The Himalayas separate our great Indian empire from that of China; and we do not at present apprehend an invasion from that quarter. The Suliman Mountains divide us from the Afghans, and the great Russian and Persian empires farther west. Still, we know that in the eleventh century a great Mahometan invasion of India took place; our own armies have more than once penetrated to Kabul. Perhaps the common garden wall separating adjacent suburban residences furnishes a suitable illustration of the great natural walls which divide, not households or families, but much larger families than these,--the nations of the world. Just as unruly boys sometimes climb over the neighbour's wall and play games in a garden which is not their own; or as burglars may surmount these obstacles to their progress, and finding a way into the house by a back door or kitchen window, commence their ravages,--so a neighbouring (but not neighbourly) nation, bent on conquest, may invade some natural garden of the world, such as India, by forcing their way through physical barriers which for ordinary purposes serve to protect those within. The Thian Shan Mountains divide Russia from China's sphere of influence. The Caucasus Mountains separate Russia from Asia Minor. Austro-Hungary is bounded by the Carpathians, Spain by the Pyrenees. The Alps of Switzerland separate four nations not very friendly to each other; and lastly, in our own country the Cheviot Hills, together with the Tweed, form the boundary between Scotland and England. Where there are no mountains or hills, rivers sometimes serve as boundaries, but of course they do not answer the purpose so well. Sometimes a nation actually builds a wall for a boundary. Of this the great wall of China and the Roman wall between the Cheviots and the Solway Firth are familiar examples. In the second place, mountains have always been a refuge and shelter for conquered races; and the primitive tribes who once lived in the plains have been forced by adverse circumstances to take to the hills. This has taken place over and over again.