This brings us to the last province, commerce, the science of interchange. The study of minerals, of the distribution of plants and animals, is of little advantage, aside from commerce and its uses to man. It is the interchange of the products of one region for those of another which has had, on the whole, the greatest influence on the human race. Think, for an instant, of the transfer of the potato from America to Europe, of maize to Asia; of the far more ancient introduction of wheat and rice from Asia into Europe; and not these alone, but almost all the fruits. Think of the carrying from Asia to America, and, in fact, to all tropical lands, such products as sugar, coffee, and cotton. Think, too, of the results of the search for gold, ivory, and slaves in the interior of Africa, and of gold in California and Australia, opening such immense districts to settlements. The search after platina has disclosed the most guarded recesses of the Cordilleras and the Ural chain; while the need of copper first gave us our complete knowledge of the great system of American lakes. Without the expeditions to secure the whale, the walrus, and the seal, as well as the fur-bearing animals, the polar world would be still untraversed. The discovery of coal on a hundred shores otherwise unknown, led to the settlement of man in colonies from India and China southward to the Antarctic Continent, and northward to Nova Zembla, Spitzbergen, and Greenland. And not the continents only, seas and oceans have been thoroughly studied, in order to secure a safe pathway for man to the regions which contain his spoils. In the furtherance of this, the highest praise must be awarded to the British government. Through its enterprise and liberality, almost every island group has been examined, a thorough study of marine currents undertaken, careful soundings made in all waters, and a most extensive chartography accomplished. The charts published by the English admiralty already are counted by thousands. Yet the French have not been backward in like investigations. Understanding the value of commerce, their Dépôt de la Marine has not been inactive. Scandinavia has also done her part. The United States has accomplished one of the most thorough coast surveys ever undertaken by any nation; its difficulties are only to be measured by its extent. In fact, the whole civilised world has sent its messengers to the ends of the earth, and have united in this grand crusade of our age, the enriching of all men by a liberal system of interchange of the commodities of all climes. COMPARATIVE GEOGRAPHY. PART I. THE SURFACE OF THE EARTH CONSIDERED IN ITS MOST GENERAL RELATIONS. The Spheroidal Form of the Earth.