it is, that throughout the table lands, the geographical position, as far as latitude is concerned, is entirely neutralized by the extreme rarefaction of the atmosphere obtained by ascending through loftier regions. Humboldt graphically declares that climates succeed each other in strata or layers, as we pass from Vera Cruz to the capital, or from the capital, descend to Acapulco or San Blas on the west coast,—beholding in our varied journey, the whole scale of vegetable life. The wild abundance of vegetation on the shore of the Gulf,—its beautiful palms whose stems are wreathed by a myriad of impenetrable parasites which grow with such rank luxuriance in the hot and humid air of the tropics,—are exchanged, as we begin to rise from the level of the sea, for hardier forest trees. At Jalapa the air is milder, though the vapors from the Gulf which concentrate and condense at about this height on the sides of the mountains, sustain the perpetual freshness of the verdure. Further on, the oak and the orange give place to the fir and pine. Here the rarefied air becomes pure, thin and perfectly transparent; but as it necessarily lacks moisture, which condenses below this region, the vegetation is neither so luxuriant nor so constantly vigorous. Great plains or basins, spread out in silent and melancholy vistas before the traveller,—many of them, cold, bleak and lonely moors, whose dreary levels sadden the heart of the spectator. The sun which comes down through the cloudless medium of an atmosphere unscreened by the usual curtain of vapor, parches and crisps the thirsty soil, whilst the winds that sweep uninterruptedly over the unbroken expanse, fill the air, during the dry season, with sand and dust. These high barren plains occupy a large portion of the centre of the country between Zacatecas, Durango and Saltillo; and such is in fact the character of large portions of the whole of Mexico, except when the comparatively level nature of the soil permits the small rivulets that filter from the Cordillera through the narrow vallies, to form themselves into rivers which may be used for irrigation. Wherever this is the case nature at once recovers her vigor under the influence of heat and moisture.
These physical features, and consequent diversities of temperature, have caused the division of Mexico, as it rises from the two Oceans, into three regions, or superficial strata, which are called, the tierras calientes, or hot lands; the tierras templadas, or temperate lands; and the tierras frias or cold lands. The tierra caliente covers chiefly that portion of the territory which lies on the borders of the Atlantic and Pacific; yet it is not confined exclusively to the coast, inasmuch as all those parts of Mexico in which there is heat and moisture enough to produce the fruits
