(9) Thus, while an empire with a developed civilisation may communicate it to uncivilised conquered peoples not too far below its own anthropological level, the secondary civilisation thus acquired is in its nature less "viable," less capable of independent evolution, than one set up by the free commerce of trading peoples. The most rapid growths of civilisation appear always to have occurred by way of the multiplying of free contacts among trading communities, and among the free colonies of such.[140] The "money economy" they introduced was a great instrument of social and industrial evolution;[141] and on such city civilisations the ancient empires themselves seem always to have proceeded.[142] (10) Every phase of civilisation has its special drawbacks, so that great retrogression may follow on great development, especially when adventitious sources of wealth are the foundation of a luxurious culture. In some cases a great development may be dependent on an exhaustible source of wealth, as in the case of Britain's coal supply, the empire of ancient Rome, the primacy of the Pope before the Reformation, or even the Periclean empire of Athens, and the trade monopolies of Venice, the Hansa Towns, and the Dutch Republic. (11) The expression "decay" as applied to a people, however, has only a relative significance: used absolutely, it stands for a delusion. Economic conditions may worsen, and military power decline; but such processes imply no physiological degeneration. All the "dead" civilisations of the past were _overthrown or absorbed by military violence_; and there is no known case of a nation physically well placed dying out.