The boundless plains or steppes of South America, and the deserts beyond the Atlas range of mountains, can only be regarded as mere local phenomena. The former are found to be covered, at least in the rainy season, with grasses and low almost herbaceous Mimosæ; while the latter are seas of sand in the interior of the Old Continent,—vast arid tracts surrounded by borders of evergreen forests. Here and there only a few isolated fan-palms remind the wanderer that these dreary solitudes are a portion of animated nature. Amid the optical delusions occasioned by the radiation of heat, we see the bases of these trees at one moment hovering in the air, at the next their inverted image reflected in the undulating strata of the atmosphere. To the west of the Peruvian Andes, on the shores of the Pacific, I have passed weeks in traversing these waterless deserts. The origin of this absence of plants over large tracts of land, in regions characterised on every side by the most exuberant vegetation, is a geological phenomenon which has hitherto received but little attention; it undoubtedly arises from former revolutions of nature, such as inundations, or from volcanic convulsions of the earth’s surface. When once a region loses its vegetable covering, if the sand is loose and devoid of springs, and if vertically ascending currents of heated air prevent the precipitation of vapour[79], thousands of years may elapse before organic life can penetrate from the green shores to the interior of the dreary waste. Those who are capable of surveying nature with a comprehensive glance, and abstract their attention from local phenomena, cannot fail to observe that organic development and abundance of vitality gradually increase from the poles towards the equator, in proportion to the increase of animating heat. But in this distribution every different climate has allotted to it some beauty peculiar to itself: to the Tropics belong variety and magnitude in vegetable forms; to the North the aspect of its meadows and the periodical renovation of nature at the first genial breath of spring. Every zone, besides its own peculiar advantages, has its own distinctive character. The primeval force of organization, notwithstanding a certain independence in the abnormal development of individual parts, binds all animal and vegetable structures to fixed ever-recurring types. For as in some individual organic beings we recognise a definite physiognomy, and as descriptive botany and zoology are, strictly speaking, analyses of animal and vegetable forms, so also there is a certain natural physiognomy peculiar to every region of the earth.