India has been divided off from the rest of Asia by walls of singular strength and height. The whole of the Indian land frontier is fenced and fortified by mountain ranges; and where, in the southwest toward the sea, the mountains subside and have an easier slope, the Indian desert is interposed between the outer frontier and the fertile midland region. It is as if Nature, knowing the richness of the land and the comparative weakness of its people, had taken the greatest possible pains to protect it; for along the whole of that vast line of mountain wall which overhangs the northwest and the northern boundaries of India there are only a very few practicable passes.
These are the outlets through Afghanistan, by which Alexander the Great and all subsequent invaders have descended upon the low country; and any one who, after traversing the interminable hills and stony valleys of Afghanistan, has seen, on mounting the last ridge, the vast plain of India spreading out before him in dusky haze like a sea, may imagine the feelings with which such a prospect was surveyed by those adventurous leaders when they first looked down on it from the edge of the Asiatic highlands. Along the whole northern line of frontier, the Himalayas are practically impassable; for the chain of towering mountains is backed by a lofty tableland, rising at its highest elevation to nearly seventeen thousand feet, which projects northward into Central Asia like the immense glacis of a fortress.
Such are the natural fortifications of India land-
