3. _The Increase of Trade and Commerce._ Between different communities there developed during the nineteenth century, upon the growth of better transportation, a great increase of trade and commerce, for along with the better transportation went a specialization in industry, on the part of both communities and classes. The modern city is often largely a product of modern transportation. We find all the great cities located at natural breaks in transportation. The cities of the Middle Ages were largely centers of trade and commerce where goods were distributed to various minor centers. The modern city has not lost this characteristic through developing into an industrial center. On the contrary, the status of the city in trade and commerce makes it at the same time a valuable center for the development of manufacturing industries. The break between land and water transportation is particularly favorable to the development of large cities. Thus, we find New York located where goods shipped to Europe must be transferred from land to water transportation; Chicago, located at the head of the water transportation of the Great Lakes; St. Louis, at the head of the navigation of the Mississippi River. Only Denver and Indianapolis among the great cities of the United States in 1910 are not located on a river or some other navigable water. _Minor Causes._ These are the chief social causes of the growth of cities, and, as we have seen, they are wholly industrial in their nature. Undoubtedly the modern city is a product of modern industry. Certain non-economic factors may also enter into the growth of cities, but these are of but slight importance; such are the greater intellectual and educational advantages which the city offers, the great opportunities for pleasure and amusement in the city and the like. Such minor and individual causes have had but little part in the growth of the great cities of the present. Social And Moral Conditions Of City Life.--Certain social conditions in our cities are worthy of attention in order that we may understand the effect of the city upon social and racial evolution.