At the beginning of each book Pliny is careful to give the names of the authors that he has consulted for it.[6] As the subjects that he treats of are very varied the total list of authorities is very large. Some of the names, such as Virgil, Archimedes, and others, are those of men still held in reverence; while many are naturally now but little known, their works having perished. As an illustration of the thoroughness of Pliny in the matter we will give an illustrative list—that which precedes his eighth book, dealing with land animals. He divides his lists always into two sections, and commences with the authors of his own country. These in this particular instance are Mutianus, Procilius, Verrius Flaccus, L. Piso, Cornelius Valerianus, Cato the Censor, Fenestella, Trogus, Actius, Columella, Virgil, Varro, Metellus Scipio, Cornelius Celsus, Nigidius, Trebius Niger, Pomponius Mela, and Manlius Sura. His foreign authorities are considerably more numerous, and are, naturally, most of them Greek writers: Polybius, Onesicritus, Isidorus, Antipater, Aristotle, Demetrius, Democritus, Theophrastus, Euanthes, Hiero, Duris, Ctesias, Philistus, Architas, Philarchus, Amphilochus the Athenian, Anaxipolis the Thasian, Apollodorus of Lemnos, Aristophanes the Milesian, Antigonus the Cymæan, and twenty-three others, whom it is needless to add to the list, as it is already quite long enough to illustrate the care with which Pliny fortified his own knowledge with the best aid that he could procure. Though, doubtless, in some cases, the bearers of these names were travellers and others who contributed but one or two items to the store of knowledge, the greater portion of the names are those of men who, to the best of their ability, were endeavouring to penetrate the secrets of nature. It is a striking fact that at this early period there should be such a body of scientific opinion to draw upon. Pliny tells us that he has dealt with twenty thousand subjects and that this has necessitated the perusal of over two thousand books.