A NEW WAVE : arrives in Castro's Cuba THE VISIT of Soviet leader Mikhail Gorbachev to Cuba this week, and the attention it has focused on Soviet-Cuban relations, provokes speculation about the future of communism in the Third World. If perestroika is truly the wave of the communist world's future, Cuba is set to be sucked down in the undertow, with its leader Fidel Castro struggling all the way to the bottom unless it radically changes its policies. Although the Cuban government has sought to minimize differences with the p , I I -I 1 11 I II W . ..r-r Soviet President Mikhail Gorbachev and Cuban President Fidel Castro wave from a motorcade in Havana Sunday night. '4 . AP Gorbachev on a tide of reform . . . Soviets, the new openness of Mr. Gorbachev's government has not been well received in Havana. In the eyes of Mr. Castro and some other Third World communist leaders, such reforms seem unworkable when many basic needs remain unmet even in Cuba, where strides in health care and education have been made. But as the Soviet Union seeks to become more competitive with the West, it has been forced to change holding last month's extraordinary elections, encouraging independent agriculture. In the process, the gap has widened between the USSR and its Third World partners who remain mired in the rhetoric and practices of outdated revolutionary communism. In the years to come, the Soviet Union may find it more difficult to straddle the fence between revolution and economic development. It may start to question, for example, its current annual $4.5-billion commitment to keep the Cuban economy moving. How long will the Soviets continue spending at that level to support a government that does not share its view of communism in the 21st Century? It is hard to imagine that there will not be pressure, however subtle, for Cuba to change. How remarkable it would be if democratic reforms in Cuba were to come as a result of Soviet, rather than American, instigation.