HISTORY IS BEING REMADE ON A REGUlar basis lately at the New York Stock Exchange. Last year the exchange ended more than two centuries as the ultimate members-only club, merging with electronic exchange operator Archipelago Holdings and becoming a public company. Then it created the first transatlantic market by acquiring Paris-based Euronext.

This past summer a different kind of history was recorded at the Big Board: Sometime in June the percentage of daily trading volume in NYSE-listed stocks that was executed by the exchange dipped below 50 percent. For the first time in its storied history, the institution that had for so long been at the epicenter of global capitalism no longer controlled its own market. The situation has only deteriorated further. In August the exchange executed just 45.8 percent of the volume in NYSE-listed stocks. It's an extraordinary turn of events, considering that the Big Board's market share floated above 90 percent for much of the 1990s and...