proximately the 120th meridian of E. long., to the west of which the flora and fauna are essentially Asiatic in their type, while to the south and east the Australian element begins to be distinctly marked, soon to become predominant. To this boundary has been given the name of Wallace s line, after the eminent naturalist who first indicated its
existence.
7. Owing to the great extent of Asia, it is not easy to obtain a correct conception of the actual form of its outline from ordinary maps, the distortions which accompany pro jections of large spherical areas on a flat surface being necessarily great and misleading. Turning, therefore, to a globe, Asia, viewed as a whole, will be seen to have the form of a great isosceles spherical triangle, having its north eastern apex at Cape Vostochnyii, in Behring s Straits ; its two equal sides, in length about a quadrant of the sphere, or 6500 miles, extending on the west to the southern point of Arabia, and on the east to the extremity of the Malay peninsula; and the base between these points, occupying about 60 of a great circle, or 4500 miles, and being deeply indented by the Arabian Sea and the Bay of Bengal on either side of the Indian peninsula. A great circle, drawn through Cape Vostochnyii and the southern point of Arabia, passes nearly along the coast line of the Arctic Ocean, over the Ural Mountains, through the western part of the Cas pian, and nearly along the boundary between Persia and Asiatic Turkey. Asia Minor and the north-western half of Arabia lie outside of such a great circle, which otherwise indicates, with fair accuracy, the north-western boundary of Asia. In like manner a great circle drawn through Cape Vostochnyii and the extremity of the Malay penin sula, passes nearly over the coasts of Manchuria, China, and Cochin-China, and departs comparatively little from the eastern boundary.
8. Although for the purposes of geographical nomenclature, boundaries formed by a coast line that is, by depressions of the earth's solid crust beloiv the ocean level are most easily recognised, and are of special convenience; and although such boundaries, from following lines on which the continuity of the land is interrupted, often necessarily indicate important differences in the conditions of adjoining countries, and of their political and physical relations, yet variations of the elevation of the surface above the sea-level frequently produce effects not less marked. The changes of temperature and climate caused by difference of elevation are quite comparable in their magnitude and effect on all organised creatures with those due to differences of latitude ; and the relative position of the high and low lands on the earth's surface, by modifying the direction of the winds, the fall of rain, and other atmospheric phenomena, produces effects in no sense less important than those due to the relative distribution of the land and sea. Hence the study of the mountain ranges of a continent is, for a proper apprehension of its physical conditions and characteristics, as essential as the examination of its extent and position in relation to the equator and poles, and the configuration of its coasts.
9. From such causes the physical conditions of a large part of Asia, and the history of its populations, have been very greatly influenced by the occurrence of the mass of mountain, which includes the Himalaya and the whole elevated area having true physical connection with that range, and occupies an area about 2000 miles in length and vary ing from 100 to 500 miles in width, between the G5th and 100th meridians east from Greenwich, and between the 28th and 35th degrees of N". lat. These mountains, which are the highest in the world, rise, along their entire length, far above the line of perpetual snow, and few of the passes across the main ridges are at a less altitude than 15,000 or 1 6,000 feet above the sea. Peaks of 20,000 feet abound along the whole chain, and the points that exceed that elevation are numerous, the highest hitherto measured being more than 29,000 feet above the sea. A mountain range such as this, attaining altitudes at which vegetable life ceases, and the support of animal life is extremely diffi cult, constitutes an almost impassible barrier against the spread of all forms of living creatures. The mountain mass, moreover, is not less important in causing a com plete separation between the atmospheric conditions on its opposite flanks, by reason of the extent to which it pene trates that stratum of the atmosphere which is in contact with the earth s surface and is effective in determining climate. The highest summits create serious obstructions to the movements of nearly three-fourths of the mass of the air resting on this part of the earth, and of nearly the whole of the moisture it contains ; the average height of the entire chain is such as to make it an almost absolute barrier to one-half of the air and three-fourths of the mois ture ; while the lower ranges also produce important atmo spheric effects, one-fourth of the air and one-half of the watery vapour it carries with it lying below 9000 feet.
10. This great mass of mountain, constituting as it does a complete natural line of division across a large part of the continent, will form a convenient basis from which to work, in proceeding, as will now be done, to give a general view of the principal countries contained in Asia.
11. The summit of the great mountain mass is occupied by Tibet, a country known by its inhabitants under the name of Bod, or Eodyul. Tibet is a rugged table-land, narrow as compared to its length, broken up by a succession of mountain ranges, which follow as a rule the direction of the length of the table-land, and commonly rise into the regions of perpetual snow ; between the flanks of these lie valleys, closely hemmed in, usually narrow, having a very moderate inclination, but at intervals opening out into wide plains, and occupied either by rivers, or frequently by lakes from which there is no outflow and the waters of which are salt. The eastern termination of Tibet is in the line of snowy mountains which flanks China on the west, between the 27th and 35th parallels of latitude, and about on the 103d meridian east. On the west the table land is prolonged beyond the political limits of Tibet, though with much the same physical features, to about the 70th meridian, beyond which it terminates ; and the ranges which are covered with perpetual snow as far west as Samarkhand thence rapidly diminish in height, and ter minate in low hills north of Bukhara.
12. The mean elevation of Tibet may be taken as 15,000 feet above the sea. The broad mountainous slope by which it is connected with the lower levels of Hindostan contains the ranges known as the Himalaya ; the names of Kouenlun, or Karakorum, have been given by some geographers to the northern slope that descends to the central plains of the Gobi, though these mountains are not locally known under those names, Kouenlun being ap parently a Chinese corruption of some Turkish or Tibetan word, and Karakorum only one of the many passes that lead from Western Tibet to the northward.
13. The extreme rigour of the climate of Tibet, which combines great cold with great drought, makes the country essentially very poor, and the chief portion of it little better than desert. The vegetation is everywhere most scanty, and anything deserving the name of a tree is hardly to be found unless in the more sheltered spots, and then artificially planted. The population in the lower and warmer valleys live in houses, and follow agriculture ; in the higher regions they are nomadic shepherds, thinly scattered over a large area.
