southeast, on the eastern border, and south and southwest on the western. In winter there is a shorter period in which the winds blow outwardly. (See article on Wind.) Rainfall.—The principal factors governing rainfall are evaporation, direction of winds, and distribution of mountains. A combination of these factors most favorable to a large rainfall is found on the west coast of Africa near the equator. Here the humid atmosphere from the Atlantic is carried landward by the winds and, becoming cooler, deposits the greater part of its moisture before passing the highland region. The maximum limit of precipitation is probably attained in Kamerun, where the total rainfall in the year may exceed 350 inches, while the Niger Delta and the coasts of Sierra Leone and Liberia also are excessively humid. On the east equatorial coast the winds from the Indian Ocean deliver considerable moisture, but not in such abundance as on the west coast. As they pass into the interior, the winds from both the Atlantic and Indian oceans are deprived of their humidity, especially in the mountains, which act as precipitating agents. Equatorial Africa, as a whole, is thus characterized by a heavy rainfall. North and south of this region, however, the conditions exhibit a striking contrast. In the north is the Sahara Desert, the largest arid region in the world, where the prevailing winds are from the northeast and are hot and dry, while the humidity of the southerly winds that may penetrate into the interior is diminished by the heat, and seldom falls as rain. A second arid region, the Kalahari Desert, is found in the southern limb of the continent, between the Zambezi and Orange rivers and the eastern and western coastal highlands. It has a small spasmodic rainfall, which is usually insufficient to support a constant growth of vegetation. The Mediterranean coast region and the extreme southern extension have a dry climate that is tempered by rains during certain seasons. Besides the continental distribution of rainfall, there is a seasonal variation in the amount received in different latitudes. In the regions near the equator rain may fall during every month of the year, but the periods of greatest precipitation occur when the sun is nearly vertical, in spring and fall. Away from the equator there is generally but one wet season. See articles on countries of Africa.
Flora. The vegetation of Africa is very diversified on account of the well marked topographic districts and the varied climatic conditions. The three zones of tropical, north temperate, and south temperate climate have their peculiar types of vegetation, the distribution of which in each zone is determined by the immediate physiographic features. Forest, steppe, savanna, and desert floras are found in each zone. The flora of the Mediterranean slope of the northern temperate zone has a general resemblance to that of southern Europe, with forests of oak and of smaller trees, as olives and figs, with also the vine and the same cereal grains. The desert regions (typified by the Sahara in the north temperate zone and the Kalahari Desert in Bechuanaland of the south temperate zone) support a scant xerophytic vegetation, which, contrasted with the flora of the North American deserts, has for its most prominent types quite leafless, thorny and fleshy euphorbias and acacias instead of cactuses. In the Sahara Desert the date palm grows often in extensive groves in the oases, and its wide distribution is probably due in large part to the dispersion of its seeds by the nomadic tribes, for whom its fruit serves as an important article of food. Bordering the Sahara and the Kalahari deserts are extensive semi-arid steppe or prairie regions, where the slight rainfall permits of the existence of a somewhat more varied flora, which combines certain of the desert and forest types. The steppe region of the southern temperate zone has, by reason of its isolation, developed a flora peculiarly its own, which is characterized both by the abundant presence of many members of the heath family (which often grow to a height exceeding 10 feet), and also by the general brilliancy of color of the flowering plants.
Those portions of Africa which have a moist climate are divisible into the savanna and forest regions. The forests are found mostly in the equatorial districts, where they are of enormous extent. Here the trees grow to great heights (often 200 feet), and, being close together, support numbers of parasitic vines, forming over vast areas a dense, tangled covering of foliage, through which the direct rays of the sun seldom penetrate. The savanna districts are uniform plains of both high and low land. On the damp low lands, reeds, especially the papyrus, abound (as, for example, in the marshy regions of the Nile and Congo valleys); on the drier high grounds good pasture grass with euphorbias forms the dominant vegetation, together with forest growths in the river valleys. The more important trees are the baobab (Adansonia) and the wine and oil palms (Raphia and Elœis). In conclusion, it may be stated that the flora of Africa is characterized by the extensive development of acacias and euphorbias over the entire continent, with the date palm in the northern (particularly in the arid) regions, and the papyrus in the marshes. See Distribution of Plants.
Fauna. The fauna of Africa is remarkable for its homogeneity, for the continental range of a great number of its groups and species, due to the absence of extensive mountain barriers, and for its remarkable alliance with the faunæ of the other divisions of the southern hemisphere. Africa—apart from the northwestern corner (the Atlas .Mountains, in which live the aoudad and certain other European forms)—is now regarded as forming, together with Arabia and Palestine, a single zoögeographical prime division called Ethiopian. Surveying its principal groups of animals, it is seen to be characterized in respect to the mammals by the preponderance of hoofed animals and the great size of many, such as the elephant, hippopotamus, and rhinoceros, by the originally vast numbers of gregarious grazers, and by their distinctive forms. Thus, there are no true oxen, but a buffalo is abundant; no camels nor llamas; no sheep nor goats; no deer (except the aberrant chevrotain) nor true swine. But it has exclusively several species of the horse family, the zebra, quagga, and wild ass; a giraffe, once ranging all the southern plains, and the okapi (q.v.); the tribe of hyraxes, and almost a hundred kinds of antelopes and gazelles, few of which range outside of Africa and Arabia. Of apes, the chimpanzee and gorilla belong to the equatorial forests alone; but more widely distributed, though exclusively African, are the baboons, various kinds of monkeys, and nearly
