In the early twentieth century, the science of sociology found supporters in the United States and Canada partly because the cities there were growing so rapidly. It often appeared that North American cities would be unable to absorb all the new comers arriving in such large numbers. Presociological thinkers like Frederick Law Olmsted, the founder of the movement to build parks and recreation areas in cities, and Jacob Riis, an advocate of slum reform, urged the nation's leaders to invest in improving the urban environment, building parks and beaches, and making better housing available to all. These reform efforts were greatly aided by sociologists who conducted empirical research on the social conditions in cities. In the early twentieth century, many sociologists lived in cities like Chicago that were characterized by rapid population growth and serious social problems. It seemed logical to use empirical research to construct theories about how cities grow and change in response to major social forces as well as more controlled urban planning. The founders of the Chicago school of sociology, Robert Park and Ernest Burgess, attempted to develop a dynamic model of the city, one that would account not only for the expansion of cities in terms of population and territory but also for the patterns of settlement and land use within cities. They identified several factors that influence the physical form of cities. As Park stated, among them are "transportation and communication, tramways and telephones, newspapers and advertising, steel construction and elevators – all things, in fact, which tend to bring about at once a greater mobility and a greater concentration of the urban populations." Park and Burgess based their model of urban growth on the concept of "natural areas" – that is, areas such as occupational suburbs or residential enclaves in which the population is relatively homogeneous and land is used in similar ways without deliberate planning. Park and Burgess saw urban expansion as occurring through a series of "invasions" of successive zones or areas surrounding the center of the city. For example, people from rural areas and other societies "invaded" areas where housing was inexpensive. Those areas: ended to be close to the places where they worked. In turn, people who could afford better housing and the cost of commuting "invaded" areas farther from the business district. Park and Burgess's model has come to be known as the "concentric-zone model" (represented by the figure). Because the model was originally based on studies of Chicago, its center is labeled "Loop," the term commonly applied to that city's central commercial zone. Surrounding the central zone is a "zone in transition," an area that is being invaded by business and light manufacturing. The third zone is inhabited by workers who do not want to live in the factory or business district but at the same time need to live reasonably close to where they work. The fourth or residential zone consists of upscale apartment buildings and single-family homes. And the outermost ring, outside the city limits, is the suburban or commuters' zone; its residents live within a 30- to 60-minute ride of the central business district. Studies by Park, Burgess, and other Chicago-school sociologists showed how new groups of immigrants tended to be concentrated in separate areas within inner-city zones, where sometimes experienced tension with other ethnic groups that had arrived earlier. Over time, however, each group was able to adjust to life in the city and to find a place for itself in the urban economy. Eventually many of the immigrants moved to unsegregated areas in outer zones; the areas they left behind were promptly occupied by new waves of immigrants. The Park and Burgess model of growth in zones and natural areas of the city can still be used to describe patterns of growth in cities that were built around a central business district and that continue to attract large numbers of immigrants. But this model is biased toward the commercial and industrial cities of North America, which have tended to form around business centers rather than around palaces or cathedrals, as is often the case in some other parts of the world. Moreover, it fails to account for other patterns of urbanization, such as the rapid urbanization that occurs along commercial transportation corridors and the rise of nearby satellite cities.