Schumpeter's Social Molecules are Families

Joseph Schumpeter, in Capitalism, Socialism, and Democracy, offers a rich idea of both social mobility and the place of family lineages in social structure:

it can be shown in all cases, first, that human molecules rise and fall within the class into which they are born, in a manner which fits the hypothesis that they do so because of their relative aptitudes; and it can also be shown, second, that they rise and fall across the boundary lines of their class in the same manner. This rise and fall into higher and lower classes as a rule takes more than one generation. These molecules are therefore families rather than individuals.