We can better measure the contributions of the Spanish travelers and historians who penetrated the jungles of the Chaco, crossed the pampas of the Plata region, scaled the Andes and followed the course of the Amazon, the Orinoco and the Paraná if we pause to consider that the vast American continent, even today, presents great, physical barriers to research. Most of the works here studied are the direct result of firsthand observation. An appreciable number, however, do not fall into this category. All of them, nevertheless, offer a rich variety of themes for they abound in descriptions of the land and climate of the United States, the region of the Californias, Mexico, Venezuela, the Guianas, the Gran Chaco, the lands of the Orinoco, Colombia, Ecuador, Peru, Chile, the countries of the Río de la Plata and the region of the Strait of Magellan. Interest in the flora and fauna of America, so prevalent during this century, is reflected in the numerous chapters devoted to detailed descriptions of plant and animal life. The study of the natural history of America, which had absorbed the interest of so many Spanish writers in preceding centuries, enjoyed a remarkable development under royal patronage during the eighteenth century. The scientific missions sent to America by the Spanish Crown, resulted in the monumental works of Hipólito Ruiz and José Pavón, Florae peruvianae et chilensis prodromus (1794) of which only three of twelve manuscript volumes have been published; Martín Sessé, Flora mexicana, unpublished, and the unpublished Flora de Santa Fe de Bogotá of José Celestino Mutis whom Baron Humboldt called the illustrious patriarch of American botanists.