As progress developed and the communities at a greater distance became acquainted with one another's wants and the various kinds of goods that certain districts supplied, this tendency to specialization and consequent exchange of goods would grow in an ever-widening circle. Instead of the tribe being a commercial unity, the zone in which the interchange of goods went on would widen as far as the geographical and other boundaries allowed it. In the same country one district would be found to be specially well adapted for agriculture, and another for pasture; another, being well supplied with metals, would naturally provide a race of smiths and producers of rough tools for industry, and the exchange of commodities between districts with these various capacities would mean that the specialization of production would go steadily further, and that a whole town or village would be found in which the great majority of the inhabitants were at work upon one particular form of industry, relying for the other kinds of commodities that they required upon the activity of a similar community living in the next valley or on the other side of the river. This widening-out process would naturally extend itself over the borders of different countries. Obstacles to this process would be found in the differences of language and probably in the difficulties of transport. On the other hand, it would be greatly stimulated by the different ideas of value that prevailed in different communities. Value depends upon the extent to which anybody wants a thing, also on what he thinks it is worth, that is to say, the number of commodities in his possession with which he is prepared to part in order to secure it. Obviously commodities coming in from foreign countries, and being unknown or rare in the country in which they are offered, if they are otherwise at all attractive, possess a certain amount of what is called scarcity value, which makes them easily saleable by adventurous merchants who arrive with the cargo.