In "The Tropical World," my additions to the labors of Dr. Hartwig have been much more considerable. Since his work was written, immense additions have been made to our knowledge of portions of the region lying within the Tropics. SQUIER has traversed the plateaus of Bolivia and Peru; and apart from the abstracts of his journeys which he has published, he has favored me with much information to be embodied in the great work upon which he has for years been engaged. HOLTON has furnished a curious book on the great table-land of Bogotá; ORTON has crossed the Andes, explored the Valley of Quito, and descended the Amazon from its upper waters to its mouth; and AGASSIZ has made large contributions to our knowledge of the natural history of the mighty Valley of the Amazon.
Our knowledge of the hitherto almost unknown parts of Africa has been more than doubled since Dr. Hartwig prepared his work. ANDERSSON and BALDWIN have told their hunting adventures in Southern Africa; BARTH has traversed the great Sahara; SPEKE and BAKER have solved the mystery of the source of the Nile; DU CHAILLU has again pierced the continent on the line of the equator, and described the mysteries of the home of the gorilla.
Perhaps the most entirely fresh account of a part of the Tropical World is WALLACE's work on the Malay Archipelago, a group of islands surpassing in extent all the inhabitable parts of Europe, and, although now almost uninhabited, capable of sustaining a population greater than that now living outside of China and India.
Of all these, and many more authorities, I have made free use; and in both parts of the work, I have steadily kept in view the leading idea of Dr. Hartwig: To describe the Polar and Tropical Worlds in their principal natural features, and to point out the influence of their respective climates upon the development of animal and vegetable life, and particularly upon human beings.
The liberality of the Publishers has placed at my disposal illustrations far exceeding in number and beauty those in the original work. They present to the eye information which words would often be inadequate to express to the ear. I trust that my own additions to the work will not be found unworthy of the foundation laid by Dr. Hartwig.