The worldwide collapse of stock prices has many victims — pension funds, insurance companies, hedge funds, financial services firms. But those are players who, if they are smart, have the wherewithal to withstand a steep sell-off. Some will even use short-sales or derivatives bets to profit on falling prices. What about the small investor, the individual who is socking away modest sums for retirement or college costs? Should small investors rush for the sidelines? Or should they view this as a buying opportunity and plough more money into the market? Neither, according to the majority of experts Knowledge at Wharton interviewed in recent days as the markets rushed downward: Wharton finance professors Jeremy Siegel , Richard Marston , Richard Herring and Franklin Allen ; Jack Bogle, founder of the Vanguard Group, the mutual fund company; and Princeton economics professor Burton J. Malkiel, author of A Random Walk Down Wall Street.…