The Wealth of Nations by Part 1 Chapter 10 Page 44

credit of a frugal and thriving man increases much faster than his stock. His trade is extended in proportion to the amount of both, and the sum or amount of his profits is in proportion to the extent of his trade, and his annual accumulation in proportion to the amount of his profits. It seldom happens, however, that great fortunes are made even in great towns by any one regular, established, and well-known branch of business, but in consequence of a long life of industry, frugality, and attention.

Sudden fortunes, indeed, are sometimes made in such places by what is called the trade of speculation. The speculative merchant exercises no one regular, established, or well-known branch of business. He is a corn merchant this year, and a wine merchant the next, and a sugar, tobacco, or tea merchant the year after.