But, fortunately for mankind, the neat rents of the land, under a system of private property, can never be diminished by the progress of cultivation. - Thomas Robert Malthus

It is an acknowledged truth in philosophy that a just theory will always be confirmed by experiment.

Every exchange which takes place in a country, effects a distribution of its produce better adapted to the wants of society....
If two districts, one of which possessed a rich copper mine, and the other a rich tin mine, had always been separated by an impassable river or mountain, there can be no doubt that an opening of a communication, a greater demand would take place, and a greater price be given for both the tin and the copper;and this greater price of both metals, though it might be only temporary, would alone go a great way towards furnishing the additional capital wanted to supply the additional demand; and the capitals of both districts, and the products of both mines, would be increased both in quantity and value to a degree which could not have taken place without the this new distribution of the produce, or some equivalent to it.

It is also very important to observe, that menial servants are absolutely necessary to make the resources of the higher and middle classes of society efficient in the demand for material products.