Inversion has always been easy to trace in Germany. Ammianus Marcellinus bears witness to its prevalence among some German tribes in later Roman days.[75] In mediæval times, as Schultz points out, references to sodomy in Germany were far from uncommon. Various princes of the German Imperial house, and of other princely families in the Middle Ages, were noted for their intimate friendships. At a later date, attention has frequently been called to the extreme emotional warmth which has often marked German friendship, even when there has been no suspicion of any true homosexual relationship.[76] The eighteenth century, in the full enjoyment of that abandonment to sentiment initiated by Rousseau, proved peculiarly favorable to the expansion of the tendency to sentimental friendship. On this basis a really inverted tendency, when it existed, could easily come to the surface and find expression. We find this well illustrated in the poet Heinrich von Kleist who seems to have been of bisexual temperament, and his feelings for the girl he wished to marry were, indeed, much cooler than those for his friend. To this friend, Ernst von Pfuël (afterward Prussian war minister), Kleist wrote in 1805 at the age of 28: "You bring the days of the Greeks back to me; I could sleep with you, dear youth, my whole soul so embraces you. When you used to bathe in the Lake of Thun I would gaze with the real feelings of a girl at your beautiful body. It would serve an artist to study from." There follows an enthusiastic account of his friend's beauty and of the Greek "idea of the love of youths," and Kleist concludes: "Go with me to Anspach, and let us enjoy the sweets of friendship.... I shall never marry; you must be wife and children to me."[77] In all social classes and in all fields of activity, Germany during the nineteenth century produced a long series of famous or notorious homosexual persons. At the one end we find people of the highest intellectual distinction, such as Alexander von Humboldt, whom Näcke, a cautious investigator, stated that he had good ground for regarding as an invert.[78] At the other end we find prosperous commercial and manufacturing people who leave Germany to find solace in the free and congenial homosexual atmosphere of Capri; of these F.A. Krupp, the head of the famous Essen factory, may be regarded as the type.[79]