CLIMATE OF SOUTH AMEEICA. 31 



Old lacustrine basins, dry watercourses, and other indications of extremely ener- 

 getic water action occur precisely on the western slopes where the work at present 

 accomplished by the moisture, under the form of dews and rare showers, is insigni- 

 ficant compared with that produced by the changes of temperature. Noteworthy 

 especially are the deep quebradas, or narrow gorges, excavated to depths of from 

 300 to 600 feet in the escarpments of the Peruvian plateaux. One asks in 

 amazement, what downpours could have scored such tremendous furrows in the 

 live rock ? They are certainly not the work of the few showers which fall every 

 thirty or forty years, as if by a miracle, in this now almost rainless region. 



The hypothesis of a formerly moist climate is confirmed by the facts drawn 

 from the domain of natural history. Various plants flourishing on the Ecuador 

 and north Peruvian uplands reappear in South Chili, but are completely absent 

 from the intervening arid Bolivian tablelands. So also with certain species of 

 animals, such as the Cervus antisensis of the Peruvian Andes, which has been 

 described by D'Orbigny and Tschudi, and which appears to be identical with the 

 guermul or CervKS chilcnsis of the southern Andes and Magellanic lands. It occurs 

 nowhere in North Chili, and the question arises, how has its range been severed 

 in two ? How does it happen that the same plants also occupy two distinct 

 domains, one cold, the other hot, while avoiding the intermediate temperate zone ? 

 The explanation is that rain and atmospheric moisture are a necessary element in 

 the evolution of these organisms. So long as the Andean plateaux were suffi- 

 ciently watered, plants and animals roamed freely over the region at present 

 occupied by the Atacama desert and neighbouring heights. But when the rains 

 failed, a solution of continuity was effected between the northern and southern 

 biological areas. In the heart of the Atacama desert, where nothing now sprouts 

 except a few almost leafless stalks, the miner's pick often turns up the roots of large 

 trees which formerly grew in forests on the now arid steppe.* 



To the increasing dryness of the climate is also due the fact that the great 

 Bolivian lake, Titicaca, has ceased to form part of the Amazons system. Formerly 

 it sent its overflow to the Beni affluent, but it is no longer able to cross the 

 parting line, and the slowly subsiding waters have left vast spaces unflooded. 

 What remains of the old inland sea is nearly fresh, doubtless because the isolation 

 of the lacustrine basin dates from a comparatively recent geographical epoch. 



Flora. 



In the relative extent of its area under timber South America is surpassed by 

 the Eastern Archipelago alone. Even Central Africa with its prodigious seas of 

 verdure, which the Stanley expedition up the Aruwimi had so much difficulty in 

 traversing, presents no such extensive space under continuous arboreal vegetation 

 as the boundless woodlands of the Amazons basin and its affluents. These wood- 

 lands comprise also the whole of the Guiana seaboard, and are continued north- 

 westwards by those of the Magdalena and Atrato valleys in Colombia. 



With the exception of the interruptions caused by rocks, lakes, swamps, and 

 * Philippi ; H. W. Bates, Stanford's South America, 



