The fourth of the group, _Gottlob König_ (1776-1849), was a practitioner without a university education, who had enjoyed the teaching and influence of Cotta whom he succeeded in Eisenach as the head of the ducal forest administration. He also founded here a private forest school, which, in 1830, became a state institution, and is still in existence. König became noted by his contributions to the scientific, especially the mathematical side of forestry, developing forest mensuration and statics. In this latter branch he was the forerunner of Pressler and of the modern school of finance. In his “_Anleitung zur Holztaxation_” (1813) he gives a complete account of forest mensuration and in the part devoted to forest valuation he develops the first soil rent formula and the methods of determining the cost value of stands. His “Forest Mathematics” (1835) in which he introduces factors of form and many other new ideas was an original contribution to science. Very different in character from these four leaders was the aggressive, sharp-witted _Friedrich Wilhelm Leopold Pfeil_ (1783-1859), who, without a university education, and in spite of his poor knowledge of mathematics and natural history, advanced himself by native wit and genius. After a brief period of employment in private service, in the province of Silesia, he accepted the position of professor of forestry at the Berlin University, in 1821, in connection with Hartig, with whom, however, he was at sword’s point. It was at his instigation, with the assistance of von Humboldt, that the school was transferred, in 1830, to Eberswalde, Pfeil becoming its director. While Hartig was a generalizer, Pfeil was an individualizer, free from dogma, and most suggestive; a free lance and a fighter. Critical in the extreme and prolific in his literary work, he domineered the forestry literature of the day by means of his _Kritische Blaetter_, a journal of much import and merit. The youngest of the group, _Karl Heyer_ (1797-1856), a thoroughly educated man, combined the professorial position in the University of Giessen (1835) with practical management of a forest district, but in 1834 abandoned the latter in order to devote himself entirely to literary work. He was one of the clearest and most systematic expounders, and both his _Waldbau_ (silviculture, 1854) and his _Waldertragsregelung_ (forest organization, 1841) are classics. The last, fifth edition of the Waldbau, appearing in 1906 in two volumes, has been brought up to date by Professor Hess. He devised one of the most rational methods of forest organization, and, imbued with the necessity of basing forest management on exact scientific inquiry, instead of on empiricism alone, he formulated instructions for forest static investigations, a subject which his son, Gustav Heyer, elaborated into a science. 4. _Progress in Silviculture._