In Europe and Asia, the first 'medium of exchange' or 'standard of value' was not gold or silver, but cattle. In other parts of the world, however, such different commodities as salt, shells, stones and coca beans were all used as 'money': They all offered advantages over the barter system, but none of them was perfect. Some were too heavy, some could not be divided into small enough parts (how much of a cow would you need to buy a piece of cheese?), and some were not permanent enough. The solution was money as we know it, that is, as coins. The invention of coins appears to have occurred almost simultaneously but quite independently in ancient Greece and in China in about 800 BC. The reason historians believe development was independent is because of the notable differences in the two systems. The first money to appear in China was made of bronze. More notably, it was not -circular, but in the shape of a knife ! The knife had a hole pierced in the handle so that it could be suspended (for example, from a string), and it generally bore an inscription. Other shapes included keys or spades, but what they had in common was the pierced hole. It was probably round 250 BC that the first Chinese money we would recognize as coins appeared, and, subsequently, the famous Ming mint produced a round coin with a square hole in it. This particular coin bore the inscription, 'Knife of Ming', but the knife itself had disappeared. It was from this coin that the famous 'cash' developed. The Chinese word, 'cash' means 'a small unit of currency'. Al though Chinese coins often have inscriptions, they virtually never had portraits, or types of any kind, until the 19th century when they were influenced by western models. It is implied in the passage that ______.