8 INTRODUCTION
form in Central Asia, it will be easy to comprehend how similar conditions of climate in other parts of the world give rise to similar topographic features, and how the two combine to determine the distribution and nature of life of all forms.
The rainfall of Central Asia is so small that the rivers fail to reach the sea. Hence the whole of a vast region, stretching three thousand miles east and west, and having an area nearly equal to that of the United States, is made up of enclosed basins, from which there is no outlet. Each consists essentially of a peripheral ring of higher land, usually mountainous, but sometimes merely a broad, gentle arch, — within which a desert plain of gravel, sand, and clay, brought from the mountains by rivers, surrounds a salt lake, or the saline beds whence the waters of an ancient lake have evaporated. Where the peripheral ring of higher land is at all mountainous, it is flanked, and often half buried, by vast slopes of barren rock-waste — typical piedmont deposits of gravel, washed out from the uplands by floods. Because of the aridity, vegetable life is scanty except along the courses of streams and in the rainy plateaus. Far less than a tenth of the country is permanently habitable: the rest is either absolute desert, or mitigated desert which supports vegetation part of the year, but is too dry among the plains, and too cold among the mountains, to allow permanent occupation. Hence the inhabitants must either live in irrigated oases along the rivers, or wander from place to place in search of pasture for their flocks. There are no manufacturing communities, either large or small ; no commercial centres except local bazaars ; and no con-
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