The next step brings us to the upper stage of barbarism, that period in which all nations of civilization go through their heroic era. It is the time of the iron sword, but also of the iron plow share and axe. The iron had become the servant of man. It is the last and most important of all raw products that play a revolutionary role in history; the last--if we except the potato. Iron brought about agriculture on a larger scale and the clearing of extensive forest tracts for cultivation. It gave to the craftsman a tool of such hardness and sharpness that no stone, no other known metal, could withstand it. All this came about gradually. The first iron was often softer than bronze. Therefore stone implements disappeared very slowly. Not only in the Hildebrand Song, but also at Hastings in 1066, stone axes were still used in fighting. But progress was now irresistible, less interrupted and more rapid. The town, inclosing houses of stone or tiles within its turreted and crested stone walls, became the central seat of the tribe or federation of tribes. It showed an astounding progress of architecture, but also an increase of danger and of the demand for protection. Wealth increased rapidly, but it was the wealth of private individuals. Weaving, metal work and other more and more differentiating industries developed an increasing variety and display of art in production. Agriculture furnished not alone grain, peas, beans and fruit, but also oil and wine, the preparation of which had now been learned. Such a diversity of action could not be displayed by any single individual. The second great division of labor took place: handicrafts separated from agriculture. The growing intensity of production and the increased productivity enhanced the value of human labor power. Slavery, which had been a rising and sporadic factor in the preceding stage, now became an essential part of the social system. The slaves ceased to be simple assistants. They were now driven in scores to the work in the fields and shops. The division of production into two great branches, agriculture and handicrafts, gave rise to production for exchange, the production of commodities. Trade arose at the same time, not only in the interior and on the tribal boundaries, but also in the form of maritime exchange. All this was as yet in a very undeveloped state. The precious metals gained preference as a universal money commodity, but still uncoined and exchanged merely by dead weight.