operations, however, can never extend beyond a mere modification; for over and above his control remain the great conditions of heat, light, moisture, &c., which ever govern the main geographical arrangements of plants and animals; and it is to these in particular that the limits of a text-book will permit us to refer.

Plant-Life-its Distribution and Governing Conditions.

251. As already mentioned, heat, light, and moisture are the principal conditions affecting the terrestrial distribution of vegetable life. These conditions have their greatest intensity near the equator; hence the greatest exuberance of vegetation within the tropics, and its gradual declension as we proceed towards either pole. As might be anticipated, however, this declension is governed more by the isotherms than by the parallels of latitude —the mean amount of annual heat being the predominant condition in vegetable distribution, though the amount of summer heat (isotheral) has also much to do with its ripening and perfection. The mean annual temperature of two places may be the same, and yet the summer temperature of the one may be 10° higher than that of the other, while the winter temperature may be 15° or even 20° lower. Much, therefore, in Botanical Geography depends upon the amount of heat which a plant receives during the period of its greatest activity, and this, in general, is regulated more by the monthly than by the annual isotherms. The zones of vegetation shading more gradually into each other than the astronomical zones (torrid, temperate, and frigid), botanists make a minuter subdivision of the earth's surface into equatorial, tropical, sub-tropical, warmer-temperate, colder-temperate, sub-arctic, arctic, and polar-each characterised by some peculiar feature, though partaking on either side of the forms that belong to the two adjacent zones.

252. The equatorial zone, bounded by the isotherm of 79°, is characterised, wherever moisture is present, by its luxuriant vegetation. It embraces the central regions of Africa from shore to shore, Ceylon, the southern portion of the Indian peninsula, Malaya, the Indian Archipelago, the northernmost parts of Australia, New Guinea, and other Pacific islands in the same latitudes, a large portion of equatorial South America, including Columbia, Peru, Guiana, and the northern parts of Brazil. Succulent stems, large and showy flowers, gigantic parasites and climbers, arborescent grasses (canes, bamboos, &c.), orchids, palms, bananas, breadfruits, custard-apples, silk-cottons, banians, Adansonias, and the

like, are typical of this belt, the greater portion of which, for reasons given in the preceding chapter, lies to the north of the equator. 253. The tropical zones, extending to the isotherms of 73° on either side of the equator, include, in the New World, Bolivia, Brazil, and Paraguay in the southern hemisphere, and the West Indies, Yucatan, Guatemala, and part of Mexico in the northern ; while in the Old World it embraces Nubia, Senegambia, Madagascar, Mauritius, and North Australia in the southern hemisphere, and South Arabia, India, Burmah, and southern China in the northern. These zones are characterised by palms, bananas, pineapples, tree-ferns, species of fig, pepper-shrubs, cinnamon, indigo, cotton, coffee, sugar-cane, &c.-there being fewer parasites, and more underwood in the forests. And it has been further remarked by the botanist, that as palms and bananas may be said to mark the equatorial zone, so may arborescent ferns and species of fig be said to predominate in the tropical.

254. The sub-tropical, bounded by the isotherm of 63°, embrace Southern Africa and Australia, Paraguay, La Plata, and Chili in the southern hemisphere; and North Africa, Egypt, Syria, North Arabia, Persia, Northern India, part of China, the Southern States of North America, Mexico, and California in the northern hemisphere. They are characterised by a luxuriant growth of magnolias, laurels, myrtles, and figs, together with certain palms, zamias, cactuses, and arborescent euphorbias. In these zones, as in the damp regions of the preceding belts, vegetation is green throughout the year, and the climate, unless where rainless, is described as delightful. They are pre-eminently the lands of the laurel and myrtle.

255. The warmer-temperate, limited by 5340, are still regions of evergreens, but are marked by the absence of palms—the dwarf palm of Europe, the palmetto of North America, and the Chilian palm, being, as it were, outlying forms from the sub-tropical zone. Deciduous forest-trees, oaks, chestnuts, &c., and figs, oranges, pomegranates, olives, and the vine, are typical of the warmer-temperate in the northern hemisphere; shrubby ferns, arborescent grasses, and araucaria in the southern. This "region of evergreen trees," as it has been called, embraces the well-known flora of southern Europe, as well as those of Asia Minor, the north of China, and Japan.

256. The colder-temperate, bounded by the mean annual temperature or isotherm of 42°, is, in the northern hemisphere, the great zone of deciduous forest-trees, or those which shed their leaves in winter-hence the seasonal contrasts unknown in warmer regions. The characteristic vegetation of this zone is well seen in

that of our own country, the north of France, and Germany— forests of coniferous trees (fir, pine, yew, &c.) and expanses of heath adding peculiar features to the area. The cultivation of wheat scarcely extends beyond this zone in the northern hemisphere; in the southern it is occupied chiefly by the oceanTierra del Fuego, Falkland Islands, and Kerguelen's Land being the only important portions.

257. The sub-arctic zone, limited by the isotherm of 39°, is characterised by coniferous trees (pine, larch, spruce, juniper, &c.), poplar, beech, grasses, and heaths; and, on its northern limits, by birch, willow, and alder, which become dwarfed, and never attain to the size of trees. "This zone," says Professor Balfour, "is of less extent than the preceding, and in the interior of Asia it is perhaps not so easily distinguishable from it as in Europe. In the northern hemisphere it is the zone of firs and willows; in the southern it embraces a few barren islands. The northern parts of Siberia and Norway, the Faröe Islands, and Iceland belong to this zone. In the Faroe Islands barley does not always ripen, but the turnip and potato generally succeed. The Amentiferæ (plants bearing catkins) in these as well as in Iceland do not become trees. Grasses, common heath, and juniper, form features in the physiognomy of Iceland; in Siberia, forests of pine, larch, spruce, poplar, and birch occur."

258. The arctic zone (which has no equivalent in the southern hemisphere, where the ocean alone prevails) is marked by the dwarf birch, alder, and willow; by occasional pines and firs; by grasses; and by numerous lichens and mosses on its northern limits. At Hammerfest, in lat. 71°, according to the authority just quoted, potatoes, turnips, cabbage, and carrots generally succeed. In the American section, rhododendron, andromeda, and azalea are not unfrequent.

259. The polar zone is characterised chiefly by its flowerless plants-lichens and mosses-though, during its brief summer, species of ranunculus, saxifrage, scurvy-grass, rush, willow, &c., make their appearance. In this zone there are no trees nor bushes, nor any cultivation of plants for food. In the cold zones it has been remarked that there are more genera and fewer species, and that while the species are few, the individuals are numerous.

260. Such, in general terms, is the characteristic distribution of vegetable life as we proceed from warmer to colder latitudes. But the student is already familiar with the fact that temperature decreases not only as we proceed from the equator towards the poles, but also as we ascend from the level of the sea into the higher regions of the atmosphere. This ascent-hypsometrical, as it is

technically termed (Gr. hypsos, height, and metron, measure)-is marked by analogous belts of vegetation; and at the equator, for instance, the traveller who ascends a lofty mountain passes through a flora much akin to that which marks the successive horizontal zones alluded to in the preceding paragraphs. "We pass," says Herschel, "through the same series of climates, so far as temperature is concerned, which we should do by travelling from the same station to the polar regions of the globe; and in a country where very great differences of level exist, we find every variety of climate arranged in zones according to the altitude (hypsometrical zones), and characterised by the vegetable productions appropriated to their habitual temperature." Thus Humboldt, in describing the South American Alpine flora, remarks:-"In the burning plains scarce raised above the level of the Southern Ocean, we find bananas, cycads, and palms in the greatest luxuriance; after them, shaded by the lofty sides of the valleys in the Andes, tree-ferns; and next in succession, bedewed by cool, misty clouds, cinchonas (Peruvian-bark trees) appear. When lofty trees cease, we come to aralias (ivies) and myrtle-leaved andromedas (heaths); these are succeeded by bejarias abounding in resin, and forming a purple belt around the mountains. In the stony region of the Paramos, the more lofty plants and showy flowering herbs disappear, and are succeeded by large meadows covered with grasses, on which the llama feeds. We now reach the bare trachytic rocks, on which the lowest tribes of plants flourish. Paramelias, lecidias, and leprarias (lichens), with their many-coloured thalli and fructification, form the flora of this inhospitable zone. Patches of recentlyfallen snow now begin to cover the last efforts of vegetable life, and then the line of eternal snow begins."

261. In the Old World similar phenomena present themselves; and out of numerous examples we may take the following account of the Himalayan vegetation, as observed by Madden and Strachey in their journey from the plains of India, through Kemaon, to Tibet:

66 Ascending, we find forms of temperate climates gradually introduced above 3000 feet, as seen in species of pine, rose, bramble, oak, berberry, primrose, &c. At 5000 feet the arboreous vegetation of the plains is altogether superseded by such trees as oak, rhododendron, andromeda, cypress, and pine. The first ridge crossed ascends to a height of 8700 feet in a distance of not more than ten or twelve miles from the termination of the plains. The European character of the vegetation is here thoroughly established; and although specific identities are comparatively rare, the representative forms are most abundant. From 7000 to 11,000 feet— the region of the Alpine forest—the trees most common are oak,

horse-chestnut, elm, maple, pine, yew, hazel (growing to a large tree), and many others. At about 11,500 feet the forest endsWebb's pine and the Bhojatran birch being usually the last trees. Shrubs continue in abundance for about 1000 feet more; and about 12,000 feet the vegetation becomes almost entirely herbaceous. On the southern face of the mountains the snow-line is probably at an elevation of 15,500 feet. The highest dicotyledonous plant noticed was at about 17,500 feet, probably a species of echinospermum. A nettle, also, is common at these heights. The snow-line here recedes to 18,500 or 19,000 feet. In Tibet itself the vegetation is scanty in the extreme, consisting chiefly of caragana, species of artemisia, potentilla, and a few grasses. The cultivation of barley extends to 14,000 feet. Turnips and radishes, on rare occasions, are cultivated at nearly 16,000 feet. Vegetation ends at 17,500 feet: scanty pasturage being found only in favoured localities at this elevation; and the highest flowering plants are corydalis, cruciferæ, sedum, and a few others."

262. In temperate latitudes, though the variety of vegetation be less, and the lower zones of tropical flora be necessarily wanting, similar phenomena present themselves. "We may begin the ascent of the Alps, for instance, in the midst of warm vineyards, and pass through a succession of oaks, sweet chestnuts, and beeches, till we gain the elevation of the more hardy pines and stunted birches, and tread on pastures fringed by borders of perpetual snow. At the height of 1950 feet the vine disappears; and at 1000 feet higher the sweet chestnuts cease to thrive; 1000 feet farther, and the oak is unable to maintain itself; the birch ceases to grow at an elevation of 4680 feet; and the spruce fir at the height of 5900 feet, beyond which no tree appears. The rhododendron then covers immense tracts to the height of 7800 feet; and the herbaceous willow creeps 200 or 300 feet higher, accompanied by a few saxifrages, gentians, and grasses; while lichens and mosses struggle up to the imperishable barrier of eternal snow." On the Pyrenees, in like manner, the following belts have been observed:-1. The zone of vine and maize cultivation, and of the chestnut woods. 2. A zone extending from the limit of the vine to about 4200 feet, at which limit the cultivation of rye ceases; here we meet with box, saxifrage, gum, &c. 3. From the limit of the cultivation of esculent vegetables at 4200 feet to the zone of the spruce-fir. 4. From the limit of the spruce fir zone, at 6000 to 7200 feet, characterised by the presence of the Scotch fir. 5. From 7200 to 8400 feet is an Alpine zone, characterised by the dwarf juniper, Saxifraga bryoides, Soldanella alpina, Juncus trifidus, &c. 6. A zone above 8400 feet exhibits a few Alpine