These different sorts of rude produce may be divided into three classes. The first comprehends those which it is scarce in the power of human industry to multiply at all. The second, those which it can multiply in proportion to the demand. The third, those in which the efficacy of industry is either limited or uncertain. In the progress of wealth and improvement, the real price of the first may rise to any degree of extravagance, and seems not to be limited by any certain boundary. That of the second, though it may rise greatly, has, however, a certain boundary beyond which it cannot well pass for any considerable time together. That of the third, though its natural tendency is to rise in the progress of improvement, yet in the same degree of improvement it may sometimes happen even to fall, sometimes to continue the same, and sometimes to rise more or less, according as different accidents render the efforts of human industry, in multiplying this sort of rude produce, more or less successful.