PDF - Wallace Online

TROPICAL NATUREpermanent springs to supply them are worthless. In thecolder partsof the temperate zones the absence of forests isnot so much felt, because the hills and uplands are naturallyclothed with a thick coating of turf or peat which absorbsmoisture and does not become overheated by the sun's rays,and the rains are seldom violent enough to strip this protectivecovering from the surface. In tropical and even inwarm -temperate countries, on the other hand, the rains areperiodical and often of excessive violence for a short period ;and when the forests are cleared away the torrents of rainsoon stripoff the vegetable soil, and thus destroy in a fewyears the fertility which has been the growth of many centuries.The bare subsoil becoming heated by the sun, everyparticle of moisture which does not flow off is evaporated, andthis again reacts on the climate, producing long -continueddroughts only relieved by sudden and violent storms, whichadd to the destruction and render all attempts at cultivationunavailing. Wide tracts of fertile land in the south of Europehave been devastated in this manner, and have become absolutelyuninhabitable. Knowingly to produce such disastrousresults would be a far more serious offence than any destructionof property which human labour has produced and canreplace ; yet we have ignorantly allowed such extensive clearingsfor coffee cultivation in India and Ceylon as to causethe destruction of much fertile soil, which human labourcannot replace, and which will surely, if not checked in time,lead to the deterioration of the climate and the permanentimpoverishment of the country. lShort Twilight of the Equatorial ZoneOne of the phenomena which markedly distinguish theequatorial from the temperate and polar zones is theshortness of the twilight and consequent rapid transitionfrom day to night and from night to day. As this dependsonly on the fact of the sun descending vertically insteadofobliquely below the horizon, the difference is mostmarked when we compare our midsummer twilight with1 For a terrible picture of the irreparable devastation caused by the recklessclearing of forests, see the third chapter of Mr. Marsh's work, The Earth asModified by Human Action.