In Mr. Andreessen’s view, there shouldn’t be 50 Silicon Valleys. Instead, there should be 50 different kinds of Silicon Valley. For example, there could be Biotech Valley, a Stem Cell Valley, a 3-D Printing Valley or a Drone Valley. As he noted, there are huge regulatory hurdles in many of these fields. If a city wanted to spur innovation around drones, for instance, it might have to remove any local legal barriers to flying unmanned aircraft.