The most obvious suggestion is that they were cut by streams when the oceanic slopes were above water.
Most geologists doubt it.
Alternatively, the continents and ocean basins might have undergone vast shifting movements that exposed the margins to river erosion. The implications of these ideas may lead to radical changes in supposedly well-established geological concepts. DDT has now made it possible to control the disease. WHO teams have been in Greece for a year, battling malaria with DDT and synthetic antimalarials. Demonstration units have just arrived in Indo-China and Siam.