hence arise winds or aërial currents,—the warm air of one locality expanding and ascending, and the colder rushing in from all sides to supply the deficiency. The winds thus generated assume various directions and physical characteristics—the chief cause of modification being the different amounts of heat received by the different zones of the earth. Thus some are said to be constant, as the trade-winds of the tropics; others periodical, as the monsoons and the sea and land breezes; and others, again, variable and irregular, as the winds of the higher latitudes. In their physical characters they are governed chiefly by locality and the nature of the region from which they blow, and are thus hot or cold, moist or dry, relaxing or invigorating-floating as a zephyr that scarcely disturbs the thistle-down, or sweeping as hurricanes that uproot forests and overturn buildings. ; The warmer the air, the greater its capacity for moisture but in every case there is a point beyond which it is incapable of sustaining more vapour in an invisible form. It is then said to be saturated; and in this state any cooling, by coming in contact with the colder earth, or by the contact and commingling of colder aërial currents, produces condensation into dews, fogs, rains, snow, hail, and other kindred phenomena. These aqueous phenomena, -whether descending as dew, from the unequal temperatures of the earth and air-as fogs and mists, from unequal temperatures of aërial strata-or as rains, by more sudden condensation,-are all essential to the vegetable and animal economies. Rains, like the winds on which they mainly depend, are almost constant within certain equatorial districts; periodical within the regions of the monsoons; and irregular in all the higher latitudes. Some tracts as the Sahara, Egypt, Arabia, Persia, Mongolia, and Peru —are rainless, or all but rainless; but in most countries the rainfall, though varying from month to month and year to year, is, in the long average, pretty regular and persistent. It will vary, of course, according to latitude, direction of prevailing winds, proximity to the ocean, direction of mountain-chains, and the like; hence its great difference, even on opposite sides of the same island,— nual falls within temperate zones varying from 10 to 80 inches, and within tropical, from 100 to 500 or 600 inches. When the temperature of the air falls below the freezing-point, fogs and mists are converted into snow, and rain into hail-snow being mainly an extratropical and winter precipitation, hail occurring in all latitudes and at all seasons of the year. Within the tropics at the level of the sea snow is unknown; in temperate zones it falls less or more during winter; but in polar regions, and at great elevations above the sea in all latitudes, its presence is perennial. The altitude -an at which snow remains unaffected by the heat of summer (the snow-line) varies with the latitude-descending from 14,000 or 16,000 feet at the equator to near the sea-level at either pole. The latitudinal limit, north and south (the snow-limit), varies also according to the distribution of sea and land-receding and advancing as either element prevails. The perpetual presence of snow and ice on lofty mountain-ridges and polar uplands gives rise to avalanches, glaciers, and icebergs-phenomena whose geological influences are not less apparent than their climatological. The causes which affect the climate or weather-conditions of any locality are thus extremely varied-latitude, altitude, distribution of land and water, proximity to the sea, prevalent winds, direction of mountain-chains, slope, nature of soil, cultivation, and the like, all more or less exerting their modifying influences. Heat and moisture, however, are the great regulators of climate; and thus, in general terms, climatological zones may be said to decrease in importance from the equator to the poles. Nevertheless, this decrease by no means coincides with the parallels of latitude; hence to determine the mean temperature of any locality, numerous thermometrical observations have to be made at hourly, daily, monthly, and yearly intervals. The results of such observations enable the meteorologist to connect places having the same summer, winter, and mean annual temperatures; and hence the scheme of exhibiting them at a glance by means of isotheral, isocheimonal or isocheimal, isothermal, and other lines. To the student who would enter more fully into the consideration of the atmosphere and its phenomena, and especially into the subject of climatology as affecting the plant-life and animal-life of the globe, we would recommend perusal of some treatise on meteorology, as that by Professor Kaemtz (an English translation of which appeared in 1845); the essay by Sir John Herschel, which has been republished from the 'Encyclopædia Britannica ;' Dr Thomson's Elements ;' or the still briefer and simpler compilation of Professor Brocklesby of Hartford, U.S.,-a work of much greater merit than pretension. XIII. LIFE ITS DISTRIBUTION AND FUNCTION. Life as affected by External Conditions. 246. HAVING noticed the general relations of the Land, Water, and Atmosphere, and the principal phenomena arising therefrom, we now turn to the Life by which they are respectively peopled. Hitherto we have dealt with the inorganic phases of nature; we have now to consider the organic. In the former case, the study involved consideration merely of chemical and mechanical forces acting from without; now we have to deal with the superaddition of vital action operating through peculiar organs from within. The material aspects and relations of nature form the themes of the physicist; the vital or organic constitute the study of the physiologist. Under the term Life is embraced all that appertains to the vegetable and animal kingdoms—subjects which belong to the domain of Botany and Zoology, and only come under the notice of Geography in as far as they are dependent on external conditions for their position or distribution on the globe. The origin, nature, and function of Life form the theme of Biology; its distribution and external relations become important considerations in the study of Physical Geography. 247. Whatever be the nature and origin of Life, it is clearly dependent for its continuance on the physical conditions by which it is surrounded. A little more heat or a little more cold, a little more moisture or a little more drought, and the plant flourishes or decays, the animal increases or dies. It is obvious, then, that, on a globe having different zones of climate, having regions of excessive humidity and regions of excessive drought-and having, moreover, different areas of land and water-life must be as diversified in its nature as the conditions under which it is destined to exist. The arrangement of plants into aquatic, terres trial and aërial—into flowering and flowerless-into trees, shrubs, herbs, grasses, mosses, lichens, sea-weeds, &c.— belongs to the province of Botany; and of animals into aquatic, terrestrial, and amphibious — into mammals, birds, reptiles, fishes, shell-fish, &c.-belongs in like manner to the province of Zoology. What the geographer has more especially to consider is their distribution over certain areas, the physical conditions apparently concerned in their restriction to these areas, and the dependence of the one kingdom upon the other, as completing the economy of nature. As it is Life that gives to creation its highest interest, so the consideration of its relations becomes the highest theme of our subject. For the sake of brevity, the term flora (Lat.) is employed to designate the plant-life of a region or epoch, and the term fauna its animal-life; hence we speak of the "Flora of South America" and the “Fauna of South America,” as well as of the Flora and Fauna of the Tertiary, the Chalk, or any other geological period. 248. So far as the eye, or the eye aided by the microscope, can perceive, life is everywhere present-in the air, on the earth, and in the water, or even parasitic on and within other plants and animals. Unless, perhaps, among the perpetual snows and ices of the poles and lofty mountain-peaks, or in the extreme depths of the ocean, its manifestations are sufficiently apparent; and even in these situations some forms unknown to observation may find a permanent or temporary home. And yet, universal as life may appear, it is confined to the merest film of the terraqueous globe. A few thousand feet above the sea-level on land, and a few thousand feet beneath it in the waters, limit this stratum of life on either hand. Thickest at the equator, it thins out towards the poles; and densest near the sea-level, it becomes rarer and rarer the more it rises above or falls beneath this line of greatest intensity. Heat, light, moisture, and nature of soil are the great regulators of life on the land; heat, light, depth, nature of bottom, and saline composition, the main regulators in the ocean. Were it not for these causes there is no reason why the same forms of life should not prevail in every region from the equator to the poles, and from the shore-line to the darkest abysses of the ocean. The influence, however, of external conditions is insuperable. The palm of the tropics would dwarf and die in the temperate zone; the whale of the Greenland seas would perish in the waters of the equator; the rush that luxuriates in the marsh would wither if transferred to the arid upland; the shell-fish that swarm within the influence of the tides would die if submerged to the depth of a few hundred fathoms; the fresh water that forms the vital ele P ment for one family becomes the poison of another that has been destined to inhabit the saline waters of the ocean. 249. Every plant and every animal is, therefore, adapted by nature to the position it occupies, and within that position continues to fulfil its function so long as the surrounding conditions remain unchanged. Why one genus or kind should differ from another genus, man may never know; and, at all events, the inquiry belongs to the general subject of biology, and not to that of geography. Again, why certain forms should only appear in certain regions—the kangaroo, for instance, in Australia, the ostrich in Africa, and the llama in South America-while other regions seem equally fitted for their residence, is a question involving considerations of origin, and of geological alternations of land and water, that lie beyond the scope of our subject. What more immediately concerns geography, are the existing distribution of plants and animals, the conditions accompanying that distribution, and the question how far they are capable of being transferred to, and acclimatised in, other districts for the luxury and necessities of man. These are the subjects of importance to the merchant, trader, and farmer-the determination of the regions of supply, the amount and quality of products, and the possibility of the profitable growth and cultivation of the plants or animals in other countries than those which they naturally occupy. 250. Some tribes have naturally a wider range than others; some, again, have a more elastic constitution, and are capable of enduring greater diversities of climate; and others, obeying the instincts of food, procreation, &c., migrate from the unfavourable season of one region to the favourable season of another. Of all animals man has the widest range — his superior intelligence enabling him to modify and overcome conditions that would be fatal to other creatures. Many, however, of the domesticated animals and cultivated plants have also considerable elasticity of constitution, and thus man has been enabled to carry them along with him over the greater part of the habitable globe. In this way we require to distinguish between the truly indigenous or native products of a country, and the exotic or imported, though many species have been so long transferred and re-transferred that it is now impossible to point to their original habitats. While, therefore, there is a natural apportioning of plants and animals to certain areas, and while external conditions are evidently the main regulators of this distribution, it must ever be borne in mind that man has already modified, and is continually modifying, this distribution by transferring, cultivating, and destroying, according as his wants and wishes may compel. His |