The ancient city was primarily a fortress, a place of refuge in time of war. The modern city, on the contrary, is primarily a convenience of commerce, and owes its existence to the market place around which it sprang up. Industrial competition and the division of labor, which have probably done most to develop the latent powers of mankind, are possible only upon condition of the existence of markets, of money, and other devices for the facilitation of trade and commerce. An old German adage declares that “city air makes men free” (_Stadt Luft macht frei_). This is doubtless a reference to the days when the free cities of Germany enjoyed the patronage of the emperor, and laws made the fugitive serf a free man if he succeeded for a year and a day in breathing city air. Law, of itself, could not, however, have made the craftsman free. An open market in which he might sell the products of his labor was a necessary incident of his freedom, and it was the application of the money economy to the relations of master and man that completed the emancipation of the serf. _Vocational classes and vocational types._—The old adage which describes the city as the natural environment of the free man still holds so far as the individual man finds in the chances, the diversity of interests and tasks, and in the vast unconscious co-operation of city life the opportunity to choose his own vocation and develop his peculiar individual talents. The city offers a market for the special talents of individual men. Personal competition tends to select for each special task the individual who is best suited to perform it. The difference of natural talents in different men is, in reality, much less than we are aware of; and the very different genius which appears to distinguish men of different professions, when grown up to maturity, is not upon many occasions so much the cause, as the effect of the division of labour. The difference between the most dissimilar characters, between a philosopher and a common street porter, for example, seems to arise not so much from nature, as from habit, custom, and education. When they came into the world, and for the first six or eight years of their existence, they were perhaps very much alike, and neither their parents nor playfellows could perceive any remarkable difference. About that age, or soon after, they come to be employed in different occupations. The difference of talents comes then to be taken notice of, and widens by degrees, till at last the vanity of the philosopher is willing to acknowledge scarce any resemblance. But without the disposition to truck, barter, and exchange, every man must have procured to himself every necessary and conveniency of life which he wanted. All must have had the same duties to perform, and the same work to do, and there could have been no such difference of employment as could alone give occasion to any great difference of talent....