While at the University of Chicago, Park continued to strengthen his theory of human ecology. Along with Ernest W. Burgess developed a program of urban research in the sociology department. They also developed a theory of urban ecology, which first appeared in their book "Introduction to the Science of Sociology" (1922). Using the city of Chicago as their they proposed that cities were environments like those found in nature. Park and Burgess suggested that cities were governed by many of the same forces of Darwinian evolution that happens in ecosystems. They felt the most significant force was competition. Competition was created by groups fighting for urban resources, like land, which led to a division of urban space into ecological niches. Within these niches people shared similar social characteristics because they were subject to the same ecological pressure.