The technique of cultivation underwent some significant changes. The animal-drawn plough seems to have been invented during this period, and from now on, some metal agricultural implements like iron sickles and iron plough-shares became more common. A fallow system was introduced so that cultivation became more intensive. Manuring of fields was already known in Shang time. It seems that the consumption of meat decreased from this period on: less mutton and beef were eaten. Pig and dog became the main sources of meat, and higher consumption of beans made up for the loss of proteins. All this indicates a strong population increase. We have no statistics for this period, but by 400 B.C. it is conceivable that the population under the control of the various individual states comprised something around twenty-five millions. The eastern plains emerge more and more as centres of production. The increased use of metal and the invention of coins greatly stimulated trade. Iron which now became quite common, was produced mainly in Shansi, other metals in South China. But what were the traders to do with their profits? Even later in China, and almost down to recent times, it was never possible to hoard large quantities of money. Normally the money was of copper, and a considerable capital in the form of copper coin took up a good deal of room and was not easy to conceal. If anyone had much money, everyone in his village knew it. No one dared to hoard to any extent for fear of attracting bandits and creating lasting insecurity. On the other hand the merchants wanted to attain the standard of living which the nobles, the landowners, used to have. Thus they began to invest their money in land. This was all the easier for them since it often happened that one of the lesser nobles or a peasant fell deeply into debt to a merchant and found himself compelled to give up his land in payment of the debt. Soon the merchants took over another function. So long as there had been many small feudal states, and the feudal lords had created lesser lords with small fiefs, it had been a simple matter for the taxes to be collected, in the form of grain, from the peasants through the agents of the lesser lords. Now that there were only a few great states in existence, the old system was no longer effectual. This gave the merchants their opportunity. The rulers of the various states entrusted the merchants with the collection of taxes, and this had great advantages for the ruler: he could obtain part of the taxes at once, as the merchant usually had grain in stock, or was himself a landowner and could make advances at any time. Through having to pay the taxes to the merchant, the village population became dependent on him. Thus the merchants developed into the first administrative officials in the provinces.