theory of external economy造句

Location theory, that had been separately developed in Germany and North America in the early 20th century, and the theory of external economies from " localized industries " ( as described in Alfred Marshall's " Principles of Economics " ( 1890 ) ) formed the theoretical basis of regional economics, which has played a central role in regional science.
It's difficult to find theory of external economy in a sentence. 用theory of external economy造句挺难的