Growth of population pressure is postulated to be a major determinant of human social evolution through the mechanism of competition for increasingly scarce subsistence resources. In terms of a model for societies practicing agriculture, inter- and intra-group competition for such natural resources is seen as leading to the evolution of more competitively successful cooperative units in descent (in classless societies) and in political structure, and to the evolution of class stratification. A scale for measuring population pressure in agricultural societies is proposed, and the prediction is made that a positive correlation should occur between the postulated population pressure scale and social forms in accordance with the theory. Tests are undertaken, in view of a reappraisal of Galton's problem, using all societies for which appropriate data are adequate in a universe of 1170 cultures. The results of the tests, presented as scatterplots, are consistent with the predictions based upon the theoretical model. The resource scarcity-competition model is successful in predicting evolution in descent as well as in political organization and class stratification.