Page:EB1911 - Volume 19.djvu/792

 therein the trade winds of a broadened torrid zone, the stormy westerly winds of middle latitudes and the irregular winds of the polar regions, are well exemplified over North America; but, as is usually the case on land, the systematic movement of the atmosphere is better seen in the drift of the clouds than in the movement of the surface winds, which are much modified by the changes from hill to valley, from mountain to plain. Nevertheless the prevalence of the general atmosphere currents has much to do with the control of certain values of annual temperature range, as well as with the distribution of rainfall. The former are small (about 20°) along a great stretch of the Pacific coast, even as far north as Alaska, where the moderating influences of the ocean are brought upon the land by the westerly winds; while a range appropriate to a continental interior (30° or 40°) is experienced over most of the eastern side of the continent in temperate latitudes, and even upon the North Atlantic ocean near the American coast, where strong seasonal changes of temperature are carried forward by the westerly winds. It is particularly in this respect that the general climatic resemblances between North America and Eurasia, above referred to, are broken; for eastern Canada and western Europe are strikingly unlike in seasonal variations of temperature. Labrador is about 10° cooler than northern Germany in July, but nearly 40° colder in January.

The distribution of rainfall is in general controlled by the prevailing course of the winds. The West Indies receive abundant rain from the passing trades. In Mexico and Central America the eastern slopes are for the most part better watered than the western, because the winds there come chiefly from the east (maximum over 100 in. in Guatemala and adjacent parts). Farther north the reverse holds true; the Pacific slope north of 40° latitude has an abundant rainfall (maximum over 100 in.), and its mountains are clothed with dense forests. There are large areas of deficient rainfall (less than 20 in.) in the interior of the continent, where the intermontane basins and the piedmont plains that slope eastward from the Rocky Mountains in middle latitudes are treeless. The areas afflicted with dryness are unsymmetrically distributed, being west of the medial meridian (95°), because of the ranges near the Pacific by which rain is withheld from the basins and from the plains farther east. The dryness is induced not only by light precipitation, but also by active evaporation in the warm season—a rule that holds true until a high latitude is reached. East of the medial meridian great profit is received from the warm and moist winds that are drawn inland from the water surface of the mediterraneans (Gulf of Mexico and Caribbean Sea) which so advantageously occupy the latitudes that are given up to the Sahara in the Old World. It is largely on this account that the central and eastern parts of the Mississippi basin enjoy a sufficient and well-distributed rainfall, producing forests or fertile prairies over great areas (rainfall over 40 in.). Regions of prevailing snowfall are chiefly in the north-west and north-east; the former includes the higher ranges of the western highlands in Canada and Alaska, where the snowfall from the Pacific winds is heavy, and extensive snowfields and glaciers are formed; the former includes Greenland, where a heavy ice-sheet shrouds the land, the snowfall of moderate measure being probably supplied mostly from the North Atlantic. In the northern continental interior snow covers the ground during the winter season, not that the snowfall is heavy but that the persistent cold weather preserves the moderate amount that falls.

The extension of the continent across the belts of the terrestrial wind system tends to turn branch winds from the westerlies toward the trades on the Pacific border, and from the trades toward the westerlies on the Atlantic border. This effect is strengthened in summer, when the higher temperature prevalent over the continent causes the air to flow away from above the lands, and to accumulate over the neighbouring oceans, on each of which a vast anticyclone is thereby established—the circulation of the atmosphere over the North Atlantic and North Pacific thus coming to simulate the circulation of the surface waters of the oceans themselves. It is partly on account of this deflection of the summer winds up the Mississippi valley that the eastern interior of the continent receives a beneficent rainfall as already stated. In winter when the inflow from the south is replaced by an outflow, little rain or snow would fall but for the indraft winds of cyclonic storms by which the outflow appropriate to the cold season of the continent is temporarily reversed. The free play of the cyclonic winds north and south over the great medial plains permits indrafts from torrid and frigid sources, which sometimes succeed each other rapidly, producing abrupt and frequent weather changes. Something of the same contrasts is produced by winds drawn in upon the eastern coast alternately from over the moist and warm waters of the Gulf Stream, and from over the moist and cold waters of the Labrador current.

The southerly flow of the branching winds along the Pacific coast gives them a drying quality, and thus still further broadens the western arid region towards the ocean until it reaches the coast in southern California and north-western Mexico (rainfall less than 10 in.), there joining the arid belt of western Mexico and presenting a strong contrast to the rainy forested coast farther north; but although unfavourably dry, the southern California coast is one of the most truly temperate regions of the world, in respect of mildness and constancy of temperature. The drying winds cover all California in summer, but they migrate southward in the winter, giving place to the stormy westerlies. Thus California has a subtropical climate of wet winters and dry summers; while north in British Columbia and Alaska there is plentiful rainfall all the year round, and farther south there is persistent aridity.

The fauna of North America (Nearctic) is more closely related to that of Europe-Asia (Palaearctic) than to that of any other zoogeographical province; the two being united by many writers in one faunal province (Holarctic). The reindeer (caribou), beaver and polar bear are common to both provinces. The moose, wapiti, bison and grizzly bear of North America are closely related to the elk, red deer or stag, aurochs and brown bear of Eurasia; and the following groups are well represented in both provinces: cats, lynxes, weasels, bears, wolves, foxes, seals, hares, squirrels, marmots, lemming, sheep and deer. On the other hand the following forms are characteristic of North America: (rodents) pouched rats or gophers, musk rat, prairie dog, Canadian porcupine; (carnivora) raccoon and skunk; (ungulates) musk ox, bighorn, Rocky Mountains goat, pronghorn; (marsupial) opossum. Among birds there is a close resemblance to those of Eurasia, with some admixture of South American forms, as in the humming birds. The forms especially characteristic of the northern continent are the Baltimore oriole, bobolink, cowbird, flycatchers, wood-warblers, Californian quail, tree grouse, sage grouse, wild turkey and turkey buzzard. The house sparrow of Europe has been introduced, and has become very common, especially in the cities, where it is known as the English sparrow. Reptilian and amphibian groups are well represented; turtles are especially numerous; salamanders are varied and large; rattlesnakes are among the more peculiar forms. Among fish, the characteristic forms are the cyprinoids (carp), sturgeon, salmon, pike and especially the suckers, sunfish, mudfish (Amia) and gar pike (Lepidosteus). The most characteristic group of invertebrates is the Unionidae or river mussels.

The floral areas of North America, limited by the geographic divisions of the continent, may be divided into five belts: the eastern forested area, the western forested area, the interior unforested area, the northern barren lands and the Gulf coast. The eastern forested area extends from the Laurentian highland in Canada to the Great Lakes, and southward east of the Mississippi to the Gulf coast. In the north and along the mountains southward, the forests are largely coniferous, with a mixture of birches, poplars and maples. Southward, especially in the interior and at low altitudes, the conifers largely disappear, and oaks, hickories, plane-trees, tulip-trees, walnuts and other valuable deciduous species abound. The western forested area begins in the eastern Rocky Mountains and extends to the Pacific. Eastward in the mountains the forests are interspersed with arid districts which increase in area southward. Northward, in Canada, the mountains of the middle Cordilleras are densely wooded with continuous forest up to the timber line. Near the middle Pacific coast the forests attain a luxuriant development, the redwood (Sequoia) of California and Oregon sometimes reaching a height of from 300 to 400 ft. The unforested area of the interior consists of two very dissimilar portions. The vast fertile prairies extend from the Great Lakes westward to the Great Plains, and southward west of the Mississippi, with occasional eastward lobes at low altitudes. On these plains grasses and other herbaceous vegetation abound, and throughout this fertile belt agriculture is largely followed, the grain and hay crops being especially important. Northward in Canada the plains become wooded, the western mountains and the eastern highlands being thus connected by a narrow strip of forest. South-westward and westward the fertile prairie gives way to a vast arid region beginning on the Great Plains and extending as far as south-eastern California, and thence southward into Mexico. On this broad desert few trees are found, although piñons grow on the cliffs and ledges, and cottonwoods occur along the watercourses; but the various ranges that surmount the desert frequently carry forests. The desert vegetation as a whole consists of cacti, agaves, sage-brush (Artemisia) and other plants adapted to arid conditions. North of the eastern forested area and east of the northern Cordilleras are the “barren lands,” with frozen subsoil, extending to the Arctic coast. The growing season here is short and the climate forbidding, so that trees cannot develop, although birches, poplars, willows and other genera, which southward attain large size, are present as dwarf shrubs. The vegetation of this northern barren district, like that of bleak mountain summits southward, is very similar in character to that of other extreme boreal regions. Blueberries, crowberries and some other small fruits are abundant, but the brief summer will not mature most crops of the temperate zone. The Gulf coast, on the other hand, supports a vegetation decidedly tropical in its nature. Somewhat developed in Florida and the other southern states, this flora becomes the prevailing one on the coast of Mexico and Central America, especially from the region of Vera Cruz southward, where the forests are largely composed of palms and live oaks, and where giant bamboos often attain a height of 40 ft. In these tropical forests many orchids and other showy plants of northern conservatories are native.

North America, with an area of about 8,000,000 sq. m. (16% of all the lands, or 4·12% of the whole earth’s surface), and a mean altitude of about 2000 ft., at present, plays a part in human history that is of greater importance than is warranted