The educational value of geography, as we have seen, was recognized in a practical manner by Newton; and towards the close of the eighteenth century physical geography was taken as a lecture-subject by the philosopher Immanuel Kant at Königsberg, and by him was given exalted rank as a “summary of nature.” Alexander von Humboldt (1769–1859) further systematized the theory of the control of land-forms and climate over the distribution and habits of plants, animals, and man, and was able to draw not only upon the collection of facts made by other travellers, but also upon his own observations. His journey in 1799–1802 in America, during which he explored the Orinoco and discovered its connection with the Amazon through the Casiquiare, and visited Cuba, Quito and Mount Chimborazo, and Mexico, was the practical foundation of his scientific career. In the course of it he collected material for his researches into temperature at different elevations, into plant geography, terrestrial magnetism, volcanic phenomena, and much besides, while he also travelled through Russia to the Yenisei in 1829. In the work of Karl Ritter (1779–1859) is found the importance of establishing comparisons and investigating differences between similar regions in different parts of the world. Oscar Peschel (1826–75) corrected Ritter’s marked tendency to give excessive prominence to historical detail. The exposition of theoretical geography was carried on by Ferdinand von Richthofen, Hermann Wagner, and Friedrich Ratzel; and with the work of these and other leaders in the school of German geographical thinkers and teachers is associated the German pre-eminence in cartography during the nineteenth century, in which connection a passing tribute should be paid to Humboldt’s introduction to cartographers of the principle of drawing upon maps lines to show areas of equal temperature (isotherms), rainfall, etc. Geography as an educational subject of widely-recognized value is coming by its rights, though the majority of the last generation may recall it as affording little else than superficial instruction in the position of countries, places, mountains, and rivers. But now, not only in Germany, but in Great Britain and elsewhere, it has been widely adopted as an examination-subject in both primary and secondary education, as well as for certain specific purposes, and geographical chairs or lectureships have been established in a number of universities. The fostering of geography as an educational subject has been one of the great tasks, and that of furthering exploratory and other research another, of the many geographical societies which have been founded throughout the civilized world in the nineteenth century and after. That of Paris in 1825, and that of Berlin in 1827, are the oldest of these now flourishing, though with the Royal Geographical Society in London (1830) was merged the older African Association.