness occurring in parallel bands all round the globe,must evidently depend on the general laws which determinethe distribution of moisture over the earth, more or lessmodified by local causes. Without going into meteorologicaldetails, some of which have been given in the precedingchapter, the main facts may be explained by the mode inwhich the great aerial currents are distributed. The tradewinds passing over the ocean from north-east to south-west,and from south-east to north-west, with an oblique tendencytowards the equator, become saturated with vapour, and areready to give out moisture whenever they are forced upwardsor in any other way have their temperature lowered. Theentire equatorial zone becomes thus charged with vapourladenair, which is the primary necessity of a luxuriantvegetation. The surplus air (produced by the meeting of thetwo trade winds) which is ever rising in the equatorial beltand giving upits store of vapour, flows off north and south asdry, cool air, and descends to the earth in the vicinity of thetropics. Here it sucks up whatever moisture it meets withand thus tends to keep this zone in an arid condition. Thetrades themselves are believed to be supplied by descendingcurrents from the temperate zones, and these are at firstequally dry and only become vapour-laden when they havepassed over some extent of moist surface. At the solsticesthe sun passes vertically over the vicinity of the tropics forseveral weeks, and this further aggravates the aridity ;andwherever the soil issandy and there are no lofty mountainchains to supply ample irrigation, the result is a more^or lessperfect desert. Analogous causes, which a study of aerialcurrents will render intelligible, have produced other great