The mystery, then, is, what were the sources of _Physica_? Some have fancied that Hildegard in preparing this made use of the writings not only of Pliny and Virgil, but also of those of Macer, Constantinus Africanus, Walafrid Strabo, Isodore of Seville, and other writers who were in great vogue during the Middle Ages. The general consensus of opinion, however, of those who have carefully studied this interesting problem is that the gentle nun was not acquainted with any of the authors named, except, possibly, Isodore of Seville, whose works were all held in high esteem, especially during the period of Hildegard's greatest literary activity. Hildegard's _Physica_ has a special value for philologists, as well as for students of natural history, for it contains the German names of plants still used by the people of the Fatherland seven hundred years after they were penned by the painstaking abbess of St. Rupert's.[164] Referring to the Saint's work entitled _De Natura Hominis, Elementorum, Diversarumque Creaturarum_--a treatise on the nature of man, the elements and divers created things--no less an authority than Dr. Charles Daremberg declares that it will always hold an important place in the history of medical art and of inanimate and animate nature--_insignis semper locus debetitur in artis medicae rerumque naturalium historia_.[165] He even goes further and affirms that Hildegard was familiar with numerous facts of science regarding which other mediaeval writers were entirely ignorant. More than this. She was acquainted with many of nature's secrets which were unknown to men of science until recent times, and which, on being disclosed by modern researches, have been proclaimed to the world as new discoveries.[166] One reason why St. Hildegard's writings on botany, zoology and mineralogy are not better known is that few students care to make the effort to master her voluminous works. They require long and assiduous study and a knowledge of her peculiarities of style and expression which is acquired only after patient and persistent labor. But the labor is not in vain, as is evidenced by the numerous monographs which have appeared in recent years, especially in Germany, on the scientific works of this marvelous nun of the twelfth century. All things considered, the Abbess of Bingen may be said to hold the same position in the natural sciences of her time as was held in the physical and mathematical sciences seven hundred years earlier by the illustrious Hypatia of Alexandria.