The occupation of the city people was chiefly that of manufacturing, trading and grafting on the farmer and on each other. The invention of steam power made it economical to assemble workmen into large factories which added another cause to the growth of cities. The use of this steam power forced the city worker out of his home and into the more economical factory, thus developing the factory system. The continual growth of cities soon filled the land with large groups of houses, crowding each other for room. As the houses were built closer and closer together, the amount of light and air was shut out, in order that the distance the workers lived from their work might not increase. At first workers went from their work to their houses on foot, later by means of the horse car, still later by steam car and now the electric car is supreme. As these transportation facilities first used to get provisions into the city and the manufactured product out of it were utilized to get the workers to and from their work, the houses began to follow the transportation lines. _Transportation Determines the Form of Cities._ As time and the expense of transportation rather than distance were the elements that governed the distance from the heart of the city that could be used for workers’ homes, the utilization of fixed lines of traffic resulted in the city building out along main streets, trolleys and railroads. Along main lines of traffic, as between two important cities, the population began to group itself into lines. This is the state of the distribution of population to be found in the world to-day. But the present distribution is imperfect. The trolleys carry people to the street corner but make no provision for getting them into their homes or for getting the meals on the sideboard, the book from the library to the center table, or the camphor from the drug store to the sick room. The means of conveying the necessities of civilization is almost wholly that of rails, pipes and wires. The former is the means of transporting people and parcels, the second of liquids and gases, the third of electricity in its various forms.