The most obvious way in which progress has been promoted was that the conquering invaders became a leisured aristocracy, having their material needs supplied by the labour of the indigenous population, which became a more or less servile class. All the ancient civilisations were thus founded upon servile labour. We may be sure that, until such a social system resulted from conquest, no people made much progress; because all individuals were fully occupied in securing their means of subsistence, either by warfare, by the tending of herds, or by agriculture. Each people was self-supporting, and knew no or few needs beyond those which their own labour was able to supply; and labour was individual, or was co-operative only among small groups, such as the communal family groups. It could, therefore, undertake no great works, whether of building or engineering, such as large public buildings, irrigation, or road making. Each family consumed what it produced, and consequently there was no large accumulation of capital; for there were no motives for storing up their primitive wealth, and generally no wealth of durable and storable form. But, as soon as a ruling class could dispose of the labour of a large part of the population, making them work for a mere subsistence wage, there was initiated that régime of capital and labour on which, up to the present time, all civilisation has been founded. Wealth was accumulated; great works, such as the pyramids, demanding enormous expenditure of human life and work, could be undertaken; and a leisured class was created, which, being freed from the necessity of bodily toil, was able to turn its energy to speculative inquiry, to the enjoyment of art and luxury, to directing and organising the labour of the multitude, to inventing the tools that render labour more effective, to studying natural phenomena such as the cycle of the seasons, a more accurate knowledge of which added to the productivity of labour; for it was in the service of wealth production, that in the main science arose, especially mathematics and mechanical and astronomical science, arithmetic and geometry through the need of a practical art of measurement, astronomical science through the need of foreseeing the seasons. The desire to enjoy art and luxury is one which feeds itself and grows, when once aroused; and it was these growing desires of the leisured and wealthy classes which created trade, or at any rate first developed it beyond the merest rudiments; and in doing so led to regular and friendly intercourse between nations.