Editor's Note: Tom Philpott's April 28 piece on the swine flu pandemic, which raised the question of whether there is a link between the virus' emergence in Mexico and the presence nearby of factory-scale pork farms, sparked a vigorous debate on the Society for Environmental Journalists listserv. Merritt Clifton was one of several writers to take issue with Tom's piece. At Grist's invitation, he put his critique into an essay form, which is posted below: -- Thirty years ago this month I knelt beside the Yamaska River in southern Quebec with a test kit -- downstream from several of the …