It would also be interesting to discover in what way the old Egyptians came to choose the vulture as a symbol of motherhood. As a matter of fact the religion and culture of Egyptians were subjects of scientific interest even to the Greeks and Romans, and long before we ourselves were able to read the Egyptian monuments we had at our disposal some communications about them from preserved works of classical antiquity. Some of these writings belonged to familiar authors like Strabo, Plutarch, Aminianus Marcellus, and some bear unfamiliar names and are uncertain as to origin and time, like the hieroglyphica of Horapollo Nilus, and like the traditional book of oriental priestly wisdom bearing the godly name Hermes Trismegistos. From these sources we learn that the vulture was a symbol of motherhood because it was thought that this species of birds had only female vultures and no males.[30] The natural history of the ancients shows a counterpart to this limitation among the scarebæus beetles which were revered by the Egyptians as godly, no females were supposed to exist.[31] But how does impregnation take place in vultures if only females exist? This is fully answered in a passage of Horapollo.[32] At a certain time these birds stop in the midst of their flight, open their vagina and are impregnated by the wind. Unexpectedly we have now reached a point where we can take something as quite probable which only shortly before we had to reject as absurd. It is quite possible that Leonardo was well acquainted with the scientific fable, according to which the Egyptians represented the idea of mother with the picture of the vulture. He was an omnivorous reader whose interest comprised all spheres of literature and knowledge. In the Codex Atlanticus we find an index of all books which he possessed at a certain time,[33] as well as numerous notices about other books which he borrowed from friends, and according to the excerpts which Fr. Richter[34] compiled from his drawings we can hardly overestimate the extent of his reading. Among these books there was no lack of older as well as contemporary works treating of natural history. All these books were already in print at that time, and it so happens that Milan was the principal place of the young art of book printing in Italy. When we proceed further we come upon a communication which may raise to a certainty the probability that Leonardo knew the vulture fable. The erudite editor and commentator of Horapollo remarked in connection with the text (p. 172) cited before: _Caeterum hanc fabulam de vulturibus cupide amplexi sunt Patres Ecclesiastici, ut ita argumento ex rerum natura petito refutarent eos, qui Virginis partum negabant; itaque apud omnes fere hujus rei mentio occurit._