Halliday (2008: 164):
There is no denying that writing is parasitic on speech; or perhaps we should say epiphytic, since presumably it does not cause injury to its host. But once writing has evolved to the point where anything in the language can be written down, it adds a new dimension to the meaning potential. There is no point in arguing about which can develop more power; each has its own mechanisms and its own strategies, related to their complementary ways of achieving and managing complexity. But they generate power of rather different kinds.