But there are plenty of other phenomena wholly or partially inexplicable. It is easy to see why Rome trended downward when great slave-tilled farms spread over what had once been a country-side of peasant proprietors, when greed and luxury and sensuality ate like acids into the fibre of the upper classes, while the mass of the citizens grew to depend not upon their own exertions, but upon the state, for their pleasures and their very livelihood. But this does not explain why the forward movement stopped at different times, so far as different matters were concerned; at one time as regards literature, at another time as regards architecture, at another time as regards city-building. There is nothing mysterious about Rome’s dissolution at the time of the barbarian invasions; apart from the impoverishment and depopulation of the Empire, its fall would be quite sufficiently explained by the mere fact that the average citizen had lost the fighting edge, an essential even under a despotism, and therefore far more essential in free, self-governing communities such as those of the English-speaking peoples of to-day. The mystery is rather that out of the chaos and corruption of Roman society during the last days of the oligarchic republic, there should have sprung an Empire able to hold things with reasonable steadiness for three or four centuries. But why, for instance, should the higher kinds of literary productiveness have ceased about the beginning of the second century, whereas the following centuries witnessed a great outbreak of energy in the shape of city-building in the provinces, not only in Western Europe, but in Africa? We cannot even guess why the springs of one kind of energy dried up, while there was yet no cessation of another kind.