Imagens das páginas

chains, again, whose summits are covered with perpetual snow, greatly reduce the temperature. The Andes, by intercepting the moisture of the trade winds as they blow from the east, cause copious rain on their east side and drought on the western, and in other ways greatly affect the general climate. The great indentation of the sea between North and South America; the huge forests, traversed by immense rivers; the absence of large sandy deserts, which easily become heated by a tropical sun; all serve to make America a cold country. A large portion of North America, moreover, expands within the Arctic circle. Sweeping over such a country, the north-west wind, which prevails in North America during the winter, becomes intensely cold; and northern blasts often make sudden inroads upon the tropical regions. Additional causes, such as the narrowing of the land, contribute to increase the rigour of the seasons in South America; especially as we approach Cape Horn. The whole of America, with little exception, is remarkable for its extreme moisture, and, consequently, its profuse vegetation.



74. Vegetables range over the whole face of the globe, from pole to pole; from the towering summit of the Andes, where the lichen creeps over the hardest rocks, to the bosom of the ocean, where we meet

with floating meadows of sea-weeds. Some vegetables delight in great heat, others flourish best in cold. Some require the extreme of moisture, others draw their sustenance from the arid rock and the burning sand. The geographical differences which vegetables present, depend almost entirely on the different degrees of heat, light, and moisture which they receive, as well as on the nature of the soil whence they derive nourishment, and the influence of ever-varying atmospheric phenomena.

75. It is in the torrid zone that vegetables are found in the greatest vigour, variety, and beauty. There the herbaceous plant of other zones becomes a shrub, and the shrub a tree. There we find the sugar-cane, the coffee-tree, the palm, the bread-fruit tree, the immense baobab, the date, cacao, vanilla, nutmeg, and the most delicate spices. The forest trees are often covered with a profusion of parasitical plants of the most brilliant colours and the most aromatic odours. At the same time, in consequence of the prodigious height to which whole regions are elevated in the tropics, and the reduced temperature of that elevation, the productions of the more temperate and colder regions are also found here. Cypresses, firs, dates, barberries, and alders, which resemble those of our own country, cover the mountainous districts of the south of Mexico, as well as the chain of the Andes under the equator.

76. Vegetation is greatly determined by latitude. In the temperate regions, as we recede from the equator, we find the rosaceous plants, as the rose itself; the raspberry and the bramble, the apple, the pear, and the mountain ash, the almond and the peach, the

apricot, the plum, the cherry, and the laurel. Here are also the cruciferous plants, as the radish, the cabbage, the turnip, cress, mustard, and rape: and the umbelliferous plants, of which some are poisonous, as hemlock and water dropwort; others are esculents, or good for food, as celery, carrots, and parsnips; many yield aromatic fruits, as caraway, coriander, and anise; a few secrete a fetid gum resin, as galbanum and assafoetida. As we travel northward, we successively meet the various dates, the chesnut, and the beech; the fir, the cedar, and the pine; the poplar and the willow, the alder and the birch. Verdant meadows of soft grasses, and smiling fields of waving corn, intersperse and beautify the landscape. The tropical parasites disappear before plants whose fleshy roots draw their sustenance from the soil; the trunks of aged trees are clothed with mosses, parasitic fungi creep round decaying vegetables of larger growth, and the waters abound with numerous species of confervæ, which float in stagnant pools or sluggish streams in the form of green entangled threads.

77. As we enter the frigid regions, all these gradually dwindle and disappear; and grasses, mosses, and lichens are the last retreats of vegetable life. The birch, which is the hardiest of trees, ceases to grow in latitude 70°; while beyond the region of lichens, vegetation is arrested by perpetual snow; although even here, if a southern aspect chance to thaw for four or five days any cleft in a rock, a few hardy specimens of ranunculus glacialis venture to bloom.

78. The isothermal lines, to which we have al

ready referred, have been chiefly determined by the growth of particular plants. Near the equator they coincide generally speaking with the parallels of latitude; but as they recede thence, their course becomes very irregular. An isothermal line of any given

temperature will recede farther from the equator in Europe than it will either in America or Asia, from the causes already mentioned under the head of climate; and, in like manner, in passing through the maritime parts of Europe, and the adjacent islands, it will recede farther from the equator than either in the continental parts or in elevated regions. The isothermal lines of 78° Fahr., 68° Fahr., 59° Fahr., 50° Fahr., 41° Fahr., and 32° Fahr. divide the earth's surface, in each hemisphere, into seven vegetable zones. 1. The region of the Spices, a regular zone extending 20° on each side of the equator, with a mean temperature, varying from 81° Fahr. at the equator to 78° on its northern and southern borders. 2. The region of the Sugar-cane, whose mean temperature ranges from 78° to 68° Fahr. 3. The region of the Olive and Fig, from 68° to 59° Fahr. 4. The region of the Wine-grape, from 59° to 50° Fahr. 5. The region of the Oak and Wheat, from 50° to 41° Fahr. 6. The region of the Fir, Pine, and Birch, from 41° to 32° Fahr. 7. The region of Lichens and Mosses, extending from the isothermal line of 32° Fahr. towards the poles, until vegetation ceases. The above trees and plants are not confined to these zones; but it is in these that they attain their greatest perfection.

79. In ascending a lofty mountain, particularly in a warm country, we may expect to meet with the

vegetables of different climates. If the mountain be near the equator, we might theoretically expect to find its base encircled by the aromatic productions of the torrid zone; on its sides, the sugar-cane and coffee-tree of the tropics; higher up, the olive and fig of Spain, Italy, and Turkey; higher still, the vines of France and Germany; next, the oaks, elms, and beeches of England and the north of Europe; then, the firs and pines of Scotland and Scandinavia ; and, lastly, the lichens and mosses of Lapland. Approximations to this theoretical distribution actually exist; as on the Peak of Teneriffe, Mount Ararat, and Mount Etna.



80. Like vegetables, animals are adapted to different climates, soils, and localities: some are confined to special regions; others are distributed over nearly the whole surface of the globe. The native animals, again, of one region often admit of transportation to other countries; where, however, they generally undergo considerable changes, and pass into numerous varieties. Subject to these varieties, the ox, the horse, and the hog are found from the equator to the polar circles; though within the frigid zones, the horse and the ox degenerate and disappear. The sheep, the goat, and the dog extend over the whole habitable globe.

81. The earth may be divided into fourteen zoo

« AnteriorContinuar »