PHYSIOGNOMY OF PLANTS. 233 



there, as on the enchanting shores of the Lake of Albano, Italy has 

 her oak forests, with glades as deeply embowered, and verdure as 

 fresh as those which we admire in the North of Europe. 



The deserts to the south of the Atlas, and the immense plains or 

 steppes of South America, must be regarded as only local phenomena. 

 The latter, the South American steppes, are clothed, in the rainy 

 season at least, with grass, and with low-growing almost herbaceous 

 mimosas. The African deserts are, indeed, at all seasons devoid of 

 vegetation ; seas of sand, surrounded by forest shores clothed with 

 perpetual verdure. A few scattered fan palms alone recall to the 

 wanderer's recollection that these awful solitudes belong to the do- 

 main of the same animated terrestrial creation which is elsewhere so 

 rich and so varied. The fantastic play of the mirage, occasioned by 

 the effects of radiant heat, sometimes causes these palm trees to ap- 

 pear divided from the ground and hovering above its surface, and some- 

 times shows their inverted image reflected in strata of air undulating 

 like the waves of the sea. On the west of the great Peruvian chain 

 of the Andes, on the coasts of the Pacific, I have passed entire weeks 

 in traversing similar deserts destitute of water. 



The origin of extensive arid tracts destitute of plants, in the. midst 

 of countries rich in luxuriant vegetation, is a geognostical problem 

 which has hitherto been but little considered, but which has doubt- 

 less depended on ancient revolutions of nature, such as inundations 

 or great volcanic changes. When once a region has lost the cover- 

 ing of plants with which it was invested, if the sands are loose and ' 

 mobile, and are destitute of springs, and if the heated atmosphere, 

 forming constantly ascending currents, prevents precipitation taking 

 place from clouds, ( 9 ) thousands of years may elapse ere organic life 

 can pass from the verdant shores to the interior of the sandy sea, 

 and repossess itself of the domain from which it had been banished. 



Those, therefore, who can view nature with a comprehensive 

 glance and apart from local phenomena, may see from the Poles to 

 the Equator organic life and vigor gradually augment with the aug- 

 mentation of vivifying heat. But, in the course of this progressive 

 increase, there are reserved to each zone its own peculiar beauties ; 

 to the tropics, variety and grandeur of vegetable forms; to the 

 north ; the aspect of its meadows and green pastures, and the periodic 



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