Where, in any prosecution for an offence against this Act, it is established that a person is in possession, custody or control of any [wild animal, captive animal], animal article, meat, trophy, uncured trophy, specified plant, or part or derivative thereof it shall be presumed, until the contrary is proved, the burden of proving which shall lie on the accused, that such person is in unlawful possession, custody or control of such [wild animal, captive animal], animal article, meat trophy, uncured trophy, specified plant, or part or derivative thereof [or scheduled specimen].