In the fifth, sixth, and seventh centuries science made no progress whatever. It was still taught that there were limits to the ocean. Thus Lactantius asserted that there could not be inhabitants beyond the line of the tropics. This Father of the Church considered it a monstrous opinion that the earth is round, that the heavens turn about it, and that all parts of the earth are inhabited. "There are some people," he says, "so extravagant as to persuade themselves that there are men who have their heads downwards and their feet upwards; that all that lies down here is hung up there; that the trees and herbs grow downwards; and that the snow and hail fall upwards.... Those people who maintain such opinions do so for no other purpose than to amuse themselves by disputation, and to show their spirit; otherwise it would be easy to prove by invincible argument that it is impossible for the heavens to be underneath the earth." (Divine Institution). Saint Augustin also, in his _City of God_, says: "There is no reason to believe in that fabulous hypothesis of the antipodes, that is to say, of men who inhabit the other side of the earth--where the sun rises when it sets with us, and who have their feet opposed to ours." ... "But even if it were demonstrated by any argument that the earth and world have a spherical form, it would be too absurd to pretend that any hardy voyagers, after having traversed the immensity of the ocean, had been able to reach that part of the world and there implant a detached branch of the primaeval human family." In the same strain wrote S. Basil, S. Ambrose, S. Justin Martyr, S. Chrysostom, Procopius of Gaza, Severianus, Diodorus Bishop of Tarsus, and the greater number of the thinkers of that epoch. Eusebius of Caesaraea was bold enough on one occasion to write in his Commentaries on the Psalms, that, "according to the opinion of some the earth is round;" but he draws back in another work from so rash an assertion. Even in the fifteenth century the monks of Salamanca and Alcala opposed the old arguments against the antipodes to all the theories of Columbus. [Illustration: FIG. 35.--THE EARTH'S SHADOW.] In the middle of the sixteenth century Gregory of Tours adopted also the opinion that the intertropical zone was uninhabitable, and, like other historians, he taught that the Nile came from the unknown land in the east, descended to the south, crossed the ocean which separated the antichthone from Africa, and then alone became: visible. The geographical and cosmographical ideas that were then prevalent may also be judged of by what S. Avitus, a Latin poet of the sixth century and nephew of the Emperor Flavius Avitus, says in his poem on the Creation, where he describes the terrestrial Paradise. "Beyond India," he writes, "_where the world commences_, where the confines of heaven and earth are joined, is an exalted asylum, inaccessible to mortals, and closed by eternal barriers, since the first sin was committed."