A science fiction author, Robert Heinlein, famously stated that “every generation thinks it invented sex, and every generation is wrong.” Something similar could be said of “The Invasion of the Body Snatchers” (1956). Director Don Siegel’s sci-fi parable about conformity and paranoia has seen three remakes — there is, apparently, another in development — and its tendrils have spread into many byways of popular culture. Every passing decade, it seems, needs to redefine body-snatching, but none have been as good as the original.