In classical and Marxian political economy economic advancement takes place in stages of development. These stages are characterized by different functional distributions of income. In this paper we show that the Cobb-Douglas Production Function is the only production function which has the property of a constant functional distribution of income of the factors of production. Furthermore it is argued that an aggregate production function is not appropriate to analysing macro-economic processes of a capitalist economy. But it is a very useful tool in the hands of a socialist planning authority.