Every improvement in agricultural industry instead of keeping men on the farm has tended to drive them from it. Scientific agriculture carried on with modern machinery necessarily lessens the need of a great proportion of the population being employed to produce the foodstuff and other raw materials which the world needs. Hence it has tended to free men from the soil and to make it possible for a larger and larger number to go to the city. Therefore the relatively diminishing importance of agriculture has been one of the prime causes of the growth of the cities in the nineteenth century; and so far as we can see this cause will continue to operate for some time to come. 2. _The Growth and Centralization of Manufacturing Industries._ This is perhaps the most vital cause of the growth of cities. The great city, as we have already said, is very largely the product of modern industrialism. Improved machinery, improved transportation, and enlarged markets, together with the increased wants of men, not only have made possible a great growth of manufacturing industries, but also these same factors have tended to centralize manufacturing industries in the cities. Let us note briefly why it is that manufacturing industries are grouped together in great cities rather than scattered throughout the rural communities. In centralizing manufacturing plants in cities, certain industrial economies are secured, such as: (1) economy in motor power, whether it be water or coal; (2) economy in machinery--it is not necessary to duplicate machines; (3) economy in wages--one superintendent, for example, can oversee a large plant; (4) utilization of by-products--when many factories are grouped together by-products, which are sometimes more valuable than the main products, can be better utilized. (5) There is economy in buying raw material and in selling finished products when many factories are grouped together. For all these reasons, along with the further reason that those who labor in factories must live close to them, manufacturing has been a prime cause of the modern city, and, so far as we can see, will continue to further urbanize our population in the future.