SINCE the beginning of this century economists have discussed business cycle and foreign trade, money and banking policy a great deal, but the centre of the social sciences, as Sir William Beveridge reminds us, is going to be the problem of population. There are three aspects of population theory which should be clearly distinguished. The first is the bioecological aspect which deals with the regional distribution and capacity of population in different countries and in larger areas; it also investigates the essential man-land balance which sets the limits of growth, permanence, and welfare of the population. The demographic aspect is concerned with the structure and variation of population; with age and sex composition; and with birth, marriage, and death rates, as well as differential trends in social classes and occupations and their effects on the numbers and quality of the population. The socio-economic aspects deal with the effects of social and economic factors and institutions, of custom, law, and political and social attitudes and sentiments on the trend of population and vice versa; with the relations between variations in population and standards of living and welfare; and with the measures of policy for regulation of the trend of population by each nation and of international population movements in the interests of world peace and progress. The above aspects have not been all fully explored nor has there been any appreciable effort to coordinate the divergent lines of inquiry. The lack of such coordination which is responsible for much barrenness of population analysis is promoted not only by inadequate development and dearth of data in scientific, vital, and social statistics but also by the watertight division of the sciences-ecology, biology, genetics, geography, economics, politics, and sociology-which ought to focus their separate and collective attention on the regional, the economic, the political, and the institutional complex within which the population movement occurs. Without an integration of the different fields of research a conmprehensive and well-rounded theory of population cannot be fashioned. Indeed, this can develop mainly through the analysis of the interrelations between ecologic, economic, political, and institutional or social-psychological factors and movements implicated in the equilibrium of population on its three levels, biological, economic, and ethical. At the bio-ecologic level the elements of the population movement which human