No part of the world can be truly understood without a knowledge of its garment of vegetation, for this determines not only the nature of the animal inhabitants but also the occupations of the majority of human beings. Although the soil has much to do with the character of vegetation, climate has infinitely more. It is temperature which causes the moss and lichens of the barren tundras in the far north to be replaced by orchids, twining vines, and mahogany trees near the equator. It is rainfall which determines that vigorous forests shall grow in the Appalachians in latitudes where grasslands prevail in the plains and deserts in the western cordillera.
Forests, grass-lands, deserts, represent the three chief types of vegetation on the surface of the earth. Each is a response to certain well-defined conditions of climate. Forests demand an abundance of moisture throughout the entire season of growth. Where this season lasts only three months the forest is very different from where it lasts twelve. But no forest can be vigorous if the ground habitually becomes dry for a considerable period during which the weather is warm enough for growth. Desert vegetation, on the other hand, which consists primarily of bushes with small, drought-resistant leaves, needs only a few irregular and infrequent showers in order to endure long periods of heat and drought. Discontinuity of moisture is the cause of deserts, just as continuity is the necessary condition of forest growth. Grasses prevail where the climatic conditions are intermediate between those of the forest and the desert. Their primary requisite is a short period of fairly abundant moisture with warmth enough to ripen their seeds. Unlike the trees of the forests, they thrive even though the wet period be only a fraction of the entire time that is warm enough for growth. Unlike the bushes of the desert, they rarely thrive unless the ground is well soaked for at least a few weeks. Most people think of forests as offering far more variety than either deserts or grass-lands. To them grass is just grass, while trees seem to possess individuality. In reality, however, the short turfy grass of the far north differs from the four-foot fronds of the bunchy saccaton grass of Arizona, and from the far taller tufts of the plumed pampas grass, much more than the pine tree differs from the palm. Deserts vary even more than either forests or grass-lands. The traveler in the Arizona desert, for example, has been jogging across a gravelly plain studded at intervals of a few yards with little bushes a foot high. The scenery is so monotonous and the noon sunshine so warm that he almost falls asleep. When he wakes from his daydream, so weird are his surroundings that he thinks he must be in one of the places to which Sindbad was carried by the roc. The trail has entered an open forest of joshuas, as the big tree yuccas are called in Arizona. Their shaggy trunks and uncouth branches are rendered doubly unkempt by swordlike, ashy-yellow dead leaves that double back on the trunk but refuse to fall to the ground. At a height of from twelve to twenty feet each arm of the many-branched candelabrum ends in a stiff rosette of gray-green spiky leaves as tough as hemp. Equally bizarre and much more imposing is a desert "stand" of giant suhuaros, great fluted tree-cacti thirty feet or more high. In spite of their size the suhuaros are desert types as truly as is sagebrush.
In America the most widespread type of forest is the evergreen coniferous woodland of the north. Its pines, firs, spruces, hemlocks, and cedars which are really junipers, cover most of Canada together with northern New England and the region south of Lakes Huron and Superior. At its northern limit the forest looks thoroughly forlorn. The gnarled and stunted trees are thickly studded with half-dead branches bent down by the weight of snow, so that the lower ones sweep the ground, while the upper look tired and discouraged from their struggle with an inclement climate. Farther south, however, the forest loses this aspect of terrific struggle. In Maine, for example, it gives a pleasant impression of comfortable prosperity. Wherever the trees have room to grow, they are full and stocky, and even where they are crowded together their slender upspringing trunks look alert and energetic. The signs of death and decay, indeed, appear everywhere in fallen trunks, dead branches, and decayed masses of wood, but moss and lichens, twinflowers and bunchberries so quickly mantle the prostrate trees that they do not seem like tokens of weakness. Then, too, in every open space thousands of young trees bank their soft green masses so gracefully that one has an ever-present sense of pleased surprise as he comes upon this younger foliage out of the dim aisles among the bigger trees.
Except on their southern borders the great northern forests are not good as a permanent home for man. The snow lies so late in the spring and the summers are so short and cool that agriculture does not prosper. As a home for the fox, marten, weasel, beaver, and many other fur-bearing animals, however, the coniferous forests are almost ideal. That is why the Hudson's Bay Company is one of the few great organizations which have persisted and prospered from colonial times to the present. As long ago as 1670 Charles II granted to Prince Rupert and seventeen noblemen and gentlemen a charter so sweeping that, aside from their own powers of assimilation, there was almost no limit to what the "Governor and Company of Adventurers of England trading into Hudson's Bay" might acquire. By 1749, nearly eighty years after the granting of the charter, however, the Company had only four or five forts on the coast of Hudson Bay, with about 120 regular employees. Nevertheless the poor Indians were so ignorant of the value of their furs and the consequent profits were so large that, after Canada had been ceded to Great Britain in 1763, a rival organization, the Northwest Fur Company of Montreal, was established. Then there began an era that was truly terrible for the Indians of the northern forest. In their eagerness to get the valuable furs the companies offered the Indians strong liquors in an abundance that ruined the poor red man, body and soul. Moreover the fur-bearing animals were killed not only in winter but during the breeding season. Many mother animals were shot and their little ones were left to die. Hence in a short time the wild creatures of the great northern forest were so scarce that the Indians well-nigh starved.
In spite of this slaughter of fur-bearing animals, the same Company still draws fat dividends from the northern forest and its furry inhabitants. If the forest had been more habitable, it would long ago have been occupied by settlers, as have its warmer, southern portions, and the Company would have ceased to exist. Aside from the regions too cold or too dry to support any vegetation whatever, few parts of the world are more deadening to civilization than the forests of the far north. Near the northern limit of the great evergreen forest of North America wild animals are so rare that a family of hunting Indians can scarcely find a living in a thousand square miles. Today the voracious maw of the daily newspaper is eating the spruce and hemlock by means of relentless saws and rattling pulp-mills. In the wake of the lumbermen settlers are tardily spreading northward from the more favored tracts in northern New England and southern Canada. Nevertheless most of the evergreen forests of the north must always remain the home of wild animals and trappers, a backward region in which it is easy for a great fur company to maintain a practical monopoly.
Outliers of the pine forest extend far down into the United States. The easternmost lies in part along the Appalachians and in part along the coastal plain from southern New Jersey to Texas. The coastal forest is unlike the other coniferous forests in two respects, for its distribution and growth are not limited by long winters but by sandy soil which quickly becomes dry. This drier southern pine forest lacks the beauty of its northern companion. Its trees are often tall and stately, but they are usually much scattered and are surrounded by stretches of scanty grass. There is no trace of the mossy carpet and dense copses of undergrowth that add so much to the picturesqueness of the forests farther north. The unkempt half-breed or Indian hunter is replaced by the prosaic gatherer of turpentine. As the man of the southern forests shuffles along in blue or khaki overalls and carries his buckets from tree to tree, he seems a dull figure contrasted with the active northern hunter who glides swiftly and silently from trap to trap on his rawhide snowshoes. Yet though the southern pine forest may be less picturesque than the northern, it is more useful to man. In spite of its sandy soil, much of this forest land is being reclaimed, and all will some day probably be covered by farms.
Two other outliers of the northern evergreen forest extend southward along the cool heights of the Rocky Mountains and of the Pacific coast ranges of the United States. In the Olympic and Sierra Nevada ranges the most western outlier of this northern band of vegetation probably contains the most inspiring forests of the world. There grow the vigorous Oregon pines, firs, and spruces, and the still more famous Big Trees or sequoias. High on the sides of the Sierra above the yuccas, the live oaks, and the deciduous forest of the lower slopes, one meets these Big Trees. To come upon them suddenly after a long, rough tramp over the sunny lower slopes is the experience of a lifetime. Upward the great trees rise sheer one hundred feet without a branch. The huge fluted trunks encased in soft, red bark six inches or a foot thick are more impressive than the columns of the grandest cathedral. It seems irreverent to speak above a whisper. Each tree is a new wonder. One has to walk around it and study it to appreciate its enormous size. Where a tree chances to stand isolated so that one can see its full majesty, the sense of awe is tempered by the feeling that in spite of their size the trees have a beauty all their own. Lifted to such heights, the branches appear to be covered with masses of peculiarly soft and rounded foliage like the piled-up banks of a white cumulus cloud before a thunderstorm. At the base of such a tree the eye is caught by the sharp, triangular outline of one of its young progeny. The lower branches sweep the ground. The foliage is harsh and rough. In almost no other species of trees is there such a change from comparatively ungraceful youth to a superbly beautiful old age.
The second great type of American forest is deciduous. The trees have broad leaves quite unlike the slender needles or overlapping scales of the northern evergreens. Each winter such forests shed their leaves. Among the mountains where the frosts come suddenly, the blaze of glory and brilliance of color which herald the shedding of the leaves are surpassed in no other part of the world. Even the colors of the Painted Desert in northern Arizona and the wonderful flowers of the California plains are less pleasing. In the Painted Desert the patches of red, yellow, gray-blue, white, pale green, and black have a garish, almost repellent appearance. In California the flame-colored acres of poppies in some places, of white or yellow daisylike flowers in others, or of purple blossoms elsewhere have a softer expression than the bare soil of the desert. Yet they lack the delicate blending and harmony of colors which is the greatest charm of the autumn foliage in the deciduous forests. Even where the forests consist of such trees as birches, beeches, aspens, or sycamores, whose leaves merely turn yellow in the fall, the contrast between this color and the green tint of summer or the bare branches of winter adds a spice of variety which is lacking in other and more monotonous forests.
From still other points of view the deciduous forest has an almost unequaled degree of variety. In one place it consists of graceful little birches whose white trunks shimmering in the twilight form just the background for ghosts. Contrast them with the oak forest half a mile away. There the sense of gracefulness gives place to a feeling of strength. The lines are no longer vertical but horizontal. The knotted elbows of the branches recall the keels of sturdy merchantmen of bygone days. The acorns under foot suggest food for the herds of half-wild pigs which roam among the trees in many a southern county. Of quite another type are the stately forests of the Appalachians where splendid magnolia and tulip trees spread their broad limbs aloft at heights of one hundred feet or more.
Deciduous forests grow in the well-balanced regions where summer and winter approach equality, where neither is unduly long, and where neither is subject to prolonged drought. They extend southward from central New England, the Great Lakes, and Minnesota, to Mississippi, Arkansas, and eastern Texas. They predominate even in parts of such prairie States as Michigan, Indiana, southern Illinois, and southeastern Missouri. No part of the continent is more populous or more progressive than the regions once covered by deciduous forests. In the United States nearly sixty per cent of the inhabitants live in areas reclaimed from such forests. Yet the area of the forests is less than a quarter of the three million square miles that make up the United States.
In their relation to human life the forests of America differ far more than do either grass-lands or deserts. In the far north, as we have seen, the pine forests furnish one of the least favorable environments. In middle latitudes the deciduous forests go to the opposite extreme and furnish the most highly favored of the homes of man. Still farther southward the increasing luxuriance of the forests, especially along the Atlantic coast, renders them less and less favorable to mankind. In southern Mexico and Yucatan the stately equatorial rain forest, the most exuberant of all types of vegetation and the most unconquerable by man, makes its appearance. It forms a discontinuous belt along the wet east coast and on the lower slopes of the mountains from southern Yucatan to Venezuela. Then it is interrupted by the grasslands of the Orinoco, but revives again in still greater magnificence in the Guianas. Thence it stretches not only along the coast but far into the little known interior of the Great Amazon basin, while southward it borders all the coast as far as southern Brazil. In the Amazon basin it reaches its highest development and becomes the crowning glory of the vegetable world, the most baffling obstacle to human progress.
Except in its evil effects on man, the equatorial rain forest is the antithesis of the forests of the extreme north. The equatorial trees are hardwood giants, broad leaved, bright flowered, and often fruit-bearing. The northern trees are softwood dwarfs, needle-leaved, flowerless, and cone-bearing. The equatorial trees are often branchless for one hundred feet, but spread at the top into a broad overarching canopy which shuts out the sun perpetually. The northern trees form sharp little pyramids with low, widely spreading branches at the base and only short twigs at the top. In the equatorial forests there is almost no underbrush. The animals, such as monkeys, snakes, parrots, and brilliant insects, live chiefly in the lofty treetops. In the northern forests there is almost nothing except underbrush, and the foxes, rabbits, weasels, ptarmigans, and mosquitoes live close to the ground in the shelter of the branches. Both forests are alike, however, in being practically uninhabited by man. Each is peopled only by primitive nomadic hunters who stand at the very bottom in the scale of civilization.
Aside from the rain forest there are two other types in tropical countries—jungle and scrub. The distinction between rain forest, jungle, and scrub is due to the amount and the season of rainfall. An understanding of this distinction not only explains many things in the present condition of Latin America but also in the history of pre-Columbian Central America. Forests, as we have seen, require that the ground be moist throughout practically the whole of the season that is warm enough for growth. Since the warm season lasts throughout the year within the tropics, dense forests composed of uniformly large trees corresponding to our oaks, maples, and beeches will not thrive unless the ground is wet most of the time. Of course there may be no rain for a few weeks, but there must be no long and regularly recurrent periods of drought. Smaller trees and such species as the cocoanut palm are much less exacting and will flourish even if there is a dry period of several months. Still smaller, bushy species will thrive even when the rainfall lasts only two or three months. Hence where the rainy season lasts most of the year, rain forest prevails; where the rainy and dry seasons do not differ greatly in length, tropical jungle is the dominant growth; and where the rainy season is short and the dry season long, the jungle degenerates into scrub or bush.
The relation of scrub, jungle, and rain forest is well illustrated in Yucatan, where the ancient Mayas reared their stately temples. On the northern coast the annual rainfall is only ten or fifteen inches and is concentrated largely in our summer months. There the country is covered with scrubby bushes six to ten feet high. These are beautifully green during the rainy season from June to October, but later in the year lose almost all their leaves. The landscape would be much like that of a thick, bushy pasture in the United States at the same season, were it not that in the late winter and early spring some of the bushes bear brilliant red, yellow, or white flowers. As one goes inland from the north coast of Yucatan the rainfall increases. The bushes become taller and denser, trees twenty feet high become numerous, and many rise thirty or forty feet or even higher. This is the jungle. Its smaller portions suggest a second growth of timber in the deciduous forests of the United States fifteen or twenty years after the cutting of the original forest, but here there is much more evidence of rapid growth. A few species of bushes and trees may remain green throughout the year, but during the dry season most of the jungle plants lose their leaves, at least in part.
With every mile that one advances into the more rainy interior, the jungle becomes greener and fresher, the density of the lower growths increases, and the proportion of large trees becomes greater until finally jungle gives place to genuine forest. There many of the trees remain green throughout the year. They rise to heights of fifty or sixty feet even on the borders of their province, and at the top form a canopy so thick that the ground is shady most of the time. Even in the drier part of the year when some of the leaves have fallen, the rays of the sun scarcely reach the ground until nine or ten o'clock in the morning. Even at high noon the sunlight straggles through only in small patches. Long, sinuous lianas, often queerly braided, hang down from the trees; epiphytes and various parasitic growths add their strange green and red to the complex variety of vegetation. Young palms grow up almost in a day and block a trail which was hewn out with much labor only a few months before. Wherever the death of old trees forms an opening, a thousand seedlings begin a fierce race to reach the light. Everywhere the dominant note is intensely vigorous life, rapid growth, and quick decay.
In their effect on man, the three forms of tropical forest are very different. In the genuine rain forest agriculture is almost impossible. Not only does the poor native find himself baffled in the face of Nature, but the white man is equally at a loss. Many things combine to produce this result. Chief among them are malaria and other tropical diseases. When a few miles of railroad were being built through a strip of tropical forest along the coast of eastern Guatemala, it was impossible to keep the laborers more than twenty days at a time; indeed, unless they were sent away at the end of three weeks, they were almost sure to be stricken with virulent malarial fevers from which many died. An equally potent enemy of agriculture is the vegetation itself. Imagine the difficulty of cultivating a garden in a place where the weeds grow all the time and where many of them reach a height of ten or twenty feet in a single year. Perhaps there are people in the world who might cultivate such a region and raise marvelous crops, but they are not the indolent people of tropical America; and it is in fact doubtful whether any kind of people could live permanently in the tropical forest and retain energy enough to carry on cultivation. Nowhere in the world is there such steady, damp heat as in these shadowy, windless depths far below the lofty tops of the rain forest. Nowhere is there greater disinclination to work than among the people who dwell in this region. Consequently in the vast rain forests of the Amazon basin and in similar small forests as far north as Central America, there are today practically no inhabitants except a mere handful of the poorest and most degraded people in the world. Yet in ancient times the northern border of the rain forest was the seat of America's most advanced civilization. The explanation of this contradiction will appear later. *
* See Chapter 5, Aztecs.
Tropical jungle borders the rain forest all the way from southern Mexico to southern Brazil. It treats man far better than does the rain forest. In marked contrast to its more stately neighbor, it contains abundant game. Wild fruits ripen at almost all seasons. A few banana plants and palm trees will well-nigh support a family. If corn is planted in a clearing, the return is large in proportion to the labor. So long as the population is not too dense, life is so easy that there is little to stimulate progress. Hence, although the people of the jungle are fairly numerous, they have never played much part in history. Far more important is the role of those living in the tropical lands where scrub is the prevailing growth. In our day, for example, few tropical lowlands are more progressive than the narrow coastal strip of northern Yucatan. There on the border between jungle and scrub the vegetation does not thrive sufficiently to make life easy for the chocolate-colored natives. Effort is required if they would make a living, yet the effort is not so great as to be beyond the capacity of the indolent people of the tropics.
Leaving the forests, let us step out into the broad, breezy grass-lands. One would scarcely expect that a journey poleward out of the forest of northern Canada would lead to an improvement in the conditions of human life, yet such is the case. Where the growing season becomes so short that even the hardiest trees disappear, grassy tundras replace the forest. By furnishing food for such animals as the musk-ox, they are a great help to the handful of scattered Indians who dwell on the northern edge of the forest. In summer, when the animals grow fat on the short nutritious grass, the Indians follow them out into the open country and hunt them vigorously for food and skins to sustain life through the long dreary winter. In many cases the hunters would advance much farther into the grass-lands were it not that the abundant musk-oxen tempt the Eskimo of the seacoast also to leave their homes and both sides fear bloody encounters.
With the growth of civilization the advantage of the northern grass-lands over the northern forests becomes still more apparent. The domestic reindeer is beginning to replace the wild musk-ox. The reindeer people, like the Indian and Eskimo hunters, must be nomadic. Nevertheless their mode of life permits them to live in much greater numbers and on a much higher plane of civilization than the hunters. Since they hunt the furbearing animals in the neighboring forests during the winter, they diminish the food supply of the hunters who dwell permanently in the forest, and thus make their life still more difficult. The northern forests bid fair to decline in population rather than increase. In this New World of ours, strange as it may seem, the almost uninhabited forest regions of the far north and of the equator are probably more than twice as large as the desert areas with equally sparse population.
South of the tundras the grass-lands have a still greater advantage over the forests. In the forest region of the Laurentian highland abundant snow lasts far into the spring and keeps the ground so wet and cold that no crops can be raised. Moreover, because of the still greater abundance of snow in former times, the largest of ice sheets, as we have seen, accumulated there during the Glacial Period and scraped away most of the soil. The grassy plains, on the contrary, are favored not only by a deep, rich soil, much of which was laid down by the ice, but by the relative absence of snow in winter and the consequent rapidity with which the ground becomes warm in the spring. Hence the Canadian plains from the United States boundary northward to latitude 57 degrees contain a prosperous agricultural population of over a million people, while the far larger forested areas in the same latitude support only a few thousand.
The question is often asked why, in a state of nature, trees are so scarce on the prairies—in Iowa, for instance—although they thrive when planted. In answer we are often told that up to the middle of the nineteenth century such vast herds of buffaloes roamed the prairies that seedling trees could never get a chance to grow. It is also said that prairie fires sweeping across the plains destroyed the little trees whenever they sprouted. Doubtless the buffaloes and the fires helped to prevent forest growth, but another factor appears to be still more important. All the States between the Mississippi River and the Rocky Mountains receive much more rain in summer than in winter. But as the soil is comparatively dry in the spring when the trees begin their growth, they are handicapped. They could grow if nothing else interfered with them, just as peas will grow in a garden if the weeds are kept out. If peas, however, are left uncared for, the weeds gain the upper hand and there are no peas the second year. If the weeds are left to contend with grass, the grass in the end prevails. In the eastern forest region, if the grass be left to itself, small trees soon spring up in its midst. In half a century a field of grass goes back to forest because trees are especially favored by the climate. In the same way in the prairies, grass is especially favored, for it is not weakened by the spring drought, and it grows abundantly until it forms the wonderful stretches of waving green where the buffalo once grew fat. Moreover the fine glacial soil of the prairies is so clayey and compact that the roots of trees cannot easily penetrate it. Since grasses send their roots only into the more friable upper layers of soil, they possess another great advantage over the trees.
Far to the south of the prairies lie the grass-lands of tropical America, of which the Banos of the Orinoco furnish a good example. Almost everywhere their plumed grasses have been left to grow undisturbed by the plough, and even grazing animals are scarce. These extremely flat plains are flooded for months in the rainy season from May to October and are parched in the dry season that follows. As trees cannot endure such extremes, grasses are the prevailing growth. Elsewhere the nature of the soil causes many other grassy tracts to be scattered among the tropical jungle and forest. Trees are at a disadvantage both in porous, sandy soils, where the water drains away too rapidly, and in clayey soil, where it is held so long that the ground is saturated for weeks or months at a time. South of the tropical portion of South America the vast pampas of Argentina closely resemble the North American prairies and the drier plains to the west of them. Grain in the east and cattle in the west are fast causing the disappearance of those great tussocks of tufted grasses eight or nine feet high which hold among grasses a position analogous to that of the Big Trees of California among trees of lower growth.
It is often said that America has no real deserts. This is true in the sense that there are no regions such as are found in Asia and Africa where one can travel a hundred miles at a stretch and scarcely see a sign of vegetation-nothing but barren gravel, graceful wavy sand dunes, hard wind-swept clay, or still harder rock salt broken into rough blocks with upturned edges. In the broader sense of the term, however, America has an abundance of deserts—regions which bear a thin cover of bushy vegetation but are too dry for agriculture without irrigation. On the north such deserts begin in southern Canada where a dry region abounding in small salt lakes lies at the eastern base of the Rocky Mountains. In the United States the deserts lie almost wholly between the Sierra Nevada and the Rocky Mountain ranges, which keep out any moisture that might come from either the west or the east. Beginning on the north with the sagebrush plateau of southern Washington, the desert expands to a width of seven hundred miles in the gray, sage-covered basins of Nevada and Utah. In southern California and Arizona the sage-brush gives place to smaller forms like the saltbush, and the desert assumes a sterner aspect. Next comes the cactus desert extending from Arizona far south into Mexico. One of the notable features of the desert is the extreme heat of certain portions. Close to the Nevada border in southern California, Death Valley, 250 feet below sea-level, is the hottest place in America. There alone among the American regions familiar to the writer does one have that feeling of intense, overpowering aridity which prevails so often in the deserts of Arabia and Central Asia. Some years ago a Weather Bureau thermometer was installed in Death Valley at Furnace Creek, where the only flowing water in more than a hundred miles supports a depressing little ranch. There one or two white men, helped by a few Indians, raise alfalfa, which they sell at exorbitant prices to deluded prospectors searching for riches which they never find. Though the terrible heat ruins the health of the white men in a year or two, so that they have to move away, they have succeeded in keeping a thermometer record for some years. No other properly exposed, out-of-door thermometer in the United States, or perhaps in the world, is so familiar with a temperature of 100 degrees F. or more. During the period of not quite fifteen hundred days from the spring of 1911 to May, 1915, a maximum temperature of 100 degrees F. or more was reached on five hundred and forty-eight days, or more than one-third of the time. On July 10, 1913, the mercury rose to 134 degrees F. and touched the top of the tube. How much higher it might have gone no one can tell. That day marks the limit of temperature yet reached in this country according to official records. In the summer of 1914 there was one night when the thermometer dropped only to 114 degrees F., having been 128 degrees F. at noon. The branches of a peppertree whose roots had been freshly watered wilted as a flower wilts when broken from the stalk.
East and south of Death Valley lies the most interesting section of the American desert, the so-called succulent desert of southern Arizona and northern Mexico. There in greatest profusion grow the cacti, perhaps the latest and most highly specialized of all the great families of plants. There occur such strange scenes as the "forests" of suhuaros, whose giant columns have already been described. Their beautiful crowns of large white flowers produce a fruit which is one of the mainstays of the Papagos and other Indians of the regions. In this same region the yucca is highly developed, and its tall stalks of white or greenish flowers make the desert appear like a flower garden. In fact this whole desert, thanks to light rains in summer as well as winter, appears extraordinarily green and prosperous. Its fair appearance has deceived many a poor settler who has vainly tried to cultivate it.
Farther south the deserts of America are largely confined to plateaus like those of Mexico and Peru or to basins sheltered on all sides from rain-bearing winds. In such basins the suddenness of the transition from one type of vegetation to another is astonishing. In Guatemala, for instance, the coast is bordered by thick jungle which quickly gives place to magnificent rain forest a few miles inland. This continues two or three score miles from the coast until a point is reached where mountains begin to obstruct the rain-bearing trade-winds. At once the rain forest gives place to jungle; in a few miles jungle in its turn is replaced by scrub; and shortly the scrub degenerates to mere desert bush. Then in another fifty miles one rises to the main plateau passing once more through scrub. This time the scrub gives place to grass-lands diversified by deciduous trees and pines which give the country a distinctly temperate aspect. On such plateaus the chief civilization of the tropical Latin-American countries now centers. In the past, however, the plateaus were far surpassed by the Maya lowlands of Yucatan and Guatemala.
We are wont to think of deserts as places where the plants are of few kinds and not much crowded. As a matter of fact, an ordinary desert supports a much greater variety of plants than does either a forest or a prairie. The reason is simple. Every desert contains wet spots near springs or in swamps. Such places abound with all sorts of water-loving plants. The deserts also contain a few valleys where the larger streams keep the ground moist at all seasons. In such places the variety of trees is as great as in many forests. Moreover almost all deserts have short periods of abundant moisture.
At such times the seeds of all sorts of little annual plants, including grasses, daisies, lupines, and a host of others, sprout quickly, and give rise to a carpet of vegetation as varied and beautiful as that of the prairie. Thus the desert has not only its own peculiar bushes and succulents but many of the products of vegetation in swamps, grasslands, and forests. Though much of the ground is bare in the desert, the plants are actually crowded together as closely as possible. The showers of such regions are usually so brief that they merely wet the surface. At a depth of a foot or more the soil of many deserts never becomes moist from year's end to year's end. It is useless for plants to send their roots deep down under such circumstances, for they might not reach water for a hundred feet. Their only recourse is to spread horizontally. The farther they spread, the more water they can absorb after the scanty showers. Hence the plants of the desert throttle one another by extending their roots horizontally, just as those of the forest kill one another by springing rapidly upward and shutting out the light.
Vegetation, whether in forests, grasslands, or deserts, is the primary source of human sustenance. Without it man would perish miserably; and where it is deficient, he cannot rise to great heights in the scale of civilization. Yet strangely enough the scantiness of the vegetation of the deserts was a great help in the ascent of man. Only in dry regions could primitive man compete with nature in fostering the right kind of vegetation. In such regions arose the nations which first practised agriculture. There man became comparatively civilized while his contemporaries were still nomadic hunters in the grasslands and the forests.
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