another, and having similar conditions of climate. In proportion as the distance between two areas increases, and their mutual accessibility diminishes, and their conditions of climate differ, the likeness of the forms of life within them becomes less, until the connection may be reduced to what is due to common descent from extremely remote
ancestors.
70. Turning to the continent of Asia, such broadly characterised similarities and differences will be seen to be well marked. The general assemblage of animals and plants found over Northern Asia resembles greatly that found in the parts of Europe which are adjacent, and which have a similar climate. Siberia, north of the 50th parallel, has a climate not much differing from the similarly situated portion of Europe, though the winters are more severe and the summers hotter. The rainfall, though moderate, is still sufficient to maintain the supply of water in the great rivers that traverse the country to the Arctic Sea, and to support an abundant vegetation. A similar affinity exists between the life of the southern parts of Europe and that in the zone of Asia extending from the Mediterranean across to the Himalaya and Northern China. This belt, which embraces Asia Minor, Northern Persia, Afghanistan, and the southern slopes of the Himalaya, from its elevation has a temperate climate, and throughout it the rainfall is sufficient to maintain a vigorous vegetation, while the summers, though hot, and the winters, though severe, are not extreme. The plants and animals along it are found to have a marked similarity of character to those of South Europe, with which region the zone is virtually continuous.
71. The extremely dry and hot tracts which constitute an almost unbroken desert from Arabia, through South Persia and Baluchistan, to Sindh, are characterised by considerable uniformity in the types of life, which closely approach to those of the neighbouring hot and dry regions of Africa. The region of the heavy periodical summer rains and high temperature, which comprises India, the Indo-Chinese peninsula, and Southern China, as well as the western part of the Malay Archipelago, is also marked by much similarity in the plants and animals throughout its extent. The area between the southern border of Siberia and the margin of the temperate alpine zone of the Himalaya and North China, comprising what are commonly called Central Asia, Turkistan, Mongolia, and Western Manchuria, is an almost rainless region, having winters of extreme severity and summers of intense heat. Its animals and plants have a special character suited to the peculiar climatal conditions, more closely allied to those of the adjacent northern Siberian tract than of the other bordering regions. The south-eastern parts of the Malay islands have much in common with the Australian con tinent, to which they adjoin, though their affinities are chiefly Indian. North China and Japan also have many forms of life in common. Much still remains to be done in the exploration of China and Eastern Asia ; but it is known that many of the special forms of this region extend to the Himalaya, while others clearly indicate a connection with North America.
72. The foregoing brief review of the principal territorial divisions according to which the forms of life are distributed in Asia, indicates how close is the dependence of this distribution on climatal conditions, and this will be made more apparent by a somewhat fuller account of the main features of the flora and fauna.
73. The flora of the whole of Northern Asia is in essen tials the same as that of Northern Europe, the differences being due rather to variations of species than of genera. The absence of the oak and of all heaths east of the Ural may be noticed. Pines, larch, birch, are the principal trees on the mountains : willows, alders, and poplars on the lower ground. The northern limit of the pine in Siberia is about lat. 70.
74. Along the warm temperate zone, from the Mediter- Warm ranean to the Himalaya, extends a flora essentially Euro- temper.-, pean in character. Many European species reach the central zone Himalaya, though few are known in its eastern parts. The genera common to the Himalaya and Europe are much more abundant, and extend throughout the chain, and to all elevations. There is also a corresponding diffusion of Japanese and Chinese forms along this zone, these being most numerous in the eastern Himalaya, and less frequent in the west.
75. The truly tropical flora of the hotter and wetter regions of Eastern India is continuous with that of the Malayan peninsula and islands, and extends along the lower ranges of the Himalaya, gradually becoming less marked, and rising to lower elevations as we go westward, where the rainfall diminishes and the winter cold increases.
76. The vegetation of the higher and therefore cooler and less rainy ranges of the Himalaya has greater uniformity of character along the whole chain, and a closer general approach to European forms is maintained; an increased number of species is actually identical, among these being found, at the greatest elevations, many alpine plants believed to be identical with species of the north Arctic regions. On reaching the Tibetan plateau, with the increased dryness the flora assumes many features of the Siberian type. Many true Siberian species are found, and more Siberian genera. Some of the Siberian forms, thus brought into proximity with the Indian flora, extend to the rainy parts of the mountains, and even to the plains of upper India. Assemblages of marine plants form another remarkable feature of Tibet, these being frequently met with growing at elevations of 14,000 to 15,000 feet above the sea, more especially in the vicinity of the many salt lakes of those regions.
77. The vegetation of the hot and dry region of the south-west of the continent consists largely of plants which are diffused over Africa, Baluchistan, and Sindh many of these extend into the hotter parts of India, and not a few common Egyptian plants are to be met with in the Indian peninsula.
78. The whole number of species of plants indigenous in the region of south-eastern Asia, which includes India and the Malayan peninsula and islands, from about the 65th to the 105th meridian, is estimated by Dr Hooker at from 12,000 to 15,000. The principal orders, arranged accord ing to their numerical importance, are as follows : Legu- minosce, RubiacecB, Orchidece, Composite, Graminece,Euphor- biacece, Acanthacece, Cyperacece, and Labiates. But within this region there is a very great variation between the vegetation of the more humid and the more arid regions, while the characteristics of the flora on the higher moun tain ranges differ wholly from those of the plains. In short, we have a somewhat heterogeneous assemblage of tropical, temperate, and alpine plants, as has been already briefly indicated, of which, however, the tropical are so far domi nant as to give their character to the flora viewed as a whole. The Indian flora contains a more general and com plete illustration of almost all the chief natural families of all parts of the world than any other country. Composita; are comparatively rare; so also Graminece and Cyperacece are in some places deficient, and Labiates, Leguminosce, and ferns in others. Euphorbiaceoe, and Scrophulariacece, and Orchidece are universally present, the last in specially large proportions.
