The goals of the Caltech Population Program are to increase understanding of the interrelationships between population growth and socioeconomic and cultural patterns throughout the world and to communicate this understanding. This series of occasional papers is one step in the process of communicating research results. The papers deal primarily with problems of population growth and the interaction of population change with such variables as resources, food supply, environment, urbanization, employment, economic development, and social and cultural values. Paper Number 2 categorizes five types of environmental problems; four of the categories are of a direct nature, while the fifth is indirect. The author feels that most attention is paid to acute problems in the direct categories, while chronic indirect problems may prove to be the most serious of all. These indirect effects on human welfare act through interference with services provided for society by natural biological systems. The paper describes man's force in the ecological system both historically and for the present and future. (LS)