"Moreover, there is an unearned increment on capital and on labor, due to the presence, around the capitalist and the laborer, of a great, industrious, and prosperous society"

This quote by William Graham Sumner is describing the idea of an "unearned increment" which is an advantage that is acquired with no effort or expense. Sumner is recommending that both capital and labor gain from the existence of a flourishing society. This indicates that the presence of a successful and productive society can lead to an increase in the worth of capital and labor, without any additional effort or cost. This boost in value is an unearned increment, and it is because of the existence of a successful society. Sumner is suggesting that this unearned increment is beneficial to both capital and labor, and it is an outcome of the existence of a thriving society.