In ancient as in later times, doubtless, attempts were made to bring back to human habitation the stricken deserts that stained Italy like a leprosy. Thus Cæsar sought early to repeople Campania from the idle populace of Rome.[217] But to maintain steady cultivation in unhealthy regions there was needed an immune stock, and that was reproducible only by the old way of savage, self-preserving persistence on the part of hardy and primitive rustics working their own land. The new imported stocks, slave or free, wilted away before the scourge of fever; and the "principle of population," weakened in the spring, failed to surmount the resistance of Nature. Under the early Empire the labour needed for the culture of the Campagna had to be brought in annually from distant districts; and when the invading Goths in the fifth century devastated the whole area there was no energy left to recover it. [The theories once current as to ancient knowledge of prophylactics in the shape of perfumes and the habitual use of woollen clothing may be dismissed as fanciful. The rational conclusion is that the early races developed a relative immunity, which was possessed neither by the eastern stocks imported in the period of conquest nor by the later invading Teutons. It is noteworthy, however, that at all times the dwellers in the tainted areas learned something of the necessary hygiene. See Dureau de la Malle, as cited. His investigation is interesting as showing how, in the early decades of the nineteenth century, long before Pasteur, biology had reached the perception that fevers come of an organic infection. It was doubtless such knowledge that led the Romans to burn their dead.]