And the polar regions, which present so many natural obstacles to our explorers, are not the only portions of the earth not yet known to men of science. It may be humiliating to our pride as men, but we feel constrained to admit that among the countries not yet known to us there are some, accessible enough as far as natural obstacles are concerned, but closed against us by our fellow-men ! There are peoples in this world, dwelling in towns, obeying laws, and having customs comparatively polished, but who choose to live in seclusion, and are as little known to us as if they were the inhabitants of some other planet. Their frontiers are closed by war and its horrors, by the practice of slavery, by religious {2} fanaticism, and even commercial jealousy. We have heard of some of these peoples by vague report, but there are others concerning whom we absolutely know nothing. And thus it happens that in this age of steam, of the printing press, of incessant and feverish activity, we still know nothing, or very little, of the centre of Africa, of a portion of Australia, of the interior of that fine and no doubt most fertile island of New Guinea, and of vast table-lands in the centre of Asia. Nay, even the country which most men of learning love to look upon as the cradle of our Aryan ancestors is known to us but very imperfectly. As regards most countries which have been visited by travellers, and figure more or less correctly upon our maps, a great amount of further research is required before our knowledge of their geography can be called complete. Years will pass ere the erroneous and contradictory statements of our explorers concerning them have been set right. A prodigious amount of labour must be performed before their climate, their hydrography, their plants and animals, can be thoroughly known to us. Minute and systematic researches have to be conducted to elucidate the slow changes in the aspects and physical phenomena of many countries. The greatest caution will have to be exercised in distinguishing between changes due to the spontaneous action of natural causes and those brought about by the hand of man. And all this knowledge we must acquire before we can boast that we know the earth, and all about it !